We present STAR measurements of the charge-dependent three-particle correlator γa,b=〈cos(ϕ1a+ϕ2b−2ϕ3)〉/v2{2} and elliptic flow v2{2} in U+U, Au+Au and p+Au collisions at RHIC. The difference ...Δγ=γ(opposite-sign)−γ(same-sign) measures charge separation across the reaction plane, a predicted signal of the Chiral Magnetic Effect (CME). Although charge separation has been observed, it has been argued that the measured separation can also be explained by elliptic flow related backgrounds. In order to separate the two effects we perform measurements of the γ-correlator where background expectations differ from magnetic field driven effects. A differential measurement of γ with the relative pseudorapidity (Δη) between the first and second particles indicate that Δγ in peripheral A+A and p+A collisions are dominated by short-range correlations in Δη. However, a relatively wider component of the correlation in Δη tends to vanish the same way as projected magnetic field as predicted by MC-Glauber simulations.
We present calculations of bulk properties and multiparticle correlations in a large variety of collision systems within a hybrid formalism consisting of IP-Glasma initial conditions, Music viscous ...relativistic hydrodynamics, and UrQMD microscopic hadronic transport. In particular, we study heavy ion collisions at the Large Hadron Collider (LHC), including Pb + Pb, Xe + Xe, and O + O collisions, and Au + Au, U + U, Ru + Ru, Zr + Zr, and O + O collisions at the Relativistic Heavy Ion Collider (RHIC). We further study asymmetric systems, including p + Au, d + Au, 3He + Au, and p + Pb collisions at various energies as well as p+p collisions at 0.5 and 13 TeV. Here, we describe experimental observables in all heavy ion systems well with one fixed set of parameters, validating the energy and system dependence of the framework. As a result, many observables in the smaller systems are also well described, although they test the limits of the model.
The STAR collaboration is currently pursuing the blind analysis of the data for isobar collisions that was performed at RHIC in the year 2018 to make a decisive test of the Chiral Magnetic Effect ...(CME). Why is it so difficult to detect signals of CME in the experiment? Do we really understand different sources of background? Why observing similar charge separation between p/d + A and A + A does not stop us from pursuing the search for CME? In this contribution, I attempt to address some of these questions and briefly outline a few recent STAR analyses based on new methods and observables to isolate the possible CME-driven signal and non-CME background contributions at the top RHIC energy. Finally, I describe the procedure for the blind analysis of the isobar data. An outstanding question remains - what happens if we go down in energy? I address this by discussing how the new event-plane detector (EPD) upgrade provides a new capability at STAR towards CME search using the data from the RHIC BES-II program.
We compute initial conditions in heavy ion collisions within the color glass condensate framework by combining the impact parameter dependent saturation model with the classical Yang-Mills ...description of initial Glasma fields. In addition to fluctuations of nucleon positions, this impact parameter dependent Glasma description includes quantum fluctuations of color charges on the length scale determined by the inverse nuclear saturation scale Q(s). The model naturally produces initial energy fluctuations that are described by a negative binomial distribution. The ratio of triangularity to eccentricity ε(3)/ε(2) is close to that in a model tuned to reproduce experimental flow data. We compare transverse momentum spectra and v(2,3,4)(p(T)) of pions from different models of initial conditions using relativistic viscous hydrodynamic evolution.
Multi-particle correlation observables in the Relativistic Heavy Ion Collider small system scan are computed in a framework that contains both initial state momentum anisotropies from the Color Glass ...Condensate effective theory and final state hydrodynamic evolution. The initial state is computed using the IP-Glasma model and coupled to viscous relativistic hydrodynamic simulations, which are followed by microscopic hadronic transport. All parameters of the calculation were previously constrained using experimental data on Au+Au collisions at the same center of mass energy. We find that the qualitative features of the experimental data, such as the system and centrality dependence of the charged hadron momentum anisotropy, can only be reproduced when final state interactions are present. On the other hand, we also demonstrate that the details of the initial state are crucially important for the quantitative description of observables in the studied small systems, as neglecting the initial transverse flow profile or the initial shear stress tensor, which contain information on the momentum anisotropy from the Color Glass Condensate, has dramatic effects on the produced final state anisotropy. We further show that the initial state momentum anisotropy is correlated with the observed elliptic flow in all small systems, with the effect increasing with decreasing multiplicity. We identify the precise measurement of v2 in d+Au and Au+Au collisions at RHIC energy at the same multiplicity as a means to reveal effects of the initial state momentum anisotropy.
Results for particle production in s=5.02TeV p+Pb collisions at the Large Hadron Collider within a combined classical Yang–Mills and relativistic viscous hydrodynamic calculation are presented. We ...emphasize the importance of sub-nucleon scale fluctuations in the proton projectile to describe the experimentally observed azimuthal harmonic coefficients vn, demonstrating their sensitivity to the proton shape. We stress that the proton shape and its fluctuations are not free parameters in our calculations. Instead, they have been constrained using experimental data from HERA on exclusive vector meson production. Including temperature dependent shear and bulk viscosities, as well as UrQMD for the low temperature regime, we present results for mean transverse momenta, harmonic flow coefficients for charged hadrons and identified particles, as well as Hanbury–Brown–Twiss radii.
In a previous paper (arXiv:1011.1895), we showed that saturation models, constrained by e+p HERA data on inclusive and diffractive cross-sections, are in good agreement with p+p data at LHC in the ...soft sector. Particularly impressive was the agreement of saturation models with the multiplicity distribution as a function of nch.. In this Letter, we extend these studies further and consider the agreement of these models with data on bulk distributions in A+A collisions. We compare our results to data on central and forward particle production in d+Au collisions at RHIC and make predictions for inclusive distributions in p+Pb collisions at the LHC.
We study here multiparticle azimuthal correlations in relativistic heavy-ion collisions at a center-of-mass energy of 200 GeV. We use the impact parameter-dependent Glasma model to initialize the ...viscous hydrodynamic simulation MUSIC and employ the UrQMD transport model for the low-temperature region of the collisions. In addition, we study effects of local charge and global momentum conservation among the sampled particles. With the exception of the lowest-order three-particle correlator ${C}_{112}$, our framework provides a good description of the existing charge-inclusive azimuthal correlation data for Au+Au and U+U collisions at the Relativistic Heavy-Ion Collider (RHIC). We also present results for charge-dependent two- and three-particle correlators in Au+Au and U+U collisions and make predictions for isobar (Ru+Ru and Zr+Zr) collisions to provide a much-needed baseline for the search for the chiral magnetic effect at RHIC.
High multiplicity events in p+p collisions are studied using the theory of the Color Glass Condensate. We show that intrinsic fluctuations of the proton saturation momentum scale are needed in ...addition to the sub-nucleonic color charge fluctuations to explain the very high multiplicity tail of distributions in p+p collisions. The origin of such intrinsic fluctuations is presumably non-perturbative in nature. Classical Yang Mills simulations using the IP-Glasma model are performed to make quantitative estimations. We find that fluctuations as large as O(1) of the average values of the saturation momentum scale can lead to rare high multiplicity events seen in p+p data at RHIC and LHC energies. Using the available data on multiplicity distributions we try to constrain the distribution of the proton saturation momentum scale and make predictions for the multiplicity distribution in 13 TeV p+p collisions.
Anisotropic flow coefficients v(1)-v(5) in heavy ion collisions are computed by combining a classical Yang-Mills description of the early time Glasma flow with the subsequent relativistic viscous ...hydrodynamic evolution of matter through the quark-gluon plasma and hadron gas phases. The Glasma dynamics, as realized in the impact parameter dependent Glasma (IP-Glasma) model, takes into account event-by-event geometric fluctuations in nucleon positions and intrinsic subnucleon scale color charge fluctuations; the preequilibrium flow of matter is then matched to the music algorithm describing viscous hydrodynamic flow and particle production at freeze-out. The IP-Glasma+MUSIC model describes well both transverse momentum dependent and integrated v(n) data measured at the Large Hadron Collider and the Relativistic Heavy Ion Collider. The model also reproduces the event-by-event distributions of v(2), v(3) and v(4) measured by the ATLAS Collaboration. The implications of our results for better understanding of the dynamics of the Glasma and for the extraction of transport properties of the quark-gluon plasma are outlined.
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