1,672 research outputs found
Heavy-ion collisions - hot QCD in a lab
High-energy heavy-ion collisions provide a unique opportunity to study the
properties of the hot and dense strongly-interacting system composed of
deconfined quarks and gluons -- the quark-gluon plasma (QGP) -- in laboratory
conditions. The formation of a QGP is predicted by lattice QCD calculations as
a crossover transition from hadronic matter (at zero baryochemical potential)
and is expected to take place once the system temperature reaches values above
155 MeV and/or the energy density above . The
nature of such a strongly coupled QGP has been linked to the early Universe at
some microseconds after the Big Bang. To characterize the physical properties
of the short-lived matter (lifetime of about ) experimental
studies at Relativistic Heavy-Ion Collider and the Large Hadron collider use
auto-generated probes, such as high-energy partons created early in the
hadronic collisions, thermally emitted photons, and a set of particle
correlations that are sensitive to the collective expansion and the dynamics of
the system. The lectures briefly introduced some of the experimental techniques
and provided a glimpse at some of the results.Comment: Proceedings of the XIV International Workshop on Hadron Physics,
Florianopolis, Brazil, March 201
Two- and three-particle azimuthal correlations of high-pt charged hadrons in Pb-Au collisions at 158A GeV/c
Azimuthal correlations of hadrons with high transverse momenta serve as a
measure to study the energy loss and the fragmentation pattern of jets emerging
from hard parton-parton interactions in heavy ion collisions. Preliminary
results from the CERES experiment on two- and three-particle correlations in
central Pb-Au collisions are presented. A strongly non-Gaussian shape on the
away-side of the two-particle correlation function is observed, indicating
significant interactions of the emerging partons with the medium. Mechanisms
like deflection of the initial partons or the evolution of a mach cone in the
medium can lead to similar modifications of the jet structure on the away-side.
An analysis based on three-particle correlations is presented which helps to
shed light on the origin of the observed away-side pattern.Comment: 4 pages, 2 figures, contribution to the Quark Matter conference 200
Two-particle azimuthal correlations at high transverse momentum in Pb - Au at 158 AGeV/c
The study of two-particle azimuthal correlations at high transverse momentum has become an important tool to investigate the interaction of hard partons with the medium formed in high-energy nucleus-nucleus collisions. At SPS energies, pioneering studies by the CERES collaboration indicated a significant modification of the away-side structure in central collisions. Here we present new results emerging from the analysis of the year 2000 data set recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency and significantly improved momentum determination
Novel subjet observables for jet quenching in heavy-ion collisions
Using a novel observable that relies on the momentum difference of the two
most energetic subjets within a jet we study the internal
structure of high-energy jets simulated by several Monte Carlo event generators
that implement the partonic energy-loss in a dense partonic medium. Based on
inclusive jet and di-jet production we demonstrate that is an
effective tool to discriminate between different models of jet modifications
over a broad kinematic range. The new quantity, while preserving the colinear
and infrared safety of modern jet algorithms, it is experimentally attractive
because of its inherent resiliance against backgrounds of heavy-ion collisions.Comment: v1: 10 pages. v2: Includes (i) additional discussion about best
discriminant by calculating the RSD (ii) new section about hadronization
effects on the reconstructed subjets; version to be published in European
Physical Journal
Two particle azimuthal correlations at high transverse momentum in Pb - Au at 158 AGeV/c
The analysis of two-particle azimuthal angular correlations at high transverse momentum from Pb+Au collisions at 158 AGeV/c at SPS reveals substantial modifications of the away-side peak as compared to the distributions from p+p reactions. The data recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency, suggest that the observed modification of the back-to-back structure implies significant re-interactions of the scattered partons within the medium. These findings at top SPS energy show features qualitatively similar to the results obtained at RHIC from Au+Au collisions at GeV/c and Cu+Cu collisions at GeV/c. We present the centrality and charge dependent conditional di-jet yields for similar transverse momentum windows as investigated by the RHIC experimentsThe analysis of two-particle azimuthal angular correlations at high transverse momentum from Pb+Au collisions at 158 AGeV/c at SPS reveals substantial modifications of the away-side peak as compared to the distributions from p+p reactions. The data recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency, suggest that the observed modification of the back-to-back structure implies significant re-interactions of the scattered partons within the medium. These findings at top SPS energy show features qualitatively similar to the results obtained at RHIC from Au+Au collisions at s = 200 GeV / c and Cu+Cu collisions at s = 62 GeV / c [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004 , M. McCumber and J. Frantz, arXiv:nucl-ex/0511048 ]. We present the centrality and charge dependent conditional di-jet yields for similar transverse momentum windows as investigated by the RHIC experiments [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004 , M. McCumber and J. Frantz, arXiv:nucl-ex/0511048 , J. Adams et al., STAR Collaboration, Phys. Rev. Lett. 95 (2005) 152301, arXiv:nucl-ex/0501016 ]
Identifying groomed jet splittings in heavy-ion collisions
Measurements of jet substructure in heavy-ion collisions may provide key
insight to the nature of jet quenching in the quark-gluon plasma. Jet grooming
techniques from high-energy physics have been applied to heavy-ion collisions
in order to isolate theoretically controlled jet observables and explore
possible modification to the hard substructure of jets. However, the grooming
algorithms used have not been tailored to the unique considerations of
heavy-ion collisions, in particular to the experimental challenge of
reconstructing jets in the presence of a large underlying event. We report a
set of simple studies illustrating the impact of the underlying event on
identifying groomed jet splittings in heavy-ion collisions, and on associated
groomed jet observables. We illustrate the importance of the selection of
grooming algorithm, as certain groomers are more robust to these effects, while
others, including those commonly used in heavy-ion collisions, are susceptible
to large background effects -- which, when uncontrolled, can mimic a jet
quenching signal. These experimental considerations, along with appropriate
theoretical motivation, provide input to the choice of grooming algorithms
employed in heavy-ion collisions.Comment: Final version after journal revie
Jets in 200 GeV p+p and d+Au collisions from the STAR experiment at RHIC
Full jet reconstruction in heavy-ion collisions is a promising tool for the
quantitative study of properties of the dense medium produced at RHIC.
Measurements of d+Au collisions are important to disentangle initial state
nuclear effects from medium-induced kT broadening and jet quenching. Study of
jet production and properties in d+Au in combination with similar studies in
p+p is an important baseline measurement needed to better understand heavy-ion
results. We present mid-rapidity inclusive jet pT spectra and di-jet
correlations (kT) in 200 GeV p+p and d+Au collisions from the 2007-2008 RHIC
run. We discuss the methods used to correct the data for detector effects and
for background in d+Au collisions.Comment: 4 pages, 5 figures. To appear in Hot Quarks 2010 conference
proceeding
Jet Observables of Parton Energy Loss in High-Energy Nuclear Collisions
While strong attenuation of single particle production and particle
correlations has provided convincing evidence for large parton energy loss in
the QGP, its application to jet tomography has inherent limitations due to the
inclusive nature of the measurements. Generalization of this suppression to
full jet observables leads to an unbiased, more differential and thus powerful
approach to determining the characteristics of the hot QCD medium created in
high-energy nuclear collisions. In this article we report on recent theoretical
progress in calculating jet shapes and the related jet cross sections in the
presence of QGP-induced parton energy loss. (i) A theoretical model of
intra-jet energy flow in heavy-ion collisions is discussed. (ii) Realistic
numerical simulations demonstrate the nuclear modification factor
evolves continuously with the jet cone size or the acceptance cut
- a novel feature of jet quenching. The anticipated broadening
of jets is subtle and most readily manifested in the periphery of the cone for
smaller cone radii.Comment: Proceedings for Quark Matter 2009, updated version with minor
correction
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