1,623 research outputs found
Lorentz-covariant deformed algebra with minimal length
The -dimensional two-parameter deformed algebra with minimal length
introduced by Kempf is generalized to a Lorentz-covariant algebra describing a
()-dimensional quantized space-time. For D=3, it includes Snyder algebra
as a special case. The deformed Poincar\'e transformations leaving the algebra
invariant are identified. Uncertainty relations are studied. In the case of D=1
and one nonvanishing parameter, the bound-state energy spectrum and
wavefunctions of the Dirac oscillator are exactly obtained.Comment: 8 pages, no figure, presented at XV International Colloquium on
Integrable Systems and Quantum Symmetries (ISQS-15), Prague, June 15-17, 200
The effect of memory on relaxation in a scalar field theory
We derive a kinetic equation with a non-Markovian collision term which
includes a memory effect, from Kadanoff-Baym equations in theory
within the three-loop level for the two-particle irreducible (2PI) effective
action. The memory effect is incorporated into the kinetic equation by a
generalized Kadanoff-Baym ansatz.Based on the kinetic equations with and
without the memory effect, we investigate an influence of this effect on decay
of a single particle excitation with zero momentum in 3+1 dimensions and the
spatially homogeneous case. Numerical results show that, while the time
evolution of the zero mode is completely unaffected by the memory effect due to
a separation of scales in the weak coupling regime, this effect leads first to
faster relaxation than the case without it and then to slower relaxation as the
coupling constant increases.Comment: 12 pages, 6 eps figure
Orthogonality Effects in Relativistic Models of Nucleon Knockout Reactions
We study the effect of wave function orthogonality in the relativistic
treatment of the nucleon removal reactions (gamma, p) and (e, e' p). The
continuum wave function describing the outgoing nucleon is made orthogonal to
the relevant bound states using the Gram-Schmidt procedure. This procedure has
the advantage of preserving the asymptotic character of the continuum wave
function and hence the elastic observables are unaffected. The orthogonality
effects are found to be negligible for (e, e' p) reactions for missing momenta
up to 700 MeV/c. This holds true for both parallel and perpendicular
kinematics. By contrast the orthogonalization of the wave functions appears to
have a more pronounced effect in the case of (gamma, p) reactions. We find that
the orthogonality effect can be significant in this case particularly for large
angles. Polarization of the outgoing protons and photon asymmetry show more
sensitivity than the cross sections. If the orthogonality condition is imposed
solely on this one hole state the effects are usually smaller.Comment: LaTeX, 7 postscript figure
Relativistic Calculations for Photonuclear Reactions (III): A Consistent Relativistic Analysis of the (e,e'p) and (gamma,p) Reactions
Relativistic calculations for the quasifree electron scattering process (e,
e'p) and the direct knockout contribution to (gamma, p) reactions are
presented. The spectroscopic factors determined from the former reaction are
used to fix the magnitude of the knockout contribution to the (gamma, p)
reaction at 60 MeV. The results obtained for several nuclei indicate that the
knockout contributions are much larger in magnitude and hence closer to the
data than predicted in an earlier comparison based on non-relativistic
calculations. We discuss the sensitivity of the results to the choice of
parameters for the binding and final state interactions. We find these
uncertainties to be more pronounced at the larger missing momenta explored by
the (gamma, p) reaction. The implications of the present results for the size
of contributions due to meson exchange currents are discussed.Comment: LaTeX, 21 pages including 5 figures, submitted to Nuc. Phys.
Nuclear Medium Effects in the Relativistic Treatment of Quasifree Electron Scattering
Non-relativistic reduction of the S-matrix for the quasifree electron
scattering process is studied in order to
understand the source of differences between non-relativistic and relativistic
models. We perform an effective Pauli reduction on the relativistic expression
for the S-matrix in the one-photon exchange approximation. The reduction is
applied to the nucleon current only; the electrons are treated fully
relativistically. An expansion of the amplitude results in a power series in
the nuclear potentials. The series is found to converge rapidly only if the
nuclear potentials are included in the nuclear current operator. The results
can be cast in a form which reproduces the non-relativistic amplitudes in the
limit that the potentials are removed from the nuclear current operator. Large
differences can be found between calculations which do and do not include the
nuclear potentials in the different orders of the nuclear current operator. In
the high missing momentum region we find that the non-relativistic calculations
with potentials included in the nuclear current up to second order give results
which are close to those of the fully relativistic calculation. This behavior
is an indication of the importance of the medium modifications of the nuclear
currents in this model, which are naturally built into the relativistic
treatment of the reaction.Comment: Latex, 26 pages including 5 uuencoded postscript figures. accepted
for publication in Phys. Rev. C
Quantum dynamics and thermalization for out-of-equilibrium phi^4-theory
The quantum time evolution of \phi^4-field theory for a spatially homogeneous
system in 2+1 space-time dimensions is investigated numerically for
out-of-equilibrium initial conditions on the basis of the Kadanoff-Baym
equations including the tadpole and sunset self-energies. Whereas the tadpole
self-energy yields a dynamical mass, the sunset self-energy is responsible for
dissipation and an equilibration of the system. In particular we address the
dynamics of the spectral (`off-shell') distributions of the excited quantum
modes and the different phases in the approach to equilibrium described by
Kubo-Martin-Schwinger relations for thermal equilibrium states. The
investigation explicitly demonstrates that the only translation invariant
solutions representing the stationary fixed points of the coupled equation of
motions are those of full thermal equilibrium. They agree with those extracted
from the time integration of the Kadanoff-Baym equations in the long time
limit. Furthermore, a detailed comparison of the full quantum dynamics to more
approximate and simple schemes like that of a standard kinetic (on-shell)
Boltzmann equation is performed. Our analysis shows that the consistent
inclusion of the dynamical spectral function has a significant impact on
relaxation phenomena. The different time scales, that are involved in the
dynamical quantum evolution towards a complete thermalized state, are discussed
in detail. We find that far off-shell 1 3 processes are responsible for
chemical equilibration, which is missed in the Boltzmann limit. Finally, we
address briefly the case of (bare) massless fields. For sufficiently large
couplings we observe the onset of Bose condensation, where our scheme
within symmetric \phi^4-theory breaks down.Comment: 77 pages, 26 figure
Relativistic versus Nonrelativistic Optical Potentials in A(e,e'p)B Reactions
We investigate the role of relativistic and nonrelativistic optical
potentials used in the analysis of () data. We find that the
relativistic calculations produce smaller () cross sections even in the
case in which both relativistic and nonrelativistic optical potentials fit
equally well the elastic proton--nucleus scattering data. Compared to the
nonrelativistic impulse approximation, this effect is due to a depletion in the
nuclear interior of the relativistic nucleon current, which should be taken
into account in the nonrelativistic treatment by a proper redefinition of the
effective current operator.Comment: Added one new figure, the formalism section has been enlarged and the
list of references updated. Added one appendix. This version will appear in
Phys. Rev. C. Revtex 3.0, 6 figures (not included). Full postscript version
of the file and figures available at
http://www.nikhefk.nikhef.nl/projects/Theory/preprints
Breakup Reactions of 11Li within a Three-Body Model
We use a three-body model to investigate breakup reactions of 11Li (n+n+9Li)
on a light target. The interaction parameters are constrained by known
properties of the two-body subsystems, the 11Li binding energy and
fragmentation data. The remaining degrees of freedom are discussed. The
projectile-target interactions are described by phenomenological optical
potentials. The model predicts dependence on beam energy and target,
differences between longitudinal and transverse momentum distributions and
provides absolute values for all computed differential cross sections. We give
an almost complete series of observables and compare with corresponding
measurements. Remarkably good agreement is obtained. The relative neutron-9Li
p-wave content is about 40%. A p-resonance, consistent with measurements at
about 0.5 MeV of width about 0.4 MeV, seems to be necessary. The widths of the
momentum distributions are insensitive to target and beam energy with a
tendency to increase towards lower energies. The transverse momentum
distributions are broader than the longitudinal due to the diffraction process.
The absolute values of the cross sections follow the neutron-target cross
sections and increase strongly for beam energies decreasing below 100 MeV/u.Comment: 19 pages, 14 figures, RevTeX, psfig.st
Interactions between cell surface protein disulphide isomerase and S-nitrosoglutathione during nitric oxide delivery
In this study, we investigated the role of protein disulphide isomerase (PDI) in rapid metabolism of S-nitrosoglutathione (GSNO) and S-nitrosoalbumin (albSNO) and in NO delivery from these compounds into cells. Incubation of GSNO or albSNO (1 μM) with the megakaryocyte cell line MEG-01 resulted in a cell-mediated removal of each compound which was inhibited by blocking cell surface thiols with 5,5′-dithiobis 2-nitrobenzoic acid (DTNB) (100 μM) or inhibiting PDI with bacitracin (5 mM). GSNO, but not albSNO, rapidly inhibited platelet aggregation and stimulated cyclic GMP (cGMP) accumulation (used as a measure of intracellular NO entry). cGMP accumulation in response to GSNO (1 μM) was inhibited by MEG-01 treatment with bacitracin or DTNB, suggesting a role for PDI and surface thiols in NO delivery. PDI activity was present in MEG-01 conditioned medium, and was inhibited by high concentrations of GSNO (500 μM). A number of cell surface thiol-containing proteins were labelled using the impermeable thiol specific probe 3-(N-maleimido-propionyl) biocytin (MPB). Pretreatment of cells with GSNO resulted in a loss of thiol reactivity on some but not all proteins, suggesting selective cell surface thiol modification. Immunoprecipitation experiments showed that GSNO caused a concentration-dependent loss of thiol reactivity of PDI. Our data indicate that PDI is involved in both rapid metabolism of GSNO and intracellular NO delivery and that during this process PDI is itself altered by thiol modification. In contrast, the relevance of PDI-mediated albSNO metabolism to NO signalling is uncertain
Astroparticle Physics with a Customized Low-Background Broad Energy Germanium Detector
The MAJORANA Collaboration is building the MAJORANA DEMONSTRATOR, a 60 kg
array of high purity germanium detectors housed in an ultra-low background
shield at the Sanford Underground Laboratory in Lead, SD. The MAJORANA
DEMONSTRATOR will search for neutrinoless double-beta decay of 76Ge while
demonstrating the feasibility of a tonne-scale experiment. It may also carry
out a dark matter search in the 1-10 GeV/c^2 mass range. We have found that
customized Broad Energy Germanium (BEGe) detectors produced by Canberra have
several desirable features for a neutrinoless double-beta decay experiment,
including low electronic noise, excellent pulse shape analysis capabilities,
and simple fabrication. We have deployed a customized BEGe, the MAJORANA
Low-Background BEGe at Kimballton (MALBEK), in a low-background cryostat and
shield at the Kimballton Underground Research Facility in Virginia. This paper
will focus on the detector characteristics and measurements that can be
performed with such a radiation detector in a low-background environment.Comment: Submitted to NIMA Proceedings, SORMA XII. 9 pages, 4 figure
- …