321 research outputs found
Photon-tagged correlations in heavy-ion collisions: kinematic requirements and a case study
Photon-tagged correlations may be useful to determine how the dense partonic
medium produced in heavy-ion collisions affects the fragmentation of
high-energy quarks and gluons into a leading hadron. In these proceedings, I
discuss the kinematic requirements for the hadron and the prompt photon
transverse momentum cuts. A case study at LHC energy, tagging on p_T > 20 GeV
and p_T > 50 GeV photons, is then briefly examined.Comment: 4 pages, 2 figures. To appear in the proceedings of the 19th
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions
(QM2006), Shanghai, China, 14-20 November 200
Quarkonium hadron interaction in QCD
The analytic continuation of the operator product expansion of the scattering
amplitude allows to compute the heavy-quarkonium hadron total cross section.
The energy dependence of the Upsilon and Upsilon' cross sections with a proton
is discussed.Comment: 4 pages, 2 figures. Talk given at XXXVIth Rencontres de Moriond: QCD
and High Energy Hadronic Interactions, Les Arcs, France, 17-24 March 200
Prompt photon in association with a heavy-quark jet in Pb-Pb collisions at the LHC
We present a phenomenological study of the associated production of a prompt
photon and a heavy quark jet (charm or bottom) in Pb-Pb collisions at the LHC.
This channel allows for estimating the amount of energy loss experienced by the
charm and bottom quarks propagating in the dense QCD medium produced in those
collisions. Calculations are carried out at next-to-leading order (NLO)
accuracy using the BDMPS-Z heavy-quark quenching weights. The quenching of the
single heavy-quark jet spectrum reflects fairly the hierarchy in the heavy
quark energy loss assumed in the perturbative calculation. On the contrary, the
single photon spectrum in heavy-ion collisions is only modified at low momenta,
for which less heavy-quark jets pass the kinematic cuts. On top of single
particle spectra, the two-particle final state provides a range of observables
(photon-jet pair momentum, jet asymmetry, among others) which are studied in
detail. The comparison of the photon-jet pair momentum, from p-p to Pb-Pb
collisions, is sensitive to the amount of energy lost by the heavy-quarks and
could therefore be used in order to better understand parton energy loss
processes in the heavy quark sector.Comment: 22 pages, 18 figures, gzipped tar file. Minor changes, 2 references
added. Version published in JHE
Electromagnetic emission from hot medium measured by the PHENIX experiment at RHIC
Electromagnetic radiation has been of interest in heavy ion collisions
because they shed light on early stages of the collisions where hadronic probes
do not provide direct information since hadronization and hadronic interactions
occur later. The latest results on photon measurement from the PHENIX
experiment at RHIC reflect thermodynamic properties of the matter produced in
the heavy ion collisions. An unexpectedly large positive elliptic flow measured
for direct photons can not be explained by any of the current models.Comment: Talk contributed to Rutherford Centennial Conference, Aug 8-12, 2011,
held in Manchester, U
Quenching of hadron spectra in DIS on nuclear targets
The multiple scatterings incurred by a hard quark produced in a nuclear
medium induce the emission of soft gluons which carry a fraction of the quark
energy and eventually affect the hadronization process. Here, the depletion of
semi-inclusive hadron spectra in DIS on various nuclei (N, Ne, Cu, Kr) is
computed as a function of nu and z to leading order in alpha_s through
medium-modified fragmentation functions. Using the transport coefficient q
previously determined from Drell-Yan production, the predictions are found to
be in good agreement with EMC and HERMES preliminary data. Calculations on Xe
targets are also presented and discussed.Comment: 20 pages, 8 figure
Systematic Study of High-pT Direct Photon Production with the PHENIX Experiment at RHIC
When studying the initial state and evolution of the matter created in
relativistic heavy ion collisions, high-pT direct photons are a powerful probe.
They are created in initial hard processes and in parton fragmentation, and
possibly in interactions of partons with the hot and dense medium. We present
systematic measurements of high-pT direct photon production in \sqrt{s_{NN}} =
200 GeV p+p and Au+Au collisions. The nuclear modification factor of direct
photons is shown for 5 < pT < 18 GeV/c, and at very high transverse momenta it
seems to be below unity in the most central Au+Au collisions.Comment: 4 pages, 4 figures. Invited parallel talk at The 19th International
Conference On Ultra relativistic Nucleus-Nucleus Collisions (Quark Matter
2006), Shanghai China, Nov. 14-20, 200
Photon tagged correlations in heavy ion collisions
A detailed study of various two-particle correlation functions involving
photons and neutral pions is presented in proton-proton and lead-lead
collisions at the LHC energy. The aim is to use these correlation functions to
quantify the effect of the medium (in lead-lead collisions) on the jet decay
properties. The calculations are carried out at the leading order in QCD but
the next-to-leading order corrections are also discussed. The competition
between different production mechanisms makes the connection between the jet
energy loss spectrum and the gamma-pi correlations somewhat indirect while the
gamma-gamma correlations have a clearer relation to the jet fragmentation
properties.Comment: 32 pages, 19 figures. Minor changes, published versio
Prompt Photon Production in p-A Collisions at LHC and the Extraction of Gluon Shadowing
International audienceA report is given on the study of using prompt photon production at the LHC to probe the gluon nuclear density, and more specifically the shadowing ratio GA/Gp that one could access in foreseen p-A runs
Non-universality of transverse Coulomb exchange at small x
Within an explicit scalar QED model we compare, at fixed x << 1, the
leading-twist K_T-dependent `quark' distribution f_q(x, K_T) probed in deep
inelastic scattering and Drell-Yan production, and show that the model is
consistent with the universality of f_q(x, K_T). The extension of the model
from the aligned-jet to the 'symmetric' kinematical regime reveals interesting
properties of the physics of Coulomb rescatterings when comparing DIS and DY
processes. At small x the transverse momentum induced by multiple
scattering on a single centre is process dependent, as well as the transverse
momentum broadening occurring in collisions on a finite size nuclear target.Comment: 28 pages, 3 eps figure
High \pt{} particle correlations in pp collisions at LHC/ALICE
Two-particle correlation triggered by high-\pt{} particles allows us to study
hard scattering phenomena when full jet reconstruction is challenging. An
analysis of the first ALICE pp data where charged and neutral particles
isolated or not are used as trigger particles is presented. The two-particle
correlation between the trigger () and the associate () particles is
studied as a function of the imbalance parameter \xe=- and interpreted in terms of jet
fragmentation function.Comment: 4 pages, 4 figures, QM2011 conference proceedings, will be published
on Journal of Physics
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