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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.
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.
Di-hadron correlations with identified leading hadrons in 200 GeV Au+Au and d+Au collisions at STAR
North-Holland Pub. Co 2015 Physics letters: B Vol.751 No.-
The STAR Collaboration presents for the first time two-dimensional di-hadron correlations with identified leading hadrons in 200 GeV central Au+Au and minimum-bias d+Au collisions to explore hadronization mechanisms in the quark gluon plasma. The enhancement of the jet-like yield for leading pions in Au+Au data with respect to the d+Au reference and the absence of such an enhancement for leading non-pions (protons and kaons) are discussed within the context of a quark recombination scenario. The correlated yield at large angles, specifically in the ridge region, is found to be significantly higher for leading non-pions than pions. The consistencies of the constituent quark scaling, azimuthal harmonic model and a mini-jet modification model description of the data are tested, providing further constraints on hadronization.
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.
North-Holland Pub. Co 2015 Physics letters: B Vol.750 No.-
The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity |y<SUB>ee</SUB>|<1 in minimum-bias Au+Au collisions at s<SUB>NN</SUB>=19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened ρ spectral function for M<SUB>ee</SUB><1.1 GeV/c<SUP>2</SUP>. The integrated dielectron excess yield at s<SUB>NN</SUB>=19.6 GeV for 0.4<M<SUB>ee</SUB><0.75 GeV/c<SUP>2</SUP>, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at s<SUB>NN</SUB>=17.3 GeV. For s<SUB>NN</SUB>=200 GeV, the normalized excess yield in central collisions is higher than that at s<SUB>NN</SUB>=17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at s<SUB>NN</SUB>=200 GeV is longer than those in peripheral collisions and at lower energies.
Long-range pseudorapidity dihadron correlations in d+Au collisions at s<sub>NN</sub>=200 GeV
North-Holland Pub. Co 2015 Physics letters. Section B Vol.747 No.-
Dihadron angular correlations in d+Au collisions at s<SUB>NN</SUB>=200 GeV are reported as a function of the measured zero-degree calorimeter neutral energy and the forward charged hadron multiplicity in the Au-beam direction. A finite correlated yield is observed at large relative pseudorapidity (Δη) on the near side (i.e. relative azimuth Δφ~0). This correlated yield as a function of Δη appears to scale with the dominant, primarily jet-related, away-side (Δφ~π) yield. The Fourier coefficients of the Δφ correlation, V<SUB>n</SUB>≤cos@?nΔφ>, have a strong Δη dependence. In addition, it is found that V<SUB>1</SUB> is approximately inversely proportional to the mid-rapidity event multiplicity, while V<SUB>2</SUB> is independent of it with similar magnitude in the forward (d-going) and backward (Au-going) directions.
J/ψ production at high transverse momenta in p+p and Au + Au collisions at s<sub>NN</sub>=200 GeV
North-Holland Pub. Co 2013 Physics letters: B Vol.722 No.1
We report J/ψ spectra for transverse momenta p<SUB>T</SUB>>5 GeV/c at mid-rapidity in p+p and Au+Au collisions at s<SUB>NN</SUB>=200 GeV. The inclusive J/ψ spectrum and the extracted B-hadron feed-down are compared to models incorporating different production mechanisms. We observe significant suppression of the J/ψ yields for p<SUB>T</SUB>>5 GeV/c in 0-30% central Au+Au collisions relative to the p+p yield scaled by the number of binary nucleon-nucleon collisions in Au+Au collisions. In 30-60% mid-central collisions, no such suppression is observed. The level of suppression is consistently less than that of high-p<SUB>T</SUB>π<SUP>+/-</SUP> and low-p<SUB>T</SUB>J/ψ at RHIC and high-p<SUB>T</SUB>J/ψ at the LHC.
Direct virtual photon production in Au+Au collisions at s<sub>NN</sub>=200 GeV
North-Holland Pub. Co 2017 Physics letters. Section B Vol.770 No.-
We report the direct virtual photon invariant yields in the transverse momentum ranges 1<p<SUB>T</SUB><3 GeV/c and 5<p<SUB>T</SUB><10 GeV/c at mid-rapidity derived from the dielectron invariant mass continuum region 0.10<M<SUB>ee</SUB><0.28 GeV/c<SUP>2</SUP> for 0-80% minimum-bias Au+Au collisions at s<SUB>NN</SUB>=200 GeV. A clear excess in the invariant yield compared to the nuclear overlap function T<SUB>AA</SUB> scaled p+p reference is observed in the p<SUB>T</SUB> range 1<p<SUB>T</SUB><3 GeV/c. For p<SUB>T</SUB>>6 GeV/c the production follows T<SUB>AA</SUB> scaling. Model calculations with contributions from thermal radiation and initial hard parton scattering are consistent within uncertainties with the direct virtual photon invariant yield.