1,279 research outputs found
A systematic comparison of jet quenching in different fluid-dynamical models
Comparing four different (ideal and viscous) hydrodynamic models for the
evolution of the medium created in 200 AGeV Au-Au collisions, combined with two
different models for the path length dependence of parton energy loss, we study
the effects of jet quenching on the emission-angle dependence of the nuclear
suppression factor R_AA(phi) and the away-side per trigger yield I_AA(phi).
Each hydrodynamic model was tuned to provide a reasonable description of the
single-particle transverse momentum spectra for all collision centralities, and
the energy loss models were adjusted to yield the same pion nuclear suppression
factor in central Au-Au collisions. We find that the experimentally measured
in-plane vs. out-of-plane spread in R_AA(phi) is better reproduced by models
that shift the weight of the parton energy loss to later times along its path.
Among the models studied here, this is best achieved by energy loss models that
suppress energy loss at early times, combined with hydrodynamic models that
delay the dilution of the medium density due to hydrodynamic expansion by
viscous heating. We were unable to identify a clear tomographic benefit of a
measurement of I_AA(phi) over that of R_AA(phi).Comment: 17 pages, 11 figure
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.
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
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
Constraining the Physics of Jet Quenching
Hard probes in the context of ultrarelativistic heavy ion collisions
represent a key class of observables studied to gain informations about the QCD
medium created in such collisions. However, in practice the so-called jet
tomography has turned out to be more difficult than expected initially. One of
the major obstacles in extracting reliable tomographic information from the
data is that neither the parton-medium interaction nor the medium geometry are
known with great precision, and thus a difference in model assumptions in the
hard perturbative Quantum Choromdynamics (pQCD) modelling can usually be
compensated by a corresponding change of assumptions in the soft bulk medium
sector and vice versa. The only way to overcome this problem is to study the
full systematics of combinations of parton-medium interaction and bulk medium
evolution models. This work presents a meta-analysis summarizing results from a
number of such systematical studies and discusses in detail how certain data
sets provide specific constraints for models. Combining all available
information, only a small group of models exhibiting certain characteristic
features consistent with a pQCD picture of parton-medium interaction is found
to be viable given the data. In this picture, the dominant mechanism is
medium-induced radiation combined with a surprisingly small component of
elastic energy transfer into the medium.Comment: 22 pages, 3 figures, 2 table
Color Transparency via Coherent Exclusive rho Production
We examine the potential of the COMPASS experiment at CERN to study color
transparency via exclusive coherent vector meson production in hard
muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity
to test this important prediction of perturbative QCD.Comment: Feasibility study for COMPASS collaboration, 3 pages, no figures, 1
table. This contribution is based on talk presented at the Workshop on Spin
Physics, Trento, Italy, July 2001. The conference www site is
http://ECTstar.ect.it/contents.html The proceedings of the workshop will be
published as a special issue of Nuclear Physics B (proc suppl), eds. S Bass,
A De Roeck and A Deshpande. A more complete 26 page feasibility study with 7
figures and 3 tables is available as A. Sandacz et al., hep-ex/0106076
Revision of 26 Sept. for Ref. 11 modificatio
The role of metrology in axSpA : does it provide unique information in assessing patients and predicting outcome? Results from the BSRBR-AS registry
ACKNOWLEDGMENTS We thank the staff who contributed to running the BSRBR-AS register and we also thank the recruiting staff at the clinical centers, details of which are available at: www.abdn.ac.uk/bsrbr-as.Peer reviewedPostprin
Cosmological constraints on decaying axion-like particles: a global analysis
Axion-like particles (ALPs) decaying into photons are known to affect a wide range of astrophysical and cosmological observables. In this study we focus on ALPs with masses in the keVâMeV range and lifetimes between 10 and 10 seconds, corresponding to decays between the end of Big Bang Nucleosynthesis and the formation of the Cosmic Microwave Background (CMB). Using the CosmoBit module of the global fitting framework GAMBIT, we combine state-of-the-art calculations of the irreducible ALP freeze-in abundance, primordial element abundances (including photodisintegration through ALP decays), CMB spectral distortions and anisotropies, and constraints from supernovae and stellar cooling. This approach makes it possible for the first time to perform a global analysis of the ALP parameter space while varying the parameters of ÎCDM as well as several nuisance parameters. We find a lower bound on the ALP mass of around m > 300 keV, which can only be evaded if ALPs are stable on cosmological timescales. Future observations of CMB spectral distortions with a PIXIE-like mission are expected to improve this bound by two orders of magnitude
Cosmological constraints on decaying axion-like particles: a global analysis
Axion-like particles (ALPs) decaying into photons are known to affect a wide range of astrophysical and cosmological observables. In this study we focus on ALPs with masses in the keV-MeV range and lifetimes between 10 and 10 seconds, corresponding to decays between the end of Big Bang Nucleosynthesis and the formation of the Cosmic Microwave Background (CMB). Using the CosmoBit module of the global fitting framework GAMBIT, we combine state-of-the-art calculations of the irreducible ALP freeze-in abundance, primordial element abundances (including photodisintegration through ALP decays), CMB spectral distortions and anisotropies, and constraints from supernovae and stellar cooling. This approach makes it possible for the first time to perform a global analysis of the ALP parameter space while varying the parameters of ÎCDM as well as several nuisance parameters. We find a lower bound on the ALP mass of around m>300keV, which can only be evaded if ALPs are stable on cosmological timescales. Future observations of CMB spectral distortions with a PIXIE-like mission are expected to improve this bound by two orders of magnitude
Final state interactions in two-particle interferometry
We reconsider the influence of two-particle final state interactions (FSI) on
two-particle Bose-Einstein interferometry. We concentrate in particular on the
problem of particle emission at different times. Assuming chaoticity of the
source, we derive a new general expression for the symmetrized two-particle
cross section. We discuss the approximations needed to derive from the general
result the Koonin-Pratt formula. Introducing a less stringent version of the
so-called smoothness approximation we also derive a more accurate formula. It
can be implemented into classical event generators and allows to calculate FSI
corrected two-particle correlation functions via modified Bose-Einstein
"weights".Comment: 12 pages RevTeX, 2 ps-figures included, submitted to Phys. Rev.
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