130 research outputs found
Transversity distributions in the nucleon in the large-N_c limit
We compute the quark and antiquark transversity distributions in the nucleon
at a low normalization point of 600 MeV in the large- limit, where the
nucleon can be described as a soliton of an effective chiral theory (chiral
quark-soliton model). The flavor-nonsinglet distributions, and , appear in leading order
of the -expansion, while the flavor-singlet distributions, and , are non-zero only in
next-to-leading order. The transversity quark and antiquark distributions are
found to be significantly different from the longitudinally polarized
distributions and , respectively, in contrast to the prediction of the naive
non-relativistic quark model. We show that this affects the predictions for the
spin asymmetries in Drell-Yan pair production in transversely polarized pp and
ppbar collisions.Comment: 45 pages, 16 figure
Moments of Nucleon Light Cone Quark Distributions Calculated in Full Lattice QCD
Moments of the quark density, helicity, and transversity distributions are
calculated in unquenched lattice QCD. Calculations of proton matrix elements of
operators corresponding to these moments through the operator product expansion
have been performed on lattices for Wilson fermions at using configurations from the SESAM collaboration and at
using configurations from SCRI. One-loop perturbative renormalization
corrections are included. At quark masses accessible in present calculations,
there is no statistically significant difference between quenched and full QCD
results, indicating that the contributions of quark-antiquark excitations from
the Dirac Sea are small. Close agreement between calculations with cooled
configurations containing essentially only instantons and the full gluon
configurations indicates that quark zero modes associated with instantons play
a dominant role. Naive linear extrapolation of the full QCD calculation to the
physical pion mass yields results inconsistent with experiment. Extrapolation
to the chiral limit including the physics of the pion cloud can resolve this
discrepancy and the requirements for a definitive chiral extrapolation are
described.Comment: 53 Pages Revtex, 26 Figures, 9 Tables. Added additional reference and
updated referenced data in Table I
High-p_T pion and kaon production in relativistic nuclear collisions
High-p_T pion and kaon production is studied in relativistic proton-proton,
proton-nucleus, and nucleus-nucleus collisions in a wide energy range. Cross
sections are calculated based on perturbative QCD, augmented by a
phenomenological transverse momentum distribution of partons (``intrinsic
k_T''). An energy dependent width of the transverse momentum distribution is
extracted from pion and charged hadron production data in
proton-proton/proton-antiproton collisions. Effects of multiscattering and
shadowing in the strongly interacting medium are taken into account.
Enhancement of the transverse momentum width is introduced and parameterized to
explain the Cronin effect. In collisions between heavy nuclei, the model
over-predicts central pion production cross sections (more significantly at
higher energies), hinting at the presence of jet quenching. Predictions are
made for proton-nucleus and nucleus-nucleus collisions at RHIC energies.Comment: 26 pages in Latex, 19 EPS figure
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
Neutron structure function and inclusive DIS from H-3 and He-3 at large Bjorken-x
A detailed study of inclusive deep inelastic scattering (DIS) from mirror A =
3 nuclei at large values of the Bjorken variable x is presented. The main
purpose is to estimate the theoretical uncertainties on the extraction of the
neutron DIS structure function from such nuclear measurements. On one hand,
within models in which no modification of the bound nucleon structure functions
is taken into account, we have investigated the possible uncertainties arising
from: i) charge symmetry breaking terms in the nucleon-nucleon interaction, ii)
finite Q**2 effects neglected in the Bjorken limit, iii) the role of different
prescriptions for the nucleon Spectral Function normalization providing baryon
number conservation, and iv) the differences between the virtual nucleon and
light cone formalisms. Although these effects have been not yet considered in
existing analyses, our conclusion is that all these effects cancel at the level
of ~ 1% for x < 0.75 in overall agreement with previous findings. On the other
hand we have considered several models in which the modification of the bound
nucleon structure functions is accounted for to describe the EMC effect in DIS
scattering from nuclei. It turns out that within these models the cancellation
of nuclear effects is expected to occur only at a level of ~ 3%, leading to an
accuracy of ~ 12 % in the extraction of the neutron to proton structure
function ratio at x ~ 0.7 -0.8$. Another consequence of considering a broad
range of models of the EMC effect is that the previously suggested iteration
procedure does not improve the accuracy of the extraction of the neutron to
proton structure function ratio.Comment: revised version to appear in Phys. Rev. C; main modifications in
Section 4; no change in the conclusion
Strangeness in the Nucleon on the Light-Cone
Strange matrix elements of the nucleon are calculated within the light-cone
formulation of the meson cloud model. The dependence of the strange
vector and axial vector form factors is computed, and the strangeness radius
and magnetic moment extracted, both of which are found to be very small and
slightly negative. Within the same framework one finds a small but non-zero
excess of the antistrange distribution over the strange at large . Kaon
loops are unlikely, however, to be the source of a large polarized strange
quark distribution.Comment: 22 pages revtex, 7 postscript figures, accepted for publication in
Phys. Rev.
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Light flavor asymmetry of nucleon sea
The light flavor antiquark distributions of the nucleon sea are calculated in
the effective chiral quark model and compared with experimental results. The
contributions of the flavor-symmetric sea-quark distributions and the nuclear
EMC effect are taken into account to obtain the ratio of Drell-Yan cross
sections , which can match well
with the results measured in the FermiLab E866/NuSea experiment. The calculated
results also match the measured from different
experiments, but unmatch the behavior of derived
indirectly from the measurable quantity
by the FermiLab E866/NuSea
Collaboration at large . We suggest to measure again
at large from precision experiments with careful experimental data
treatment. We also propose an alternative procedure for experimental data
treatment.Comment: 10 pages, 8 figures, final version to appear in EPJ
Dynamic stability of vortex solutions of Ginzburg-Landau and nonlinear Schrödinger equations
The dynamic stability of vortex solutions to the Ginzburg-Landau and nonlinear Schrödinger equations is the basic assumption of the asymptotic particle plus field description of interacting vortices. For the Ginzburg-Landau dynamics we prove that all vortices are asymptotically nonlinearly stable relative to small radial perturbations. Initially finite energy perturbations of vortices decay to zero in L p (â 2 ) spaces with an algebraic rate as time tends to infinity. We also prove that under general (nonradial) perturbations, the plus and minus one-vortices are linearly dynamically stable in L 2 ; the linearized operator has spectrum equal to (ââ, 0] and generates a C 0 semigroup of contractions on L 2 (â 2 ). The nature of the zero energy point is clarified; it is resonance , a property related to the infinite energy of planar vortices. Our results on the linearized operator are also used to show that the plus and minus one-vortices for the Schrödinger (Hamiltonian) dynamics are spectrally stable, i.e. the linearized operator about these vortices has ( L 2 ) spectrum equal to the imaginary axis. The key ingredients of our analysis are the Nash-Aronson estimates for obtaining Gaussian upper bounds for fundamental solutions of parabolic operator, and a combination of variational and maximum principles.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46494/1/220_2005_Article_BF02099719.pd
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