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  • 1.
    Aissaoui, Nesrine
    et al.
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg, Sweden.
    Moth-Poulsen, Kasper
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg, Sweden.
    Käll, Mikael
    Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Johansson, Peter
    Örebro University, School of Science and Technology.
    Wilhelmsson, L. Marcus
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg, Sweden.
    Albinsson, Bo
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg, Sweden.
    FRET enhancement close to gold nanoparticles positioned in DNA origami constructs2017In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 9, no 2, 673-683 p.Article in journal (Refereed)
    Abstract [en]

    Here we investigate the energy transfer rates of a Förster resonance energy transfer (FRET) pair positioned in close proximity to a 5 nm gold nanoparticle (AuNP) on a DNA origami construct. We study the distance dependence of the FRET rate by varying the location of the donor molecule, D, relative to the AuNP while maintaining a fixed location of the acceptor molecule, A. The presence of the AuNP induces an alteration in the spontaneous emission of the donor (including radiative and non-radiative rates) which is strongly dependent on the distance between the donor and AuNP surface. Simultaneously, the energy transfer rates are enhanced at shorter D-A (and D-AuNP) distances. Overall, in addition to the direct influence of the acceptor and AuNP on the donor decay there is also a significant increase in decay rate not explained by the sum of the two interactions. This leads to enhanced energy transfer between donor and acceptor in the presence of a 5 nm AuNP. We also demonstrate that the transfer rate in the three "particle" geometry (D + A + AuNP) depends approximately linearly on the transfer rate in the donor-AuNP system, suggesting the possibility to control FRET process with electric field induced by 5 nm AuNPs close to the donor fluorophore. It is concluded that DNA origami is a very versatile platform for studying interactions between molecules and plasmonic nanoparticles in general and FRET enhancement in particular.

  • 2. Alegret, Joan
    et al.
    Johansson, Peter
    Örebro University, School of Science and Technology.
    Käll, Mikael
    Green’s tensor calculations of plasmon resonances of single holes and hole pairs in thin gold films2008In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 10, no 105004Article in journal (Refereed)
    Abstract [en]

    We present numerical calculations of the plasmon properties of single-hole and hole-pair structures in optically thin gold films obtained with the Green’s tensor formalism for stratified media. The method can be used to obtain the optical properties of a given hole system, without problems associated with the truncation of the infinite metal film. The calculations are compared with previously published experimental data and an excellent agreement is found. In particular, the calculations are shown to reproduce the evolution of the hole plasmon resonance spectrum as a function of hole diameter, film thickness and hole separation.

  • 3. Alegret, Joan
    et al.
    Käll, Mikael
    Johansson, Peter
    Örebro University, Department of Natural Sciences.
    Top-down extended meshing algorithm and its applications to Green's tensor nano-optics calculations2007In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 75, no 4Article in journal (Refereed)
    Abstract [en]

    We present a computational algorithm which speeds up Green's tensor nano-optics calculations by means of optimizing the mesh that represents the system we want to investigate. The algorithm automates the process of creating a variable-size mesh that describes an arbitrary nanostructure. The total number of elements of this mesh is smaller than that of a regular mesh representing the same structure, and thus the Green's tensor calculations can be performed faster. Precision, however, is kept at a similar level than for the regular mesh. Typically, the algorithm yields a mesh that speeds up Green's tensor calculations by a factor of 4, while giving a maximum error in the field magnitude of about 5%. The speed-up factor makes it very suitable for otherwise lengthy calculations, and the error should be acceptable for most applications.

  • 4.
    Alhulaimi, Bassemah
    et al.
    Dalhousie University, Halifax, Canada .
    Coley, Alan
    Dalhousie University, Halifax, Canada .
    Sandin, Patrik
    Dalhousie University, Halifax, Canada; Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Potsdam, Germany.
    Anisotropic Einstein-aether cosmological models2013In: Journal of Mathematical Physics, ISSN 0022-2488, E-ISSN 1089-7658, Vol. 54, no 4Article in journal (Refereed)
    Abstract [en]

    We investigate a class of spatially anisotropic cosmological models in Einstein-aether theory with a scalar field in which the self-interaction potential depends on the timelike aether vector field through the expansion and shear scalars. We derive the evolution equations in terms of expansion-normalized variables, which reduce to a dynamical system. We study the local stability of the equilibrium points of the dynamical system corresponding to physically realistic solutions, and find that there are always ranges of values of the parameters of the models for which there exists an inflationary attractor. © 2013 AIP Publishing LLC.

  • 5.
    Allasia, D
    et al.
    Università di Torino, INFN, Turin, Italy.
    Lettenström, Frans
    University of California, Santa Cruz CA, United States.
    Windmolders, R
    Université de l'Etat Mons, Mons, Belgium.
    Inelastic J ψ production in deep inelastic scattering from hydrogen and deuterium and the gluon distribution of free nucleons1991In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 258, no 3-4, 493-498 p.Article in journal (Refereed)
    Abstract [en]

    We present results on inelastic J/PSI-production from muon interactions with hydrogen and deuterium at an incident muon energy of 280 GeV. The measured cross section ratio per nucleon for muon-induced J/PSI-production in deuterium and hydrogen was found to be R(D2/H2) = 1.01 +/- 0.15. The colour singlet model is shown to provide a good description of the observed differential cross section apart from a normalisation factor. The comparison between the observed cross section and the colour singlet model prediction allows the extraction of the gluon structure function G(chi) of the nucleon. The momentum fraction-chi of the nucleon carried by the gluon is measured in the range of chi = [0.02, 0.30]. The normalised gluon distribution of free nucleons thus found can be parametrised as chi-G(chi) = 1/2 (eta + 1)(1 - chi)eta, with eta = 5.1 +/- 0.9 (stat).

  • 6.
    Allasia, D
    et al.
    Università di Torino, INFN, Turin, Italy.
    Lettenström, Frans
    University of Uppsala, Uppsala, Sweden.
    Windmolders, R
    Université de l'Etat Mons, Mons, Belgium.
    Measurement of the neutron and the proton F2 structure function ratio1990In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 249, no 2, 366-372 p.Article in journal (Refereed)
  • 7.
    Amaudruz, P
    et al.
    Paul Scherrer Institut, Villigen, Switzerland.
    Lettenström, Frans
    University of California, Santa Cruz, United States.
    Zetsche, F
    Max-Planck Institut für Kernphysik, Heidelberg, Germany.
    A re-evaluation of the nuclear structure function ratios for D, He, 6Li, C and Ca1995In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 441, no 1-2, 3-11 p.Article in journal (Refereed)
    Abstract [en]

    We present a re-evaluation of the structure function ratios F-2(He)/F-2(D), F-2(C)/F-2(D) and F-2(Ca)/F-2(D) measured in deep inelastic muon-nucleus scattering at an incident muon momentum of 200 GeV. We also present the ratios F-2(C)/F-2(Li), F-2(Ca)/F-2(Li) and F-2(Ca)/F-2(C) measured at 90 GeV. The results are based on data already published by NMC; the main difference in the analysis is a correction for the masses of the deuterium targets and an improvement in the radiative corrections. The kinematic range covered is 0.0035 < x < 0.65, 0.5 < Q(2) < 90 GeV2 for the He/D, C/D and Ca/D data and 0.0085 < x < 0.6, 0.84 < Q(2) < 17 GeV2 for the Li/C/Ca ones.

  • 8. Amaudruz, P
    et al.
    Lettenström, Frans
    University of California, Santa Cruz CA, United States.
    Zetsche, F
    Gottfried sum from the ratio F2n/F2p1991In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 66, no 21, 2712-2715 p.Article in journal (Refereed)
    Abstract [en]

    Experimental results obtained at the CERN Super Proton Synchrotron on the structure-function ratio F2n/F2p in the kinematic range 0.004 < x < 0.8 and 0.4 < Q2 < 190 GeV2, together with the structure function F2d determined from a fit to published data, are used to derive the difference F2p(x) - F2n(x). The value of the Gottfried sum integral (F2p - F2n) dx/x = 0.240 +/- 0.016 is below the quark-parton-model expectation of 1/3.

  • 9.
    Amaudruz, P
    et al.
    Paul Scherrer Institute, Villigen, Switzerland.
    Lettenström, Frans
    Institute for Particle Physics, University of California, Santa Cruz CA, United States.
    Zetsche, F
    Max Planck Institut für Kernphysik, Heidelberg, Germany.
    Measurements of Rd-Rp and RCa-Rc in deep inelastic muon scattering1992In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 294, no 1, 120-126 p.Article in journal (Refereed)
    Abstract [en]

    Results are presented on the difference in R, the ratio of longitudinally to transversely polarised virtual photon absorption cross sections, for the deuteron and the proton. They are obtained by comparing the ratio of cross sections for the deep inelastic scattering of muons from deuterium and hydrogen targets at 90 and 280 GeV incident energy. The results cover the range x = 0.01 -0.30, at an average Q2 of 9 GeV2. The measured difference R(d)-R(p) shows no significant x dependence and is compatible with zero, as well as with expectations from perturbative QCD. We use the same method to obtain the difference R(Ca)-R(C) from cross section ratios measured on carbon and calcium targets at 90 and 200 GeV incident energy.

  • 10.
    Amaudruz, P
    et al.
    Paul Scherrer Institute, Villingen, Switzerland.
    Lettenström, Frans
    University of California, Santa Cruz CA, United States.
    Zetsche, F
    MPI Heidelberg, Heidelberg, Germany.
    Precision measurement of structure function ratios for 6Li, 12C and 40Ca1992In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 53, no 1, 73-77 p.Article in journal (Refereed)
    Abstract [en]

    The structure function ratios F2C/F2Li, F2Ca/F2Li and F2Ca/F2C were measured in deep inelastic muon-nucleus scattering at an incident muon energy of 90 GeV, covering the kinematic range 0.0085 < x < 0.6 and 0.8 < Q2 < 17 GeV2. The sensitivity of the nuclear structure functions to the size and mean density of the target nucleus is discussed.

  • 11.
    Amaudruz, P
    et al.
    Paul Scherrer Institute, Villingen, Switzerland.
    Lettenström, Frans
    University of California, Santa Cruz CA, United States.
    Zetsche, F
    MPI Heidelberg, Heidelberg, Germany.
    Precision measurement of the structure function ratios F2He/F2D, F2C/F2D and F2Ca/F2D1991In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 51, no 3, 387-393 p.Article in journal (Refereed)
    Abstract [en]

    We present the structure function ratios F2He/F2D, F2C/F2D and F2Ca/F2D measured in deep inelastic muon-nucleus scattering at an incident muon momentum of 200 GeV. The kinematic range 0.0035 < x < 0.65 and 0.5 < Q2 < 90 GeV2 is covered. At low x the three ratios are significantly smaller than unity and the size of the depletion grows with decreasing x and increasing mass number A. At intermediate x the ratios show an enhancement of about 2% above unity for C/D and Ca/D, possibly less for He/D. There are indications of some Q2 dependence in the Ca/D data. The integrals of the structure function differences F2A-F2D are discussed.

  • 12.
    Amaudruz, P
    et al.
    Paul Scherrer Institute, Villigen, Switzerland.
    Lettenström, Frans
    Physics Department, Freiburg University, Freiburg, Germany.
    Zetsche, F
    Max Planck Institut für Kernphysik, Heidelberg, Germany.
    Proton and deuteron F2 structure functions in deep inelastic muon scattering1992In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 295, no 1-2, 159-168 p.Article in journal (Refereed)
    Abstract [en]

    The structure functions F2p and F2d measured by deep inelastic muon scattering at incident energies of 90 and 280 GeV are presented. These measurements cover a large kinematic range, 0.006 less-than-or-equal-to x less-than-or-equal-to 0.6 and 0.5 less-than-or-equal-to Q2 less-than-or-equal-to 55 GeV2, and include the first precise data at small x, where large scaling violations are observed. The data agree with earlier results from SLAC and BCDMS but exhibit differences with respect to those of EMC-NA2. Extrapolations to small x of recent phenomenological parton distributions are shown to disagree with the present results.

  • 13.
    Amaudruz, P
    et al.
    Paul Scherrer Institute, Villigen, Switzerland.
    Lettenström, Frans
    University of California, Institute for Particle Physics, Santa Cruz CA, United States.
    Zetsche, F
    Max-Planck Institut für Kernphysik, Heidelberg, Germany.
    The ratio F2 n/F2 p in deep inelastic muon scattering1992In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 371, no 1-2, 3-31 p.Article in journal (Refereed)
    Abstract [en]

    Results are presented on the ratio of neutron and proton structure functions, F2n/F2p, deduced from deep inelastic scattering of muons from hydrogen and deuterium. The data, which were obtained at the CERN muon beam at 90 and 280 GeV incident energy, cover the kinematic range x = 0.002-0.80 and Q2 = 0.1-190 GeV2. The measured structure function ratios have small statistical and systematic errors, particularly at small and intermediate x. The observed Q2 dependence in the range x = 0.1-0.4 is stronger than predicted by perturbative QCD. From the present data together with results from other experiments it is suggested that the twist-four coefficient for the proton is smaller than that for the neutron for x larger than 0.2.

  • 14.
    Amaudruz, P
    et al.
    Paul Scherrer Institute, Villigen, Switzerland.
    Lettenström, Frans
    University of California, Santa Cruz CA, United States.
    Zetsche, F
    MPI für Kernphysik, Heidelberg, Germany.
    Transverse momentum distributions for exclusive ρ0 muoproduction1992In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 54, no 2, 239-245 p.Article in journal (Refereed)
    Abstract [en]

    We have studied transverse momentum distributions for exclusive rho(0) muoproduction on protons and heavier nuclei at 2 < Q2 < 25 GeV2. The Q2 dependence of the slopes of the p(t)2 and t' distributions is discussed. The influence of the non-exclusive background is investigated. The p(t)2-slope for exclusive events is 4.3 +/- 0.6 +/- 0.7 GeV-2 at large Q2. The p(t)2 spectra are much softer than inclusive p(t)2 spectra of leading hadrons produced in deep inelastic scattering.

  • 15.
    Andersen, Jørgen Ellegaard
    et al.
    Center for Quantum Geometry of Moduli Spaces, University of Aarhus, Århus, Denmark.
    Fjelstad, Jens
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Reducibility of quantum representations of mapping class groups2010In: Letters in Mathematical Physics, ISSN 0377-9017, E-ISSN 1573-0530, Vol. 91, no 3, 215-239 p.Article in journal (Refereed)
    Abstract [en]

    In this paper we provide a general condition for the reducibility of the Reshetikhin–Turaev quantum representations of the mapping class groups. Namely, for any modular tensor category with a special symmetric Frobenius algebra with a non-trivial genus one partition function, we prove that the quantum representations of all the mapping class groups built from the modular tensor category are reducible. In particular, for SU(N) we get reducibility for certain levels and ranks. For the quantum SU(2) Reshetikhin–Turaev theory we construct a decomposition for all even levels. We conjecture this decomposition is a complete decomposition into irreducible representations for high enough levels.

  • 16.
    Andrén, Daniel
    et al.
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Shao, Lei
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Länk, Nils Odebo
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Acimovic, Srdjan S.
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Johansson, Peter
    Örebro University, School of Science and Technology. Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Käll, Mikael
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Probing Photothermal Effects on Optically Trapped Gold Nanorods by Simultaneous Plasmon Spectroscopy and Brownian Dynamics Analysis2017In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 11, no 10, 10053-10061 p.Article in journal (Refereed)
    Abstract [en]

    Plasmonic gold nanorods are prime candidates for a variety of biomedical, spectroscopy, data storage, and sensing applications. It was recently shown that gold nanorods optically trapped by a focused circularly polarized laser beam can function as extremely efficient nanoscopic rotary motors. The system holds promise for-applications ranging from nanofluidic flow control and nanorobotics to biomolecular actuation and analysis. However, to fully exploit this potential, one needs to be able to control and understand heating effects associated with laser trapping. We investigated photothermal heating of individual rotating gold nanorods by simultaneously probing their localized surface plasmon resonance spectrum and rotational Brownian dynamics over extended periods of time. The data reveal an extremely slow nanoparticle reshaping process, involving migration of the order of a few hundred atoms per minute, for moderate laser powers and a trapping wavelength close to plasmon resonance. The plasmon spectroscopy and Brownian analysis allows for separate temperature estimates based on the refractive index and the viscosity of the water surrounding a trapped nanorod. We show that both measurements yield similar effective temperatures, which correspond to the actual temperature at a distance of the order 10-15 nm from the particle surface. Our results shed light on photothermal processes on the nanoscale and will be useful in evaluating the applicability and performance of nanorod motors and optically heated nanoparticles for a variety of applications.

  • 17.
    Andrén, Daniel
    et al.
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Shao, Lei
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Odebo Länk, Nils
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Aćimović, Srdjan S
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Johansson, Peter
    Örebro University, School of Science and Technology. Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Käll, Mikael
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Probing Photothermal Effects on Optically Trapped Gold Nanorods by Simultaneous Plasmon Spectroscopy and Brownian Dynamics Analysis2017In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 11, no 10, 10053-10061 p.Article in journal (Refereed)
    Abstract [en]

    Plasmonic gold nanorods are prime candidates for a variety of biomedical, spectroscopy, data storage, and sensing applications. It was recently shown that gold nanorods optically trapped by a focused circularly polarized laser beam can function as extremely efficient nanoscopic rotary motors. The system holds promise for applications ranging from nanofluidic flow control and nanorobotics to biomolecular actuation and analysis. However, to fully exploit this potential, one needs to be able to control and understand heating effects associated with laser trapping. We investigated photothermal heating of individual rotating gold nanorods by simultaneously probing their localized surface plasmon resonance spectrum and rotational Brownian dynamics over extended periods of time. The data reveal an extremely slow nanoparticle reshaping process, involving migration of the order of a few hundred atoms per minute, for moderate laser powers and a trapping wavelength close to plasmon resonance. The plasmon spectroscopy and Brownian analysis allows for separate temperature estimates based on the refractive index and the viscosity of the water surrounding a trapped nanorod. We show that both measurements yield similar effective temperatures, which correspond to the actual temperature at a distance of the order 10-15 nm from the particle surface. Our results shed light on photothermal processes on the nanoscale and will be useful in evaluating the applicability and performance of nanorod motors and optically heated nanoparticles for a variety of applications.

  • 18.
    Ashman, J
    et al.
    Department of Physics, University of Sheffield, Sheffield, United Kingdom; Faculté des Sciences, Université de L'Etat à Mons, Mons, Belgium.
    Lettenström, Frans
    Istituto di Fisica, Università di Torino, Turin, Italy; Faculté des Sciences, Université de L'Etat à Mons, Mons, Belgium.
    Womersley, W J
    Department of Physics, University of Lancaster, Lancaster, United Kingdom; Faculté des Sciences, Université de L'Etat à Mons, Mons, Belgium.
    Measurement of the ratios of deep inelastic muon-nucleus cross sections on various nuclei compared to deuterium1988In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 202, no 4, 603-610 p.Article in journal (Refereed)
  • 19.
    Ashman, J
    et al.
    Department of Physics, University, Sheffield, United Kingdom.
    Lettenström, Frans
    Gustav Werners Institut, University, Uppsala, Sweden.
    Womersley, WJ
    Nuclear Physics Laboratory, University, Oxford, United Kingdom.
    Exclusive ρ0 and Φ production in deep inelastic muon scattering1988In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 39, no 2, 169-175 p.Article in journal (Refereed)
    Abstract [en]

    Data are presented on exclusive ρ0 and φ{symbol} production in deep inelastic muon scattering from a target consisting mainly of nitrogen. The ratio of the total cross sections for ρ0 and φ{symbol} production is found to be 9:(1.6±0.4) at 〈Q2〉=7.5 GeV2, consistent with the SU(3) prediction of 9:2. The t dependence for exclusive ρ0 production is found to become shallover as Q2 increases and, for large Q2, the t dependence is typical of that for a hard scattering process. Furthermore, the ratio of the cross sections for coherent: incoherent production from nitrogen is found to decrease rapidly with Q2. Such behaviour indicates that even for exclusive vector meson production the virtual photon behaves predominantly as an electromagnetic probe.

  • 20.
    Ashman, J
    et al.
    Department of Physics, University of Sheffield, Sheffield, United Kingdom.
    Lettenström, Frans
    Department of Radiation Science, University of Uppsala, Uppsala, Sweden.
    Ziemons, K
    III Physikalisches Institut A, Physikzentrum, RWTH, Aachen, Germany.
    A measurement of the ratio of the nucleon structure function in copper and deuterium1993In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 57, no 2, 211-218 p.Article in journal (Refereed)
    Abstract [en]

    Results are presented on the ratios of the nucleon structure function in copper to deuterium from two separate experiments. The data confirm that the nucleon structure function, F2, is different for bound nucleons than for the quasi-free ones in the deuteron. The redistribution in the fraction of the nucleon's momentum carried by quarks is investigated and it is found that the data are compatible with no integral loss of quark momenta due to nuclear effects.

  • 21.
    Ashman, J
    et al.
    Department of Physics, University of Sheffield, Sheffield, United Kingdom.
    Lettenström, Frans
    Istituto di Fisica, Università di Torino, Turin, Italy.
    Ziemons, K
    Fachbereich Physik, Universität Wuppertal, Wuppertal, Germany.
    A measurement of the spin asymmetry and determination of the structure function g1 in deep inelastic muon-proton scattering1988In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 206, no 2, 364-370 p.Article in journal (Refereed)
  • 22.
    Ashman, J
    et al.
    Department of Physics, University of Sheffield, Sheffield, United Kingdom.
    Lettenström, Frans
    Department of Radiation Science, University of Uppsala, Uppsala, Sweden.
    Ziemons, K
    III. Physikalisches Institut A, Physikzentrum, RWTH, Aachen, FRG.
    An investigation of the spin structure of the proton in deep inelastic scattering of polarised muons on polarised protons1989In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 328, no 1, 1-35 p.Article in journal (Refereed)
  • 23.
    Ashman, J
    et al.
    Department of Physics, University of Sheffield, Sheffield, United Kingdom.
    Lettenström, Frans
    Department of Radiation Science, University of Uppsala, Uppsala, Sweden.
    Ziemons, K
    III Physikalisches Institut A, Physikzentrum, RWTH, Aachen, Germany.
    Forward produced hadrons in μp and μd scattering and investigation of the charge structure of the nucleon1991In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 52, no 3, 361-387 p.Article in journal (Refereed)
    Abstract [en]

    Final data measured with the EMC forward spectrometer are presented on the production of forward charged hadrons in mu-p and mu-d scattering at incident beam energies between 100 and 280 GeV. The large statistic of 373000 events allows a study of the semi-inclusive hadron production as a function of z, p(T)2 and <p(T)2> in small Q2, x(Bj) and W bins. Charge multiplicity ratios and differences as a function of z and x(Bj) are given for p, d and n-targets. From the differences of charge multiplicities the ratio of the valence quark distributions of the proton d-upsilon(x)/u-upsilon(x) is determined for the first time in charged lepton scattering. The Gronau et al. sum rule is tested, the measured sum being 0.31 +/- 0.06 stat. +/- 0.05 syst., compared with the theoretical expectation of 2/7 almost-equal-to 0.286. The measured sum corresponds to an absolute value of the ratio of the d and u quark charge of 0.44 +/- 0.10 stat. +/- 0.08 syst.

  • 24.
    Ashman, J
    et al.
    Department of Physics, University of Sheffield, Sheffield, United Kingdom.
    Lettenström, Frans
    Department of Radiation Science, University of Uppsala, Uppsala, Sweden.
    Ziemons, K
    III Physikalisches Institut A, Physikzentrum, RWTH, Aachen, Germany.
    Muoproduction of J/ψ and the gluon distribution of the nucleon1992In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 56, no 1, 21-28 p.Article in journal (Refereed)
    Abstract [en]

    Measurements are presented of the inclusive distributions of the J/psi meson produced by muons of energy 200 GeV from an ammonia target. The gluon distribution of the nucleon has been derived from the data in the range 0.04 < x < 0.36 using a technique based on the colour singlet model. An arbitrary normalisation factor is required to obtain a reasonable integral of the gluon distribution. Some comments are made on the use of J/psi production by virtual photons to extract the gluon distribution at HERA.

  • 25. Billnert, R.
    et al.
    Oberstedt, S.
    Andreotti, E.
    Hult, M.
    Marissens, G.
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    New information on the characteristics of 1 in. x 1 in. cerium bromide scintillation detectors2011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 647, no 1, 94-99 p.Article in journal (Refereed)
    Abstract [en]

    In view of highly demanded new and accurate data on prompt gamma-ray emission in nuclear fission a major part of investigations is directed towards the selection of suitable detector systems. Here we have studied a new type of crystal scintillation detectors made from cerium bromide (CeBr(3)). For the first time a full characterization of such a detector is presented in terms of energy resolution, pulse-height linearity, intrinsic activity and intrinsic timing resolution. In particular the latter one is very important for prompt fission gamma-ray studies, because the presence of fast neutrons, emitted in fission too, requires the time-of-flight method for their discrimination. The energy resolution has been found to be comparable to that of cerium-doped LaCl(3):Ce detectors at an efficiency comparable to the one of a LaBr(3):Ce detector of the same size. The intrinsic activity of the CeBr(3) crystal was observed to be much lower compared to lanthanum halide crystals. The intrinsic timing resolution of a coaxial 1 in. x 1 in. sized detector was measured relative to that of a previously characterized LaCl(3):Ce detector and found to be (326 +/- 7) Ps at (60)Co energies, which is in between those of a LaBr(3):Ce and a LaCl(3):Ce detector of same size. (C) 2011 Elsevier B.V. All rights reserved.

  • 26.
    Birgersson, Evert
    Örebro University, Department of Natural Sciences.
    Determination of binary fission-fragment yields in the reaction 251Cf(nth, f) and Verification of nuclear reaction theory predictions of fission-fragment distributions in the reaction 238U(n, f)2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Neutron-induced fission has been studied at different excitation energies of the compound nucleus by measurements on the two fissioning systems, 252Cf* and 239U*.

    For the first time, the light fission fragment yields from the reaction 251Cf(nth, f) have been measured with high resolution. This experiment was performed with the recoil mass spectrometer LOHENGRIN at ILL in Grenoble, France. When the results from this work, where the compound nucleus is at thermal excitation, are compared to the spontaneous fission of 252Cf, enhanced emission yields as well as an increased mean kinetic energy is observed around A = 115. This suggests the existence of an additional super-deformed fission mode in 252Cf.

    The reaction 238U(n, f) was studied using the 2E-technique with a double Frisch grid ionization chamber. Fission fragment mass, energy and angular distributions were determined for incident neutron energies between 0.9 and 2.0 MeV. The experiments were performed at the Van de Graaff accelerator of IRMM in Geel, Belgium. This is the first measurement of the mass distribution for incident neutron energies around 0.9 MeV. The motivation for studying 238U(n, f) was to verify theoretical predictions of the mass distribution at the vibrational resonance in the fission cross section at 0.9 MeV. However, the predicted changes in fission fragment distributions could not be confirmed.

    A precise modelling of the fission process for the minor actinides becomes very important for future generation IV and accelerator driven nuclear reactors. Since fission fragment distributions depend on the excitation of the fissioning system, so does the number of delayed neutrons, which are one of the safety parameters in a reactor.

    List of papers
    1. Binary fission-fragment yields from the reaction 251Cf(nth, f)
    Open this publication in new window or tab >>Binary fission-fragment yields from the reaction 251Cf(nth, f)
    Show others...
    2005 (English)In: Nuclear fission and fission-product spectroscopy: 3rd international workshop on nuclear fission and fission-product spectroscopy, American Institute of Physics , 2005, 349-352 p.Conference paper, Published paper (Refereed)
    Abstract [en]

    The recoil mass spectrometer LOHENGRIN of the Laue-Langevin Institute, Grenoble has been used to measure the light fission-fragment mass yield and kinetic energy distributions from neutron-induced 252Cf*, using 251Cf as target material. ©2005 American Institute of Physics

    Place, publisher, year, edition, pages
    American Institute of Physics, 2005
    National Category
    Subatomic Physics Natural Sciences Physical Sciences
    Research subject
    Physics
    Identifiers
    urn:nbn:se:oru:diva-11214 (URN)10.1063/1.2137266 (DOI)0-7354-0288-4 (ISBN)
    Conference
    The 3rd International Workshop on NUCLEAR FISSION AND FISSION-PRODUCT SPECTROSCOPY, Château de Cadarache, Saint Paul lez Durance, France, May 11-14, 2005
    Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-10-18Bibliographically approved
    2. Light fission-fragment mass distribution from the reaction 251Cf(nth, f)
    Open this publication in new window or tab >>Light fission-fragment mass distribution from the reaction 251Cf(nth, f)
    Show others...
    2007 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 791, no 1-2, 1-23 p.Article in journal (Refereed) Published
    Abstract [en]

    For mass numbers A = 80 to 124 the recoil mass spectrometer LOHENGRIN of the Institute Laue-Langevin in Grenoble was used to measure with high resolution the light fission-fragment mass yields and kinetic energy distributions from thermal-neutron induced fission of 252Cf* for the first time, using 251Cf as target material. The obtained mean light fragment mass AL = (107 ± 2) and the corresponding mean kinetic energy Ek,L = (103±2) MeV are within the expected trend. Emission yields around A = 115 are enhanced and the corresponding mean kinetic energy is higher compared to spontaneous fission of 252Cf. This could be explained by the existence of an additional super-deformed fission mode.

    Place, publisher, year, edition, pages
    Amsterdam: Elsevier, 2007
    Keyword
    Nuclear reactions, 251Cf(nth, f ), Californium-251, Neutron-induced fission, Fission modes, Minor Actinides, Transmutation, Fission-fragment spectroscopy
    National Category
    Subatomic Physics Natural Sciences Physical Sciences
    Research subject
    Physics
    Identifiers
    urn:nbn:se:oru:diva-4080 (URN)10.1016/j.nuclphysa.2007.04.018 (DOI)
    Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2017-12-14Bibliographically approved
    3. Properties of the reaction 238U(n, f) at the vibrational resonances
    Open this publication in new window or tab >>Properties of the reaction 238U(n, f) at the vibrational resonances
    2009 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 817, no 1-4, 1-34 p.Article in journal (Refereed) Published
    Abstract [en]

    Recent fission cross-section calculations for the reaction 238U(n, f ), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy En = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from En = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at En = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around En = 1.2 and 1.6 MeV wasobserved at En = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at En = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.

    Place, publisher, year, edition, pages
    Amsterdam: Elsevier, 2009
    Keyword
    Nuclear reactions 238U(n, f ), E = 0.9–2.0 MeV; measured/analyzed fission fragment mass yields and
    National Category
    Subatomic Physics Physical Sciences
    Research subject
    Physics
    Identifiers
    urn:nbn:se:oru:diva-5485 (URN)10.1016/j.nuclphysa.2008.12.001 (DOI)000263449200001 ()2-s2.0-58149293406 (Scopus ID)
    Available from: 2009-02-12 Created: 2009-02-12 Last updated: 2017-12-14Bibliographically approved
  • 27.
    Birgersson, Evert
    et al.
    Örebro University, School of Science and Technology.
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Oberstedt, Stephan
    Hambsch, Franz-Josef
    Properties of the reaction 238U(n, f) at the vibrational resonances2009In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 817, no 1-4, 1-34 p.Article in journal (Refereed)
    Abstract [en]

    Recent fission cross-section calculations for the reaction 238U(n, f ), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy En = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from En = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at En = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around En = 1.2 and 1.6 MeV wasobserved at En = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at En = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.

  • 28.
    Birgersson, Evert
    et al.
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Rochman, Dimitri
    Tsekhanovich, Igor
    Raman, Subramanian
    Light fission-fragment mass distribution from the reaction 251Cf(nth, f)2007In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 791, no 1-2, 1-23 p.Article in journal (Refereed)
    Abstract [en]

    For mass numbers A = 80 to 124 the recoil mass spectrometer LOHENGRIN of the Institute Laue-Langevin in Grenoble was used to measure with high resolution the light fission-fragment mass yields and kinetic energy distributions from thermal-neutron induced fission of 252Cf* for the first time, using 251Cf as target material. The obtained mean light fragment mass AL = (107 ± 2) and the corresponding mean kinetic energy Ek,L = (103±2) MeV are within the expected trend. Emission yields around A = 115 are enhanced and the corresponding mean kinetic energy is higher compared to spontaneous fission of 252Cf. This could be explained by the existence of an additional super-deformed fission mode.

  • 29.
    Birgersson, Evert
    et al.
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Rochman, Dimitri
    Tsekhanovitsch, Igor
    Binary fission-fragment yields from the reaction 251Cf(nth, f)2005In: Nuclear fission and fission-product spectroscopy: 3rd international workshop on nuclear fission and fission-product spectroscopy, American Institute of Physics , 2005, 349-352 p.Conference paper (Refereed)
    Abstract [en]

    The recoil mass spectrometer LOHENGRIN of the Laue-Langevin Institute, Grenoble has been used to measure the light fission-fragment mass yield and kinetic energy distributions from neutron-induced 252Cf*, using 251Cf as target material. ©2005 American Institute of Physics

  • 30.
    Björnsson, Jonas
    et al.
    Department of Applied Mathematics and Theoretical Physics, Cambridge, UK.
    Fjelstad, Jens
    Department of Physics, Nanjing University, Nanjing, China.
    Modular invariant partition functions for noncompact G/Ad(H) models2011In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 83, no 8, 086007Article in journal (Refereed)
    Abstract [en]

    We propose a spectrum for a class of gauged non-compact G/Ad(H) WZNW models, including spectrally flowed images of highest, lowest, and mixed extremal weight modules. These are combined into blocks whose characters, due to the Lorentzian signature of the target space, are divergent and treated as formal expressions in need of regularisation. Assuming that this is possible, we show that these extended characters transform linearly under modular transformations, and can be used to write down modular invariant partition functions.

  • 31.
    Chen, Gang
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Cheung, Yeuk-Kwan E.
    Department of Physics, Nanjing University, Nanjing, China.
    Fan, Zheyong
    Department of Physics, Nanjing University, Nanjing, China.
    Fjelstad, Jens
    Faculty of Technology and Science, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Faculty of Technology and Science, Karlstad University, Karlstad, Sweden.
    No-ghost theorem for the bosonic Nappi-Witten string2009In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 80, no 8, 086003Article in journal (Refereed)
    Abstract [en]

    We prove a no-ghost theorem for a bosonic string propagating in Nappi-Witten spacetime. This is achieved in two steps. We first demonstrate unitarity for a class of NW/U(1) modules: the norm of any state which is primary with respect to a chosen timelike U(1) is non-negative. We then show that physical states - states satisfying the Virasoro constraints - in a class of modules of an affinisation of the Nappi-Witten algebra are contained in the NW/U(1) modules. Similar to the case of strings on AdS3, in order to saturate the spectrum obtained in light-cone quantization we are led to include modules with energy not bounded from below, which are related to modules with energy bounded from below by spectral flow automorphisms.

  • 32.
    Coley, Alan A.
    et al.
    Department of Mathematics and Statistics, Dalhousie University, Halifax, Nova Scotia, Canada.
    Leon, Genly
    Instituto de Física, Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile.
    Sandin, Patrik
    Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Potsdam, Germany.
    Latta, Joey
    Department of Mathematics and Statistics, Dalhousie University, Halifax, Nova Scotia, Canada.
    Spherically symmetric Einstein-aether perfect fluid models2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 12, 010Article in journal (Refereed)
    Abstract [en]

    We investigate spherically symmetric cosmological models in Einstein-act her theory with a tilted (non-comoving) perfect fluid source. We use a 1+3 frame formalism and adopt the comoving aether gauge to derive the evolution equations, which form a well-posed system of first order partial differential equations in two variables. We then introduce normalized variables. The formalism is particularly well-suited for numerical computations and the study of the qualitative properties of the models, which are also solutions of Horava gravity. We study the local stability of the equilibrium points of the resulting dynamical system corresponding to physically realistic in homogeneous cosmological models and astrophysical objects with values for the parameters which are consistent. with current constraints. In particular, we consider dust models in (beta-) normalized variables and derive a reduced (closed) evolution system and we obtain the general evolution equations for the spatially homogeneous Kantowski-Sachs models using, appropriate bounded normalized variables. We then analyse these models, with special emphasis on the future asymptotic behaviour for different values of the parameters. Finally, we investigate static models for a mixture of a (necessarily non-tilted) perfect fluid with a barotropic equations of state and a scalar field.

  • 33.
    Fjelstad, Jens
    Örebro University, School of Science and Technology.
    Conformal Field Theory and the Holgoraphic Principle2002Doctoral thesis, monograph (Other academic)
    Abstract [en]

    This thesis is based on five research papers with string theory as the least common denominator. The text is divided in two parts. The first part is an introduction to the constructions used in the papers; string theory, conformal field theory, topological field theory and three-dimensional gravity. The second part is a review of a concept believed to be important in any theory of quantum gravity, the so called holographic principle.A relation between three-dimensional topological field theory and two-dimensional conformal field theory is re-established in a classical formulation. This result is then used to investigate holography in a toy model, gravity in three dimensions. A construction is proposed which yields the Bekenstein-Hawking entropy of a three-dimensional black hole. The same mechanism is shown not to be responsible for the entropy of Kerr-de Sitter spacetime.Similar methods yields a relation between topological field theories in five and seven dimensions and chiral conformal field theories in four and six dimensions.There presumably exists a class of conformal field theories with novel properties, so called logarthmic conformal field theories. By starting from a conventional theory, a systematic way to deform this to a logarithmic theory is developed.

  • 34.
    Fjelstad, Jens
    Department of Physics, Karlstad University, Karlstad, Sweden.
    On 7-D TFT and 6-D chiral CFT2002Manuscript (preprint) (Other academic)
    Abstract [en]

    We perform a canonical and BRST analysis of a seven-dimensional Chern-Simons theory on a manifold with boundary. The main result is that the 7D theory induces for consistency a chiral two-form on the 6D boundary. We also comment on similar behaviour in a five-dimensional Chern-Simons theory relevant for $\N=4$ supersymmetric Yang-Mills theory in four dimensions.

  • 35.
    Fjelstad, Jens
    Department of Physics, Nanjing University, Nanjing, China.
    On duality and extended chiral symmetry in the SL(2,R) WZW model2011In: Journal of Physics A: Mathematical and General, ISSN 0305-4470, E-ISSN 1361-6447, Vol. 44, no 23, 235404Article in journal (Refereed)
    Abstract [en]

    Two chiral aspects of the SL(2,R) WZW model in an operator formalism are investigated. First, the meaning of duality, or conjugation, of primary fields is clarified. On a class of modules obtained from the discrete series it is shown, by looking at spaces of two-point conformal blocks, that a natural definition of contragredient module provides a suitable notion of conjugation of primary fields, consistent with known two-point functions. We find strong indications that an apparent contradiction with the Clebsch-Gordan series of SL(2,R), and proposed fusion rules, is explained by nonsemisimplicity of a certain category. Second, results indicating an infinite cyclic simple current group, corresponding to spectral flow automorphisms, are presented. In particular, the subgroup corresponding to even spectral flow provides part of a hypothetical extended chiral algebra resulting in proposed modular invariant bulk spectra.

  • 36.
    Fjelstad, Jens
    Department of Physics, Karlstad University, Karlstad, Sweden.
    The Holographic Principle2001Report (Other academic)
  • 37.
    Fjelstad, Jens
    et al.
    Karlstad University, Karlstad, Swede.
    Fuchs, J.
    Karlstad University, Karlstad, Swede.
    Hwang, S.
    Karlstad University, Karlstad, Swede.
    Semikhatov, A. M.
    Lebedev Institute, Moscow, Russian Federation.
    Tipunin, I. Yu.
    Lebedev Institute, Moscow, Russian Federation.
    Logarithmic conformal field theories via logarithmic deformations2002In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 633, no 3, 379-413 p.Article in journal (Refereed)
    Abstract [en]

    We construct logarithmic conformal field theories starting from an ordinary conformal field theory -- with a chiral algebra C and the corresponding space of states V -- via a two-step construction: i) deforming the chiral algebra representation on V\tensor End K[[z,1/z]], where K is an auxiliary finite-dimensional vector space, and ii) extending C by operators corresponding to the endomorphisms End K. For K=C^2, with End K being the two-dimensional Clifford algebra, our construction results in extending C by an operator that can be thought of as \partial^{-1}E, where \oint E is a fermionic screening. This covers the (2,p) Virasoro minimal models as well as the sl(2) WZW theory.

  • 38.
    Fjelstad, Jens
    et al.
    Institutionen för Fysik, Karlstads Universitet, Karlstad, Sweden.
    Fuchs, Jurgen
    Institutionen för Fysik, Karlstads Universitet, Karlstad, Sweden.
    Runkel, Ingo
    Max-Planck-Institut für Gravitationsphysik, Albert-Einstein-Institut, Golm, Germany.
    Schweigert, Christoph
    Fachbereich Mathematik, Universität Hamburg Schwerpunkt Algebra und Zahlentheorie, Zentrum für Mathematische Physik, Hamburg, Germany.
    TFT construction of RCFT correlators V: Proof of modular invariance and factorisation2006In: Theory and Applications of Categories, ISSN 1201-561X, E-ISSN 1201-561X, Vol. 16, no 16, 342-433 p.Article in journal (Refereed)
    Abstract [en]

    The correlators of two-dimensional rational conformal field theories that are obtained in the TFT construction of [FRSI,FRSII,FRSIV] are shown to be invariant under the action of the relative modular group and to obey bulk and boundary factorisation constraints. We present results both for conformal field theories defined on oriented surfaces and for theories defined on unoriented surfaces. In the latter case, in particular the so-called cross cap constraint is included.

  • 39.
    Fjelstad, Jens
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Fuchs, Jurgen
    Teoretisk Fysik, Karlstads Universitet, Karlstad, Sweden.
    Stigner, Carl
    Teoretisk Fysik, Karlstads Universitet, Karlstad, Sweden.
    RCFT with defects: Factorization and fundamental world sheets2012In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 863, no 1, 213-259 p.Article in journal (Refereed)
    Abstract [en]

    It is known that for any full rational conformal field theory, the correlation functions that are obtained by the TFT construction satisfy all locality, modular invariance and factorization conditions, and that there is a small set of fundamental correlators to which all others are related via factorization - provided that the world sheets considered do not contain any non-trivial defect lines. In this paper we generalize both results to oriented world sheets with an arbitrary network of topological defect lines.

  • 40.
    Fjelstad, Jens
    et al.
    Cardiff School of Mathematics, Cardiff University, United Kingdom; Avdelning Fysik, Karlstads Universitet, Karlstad, Sweden.
    Fuchs, Jürgen
    Avdelning Fysik, Karlstads Universitet, Karlstad, Sweden.
    Runkel, Ingo
    Department of Mathematics, King's College London, United Kingdom.
    Schweigert, Christoph
    Organisationseinheit Mathematik, Universität Hamburg, Schwerpunkt Algebra und Zahlentheorie, Hamburg, Germany.
    Uniqueness of open/closed rational CFT with given algebra of open states2008In: Advances in Theoretical and Mathematical Physics, ISSN 1095-0761, E-ISSN 1095-0753, Vol. 12, no 6, 1283-1375 p.Article in journal (Refereed)
    Abstract [en]

    We study the sewing constraints for rational two-dimensional conformal field theory on oriented surfaces with possibly non-empty boundary. The boundary condition is taken to be the same on all segments of the boundary. The following uniqueness result is established: For a solution to the sewing constraints with nondegenerate closed state vacuum and nondegenerate two-point correlators of boundary fields on the disk and of bulk fields on the sphere, up to equivalence all correlators are uniquely determined by the one-, two,- and three-point correlators on the disk.

    Thus for any such theory every consistent collection of correlators can be obtained by the TFT approach of hep-th/0204148, hep-th/0503194. As morphisms of the category of world sheets we include not only homeomorphisms, but also sewings; interpreting the correlators as a natural transformation then encodes covariance both under homeomorphisms and under sewings of world sheets.

  • 41.
    Fjelstad, Jens
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Fuchs, Jürgen
    Teoretisk fysik, Karlstad University, Karlstad, Sweden.
    Stigner, Carl
    Teoretisk fysik, Karlstad University, Karlstad, Sweden.
    Schweigert, Christoph
    Algebra und Zahlentheorie, Hamburg University, Hamburg, Germany.
    Partition functions, mapping class groups and Drinfeld doubles2012In: Symmetries and Groups in Contemporary Physics: Proceedings of the XXIX International Colloquium on Group-Theoretical Methods in PhysicsTianjin, China, 20 – 26 August 2012, 2012, 405-410 p.Conference paper (Refereed)
    Abstract [en]

    Higher genus partition functions of two-dimensional conformal field theories have to be invariants under linear actions of mapping class groups. We illustrate recent results [4,6] on the construction of such invariants by concrete expressions obtained for the case of Drinfeld doubles of finite groups. The results for doubles are independent of the characteristic of the underlying field, and the general results do not require any assumptions of semisimplicity.

  • 42.
    Fjelstad, Jens
    et al.
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Equivalence of Chern-Simons gauge theory and WZNW model using a BRST symmetry1999In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 466, no 2-4, 227-233 p.Article in journal (Refereed)
    Abstract [en]

    The equivalence between the Chern-Simons gauge theory on a three-dimensional manifold with boundary and the WZNW model on the boundary is established in a simple and general way using the BRST symmetry. Our approach is based on restoring gauge invariance of the Chern-Simons theory in the presence of a boundary. This gives a correspondence to the WZNW model that does not require solving any constraints, fixing the gauge or specifying boundary conditions.

  • 43.
    Fjelstad, Jens
    et al.
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Sectors of solutions in three-dimensional gravity and black holes2002In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 628, no 1-2, 331-360 p.Article in journal (Refereed)
    Abstract [en]

    We examine the connection between three dimensional gravity with negative cosmological constant and two-dimensional CFT via the Chern-Simons formulation. A set of generalized spectral flow transformations are shown to yield new sectors of solutions. One implication is that the microscopic calculation of the entropy of the Banados-Teitelboim-Zanelli (BTZ) black hole is corrected by a multiplicative factor with the result that it saturates the Bekenstein-Hawking expression.

  • 44.
    Fjelstad, Jens
    et al.
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Månsson, Teresia
    Institute of Theoretical Physics, Stockholm, Sweden.
    CFT description of three-dimensional Kerr-de Sitter space-time2002In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 641, no 1-2, 376-392 p.Article in journal (Refereed)
    Abstract [en]

    We describe three-dimensional Kerr-de Sitter space using similar methods as recently applied to the BTZ black hole. A rigorous form of the classical connection between gravity in three dimensions and two-dimensional conformal field theory is employed, where the fundamental degrees of freedom are described in terms of two dependent SL(2,C) currents. In contrast to the BTZ case, however, quantization does not give the Bekenstein-Hawking entropy connected to the cosmological horizon of Kerr-de Sitter space.

  • 45.
    Fjelstad, Jens
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Månsson, Teresia
    Department of Theoretical Physics, School of Engineering Sciences, Royal Institute of Technology (KTH), Stockholm, Sweden.
    New symmetries of the chiral Potts model2012In: Journal of Physics A: Mathematical and General, ISSN 0305-4470, E-ISSN 1361-6447, Vol. 45, no 15, 155208Article in journal (Refereed)
    Abstract [en]

    In this paper a hithertho unknown symmetry of the three-state chiral Potts model is found consisting of two coupled Temperley-Lieb algebras. From these we can construct new superintegrable models. One realisation is in terms of a staggered isotropic XY spin chain. Further we investigate the importance of the algebra for the existence of mutually commuting charges. This leads us to a natural generalisation of the boost-operator, which generates the charges.

  • 46. Geijer, Håkan
    et al.
    Norrman, Eva
    Örebro University, Department of Natural Sciences.
    Persliden, Jan
    Optimising the tube potential for lumbar spine radiography using a flat-panel digital detectorManuscript (preprint) (Other academic)
  • 47. Geijer, Håkan
    et al.
    Norrman, Eva
    Örebro University, Department of Natural Sciences.
    Persliden, Jan
    Optimizing the tube potential for lumbar spine radiography with a flat-pane digital detector2009In: British Journal of Radiology, ISSN 0007-1285, E-ISSN 1748-880X, Vol. 82, no 973, 62-68 p.Article in journal (Refereed)
    Abstract [en]

    The purpose of this study was to find the optimal settings for lumbar spine radiography with a flat-panel detector. A CDRAD contrast-detail phantom was imaged at various tube potentials, system speeds and filtration settings. Factorial experiments yielded a range of optimized exposure settings, which were submitted to visual grading analysis with images of an Alderson phantom. The first optimized settings involved a system speed increase from 400 to 800. For anteroposterior projection, the optimal tube potential was reduced from the default of 77 kV to 60 kV to give the best image quality without increasing the effective dose, or to 66 kV to give the lowest dose without reducing image quality. For lateral projection, the tube potential was similarly reduced from the default of 90 kV to 70 kV or 77 W. Visual grading analysis confirmed the results, with significantly better image quality when optimizing for image quality. The study thus shows that the tube potential can be reduced as long as the system speed is increased simultaneously. This leads to a lower effective dose and/or increased image quality depending on the settings chosen. The factorial experiments provided a powerful way to evaluate several parameters concomitantly.

  • 48. Göök, A.
    et al.
    Barday, R.
    Chernykh, M.
    Eckardt, C.
    Eichhorn, R.
    Enders, J.
    Hambsch, F. -J
    Von Neumann-Cosef, P.
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Oberstedt, S.
    Poltoratska, Y.
    Richter, A.
    Wagner, M.
    Photo-fission at the S-DALINAC2009In: 4th international workshop on nuclear fission and fission-product spectroscopy / [ed] Audrey Chatillon ..., American Institute of Physics (AIP), 2009, Vol. 1175, 351-354 p.Conference paper (Other academic)
    Abstract [en]

    Experiments on photo-induced fission of 238U and 234U using bremsstrahlung with endpoint energies between 6 MeV and 9 MeV have been carried out at the superconducting Darmstadt electron linear accelerator S-DALINAC. A twin Frisch grid ionization chamber has been used to determine energy and mass distributions via the 2E-technique. The fission fragment emission angle, which is used to correct for energy loss in the target and backing material for a correct determination of fragment energy and mass, is deduced from the time difference between cathode and anode signals. The research program including the foreseen search for parity violation in photo-induced fission and first experimental results will be presented. © 2009 American Institute of Physics.

  • 49.
    Göök, A.
    et al.
    Örebro University, School of Science and Technology.
    Chernykh, M.
    Eckardt, C.
    Enders, J.
    von Neumann-Cosel, P.
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Oberstedt, S.
    Richter, A.
    Fragment characteristics from fission of (238)U and (234)U induced by 6.5-9.0 MeV bremsstrahlung2011In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 851, no 1, 1-16 p.Article in journal (Refereed)
    Abstract [en]

    Fission of (238)U and (234)U induced by bremsstrahlung of 6.5-9.0 MeV endpoint energy has been investigated at the superconducting Darmstadt electron linear accelerator S-DALINAC. Using a twin Frisch grid ionization chamber, fission-fragment energy and mass distributions have been determined by means of the double kinetic-energy technique. Results on the fission-fragment characteristics from (238)U(gamma, f) are in agreement with results from the literature. In addition fission-fragment mass and energy distributions from (234)U(gamma, f) are presented for the first time in this energy region. An analysis of fission modes within the Brosa model has been performed. The relative yield of the S 1 mode was found to be (13 +/- 3)% in (234)U and (35 +/- 2)% in (238)U. (C) 2010 Elsevier B.V. All rights reserved.

  • 50.
    Göök, A.
    et al.
    Tech Univ Darmstadt, Inst Kernphys, D-64289 Darmstadt, Germany; Akademin för Naturvetenskap och Teknik, Örebro Universitet, Örebro, Sweden.
    Chernykh, M.
    Tech Univ Darmstadt, Inst Kernphys, D-64289 Darmstadt, Germany.
    Enders, J.
    Tech Univ Darmstadt, Inst Kernphys, D-64289 Darmstadt, Germany.
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Oberstedt, S.
    EC JRC Inst Reference Mat & Measurements IRMM, B-2440 Geel, Belgium.
    Particle emission angle determination in Frisch grid ionization chambers by electron drift-time measurements2010In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 621, no 1-3, 401-405 p.Article in journal (Refereed)
    Abstract [en]

    The double kinetic energy measurement of fission fragments with a double-sided Frisch grid ionization chamber allows a careful determination of the emission angle, which is essential in order to apply appropriate energy-loss corrections. We present a drift-time method, which uses the time that free electrons need to drift from the location of their creation, e.g. by a fission fragment in the counting gas, to the grid, before inducing a signal on the anode. Such a measurement leaves energy and angular information fully decoupled. We demonstrate the applicability of the drift-time method for the example of the U-234,U-238 (gamma,f) reactions performed at the superconducting Darmstadt electron linear accelerator. The angular resolutions achieved with this method are comparable to those obtained with other methods. (C) 2010 Elsevier B.V. All rights reserved.

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