232 research outputs found
Upsilon production in pp and pA collisions: from RHIC to the LHC
I discuss Upsilon production in pp collisions at RHIC, Tevatron and LHC
energies, in particular the behaviour of the differential cross section in
rapidity and the impact of QCD corrections on the P_T differential cross
section. I also emphasise the very good agreement between the parameter-free
predictions of the Colour-Singlet Model (CSM) and the first LHC data,
especially in the region of low transverse momenta, which is the most relevant
one for heavy-ion studies. I also show that the CSM predicts Upsilon
cross-section ratios in agreement with the most recent LHC data. I then briefly
discuss the nuclear-matter effects on Upsilon production at RHIC and the LHC in
p(d)A collisions and, by extension, in AA collisions. I argue that a) the
Upsilon break-up probability can be neglected, at RHIC and the LHC, b) gluon
shadowing --although non-negligible-- is not strong enough to describe forward
RHIC data, c) backward RHIC data hints at a gluon EMC effect, possibly stronger
than the quark one. Outlooks for the LHC pPb run are also presented.Comment: Contribution to the 5th International Conference On Hard And
Electromagnetic Probes Of High-Energy Nuclear Collisions (HP2012), 27 May - 1
June 2012, Cagliari, Italy. 4 pages, 5 figures, LaTeX, uses ecrc.sty
(included). v2: version to appear in Nucl. Phys. A (A few typos corrected, 2
refs. added and fig. 1(a) updated with the new STAR point
A path integral for heavy-quarks in a hot plasma
We propose a model for the propagation of a heavy-quark in a hot plasma, to
be viewed as a first step towards a full description of the dynamics of heavy
quark systems in a quark-gluon plasma, including bound state formation. The
heavy quark is treated as a non relativistic particle interacting with a
fluctuating field, whose correlator is determined by a hard thermal loop
approximation. This approximation, which concerns only the medium in which the
heavy quark propagates, is the only one that is made, and it can be improved.
The dynamics of the heavy quark is given exactly by a quantum mechanical path
integral that is calculated in this paper in the Euclidean space-time using
numerical Monte Carlo techniques. The spectral function of the heavy quark in
the medium is then reconstructed using a Maximum Entropy Method. The path
integral is also evaluated exactly in the case where the mass of the heavy
quark is infinite; one then recovers known results concerning the complex
optical potential that controls the long time behavior of the heavy quark. The
heavy quark correlator and its spectral function is also calculated
semi-analytically at the one-loop order, which allows for a detailed
description of the coupling between the heavy quark and the plasma collective
modes
Cold nuclear matter effects on J/psi production: intrinsic and extrinsic transverse momentum effects
Cold nuclear matter effects on J/psi production in proton-nucleus and
nucleus-nucleus collisions are evaluated taking into account the specific J/psi
production kinematics at the partonic level, the shadowing of the initial
parton distributions and the absorption in the nuclear matter. We consider two
different parton processes for the c-cbar pair production: one with collinear
gluons and a recoiling gluon in the final state and the other with initial
gluons carrying intrinsic transverse momentum. Our results are compared to RHIC
observables. The smaller values of the nuclear modification factor R_AA in the
forward rapidity region (with respect to the mid rapidity region) are partially
explained, therefore potentially reducing the need for recombination effects.Comment: 7 pages, 11 figures, LaTeX, uses elsarticle.cls (included).v2:
version (with minor text revisions and Fig 2 and 4a modified) to appear in
Phys.Lett.
Quasiparticle Description of the QCD Plasma, Comparison with Lattice Results at Finite T and Mu
We compare our 2+1 flavor, staggered QCD lattice results with a quasiparticle
picture. We determine the pressure, the energy density, the baryon density, the
speed of sound and the thermal masses as a function of T and . For the
available thermodynamic quantities the difference is a few percent between the
results of the two approaches. We also give the phase diagram on the --T
plane and estimate the critical chemical potential at vanishing temperature.Comment: 13 pages, 10 figure
Dilepton Production at SPS-energy Heavy Ion Collisions
The production of dileptons is studied within a hadronic transport model. We
investigate the sensitivity of the dilepton spectra to the initial
configuration of the hadronic phase in a ultrarelativistic heavy ion collision.
Possible in medium correction due to the modifications of pions and the pion
form factor in a hadronic gas are discussed.Comment: Dedicated to Gerry Brown in honor of the 32nd celebration of his 39th
birthday. 31 pages Latex including 13 eps-figures, uses psfig.sty and
epsf.st
Event-by-event fluctuations of the mean transverse momentum in 40, 80, and 158 A GeV/c Pb-Au collisions
Measurements of event-by-event fluctuations of the mean transverse momentum
in Pb-Au collisions at 40, 80, and 158 A GeV/c are presented. A significant
excess of mean p_T fluctuations at mid-rapidity is observed over the
expectation from statistically independent particle emission. The results are
somewhat smaller than recent measurements at RHIC. A possible non-monotonic
behaviour of the mean p_T fluctuations as function of collision energy, which
may have indicated that the system has passed the critical point of the QCD
phase diagram in the range of mu_B under investigation, has not been observed.
The centrality dependence of mean p_T fluctuations in Pb-Au is consistent with
an extrapolation from pp collisions assuming that the non-statistical
fluctuations scale with multiplicity. The results are compared to calculations
by the RQMD and UrQMD event generators.Comment: 28 pages, 10 figure
Do Instantons Like a Colorful Background?
We investigate chiral symmetry breaking and color symmetry breaking in QCD.
The effective potential of the corresponding scalar condensates is discussed in
the presence of non-perturbative contributions from the semiclassical
one-instanton sector. We concentrate on a color singlet scalar background which
can describe chiral condensation, as well as a color octet scalar background
which can generate mass for the gluons. Whereas a non-vanishing singlet chiral
field is favored by the instantons, we have found no indication for a
preference of color octet backgrounds.Comment: 25 pages, 7 figure
Lattice QCD Constraints on the Nuclear Equation of State
Based on the quasi-particle description of the QCD medium at finite
temperature and density we formulate the phenomenological model for the
equation of state that exhibits crossover or the first order deconfinement
phase transition. The models are constructed in such a way to be
thermodynamically consistent and to satisfy the properties of the ground state
nuclear matter comply with constraints from intermediate heavy--ion collision
data. Our equations of states show quite reasonable agreement with the recent
lattice findings on temperature and baryon chemical potential dependence of
relevant thermodynamical quantities in the parameter range covering both the
hadronic and quark--gluon sectors. The model predictions on the isentropic
trajectories in the phase diagram are shown to be consistent with the recent
lattice results. Our nuclear equations of states are to be considered as an
input to the dynamical models describing the production and the time evolution
of a thermalized medium created in heavy ion collisions in a broad energy range
from SIS up to LHC.Comment: 13 pages, 11 figure
Hadron Production in Heavy Ion Collisions
We review hadron production in heavy ion collisions with emphasis on pion and
kaon production at energies below 2 AGeV and on partonic collectivity at RHIC
energies.Comment: 31 pages, 26 figures, accepted for publication in Landolt-Boernstein
Volume 1-23
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
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