Showing papers by "A. V. Alakhverdyants published in 2011"

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.

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.

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.

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.

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.

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.