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Register a new userSystem size and energy dependence of near-side di-hadron correlations
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Two-particle azimuthal ($Deltaphi$) and pseudorapidity ($Deltaeta$) correlations using a trigger particle with large transverse momentum ($p_T$) in $d$+Au, Cu+Cu and Au+Au collisions at $sqrt{s_{{NN}}}$ =xspace 62.4 GeV and 200~GeV from the STAR experiment at RHIC are presented. The s correlation is separated into a jet-like component, narrow in both $Deltaphi$ and $Deltaeta$, and the ridge, narrow in $Deltaphi$ but broad in $Deltaeta$. Both components are studied as a function of collision centrality, and the jet-like correlation is studied as a function of the trigger and associated $p_T$. The behavior of the jet-like component is remarkably consistent for different collision systems, suggesting it is produced by fragmentation. The width of the jet-like correlation is found to increase with the system size. The ridge, previously observed in Au+Au collisions at $sqrt{s_{{NN}}}$ = 200 GeV, is also found in Cu+Cu collisions and in collisions at $sqrt{s_{{NN}}}$ =xspace 62.4 GeV, but is found to be substantially smaller at $sqrt{s_{{NN}}}$ =xspace 62.4 GeV than at $sqrt{s_{{NN}}}$ = 200 GeV for the same average number of participants ($ langle N_{mathrm{part}}rangle$). Measurements of the ridge are compared to models.
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A discussion of results for short and long-range multiplicity correlations (forward-backward) are presented for several systems (Au+Au, Cu+Cu, and pp) and energies (e.g. $sqrt{s_{NN}}$ = 200, 62.4, and $approx$ 20 GeV). These correlations are measured with increasing values of a gap in pseudorapidity, from no gap at midrapidity to a separation of 1.6 units ($|eta|$ = 0.8). For the highest energy, central A+A collisions, the forward-backward correlation strength maintains a constant value across the measurement region. In peripheral collisions, at lower energies, and in pp data, the maximum appears at midrapidity. This result may indicate the possible formation of high density matter for central A+A collisions at $sqrt{s_{NN}}$ = 200 GeV.
This paper presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.
We present a system size and energy dependence of $phi$ meson production in Cu+Cu and Au+Au collisions at $sqrt{s_{NN}}$=62.4 GeV and 200 GeV measured by the STAR experiment at RHIC. We find that the number of participant scaled $phi$ meson yields in heavy ion collisions over that of p+p collisions are larger than 1 and increase with collision energy. We compare the results with those of open-strange particles and discuss the physics implication.
We present measurements of net charge fluctuations in $Au + Au$ collisions at $sqrt{s_{NN}} = $ 19.6, 62.4, 130, and 200 GeV, $Cu + Cu$ collisions at $sqrt{s_{NN}} = $ 62.4, 200 GeV, and $p + p$ collisions at $sqrt{s} = $ 200 GeV using the dynamical net charge fluctuations measure $ u_{+-{rm,dyn}}$. We observe that the dynamical fluctuations are non-zero at all energies and exhibit a modest dependence on beam energy. A weak system size dependence is also observed. We examine the collision centrality dependence of the net charge fluctuations and find that dynamical net charge fluctuations violate $1/N_{ch}$ scaling, but display approximate $1/N_{part}$ scaling. We also study the azimuthal and rapidity dependence of the net charge correlation strength and observe strong dependence on the azimuthal angular range and pseudorapidity widths integrated to measure the correlation.
In the BDMPSZ model, the energy loss of an outgoing parton in a medium $-dE/dx$ is the transport coefficient $hat{q}$ times $L$ the length traveled. This results in jet quenching, which is well established. However BDMPSZ also predicts an azimuthal broadening of di-jets also proportional to $hat{q}L$ which has so far not been observed. The broadening should produce a larger $k_T$ in A$+$A than in p$+$p collisions. This presentation introduces the observation that the $k_T$ measured in p$+$p collisions for di-hadrons with $p_{Tt}$ and $p_{Ta}$ must be reduced to compensate for the energy loss of both the trigger and away parent partons when comparing to the $k_T$ measured with the same di-hadron $p_{Tt}$ and $p_{Ta}$ in A$+$A collisions. This idea is applied to a recent STAR di-hadron measurement in Au$+$Au at $sqrt{s_{NN}}$=200 GeV, [Phys. Lett. B760 (2016) 689], with result $<{hat{q}L}>=2.1pm 0.6$ GeV$^2$. This is more precise but in agreement with a theoretical calculation of $<{hat{q}L}>=14^{+42}_{-14}$ GeV$^2$ using the same data. Assuming a length $<{L}>approx 7$ fm for central Au$+$Au collisions the present result gives $hat{q}approx 0.30pm 0.09$ GeV$^2$/fm, in fair agreement with the JET collaboration result from single hadron suppression of $hat{q}approx 1.2pm 0.3$ GeV$^2$/fm at an initial time $tau_0=0.6$ fm/c in Au$+$Au collisions at $sqrt{s_{NN}}=200$ GeV. There are several interesting details to be discussed: for a given $p_{Tt}$ the $<{hat{q}L}>$ seems to decrease then vanish with increasing $p_{Ta}$; the di-jet spends a much longer time in the medium ($approx 7$ fm/c) then $tau_0=0.6$ fm/c which likely affects the value of $hat{q}$ that would be observed.
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