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STAR Collaboration,Agakishiev, G.,Aggarwal, M.M.,Ahammed, Z.,Alakhverdyants, A.V.,Alekseev, I.,Alford, J.,Anderson, B.D.,Anson, C.D.,Arkhipkin, D.,Averichev, G.S.,Balewski, J.,Beavis, D.R.,Behera, N.K North-Holland Pub. Co 2011 Physics letters: B Vol.704 No.5
We present first measurements of the evolution of the differential transverse momentum correlation function, C, with collision centrality in Au+Au interactions at s<SUB>NN</SUB>=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, η/s, of the matter formed in central Au+Au interactions. We obtain an upper limit estimate of η/s that suggests that the produced medium has a small viscosity per unit entropy.
Single spin asymmetry A<sub>N</sub> in polarized proton-proton elastic scattering at s=200 GeV
STAR Collaboration,Adamczyk, L.,Agakishiev, G.,Aggarwal, M.M.,Ahammed, Z.,Alakhverdyants, A.V.,Alekseev, I.,Alford, J.,Anson, C.D.,Arkhipkin, D.,Aschenauer, E.,Averichev, G.S.,Balewski, J.,Banerjee, A North-Holland Pub. Co 2013 Physics letters: B Vol.719 No.1
We report a high precision measurement of the transverse single spin asymmetry A<SUB>N</SUB> at the center of mass energy s=200 GeV in elastic proton-proton scattering by the STAR experiment at RHIC. The A<SUB>N</SUB> was measured in the four-momentum transfer squared t range 0.003≤|t|≤0.035 (GeV/c)<SUP>2</SUP>, the region of a significant interference between the electromagnetic and hadronic scattering amplitudes. The measured values of A<SUB>N</SUB> and its t-dependence are consistent with a vanishing hadronic spin-flip amplitude, thus providing strong constraints on the ratio of the single spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated by the Pomeron amplitude at this s, we conclude that this measurement addresses the question about the presence of a hadronic spin flip due to the Pomeron exchange in polarized proton-proton elastic scattering.
Abelev, B.,Adamczyk, L.,Adkins, J. K.,Agakishiev, G.,Aggarwal, M. M.,Ahammed, Z.,Alekseev, I.,Aparin, A.,Arkhipkin, D.,Aschenauer, E. C.,Ashraf, M. U.,Attri, A.,Averichev, G. S.,Bai, X.,Bairathi, V.,B American Physical Society 2016 Physical Review C Vol.94 No.1
<P>We present measurements of the near side of triggered di-hadron correlations using neutral strange baryons ( A, (A) over bar) and mesons (K-s(0)) at intermediate transverse momentum ( 3 < p(T) < 6 GeV/c) to look for possible flavor and baryon-meson dependence. This study is performed in d+Au, Cu+Cu, and Au+Au collisions at root S-NN = 200 GeV measured by the STAR experiment at RHIC. The near-side di-hadron correlation contains two structures, a peak which is narrow in azimuth and pseudorapidity consistent with correlations from jet fragmentation, and a correlation in azimuth which is broad in pseudorapidity. The particle composition of the jet-like correlation is determined using identified associated particles. The dependence of the conditional yield of the jet-like correlation on the trigger particle momentum, associated particle momentum, and centrality for correlations with unidentified trigger particles are presented. The neutral strange particle composition in jet-like correlations with unidentified charged particle triggers is not well described by PYTHIA. However, the yield of unidentified particles in jet-like correlations with neutral strange particle triggers is described reasonably well by the same model.</P>
System size dependence of associated yields in hadron-triggered jets
STAR Collaboration,Abelev, B.I.,Aggarwal, M.M.,Ahammed, Z.,Anderson, B.D.,Arkhipkin, D.,Averichev, G.S.,Balewski, J.,Barannikova, O.,Barnby, L.S.,Baudot, J.,Baumgart, S.,Beavis, D.R.,Bellwied, R.,Bene North-Holland Pub. Co 2010 Physics letters: B Vol.683 No.2
We present results on the system size dependence of high transverse momentum di-hadron correlations at s<SUB>NN</SUB>=200 GeV as measured by STAR at RHIC. Measurements in d+Au, Cu+Cu and Au+Au collisions reveal similar jet-like near-side correlation yields (correlations at small angular separation -Δφ∼0, Δη∼0) for all systems and centralities. Previous measurements have shown that the away-side (Δφ∼π) yield is suppressed in heavy-ion collisions. We present measurements of the away-side suppression as a function of transverse momentum and centrality in Cu+Cu and Au+Au collisions. The suppression is found to be similar in Cu+Cu and Au+Au collisions at a similar number of participants. The results are compared to theoretical calculations based on the parton quenching model and the modified fragmentation model. The observed differences between data and theory indicate that the correlated yields presented here will further constrain dynamic energy loss models and provide information about the dynamic density profile in heavy-ion collisions.
Adams, J,Aggarwal, M M,Ahammed, Z,Amonett, J,Anderson, B D,Anderson, M,Arkhipkin, D,Averichev, G S,Bai, Y,Balewski, J,Barannikova, O,Barnby, L S,Baudot, J,Bekele, S,Belaga, V V,Bellingeri-Laurikainen, Institute of Physics Publishing 2007 Journal of Physics. G, Nuclear and Particle Physic Vol.34 No.3
<P>We present the first study of the energy dependence of <I>p<SUB>t</SUB></I> angular correlations inferred from event-wise mean transverse momentum 〈<I>p<SUB>t</SUB></I>〉 fluctuations in heavy ion collisions. We compare our large-acceptance measurements at CM energies <img SRC='http://ej.iop.org/images/0954-3899/34/3/004/jpg233147ieqn1.gif' ALIGN='MIDDLE' ALT='\sqrt{s_{\rm NN}} = 19.6'/>, 62.4, 130 and 200 GeV to SPS measurements at 12.3 and 17.3 GeV. <I>p<SUB>t</SUB></I> angular correlation structure suggests that the principal source of <I>p<SUB>t</SUB></I> correlations and fluctuations is minijets (minimum-bias parton fragments). We observe a dramatic increase in correlations and fluctuations from SPS to RHIC energies, increasing linearly with <img SRC='http://ej.iop.org/images/0954-3899/34/3/004/jpg233147ieqn2.gif' ALIGN='MIDDLE' ALT='\ln \sqrt{s_{\rm NN}} '/> from the onset of observable jet-related 〈<I>p<SUB>t</SUB></I>〉 fluctuations near 10 GeV.</P>
J / ψ polarization in p + p collisions at s = 200 GeV in STAR
Adamczyk, L.,Adkins, J.K.,Agakishiev, G.,Aggarwal, M.M.,Ahammed, Z.,Alekseev, I.,Alford, J.,Anson, C.D.,Aparin, A.,Arkhipkin, D.,Aschenauer, E.C.,Averichev, G.S.,Balewski, J.,Banerjee, A.,Barnovska, Z North-Holland Pub. Co 2014 Physics letters. Section B Vol.739 No.-
<P><B>Abstract</B></P> <P>We report on a polarization measurement of inclusive J / ψ mesons in the di-electron decay channel at mid-rapidity at 2 < <SUB> p T </SUB> < 6 GeV / c in p + p collisions at s = 200 GeV . Data were taken with the STAR detector at RHIC. The J / ψ polarization measurement should help to distinguish between different models of the J / ψ production mechanism since they predict different <SUB> p T </SUB> dependences of the J / ψ polarization. In this analysis, J / ψ polarization is studied in the helicity frame. The polarization parameter <SUB> λ θ </SUB> measured at RHIC becomes smaller towards high <SUB> p T </SUB> , indicating more longitudinal J / ψ polarization as <SUB> p T </SUB> increases. The result is compared with predictions of presently available models.</P>
Adamczyk, L.,Adkins, J. K.,Agakishiev, G.,Aggarwal, M. M.,Ahammed, Z.,Alekseev, I.,Aparin, A.,Arkhipkin, D.,Aschenauer, E. C.,Attri, A.,Averichev, G. S.,Bai, X.,Bairathi, V.,Bellwied, R.,Bhasin, A.,Bh American Physical Society 2016 Physical review. C Vol.94 No.3
<P>We present measurements of second-order azimuthal anisotropy (v(2)) at midrapidity (vertical bar y vertical bar < 1.0) for light nuclei d, t, He-3 (for root s(NN) = 200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV) and antinuclei <(d)over bar> (root s(NN) = 200, 62.4, 39, 27, and 19.6 GeV) and (3)(He) over bar (root s(NN) = 200 GeV) in the STAR (Solenoidal Tracker at RHIC) experiment. The v(2) for these light nuclei produced in heavy-ion collisions is compared with those for p and (p) over bar. We observe mass ordering in nuclei v(2)(p(T)) at low transverse momenta (p(T) < 2.0 GeV/c). We also find a centrality dependence of v(2) for d and <(d)over bar>. The magnitude of v(2) for t and He-3 agree within statistical errors. Light-nuclei v(2) are compared with predictions from a blast-wave model. Atomic mass number (A) scaling of light-nuclei v(2)(p(T)) seems to hold for p(T)/A < 1.5 GeV/c. Results on light-nuclei v(2) from a transport-plus-coalescence model are consistent with the experimental measurements.</P>
Adamczyk, L.,Adkins, J. K.,Agakishiev, G.,Aggarwal, M. M.,Ahammed, Z.,Alekseev, I.,Alford, J.,Aparin, A.,Arkhipkin, D.,Aschenauer, E. C.,Averichev, G. S.,Banerjee, A.,Bellwied, R.,Bhasin, A.,Bhati, A. American Physical Society 2016 Physical Review C Vol.94 No.2
<P>Balance functions have been measured in terms of relative pseudorapidity (Delta(eta)) for charged particle pairs at the BNL Relativistic Heavy Ion Collider from Au + Au collisions at root s(NN) = 7.7 GeV to 200 GeV using the STAR detector. These results are compared with balance functions measured at the CERN Large Hadron Collider from Pb + Pb collisions at root s(NN) = 2.76 TeV by the ALICE Collaboration. The width of the balance function decreases as the collisions become more central and as the beam energy is increased. In contrast, the widths of the balance functions calculated using shuffled events show little dependence on centrality or beam energy and are larger than the observed widths. Balance function widths calculated using events generated by UrQMD are wider than the measured widths in central collisions and show little centrality dependence. The measured widths of the balance functions in central collisions are consistent with the delayed hadronization of a deconfined quark gluon plasma (QGP). The narrowing of the balance function in central collisions at root s(NN) = 7.7 GeV implies that a QGP is still being created at this relatively low energy.</P>
Adamczyk, L.,Adkins, J.K.,Agakishiev, G.,Aggarwal, M.M.,Ahammed, Z.,Alekseev, I.,Alford, J.,Aparin, A.,Arkhipkin, D.,Aschenauer, E.C.,Averichev, G.S.,Banerjee, A.,Bellwied, R.,Bhasin, A.,Bhati, A.K.,B Elsevier 2015 Physics letters: B Vol.743 No.-
<P><B>Abstract</B></P> <P>Dihadron correlations are analyzed in <SUB> s NN </SUB> = 200 GeV d + Au collisions classified by forward charged particle multiplicity and zero-degree neutral energy in the Au-beam direction. It is found that the jetlike correlated yield increases with the event multiplicity. After taking into account this dependence, the non-jet contribution on the away side is minimal, leaving little room for a back-to-back ridge in these collisions.</P>
Forward Neutral Pion Production inp+pandd+AuCollisions atsNN=200 GeV
Adams, J.,Aggarwal, M. M.,Ahammed, Z.,Amonett, J.,Anderson, B. D.,Arkhipkin, D.,Averichev, G. S.,Badyal, S. K.,Bai, Y.,Balewski, J.,Barannikova, O.,Barnby, L. S.,Baudot, J.,Bekele, S.,Belaga, V. V.,Be American Physical Society 2006 Physical review letters Vol.97 No.15