24 research outputs found
Space charge in drift chambers operated with the Xe,CO2(15%) mixture
Using prototype modules of the ALICE Transition Radiation Detector we
investigate space charge effects and the dependence of the pion rejection
performance on the incident angle of the ionizing particle. The average pulse
height distributions in the drift chambers operated with the Xe,CO2(15%)
mixture provide quantitative information on the gas gain reduction due to space
charge accumulating during the drift of the primary ionization. Our results
demonstrate that the pion rejection performance of a TRD is better for tracks
which are not at normal incidence to the anode wires. We present detailed
simulations of detector signals, which reproduce the measurements and lend
strong support to our interpretation of the measurements in terms of space
charge effects.Comment: 18 pages, 10 figures, accepted for publication in Nucl.Instrum.Meth.
A. Data files available at http://www-alice.gsi.de/tr
Azimuthal anisotropy of neutral pion production in Au+Au collisions at sqrt(s_NN) = 200 GeV: Path-length dependence of jet quenching and the role of initial geometry
We have measured the azimuthal anisotropy of pi0's for 1 < pT < 18 GeV/c for
Au+Au collisions at sqrt s_NN = 200 GeV. The observed anisotropy shows a
gradual decrease in 3 < pT < 7 - 10 GeV/c, but remains positive beyond 10
GeV/c. The magnitude of this anisotropy is under-predicted, up to at least 10
GeV/c, by current perturbative QCD (pQCD) energy-loss model calculations. An
estimate of the increase in anisotropy expected from initial-geometry
modification due to gluon saturation effects and initial-geometry fluctuations
is insufficient to account for this discrepancy. Calculations which implement a
path length dependence steeper than what is implied by current pQCD energy-loss
models, show reasonable agreement with the data.Comment: 384 authors, 6 pages text, 3 figures. Submitted to Phys. Rev. Lett.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Transverse momentum and centrality dependence of dihadron correlations in Au+Au collisions at sqrt(s_NN)=200 GeV: Jet-quenching and the response of partonic matter
Azimuthal angle \Delta\phi correlations are presented for charged hadrons
from dijets for 0.4 < p_T < 10 GeV/c in Au+Au collisions at sqrt(s_NN) = 200
GeV. With increasing p_T, the away-side distribution evolves from a broad to a
concave shape, then to a convex shape. Comparisons to p+p data suggest that the
away-side can be divided into a partially suppressed "head" region centered at
Delta\phi ~ \pi, and an enhanced "shoulder" region centered at Delta\phi ~ \pi
+/- 1.1. The p_T spectrum for the "head" region softens toward central
collisions, consistent with the onset of jet quenching. The spectral slope for
the "shoulder" region is independent of centrality and trigger p_T, which
offers constraints on energy transport mechanisms and suggests that the
"shoulder" region contains the medium response to energetic jets.Comment: 420 authors from 58 institutions, 6 pages, 4 figures. Submitted to
Physical Review Letters. Plain text data tables for the points plotted in
figures for this and previous PHENIX publications are (or will be) publicly
available at http://www.phenix.bnl.gov/papers.htm
Transverse momentum dependence of J/psi polarization at midrapidity in p+p collisions at sqrt(s)=200 GeV
We report the measurement of the transverse momentum dependence of inclusive
J/psi polarization in p+p collisions at sqrt(s)=200 GeV performed by the PHENIX
Experiment at RHIC. The polarization is studied in the helicity,
Gottfried-Jackson, and Collins-Soper frames for p_T < 5 GeV/c and |y| < 0.35.
The J/psi polarization in the helicity and Gottfried-Jackson frames is
consistent with zero for all transverse momenta, with a slight (1.8 sigma)
trend towards longitudinal polarization for transverse momenta above 2 GeV/c.
No conclusion is allowed due to the limited acceptance in the Collins-Soper
frame and the uncertainties of the current data. The results are compared to
observations for other collision systems and center of mass energies and to
different quarkonia production models.Comment: 384 authors from 62 institutions, 11 pages, 9 figures, 2 tables. v2
is expanded version submitted to Physical Review D. Plain text data tables
for the points plotted in figures are available at
http://www.phenix.bnl.gov/papers.htm
Particle-species dependent modification of jet-induced correlations in Au+Au collisions at sqrt(s_NN) = 200 GeV
We report PHENIX measurements of the correlation of a trigger hadron at
intermediate transverse momentum (2.5<p_{T,trig}<4 GeV/c), with associated
mesons or baryons at lower p_{T,assoc}, in Au+Au collisions at sqrt(s_NN) = 200
GeV. The jet correlations for both baryons and mesons show similar shape
alterations as a function of centrality, characteristic of strong modification
of the away-side jet. The ratio of jet-associated baryons to mesons for this
jet increases with centrality and p_{T,assoc} and, in the most central
collisions, reaches a value similar to that for inclusive measurements. This
trend is incompatible with in-vacuum fragmentation, but could be due to
jet-like contributions from correlated soft partons which recombine upon
hadronization.Comment: 344 authors, 4 pages text, RevTeX, 4 figures. Submitted to Physical
Review Letters. Plain text data tables for the points plotted in figures for
this and previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
High p_T Direct Photon and pi^0 Triggered Azimuthal Jet Correlations in sqrt(s)=200 GeV p+p Collisions
Correlations of charged hadrons of 1 < pT < 10 GeV/c with high pT direct
photons and pi^ 0 mesons in the range 5 <pT < 15 GeV/c are used to study jet
fragmentation in the photon+jet and di-jet channels, respectively. The
magnitude of the partonic transverse momentum, kT, is obtained by comparing to
a model incorporating a Gaussian kT smearing. The sensitivity of the associated
charged hadron spectra to the underlying fragmentation function is tested and
the data are compared to calculations using recent global fit results. The
shape of the direct photon-associated hadron spectrum as well as its charge
asymmetry are found to be consistent with a sample dominated by quark-gluon
Compton scattering. No significant evidence of fragmentation photon correlated
production is observed within experimental uncertainties.Comment: 431 authors, 18 pages, 18 figures, 4 tables, RevTeX-4. Submitted to
Physical Review D. Plain text data tables for the points plotted in figures
for this and previous PHENIX publications are (or will be) publicly available
at http://www.phenix.bnl.gov/papers.htm
The gluon spin contribution to the proton spin from the double helicity asymmetry in inclusive pi^0 production in polarized p+p collisions at sqrt(s)=200 GeV
The double helicity asymmetry in neutral pion production for p_T = 1 to 12
GeV/c has been measured with the PHENIX experiment in order to access the gluon
spin contribution, Delta-G, to the proton spin. Measured asymmetries are
consistent with zero, and at a theory scale of \mu^2 = 4 GeV^2 give
Delta-G^[0.02,0.3] = 0.1 to 0.2, with a constraint of -0.7 < Delta-G^[0.02,0.3]
< 0.5 at Delta-chi^2 = 9 (~3 sigma) for our sampled gluon momentum fraction (x)
range, 0.02 to 0.3. The results are obtained using predictions for our measured
asymmetries generated from four representative fits to polarized deep inelastic
scattering data. We also consider the dependence of the Delta-G constraint on
the choice of theoretical scale, a dominant uncertainty in these predictions.Comment: 386 authors from 63 institutions, 6 pages, 4 figures, 1 table.
Submitted to Physical Review Letters. Plain text data tables for the points
plotted in figures for this and previous PHENIX publications are (or will be)
publicly available at http://www.phenix.bnl.gov/papers.htm
Suppression pattern of neutral pions at high transverse momentum in Au+Au collisions at sqrt(s_NN) = 200 GeV and constraints on medium transport coefficients
For Au + Au collisions at 200 GeV we measure neutral pion production with
good statistics for transverse momentum, p_T, up to 20 GeV/c. A fivefold
suppression is found, which is essentially constant for 5 < p_T < 20 GeV/c.
Experimental uncertainties are small enough to constrain any model-dependent
parameterization for the transport coefficient of the medium, e.g. \mean(q^hat)
in the parton quenching model. The spectral shape is similar for all collision
classes, and the suppression does not saturate in Au+Au collisions; instead, it
increases proportional to the number of participating nucleons, as N_part^2/3.Comment: 422 authors, 6 pages text, RevTeX-4, 4 figures, 1 Table. Submitted to
Physical Review Letters. Plain text data tables for the points plotted in
figures for this and previous PHENIX publications are (or will be) publicly
available at http://www.phenix.bnl.gov/papers.htm
Transverse momentum dependence of eta meson suppression in Au+Au collisions at sqrt(s_NN) = 200 GeV
New measurements by the PHENIX experiment at RHIC for eta production at
midrapidity as a function of transverse momentum (p_T) and collision centrality
in sqrt(s_NN) = 200 GeV Au+Au and p+p collisions are presented. They indicate
nuclear modification factors (R_AA) that are similar both in magnitude and
trend to those found in earlier pi^0 measurements. Linear fits to R_AA in the
5--20 GeV/c p_T region show that the slope is consistent with zero within two
standard deviations at all centralities although a slow rise cannot be
excluded. Having different statistical and systematic uncertainties the pi^0
and eta measurements are complementary at high p_T; thus, along with the
extended p_T range of these data they can provide additional constraints for
theoretical modeling and the extraction of transport properties.Comment: 427 authors, 7 pages, 6 figures, 3 tables. Submitted to Phys. Rev. C,
Rapid Communications. Plain text data tables for the points plotted in
figures for this and previous PHENIX publications are (or will be) publicly
available at http://www.phenix.bnl.gov/papers.htm
Direct photon production in p plus p collisions at root s=200 GeV at midrapidity
The differential cross section for the production of direct photons in p + p collisions at root s = 200 GeV at midrapidity was measured in the PHENIX detector at the Relativistic Heavy Ion Collider. Inclusive direct photons were measured in the transverse momentum range from 5: 5-25 GeV/c, extending the range beyond previous measurements. Event structure was studied with an isolation criterion. Next-to-leading-order perturbative-quantum-chromodynamics calculations give a good description of the spectrum. When the cross section is expressed versus x(T), the PHENIX data are seen to be in agreement with measurements from other experiments at different center-of-mass energies.Office of Nuclear Physics in the Office of Science of the Department of EnergyNational Science FoundationRenaissance Technologies LLCAbilene Christian University Research CouncilResearch Foundation of SUNYCollege of Arts and Sciences, Vanderbilt University (U.S.A)Ministry of Education, Culture, Sports, Science, and TechnologyJapan Society for the Promotion of Science (Japan)Conselho Nacional de Desenvolvimento Cientifico e TecnologicoFundacao de Amparo a Pesquisa do Estado de Sao Paulo (Brazil)Natural Science Foundation of China (P.R. China)Ministry of Education, Youth and Sports (Czech Republic)Centre National de la Recherche ScientifiqueCommissariat a l'Energie AtomiqueInstitut National de Physique Nucleaire et de Physique des Particules (France)Ministry of Industry, Science and TekhnologiesBundesministerium fur Bildung und ForschungDeutscher Akademischer Austausch DienstAlexander von Humboldt Stiftung (Germany)Hungarian National Science Fund, OTKA (Hungary)Department of Atomic EnergyDepartment of Science and Technology (India)Israel Science Foundation (Israel)National Research FoundationWCU program of the Ministry Education Science and Technology (Korea)Ministry of Education and ScienceRussian Academy of SciencesFederal Agency of Atomic Energy (Russia)V.R.Wallenberg Foundation (Sweden)U.S. Civilian Research and Development Foundation for the Independent States of the Former Soviet UnionU.S.-Hungarian Fulbright Foundation for Educational ExchangeU.S.Israel Binational Science Foundatio