arXiv subjects
Publications and source records attributed to Sergei A. Voloshin.
Recent developments in the field of anisotropic flow in nuclear collision are reviewed. The results from the top AGS energy to the top RHIC energy are discussed with emphasis on techniques, interpretation, and uncertainties in the measurements.
In non-central relativistic heavy ion collisions, ¶-odd domains, which might be created in the process of the collision, are predicted to lead to charge separation along the system orbital momentum \cite{Kharzeev:2004ey}. An observable, ¶-even, but directly sensitive to the charge separation effect, has been proposed in \cite{Voloshin:2004vk} and is based on 3-particle mixed harmonics azimuthal correlations. We report the STAR measurements using this observable for Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 and 62 GeV. The results are reported as function of collision centrality, particle separation in rapidity, and particle transverse momentum. Effects that are not related to parity violation but might contribute to the signal are discussed.
Anisotropic flow measurements play a crucial role in understanding the physics and bulk properties of the system created in heavy ion collisions. In this talk I briefly review the most important results obtained so far, recent developments in the analysis techniques and the interpretation of the results, and what should we expect next, both at RHIC and LHC. I also discuss event anisotropies sensitive to the strong parity violation effects.
We use a simple a transverse radial boost scenario coupled to PYTHIA events to illustrate the impact radial flow may have on two- and three-particle correlation functions measured in heavy-ion collisions. We show that modest radial velocities can impart strong modifications to the correlation functions, some of which may be interpreted as same side ridge and away side structure that can mimic conical emission.
We discuss a specific model of elliptic flow fluctuations due to Gaussian fluctuations in the initial spatial $x$ and $y$ eccentricity components $\left\{\mean{(σ_y^2-σ_x^2)/(σ_x^2+σ_y^2)}, \mean{2σ_{xy}/(σ_x^2+σ_y^2)} \right\}$. We find that in this model $\vfour$, elliptic flow determined from 4-particle cumulants, exactly equals the average flow value in the reaction plane coordinate system, $\mean{v_{RP}}$, the relation which, in an approximate form, was found earlier by Bhalerao and Ollitrault in a more general analysis, but under the same assumption that $v_2$ is proportional to the initial system eccentricity. We further show that in the Gaussian model all higher order cumulants are equal to $\vfour$. Analysis of the distribution in the magnitude of the flow vector, the $Q-$distribution, reveals that it is totally defined by two parameters, $\vtwo$, the flow from 2-particle cumulants, and $\vfour$, thus providing equivalent information compared to the method of cumulants. The flow obtained from the $Q-$distribution is again $\vfour=\mean{v_{RP}}$.
A new method is presented for determining event-by-event fluctuations of elliptic flow, $v_2$, using first-order event planes. By studying the event-by-event distributions of $v_2$ observables and first-order event-plane observables, average flow $\avg{v_2}$ and event-by-event flow fluctuations can be separately determined, making appropriate allowance for the effects of finite multiplicity and non-flow. The method has been tested with Monte Carlo simulations. The connection between flow fluctuations and fluctuations of the initial-state participant eccentricity is discussed.
We report measurements of charged particle elliptic flow %($v_2$) at mid-rapidity in Au+Au and Cu+Cu collisions at $\sqrt{s_{_{NN}}}=62$ and 200 GeV. Using correlations between main STAR TPC and Forward TPCs ensures minimal bias due to non-flow effects. We further investigate the effect of flow fluctuations on $v_2/\eps$ scaling studying initial geometry eccentricity fluctuations in Monte-Carlo Glauber model, consistent with STAR direct measurements of elliptic flow fluctuations. It is found that accounting for the effect of flow fluctuations improves $v_2/\eps$ scaling.
In this talk I present new STAR results on measurements of integrated elliptic flow at midrapidity in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}=$200 and 62 GeV energies. These results have been obtained from azimuthal correlations between particles in the main STAR TPC and two forward TPCs, and are to a large extent free from so-called non-flow correlations. These results along with the previously reported values of ``participant'' eccentricity taking into account eccentricity fluctuations are used for testing the $v_2/\eps$ scaling, which is found to hold relatively well.
In this talk I concentrate on two topics closely related to the understanding of the energy and the system size dependence of the elliptic flow: determination of the elliptic flow ($v_2$) ``free'' from the effects of non-flow and flow fluctuations, and the role of fluctuations in the initial eccentricity of the overlap region. I introduce a new approach for the analysis of the distribution in flow vector, $dP/dq_n$, namely, I propose to use the Bessel Transform of this ditribution. I show that the Bessel Transform method is similar to the Lee-Yang Zeroes method, and is very transparent in its meaning and applications.
At the very first stage of an ultra-relativistic nucleus-nucleus collision new particles are produced in individual nucleon-nucleon collisions. In the transverse plane, all particles from a single $NN$ collision are initially located at the same position. The subsequent thermalization and transverse radial expansion of the system create strong position-momentum correlations and lead to characteristic rapidity, transverse momentum, and azimuthal correlations among the produced particles.
Correlations and fluctuations (the latter are directly related to the 2-particle correlations) is one of the important directions in analysis of heavy ion collisions. At the current stage of RHIC exploration, when the details matter, basically any physics question is addressed with help of correlation techniques. In this talk I start with a general introduction to the correlation and fluctuation formalism and discuss weak and strong sides of different type of observables. In more detail, I discuss the two-particle $p_t$ correlations/$\mpt$ fluctuations. In spite of not observing any dramatic changes in the event-by-event fluctuations with energy, which would indicate a possible phase transition, such correlations measurements remain an interesting and important subject, bringing valuable information. Lastly, I show how radial flow can generate characteristic azimuthal, transverse momentum and rapidity correlations, which could qualitatively explain many of recently observed phenomena in nuclear collisions.
In this short note I speculate on some consequences of the high energy collision picture in which the orbital angular momentum of the colliding hadrons can be converted into secondary particle angular spin momentum via some spin-orbital interaction. In particular I discuss a possibility to observe a non-zero polarization of secondary particles (e.g. hyperons) at midrapidity ($x_F=0$) and at low transverse momentum. I also speculate that such effects could contribute to the produced particle directed and elliptic flow observed in relativistic nuclear collisions.
At the very first stage of an ultra-relativistic nucleus-nucleus collision new particles are produced in individual nucleon-nucleon collisions. In the transverse plane, all particles from a single $NN$ collision are initially located at the same position. The subsequent transverse radial expansion of the system creates strong position-momentum correlations and leads to characteristic rapidity, transverse momentum, and azimuthal correlations among the produced particles.
In a recent paper (arXive:hep-ph/0406125) entitled {\em Parity violation in hot QCD: why it can happen, and how to look for it}, D. Kharzeev argues for the possibility of P- and/or CP- violation effects in heavy-ion collisions, the effects that can manifest themselves via asymmetry in $π^{\pm}$ production with respect to the direction of the system angular momentum. Here we present an experimental observable that can be used to detect and measure the effects.
We show that hadronization via quark coalescence enhances hadron elliptic flow at large pT relative to that of partons at the same transverse momentum. Therefore, compared to earlier results based on covariant parton transport theory, more moderate initial parton densities dN/dη(b=0) ~ 1500-3000 can explain the differential elliptic flow v_2(pT) data for Au+Au reactions at s^1/2=130 and 200 AGeV from RHIC. In addition, v2(pT) could saturate at about 50% higher values for baryons than for mesons. If strange quarks have weaker flow than light quarks, hadron v_2 at high pT decreases with relative strangeness content.
Recent results on event-by-event mean transverse momentum, $\mpt$, fluctuations in ultra-relativistic heavy ion collisions are briefly reviewed. We conclude that the observed fluctuations are in a rough agreement with that expected for the independent superposition of nucleon-nucleon collisions. We further discuss the possibility of the use of the forth order cumulants of the particle transverse momentum distribution in order to access the fluctuations related to collective phenomena.
Strong modification of event-by-event fluctuations in global observables was proposed by several authors as an indicator of the phase transition in system evolution. We report the results of the analysis of STAR TPC data on the charged multiplicity and mean transverse momentum fluctuations in Au+Au collisions at $\sqrt{s_{NN}}$=130 GeV. We discuss the centrality dependence of the observed fluctuations and compare the results with available data from lower collision energies and ISR data.
It was recently argued that in heavy ion collision the parity could be broken. This Note addresses the question of possibility of the experimental detection of the effect. We discuss how parity violating effects would modify the final particle distributions and how one could construct variables sensitive to the effect, and which measurement would be the (most) conclusive. Discussing different observables we also discuss the question if the ``signals'' can be faked by ``conventional'' effects (such as anisotropic flow, etc.) and make estimates of the signals.