26 research outputs found
Statistical Physics and Light-Front Quantization
Light-front quantization has important advantages for describing relativistic
statistical systems, particularly systems for which boost invariance is
essential, such as the fireball created in a heavy ion collisions. In this
paper we develop light-front field theory at finite temperature and density
with special attention to quantum chromodynamics. We construct the most general
form of the statistical operator allowed by the Poincare algebra and show that
there are no zero-mode related problems when describing phase transitions. We
then demonstrate a direct connection between densities in light-front thermal
field theory and the parton distributions measured in hard scattering
experiments. Our approach thus generalizes the concept of a parton distribution
to finite temperature. In light-front quantization, the gauge-invariant Green's
functions of a quark in a medium can be defined in terms of just 2-component
spinors and have a much simpler spinor structure than the equal-time fermion
propagator. From the Green's function, we introduce the new concept of a
light-front density matrix, whose matrix elements are related to forward and to
off-diagonal parton distributions. Furthermore, we explain how thermodynamic
quantities can be calculated in discretized light-cone quantization, which is
applicable at high chemical potential and is not plagued by the
fermion-doubling problem.Comment: 30 pages, 3 figures; v2: Refs. added, minor changes, accepted for
publication in PR
Relating parton model and color dipole formulation of heavy quark hadroproduction
At high center of mass energies, hadroproduction of heavy quarks can be
expressed in terms of the same color dipole cross section as low Bjorken-x deep
inelastic scattering. We show analytically that at leading order, the dipole
formulation is equivalent to the gluon-gluon fusion mechanism of the
conventional parton model. In phenomenological application, we employ a
parameterization of the dipole cross section which also includes higher order
and saturation effects, thereby going beyond the parton model. Numerical
calculations in the dipole approach agree well with experimental data on open
charm production over a wide range of energy. Dipole approach and next to
leading order parton model yield similar values for open charm production, but
for open bottom production, the dipole approach tends to predict somewhat
higher cross sections than the parton model.Comment: 16 pages, 4 figure
Nuclear effects in the Drell-Yan process at very high energies
We study Drell-Yan (DY) dilepton production in proton(deuterium)-nucleus and
in nucleus-nucleus collisions within the light-cone color dipole formalism.
This approach is especially suitable for predicting nuclear effects in the DY
cross section for heavy ion collisions, as it provides the impact parameter
dependence of nuclear shadowing and transverse momentum broadening, quantities
that are not available from the standard parton model. For p(D)+A collisions we
calculate nuclear shadowing and investigate nuclear modification of the DY
transverse momentum distribution at RHIC and LHC for kinematics corresponding
to coherence length much longer than the nuclear size. Calculations are
performed separately for transversely and longitudinally polarized DY photons,
and predictions are presented for the dilepton angular distribution.
Furthermore, we calculate nuclear broadening of the mean transverse momentum
squared of DY dileptons as function of the nuclear mass number and energy. We
also predict nuclear effects for the cross section of the DY process in heavy
ion collisions. We found a substantial nuclear shadowing for valence quarks,
stronger than for the sea.Comment: 46 pages, 18 figures, title changed and some discussion added,
accepted for publication in PR
Resummation of nuclear enhanced higher twist in the Drell Yan process
We investigate higher twist contributions to the transverse momentum
broadening of Drell Yan pairs in proton nucleus collisions. We revisit the
contribution of matrix elements of twist-4 and generalize this to matrix
elements of arbitrary twist. An estimate of the maximal nuclear broadening
effect is derived. A model for nuclear enhanced matrix elements of arbitrary
twist allows us to give the result of a resummation of all twists in closed
form. Subleading corrections to the maximal broadening are discussed
qualitatively.Comment: 10 pages, 5 figures; v2: minor changes in text, acknowledgement
added; v3: mistake in fig. 1 correcte
The QCD Pomeron in ultraperipheral heavy ion collisions: III. Photonuclear production of heavy quarks
We calculate the photonuclear production of heavy quarks in ultraperipheral
heavy ion collisions. The integrated cross section and the rapidity
distribution are computed employing sound high energy QCD formalisms as the
collinear and semihard approaches as well as the saturation model. In
particular, the color glass condensate (CGC) formalism is also considered using
a simple phenomenological parameterization for the color field correlator in
the medium, which allow us to obtain more reliable estimates for charm and
bottom production at
LHC energies.Comment: 15 pages, 2 figures. Extended version to be published in Eur. Phys.
J.
Physics of the Muon Spectrometer of the ALICE Experiment
The main goal of the Muon spectrometer of the ALICE experiment at LHC is the
measurement of heavy quark production in p+p, p+A and A+A collisions at LHC
energies, via the muonic channel. Physics motivations and expected performances
have been presented in this talk.Comment: 10 pages and 4 figures. Talk presented in the ICPAQGP Conference,
February 8-12, 2005, Salt Lake City, Kolkata, India. Web page of the
conference : http://www.veccal.ernet.in/~icpaqgp
Gluon Shadowing in DIS off Nuclei
Within a light-cone quantum-chromodynamics dipole formalism based on the
Green function technique, we study nuclear shadowing in deep-inelastic
scattering at small Bjorken xB < 0.01. Such a formalism incorporates naturally
color transparency and coherence length effects. Calculations of the nuclear
shadowing for the \bar{q}q Fock component of the photon are based on an exact
numerical solution of the evolution equation for the Green function, using a
realistic form of the dipole cross section and nuclear density function. Such
an exact numerical solution is unavoidable for xB > 0.0001, when a variation of
the transverse size of the \bar{q}q Fock component must be taken into account.
The eikonal approximation, used so far in most other models, can be applied
only at high energies, when xB < 0.0001 and the transverse size of the \bar{q}q
Fock component is "frozen" during propagation through the nuclear matter. At xB
< 0.01 we find quite a large contribution of gluon suppression to nuclear
shadowing, as a shadowing correction for the higher Fock states containing
gluons. Numerical results for nuclear shadowing are compared with the available
data from the E665 and NMC collaborations. Nuclear shadowing is also predicted
at very small xB corresponding to LHC kinematical range. Finally the model
predictions are compared and discussed with the results obtained from other
models.Comment: 29 pages including 7 figures; Fig.7 modified, some references and
corresponding discussion adde