1,212 research outputs found
What to learn from dilepton transverse momentum spectra in heavy-ion collisions?
Recently the NA60 collaboration has presented high precision measurements of
dimuon spectra double differential in invariant mass and transverse pair
momentum in In-In collisions at . While the
-dependence is important for an understanding of in-medium changes of light
vector mesons and is integrated insensitive to collective expansion, the
-dependence arises from an interplay between emission temperature and
collective transverse flow. This fact can be exploited to derive constraints on
the evolution model and in particular on the contributions of different phases
of the evolution to dimuon radiation into a given window. We present
arguments that a thermalized evolution phase with leaves
its imprint on the spectra.Comment: Contributed to 19th International Conference on Ultrarelativistic
Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14-
20 Nov 200
What does the rho-meson do? In-medium mass shift scenarios versus hadronic model calculations
The NA60 experiment has studied low-mass muon pair production in In-In
collisions at with unprecedented precision. With these results
there is hope that the in-medium modifications of the vector meson spectral
function can be constrained more thoroughly than before. We investigate in
particular what can be learned about collisional broadening by a hot and dense
medium and what constrains the experimental results put on in-medium mass shift
scenarios. The data show a clear indication of considerable in-medium
broadening effects but disfavor mass shift scenarios where the -meson
mass scales with the square root of the chiral condensate. Scaling scenarios
which predict at finite density a dropping of the -meson mass that is
stronger than that of the quark condensate are clearly ruled out since they are
also accompanied by a sharpening of the spectral function.Comment: Proceeding contribution, Talk given by J. Ruppert at Workshop for
Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus
Collisions (Hot Quarks 2006), Villasimius, Sardinia, Italy, 15-20 May 2006.
To appear in EPJ
Systematics of parton-medium interaction from RHIC to LHC
Despite a wealth of experimental data for high-P_T processes in heavy-ion
collisions, discriminating between different models of hard parton-medium
interactions has been difficult. A key reason is that the pQCD parton spectrum
at RHIC is falling so steeply that distinguishing even a moderate shift in
parton energy from complete parton absorption is essentially impossible. In
essence, energy loss models are effectively only probed in the vicinity of zero
energy loss and, as a result, at RHIC energies only the pathlength dependence
of energy loss offers some discriminating power. At LHC however, this is no
longer the case: Due to the much flatter shape of the parton p_T spectra
originating from 2.76 AGeV collisions, the available data probe much deeper
into the model dynamics. A simultaneous fit of the nuclear suppression at both
RHIC and LHC energies thus has great potential for discriminating between
various models that yield equally good descriptions of RHIC data alone.Comment: Talk given at Quark Matter 2011, 22-28 May 2011, Annecy, Franc
Comparing different freeze-out scenarios in azimuthal hadron correlations induced by fast partons
I review the linearized hydrodynamical treatment of a fast parton traversing
a perturbative quark-gluon plasma. Using numerical solutions for the medium's
response to the fast parton, I obtain the medium's distribution function which
is then used in a Cooper-Frye freeze-out prescription to obtain an azimuthal
particle spectrum. Two different freeze-out scenarios are considered which
yield significantly different results. I conclude that any meaningful
comparison of azimuthal hadron correlation functions to RHIC data requires
implementing a realistic freeze-out scenario in an expanding medium.Comment: Contribution to the Proceedings for 2008 Hot Quarks in Estes Park,
CO, as accepted for publication in EPJ-
A comprehensive description of multiple observables in heavy-ion collisions at SPS
Combining and expanding on work from previous publications, a model for the
evolution of ultrarelativistic heavy-ion collisions at the CERN SPS for 158
AGeV beam energy is presented. Based on the assumption of thermalization and a
parametrization of the space-time expansion of the produced matter, this model
is able to describe a large set of observables including hadronic momentum
spectra, correlations and abundancies, the emission of real photons, dilepton
radiation and the suppression pattern of charmonia. Each of these obervables
provides unique capabilities to study the reaction dynamics and taken together
they form a strong and consistent picture of the evolving system. Based on the
emission of hard photons, we argue that a strongly interacting, hot and dense
system with temperatures above 250 MeV has to be created early in the reaction.
Such a system is bound to be different from hadronic matter and likely to be a
quark-gluon plasma, and we find that this assumption is in line with the
subsequent evolution of the system that is reflected in other observables.Comment: 21 pages, 10 figures, submitted to J. Phys.
Thermal photons from fluctuating initial conditions
Event-by-event fluctuations of initial QCD-matter density produced in
heavy-ion collisions at RHIC enhance the production of thermal photons
significantly in the region GeV/ compared to a smooth
initial-state averaged profile in the ideal hydrodynamic calculation. This
enhancement is a an early time effect due to the presence of hotspots or
over-dense regions in the fluctuating initial state. The effect of fluctuations
is found to be stronger in peripheral than in central collisions.Comment: 4 pages, 3 figures. Talk given at Quark Matter 2011, 22-28 May 2011,
Annecy, Franc
Angular hadron correlations probing the early medium evolution
Hard processes are a well calibrated probe to study heavy-ion collisions.
However, the information to be gained from the nuclear suppression factor R_AA
is limited, hene one has to study more differential observables to do medium
tomography. The angular correlations of hadrons associated with a hard trigger
appear suitable as they show a rich pattern when going from low p_T to high
p_T. Of prime interest is the fate of away side partons with an in-medium
pathlength O(several fm). At high p_T the correlations become dominated by the
punchtrough of the away side parton with subsequent fragmentation. We discuss
what information about the medium density can be gained from the data.Comment: Talk given at the 19th International Conference on Ultrarelativistic
Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China,
14-20 Nov 200
Hard and soft probe - medium interactions in a 3D hydro+micro approach at RHIC
We utilize a 3D hybrid hydro+micro model for a comprehensive and consistent
description of soft and hard particle production in ultra-relativistic
heavy-ion collisions at RHIC. In the soft sector we focus on the dynamics of
(multi-)strange baryons, where a clear strangeness dependence of their
collision rates and freeze-out is observed. In the hard sector we study the
radiative energy loss of hard partons in a soft medium in the multiple soft
scattering approximation. While the nuclear suppression factor does
not reflect the high quality of the medium description (except in a reduced
systematic uncertainty in extracting the quenching power of the medium), the
hydrodynamical model also allows to study different centralities and in
particular the angular variation of with respect to the reaction
plane, allowing for a controlled variation of the in-medium path-length.Comment: 5 pages, 4 figures, Quark Matter 2006 proceedings, to appear in
Journal of Physics
Emission of thermal photons and the equilibration time in Heavy-Ion collisions
The emission of hard real photons from thermalized expanding hadronic matter
is dominated by the initial high-temperature expansion phase. Therefore, a
measurement of photon emission in ultrarelativistic heavy-ion collisions
provides valuable insights into the early conditions realized in such a
collision. In particular, the initial temperature of the expanding fireball or
equivalently the equilibration time of the strongly interacting matter are of
great interest. An accurate determination of these quantities could help to
answer the question whether or not partonic matter (the quark gluon plasma) is
created in such collisions. In this work, we investigate the emission of real
photons using a model which is based on the thermodynamics of QCD matter and
which has been shown to reproduce a large variety of other observables. With
the fireball evolution fixed beforehand, we are able to extract limits for the
equilibration time by a comparison with photon emission data measured by WA98.Comment: 12 pages, 5 figures, accepted for publication at Phys. Rev.
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