Showing papers by "J. Pluta published in 2011"

Centrality Dependence of the Charged-Particle Multiplicity Density at Midrapidity in Pb-Pb Collisions at root s(NN)=2.76 TeV

Abstract: The centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at root s(NN) = 2: 76 TeV is presented. The charged-particle density normalized per participating nucleon pair increases by about a factor of 2 from peripheral (70%-80%) to central (0%-5%) collisions. The centrality dependence is found to be similar to that observed at lower collision energies. The data are compared with models based on different mechanisms for particle production in nuclear collisions.

Higher harmonic anisotropic flow measurements of charged particles in Pb-Pb collisions at root s(NN)=2.76 TeV

Abstract: We report on the first measurement of the triangular nu(3), quadrangular nu(4), and pentagonal nu(5) charged particle flow in Pb-Pb collisions at root s(NN) = 2.76 TeV measured with the ALICE detector at the CERN Large Hadron Collider. We show that the triangular flow can be described in terms of the initial spatial anisotropy and its fluctuations, which provides strong constraints on its origin. In the most central events, where the elliptic flow nu(2) and nu(3) have similar magnitude, a double peaked structure in the two-particle azimuthal correlations is observed, which is often interpreted as a Mach cone response to fast partons. We show that this structure can be naturally explained from the measured anisotropic flow Fourier coefficients.

Production of pions, kaons and protons in pp collisions at root s=900 GeV with ALICE at the LHC

Abstract: The production of π+, π−, K+, K−, p, and \(\overline{\mathrm{p}}\) at mid-rapidity has been measured in proton-proton collisions at \(\sqrt{s} = 900~\mathrm{GeV}\) with the ALICE detector. Particle identification is performed using the specific energy loss in the inner tracking silicon detector and the time projection chamber. In addition, time-of-flight information is used to identify hadrons at higher momenta. Finally, the distinctive kink topology of the weak decay of charged kaons is used for an alternative measurement of the kaon transverse momentum (pt) spectra. Since these various particle identification tools give the best separation capabilities over different momentum ranges, the results are combined to extract spectra from pt=100 MeV/c to 2.5 GeV/c. The measured spectra are further compared with QCD-inspired models which yield a poor description. The total yields and the mean pt are compared with previous measurements, and the trends as a function of collision energy are discussed.

Strange particle production in proton-proton collisions at root s=0.9 TeV with ALICE at the LHC

Abstract: The production of mesons containing strange quarks ($\Kzs$, $\phi$) and both singly and doubly strange baryons ($\rmLambda$, $\rmAlambda$, and $\Xis$) are measured at central rapidity in pp collisions at $\sqrt{s}$ = 0.9 $\tev$ with the ALICE experiment at the LHC. The results are obtained from the analysis of about 250 k minimum bias events recorded in 2009. Measurements of yields (dN/dy) and transverse momentum spectra at central rapidities for inelastic pp collisions are presented. For mesons, we report yields ($ $) of $0.184 \pm 0.002 \stat \pm 0.006 \syst$ for $\Kzs$ and $0.021 \pm 0.004 \stat \pm 0.003 \syst$ for $\phi$. For baryons, we find $ = 0.048 \pm 0.001 \stat \pm 0.004 \syst$ for $\rmLambda$, $0.047 \pm 0.002 \stat \pm 0.005 \syst$ for $\rmAlambda$ and $0.0101 \pm 0.0020 \stat \pm 0.0009 \syst$ for $\Xis$. The results are also compared with predictions for identified particle spectra from QCD-inspired models and provide a baseline for comparisons with both future pp measurements at higher energies and heavy-ion collisions.

Observation of the antimatter helium-4 nucleus

Abstract: High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang(1); in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high-energy accelerator of heavy nuclei provides an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus ((4)(He) over bar), also known as the anti-alpha ((alpha) over bar), consists of two antiprotons and two antineutrons (baryon number B = -4). It has not been observed previously, although the alpha-particle was identified a century ago by Rutherford and is present in cosmic radiation at the ten per cent level(2). Antimatter nuclei with B -1 have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1,000 with each additional antinucleon(3-5). Here we report the observation of (4)<(He) over bar, the heaviest observed antinucleus to date. In total, 18 (4)(He) over bar counts were detected at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC; ref. 6) in 10(9) recorded gold-on-gold (Au+Au) collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon-nucleon pair. The yield is consistent with expectations from thermodynamic(7) and coalescent nucleosynthesis(8) models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of (4)(He) over bar in cosmic radiation.

Measurements of Cross Sections and Charged Pion Spectra in Proton-Carbon Interactions at 31 GeV/c

Abstract: Interaction cross sections and charged pion spectra in p+C interactions at 31 GeV/c were measured with the large-acceptance NA61/SHINE spectrometer at the CERN SPS. These data are required to improve predictions of the neutrino flux for the T2K long-baseline neutrino oscillation experiment in Japan. A set of data collected during the first NA61/SHINE run in 2007 with an isotropic graphite target with a thickness of 4% of a nuclear interaction length was used for the analysis. The measured p+C inelastic and production cross sections are 257.2 {+-} 1.9 {+-} 8.9 and 229.3 {+-} 1.9 {+-} 9.0 mb, respectively. Inclusive production cross sections for negatively and positively charged pions are presented as functions of laboratory momentum in ten intervals of the laboratory polar angle covering the range from 0 up to 420 mrad. The spectra are compared with predictions of several hadron production models.

Femtoscopy Of Pp Collisions At √s=0.9 And 7 Tev At The Lhc With Two-pion Bose-einstein Correlations

Abstract: We report on the high statistics two-pion correlation functions from pp collisions at root s = 0.9 TeV and root s = 7 TeV, measured by the ALICE experiment at the Large Hadron Collider. The correlation functions as well as the extracted source radii scale with event multiplicity and pair momentum. When analyzed in the same multiplicity and pair transverse momentum range, the correlation is similar at the two collision energies. A three-dimensional femtoscopic analysis shows an increase of the emission zone with increasing event multiplicity as well as decreasing homogeneity lengths with increasing transverse momentum. The latter trend gets more pronounced as multiplicity increases. This suggests the development of space-momentum correlations, at least for collisions producing a high multiplicity of particles. We consider these trends in the context of previous femtoscopic studies in high-energy hadron and heavy-ion collisions and discuss possible underlying physics mechanisms. Detailed analysis of the correlation reveals an exponential shape in the outward and longitudinal directions, while the sideward remains a Gaussian. This is interpreted as a result of a significant contribution of strongly decaying resonances to the emission region shape. Significant nonfemtoscopic correlations are observed, and are argued to be the consequence of "mini-jet"-like structures extending to low p(t). They are well reproduced by the Monte-Carlo generators and seen also in pi(+)pi(-) correlations.

Measurement of charm production at central rapidity in proton-proton collisions at sqrt(s) = 7 TeV

Abstract: The pt-differential inclusive production cross sections of the prompt charmed mesons D0, D+, and D*+ in the rapidity range |y| K-pi+, D+->K-pi+pi+, D*+->D0pi+, and their charge conjugates, about 8,400 D0, 2,900 D+, and 2,600 D*+ mesons with 1

Strange and multistrange particle production in Au + Au collisions at √sNN=62.4 GeV

Abstract: We present results on strange and multistrange particle production in Au + Au collisions at root s(NN) = 62.4 GeV as measured with the STAR detector at RHIC. Midrapidity transverse momentum spectra and integrated yields of K-S(0), Lambda, Xi, and Omega and their antiparticles are presented for different centrality classes. The particle yields and ratios follow a smooth energy dependence. Chemical freeze-out parameters, temperature, baryon chemical potential, and strangeness saturation factor obtained from the particle yields are presented. Intermediate transverse momentum (p(T)) phenomena are discussed based on the ratio of the measured baryon-to-meson spectra and nuclear modification factor. The centrality dependence of various measurements presented show a similar behavior as seen in Au + Au collisions at root s(NN) = 200 GeV.

K*0 production in Cu + Cu and Au + Au collisions at √sNN=62.4 GeV and 200 GeV

Abstract: We report on K*(0) production at midrapidity in Au + Au and Cu + Cu collisions at root s(NN) = 62.4 and 200 GeV collected by the Solenoid Tracker at the Relativistic Heavy Ion Collider detector. The K*(0) is reconstructed via the hadronic decays K*(0) -> K+pi(-) and (K*(0)) over bar -> K+pi(-). Transverse momentum, p(T), spectra are measured over a range of p(T) extending from 0.2 GeV/c up to 5 GeV/c. The center-of-mass energy and system size dependence of the rapidity density, dN/dy, and the average transverse momentum, , are presented. The measured N(K*(0))/N(K) and N(phi)/N(K*(0)) ratios favor the dominance of rescattering of decay daughters of K*(0) over the hadronic regeneration for the K*(0) production. In the intermediate p(T) region (2.0 < p(T) < 4.0 GeV/c), the elliptic flow parameter, v(2), and the nuclear modification factor, R-CP, agree with the expectations from the quark coalescence model of particle production.

Centrality dependence of proton and antiproton spectra in Pb +Pb collisions at 40A GeV and 158A GeV measured at the CERN Super Proton Synchrotron

Abstract: The yields of (anti)protons were measured by the NA49 Collaboration in centrality-selected Pb + Pb collisions at 40A and 158A GeV. Particle identification was obtained in the laboratory momentum range from 5 to 63 GeV/c by measuring the energy loss dE/dx in the time projection chamber detector gas. The corresponding rapidity coverage extends 1.6 units from midrapidity into the forward hemisphere. Transverse-mass spectra, the rapidity dependences of the average transverse mass, and rapidity density distributions were studied as a function of collision centrality. The values of the average transverse mass as well as the midrapidity yields of protons normalized to the number of wounded nucleons show only modest centrality dependences. In contrast, the shape of the rapidity distribution changes significantly with collision centrality, especially at 40A GeV. The experimental results are compared to calculations of the hadron-string dynamics and the ultrarelativistic quantum-molecular-dynamics transport models.

Erratum: Transverse momentum and centrality dependence of high-p T nonphotonic electron suppression in Au+Au collisions at √sNN=200GeV (Physical Review Letters (2007) 98 (192301))

Abstract: The STAR collaboration at the BNL Relativistic Heavy-Ion Collider (RHIC) reports measurements of the inclusive yield of nonphotonic electrons, which arise dominantly from semileptonic decays of heavy flavor mesons, over a broad range of transverse momenta (1.2

Pion femtoscopy in p+p collisions at √s=200 GeV

Abstract: The STAR Collaboration at the BNL Relativistic Heavy Ion Collider has measured two-pion correlation functions from p + p collisions at root s = 200 GeV. Spatial scales are extracted via a femtoscopic analysis of the correlations, though this analysis is complicated by the presence of strong nonfemtoscopic effects. Our results are put into the context of the world data set of femtoscopy in hadron-hadron collisions. We present the first direct comparison of femtoscopy in p + p and heavy ion collisions, under identical analysis and detector conditions.

Measurement of the parity-violating longitudinal single-spin asymmetry for W± boson production in polarized proton-proton collisions at √s=500GeV

Abstract: We report the first measurement of the parity-violating single-spin asymmetries for midrapidity decay positrons and electrons from W+ and W- boson production in longitudinally polarized proton-proton collisions at root s = 500 GeV by the STAR experiment at RHIC. The measured asymmetries, A(L)(W+) = -0.27 +/- 0.10(stat.) +/- 0.02(syst.) +/- 0.03(norm.) and A(L)(W-) = 0.14 +/- 0.19(stat.) +/- 0.02(syst.) +/- 0.01(norm.), are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized deep-inelastic scattering measurements.

K*(892)0 and K*(892)0 production in central Pb + Pb, Si + Si, C + C, and inelastic p + p collisions at 158A GeV

Abstract: Production of the K*(892)0 and K*(892)⁰ resonances was studied via their K⁺π⁻ and K⁻π⁺ decay modes in central Pb+Pb, Si+Si, C+C, and inelastic p+p collisions at 158A GeV(√(sNN)=17.3 GeV) with the NA49 detector at the CERN SPS. Transverse momentum and rapidity distributions were measured and total yields were estimated. The yield of K* exceeds that of K* by about a factor of two in nucleus-nucleus reactions. The total yield ratios ⟨K*⟩/⟨K+⟩ and ⟨K*⟩/⟨K-⟩ are strongly suppressed in central Pb+Pb compared to p+p, C+C, and Si+Si collisions, in agreement with the expected attenuation of these short-lived resonance states in the hadronic phase of the expanding fireball. The UrQMD model, although incorporating such a scenario, does not provide a quantitative description of the experimental results. The statistical hadron gas model assuming the same freeze-out parameters for stable hadrons and resonances overestimates the ⟨K*⟩/⟨K⟩ ratios in central Pb+Pb collisions by about a factor of 2.5.

Scaling properties at freeze-out in relativistic heavy-ion collisions

Abstract: Identified charged pion, kaon, and proton spectra are used to explore the system size dependence of bulk freeze-out properties in Cu + Cu collisions at root s(NN) = 200 and 62.4 GeV. The data are studied with hydrodynamically motivated blast-wave and statistical model frameworks in order to characterize the freeze-out properties of the system. The dependence of freeze-out parameters on beam energy and collision centrality is discussed. Using the existing results from Au + Au and pp collisions, the dependence of freeze-out parameters on the system size is also explored. This multidimensional systematic study furthers our understanding of the QCD phase diagram revealing the importance of the initial geometrical overlap of the colliding ions. The analysis of Cu + Cu collisions expands the system size dependence studies from Au + Au data with detailed measurements in the smaller system. The systematic trends of the bulk freeze-out properties of charged particles is studied with respect to the total charged particle multiplicity at midrapidity, exploring the influence of initial state effects.

High pT nonphotonic electron production in p+p collisions at √s=200GeV

Abstract: We present the measurement of nonphotonic electron production at high transverse momentum (p(T) > 2.5 GeV/c) in p + p collisions at root s = 200 GeV using data recorded during 2005 and 2008 by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The measured cross sections from the two runs are consistent with each other despite a large difference in photonic background levels due to different detector configurations. We compare the measured nonphotonic electron cross sections with previously published RHIC data and perturbative quantum chromodynamics calculations. Using the relative contributions of B and D mesons to nonphotonic electrons, we determine the integrated cross sections of electrons (e++e-2/2) at 3 GeV/c e)+(B -> D -> e))/(dy(e))](ye=0) 4.0 +/- 0.5(stat) +/- 1.1(syst) nb and [(d sigma(D -> e))/(dy(e))](ye=0) = 6.2 +/- 0.7(stat) +/- 1.5(syst) nb, respectively.

Pion femtoscopy inp+pcollisions ats=200GeV

Abstract: The STAR Collaboration at the BNL Relativistic Heavy Ion Collider has measured two-pion correlation functions from $p+p$ collisions at $\sqrt{s}=200$ GeV. Spatial scales are extracted via a femtoscopic analysis of the correlations, though this analysis is complicated by the presence of strong nonfemtoscopic effects. Our results are put into the context of the world data set of femtoscopy in hadron-hadron collisions. We present the first direct comparison of femtoscopy in $p+p$ and heavy ion collisions, under identical analysis and detector conditions.

HighpTnonphotonic electron production inp+pcollisions ats=200GeV

Abstract: We present the measurement of nonphotonic electron production at high transverse momentum (${p}_{T}g2.5\text{ }\text{ }\mathrm{GeV}/c$) in $p+p$ collisions at $\sqrt{s}=200\text{ }\text{ }\mathrm{GeV}$ using data recorded during 2005 and 2008 by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The measured cross sections from the two runs are consistent with each other despite a large difference in photonic background levels due to different detector configurations. We compare the measured nonphotonic electron cross sections with previously published RHIC data and perturbative quantum chromodynamics calculations. Using the relative contributions of B and D mesons to nonphotonic electrons, we determine the integrated cross sections of electrons ($\frac{{e}^{+}+{e}^{\ensuremath{-}}}{2}$) at $3\text{ }\text{ }\mathrm{GeV}/cl{p}_{T}l10\text{ }\text{ }\mathrm{GeV}/c$ from bottom and charm meson decays to be $[(d{\ensuremath{\sigma}}_{(B\ensuremath{\rightarrow}e)+(B\ensuremath{\rightarrow}D\ensuremath{\rightarrow}e)})/(d{y}_{e}){]}_{{y}_{e}=0}=4.0\ifmmode\pm\else\textpm\fi{}0.5(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}1.1(\mathrm{syst})\text{ }\text{ }\mathrm{nb}$ and $[(d{\ensuremath{\sigma}}_{D\ensuremath{\rightarrow}e})/(d{y}_{e}){]}_{{y}_{e}=0}=6.2\ifmmode\pm\else\textpm\fi{}0.7(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}1.5(\mathrm{syst})\text{ }\text{ }\mathrm{nb}$, respectively.

Energy dependence of kaon-to-proton ratio fluctuations in central Pb+Pb collisions from √[sNN]=6.3 to 17.3 GeV

Abstract: Kaons and protons carry large parts of two conserved quantities: strangeness and baryon number. It is argued that their correlation and thus also fluctuations are sensitive to conditions prevailing at the anticipated parton-hadron phase boundary. Fluctuations of the (K++K-)/(p+p ) and K+/p ratios have been measured for the first time by NA49 in central Pb+Pb collisions at five CERN Super Proton Synchrotron energies between √sNN=6.3 and √sNN=17.3 GeV. Both ratios exhibit a change of sign in σdyn, a measure of nonstatistical fluctuations, around √sNN=8 GeV. Below this energy, σdyn is positive, indicating higher fluctuation compared to a mixed event background sample, while for higher energies, σdyn is negative, indicating correlated emission of kaons and protons. The results are compared to hadronic transport model calculations which fail to reproduce the energy dependence.

Experimental studies of di-jet survival and surface emission bias in Au+Au collisions via angular correlations with respect to back-to-back leading hadrons

Abstract: We report first results from an analysis based on a new multi-hadron correlation technique, exploring jet-medium interactions and di-jet surface emission bias at the BNL Relativistic Heavy Ion Collider (RHIC). Pairs of back-to-back high-transverse-momentum hadrons are used for triggers to study associated hadron distributions. In contrast with two-and three-particle correlations with a single trigger with similar kinematic selections, the associated hadron distribution of both trigger sides reveals no modification in either relative pseudorapidity Delta eta or relative azimuthal angle Delta phi from d + Au to central Au + Au collisions. We determine associated hadron yields and spectra as well as production rates for such correlated back-to-back triggers to gain additional insights on medium properties.

Evolution of the differential transverse momentum correlation function with centrality in Au+Au collisions at √sNN = 200 GeV

Abstract: We present first measurements of the evolution of the differential transverse momentum correlation function, C, with collision centrality in Au + Au interactions at root s(NN) = 200 GeV. This observable exhibits a strong dependence on collision centrality that is qualitatively similar to that of number correlations previously reported. We use the observed longitudinal broadening of the near-side peak of C with increasing centrality to estimate the ratio of the shear viscosity to entropy density, eta/s, of the matter formed in central Au + Au interactions. We obtain an upper limit estimate of eta/s that suggests that the produced medium has a small viscosity per unit entropy. (C) 2011 Elsevier B.V. All rights reserved.

Energy dependence of kaon-to-proton ratio fluctuations in central Pb+Pb collisions from [sqrt]sNN=6.3 to 17.3 GeV

Abstract: Kaons and protons carry large parts of two conserved quantities: strangeness and baryon number. It is argued that their correlation and thus also fluctuations are sensitive to conditions prevailing at the anticipated parton-hadron phase boundary. Fluctuations of the (K++K-)/(p+p ) and K+/p ratios have been measured for the first time by NA49 in central Pb+Pb collisions at five CERN Super Proton Synchrotron energies between √sNN=6.3 and √sNN=17.3 GeV. Both ratios exhibit a change of sign in σdyn, a measure of nonstatistical fluctuations, around √sNN=8 GeV. Below this energy, σdyn is positive, indicating higher fluctuation compared to a mixed event background sample, while for higher energies, σdyn is negative, indicating correlated emission of kaons and protons. The results are compared to hadronic transport model calculations which fail to reproduce the energy dependence.

Search for the Critical Point of Strongly Interacting Matter at the CERN SPS

Abstract: The study of central collisions of heavy nuclei at CERN SPS energies by NA49 provided evidence for the onset of deconfinement around 30A GeV. Theoretical considerations predict a critical point of strongly interacting matter accessible in the SPS energy range. A search for the expected fluctuations has not yet found convincing signals. The strategy and plans for the continuation of this program at the SPS by NA61 with lighter nuclei is discussed.

High pT [p subscript T] nonphotonic electron production in p+p collisions at √s=200 [square root of s=200] GeV

Abstract: Citation Agakishiev, H. et al. (STAR Collaboration) "High pT nonphotonic electron production in p+p collisions at s=200GeV" Phys. Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. High p T nonphotonic electron production in p þ p collisions at ffiffi ffi s p ¼ 200 GeV We present the measurement of nonphotonic electron production at high transverse momentum (p T > 2:5 GeV=c) in p þ p collisions at ffiffi ffi s p ¼ 200 GeV using data recorded during 2005 and 2008 by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The measured cross sections from the two runs are consistent with each other despite a large difference in photonic background levels due to different detector configurations. We compare the measured nonphotonic electron cross sections with previously published RHIC data and perturbative quantum chromodynamics H. AGAKISHIEV et al. calculations. Using the relative contributions of B and D mesons to nonphotonic electrons, we determine the integrated cross sections of electrons (e þ þe À 2) at 3 GeV=c < p T < 10 GeV=c from bottom and charm meson decays to be ½ðd ðB!eÞþðB!D!eÞ Þ=ðdy e ފ y e ¼0 ¼ 4:0 AE 0:5ðstatÞ AE 1:1ðsystÞ nb and ½ðd D!e Þ=ðdy e ފ y e ¼0 ¼ 6:2 AE 0:7ðstatÞ AE 1:5ðsystÞ nb, respectively.