AGeV Au Research Articles

The direct flow of charged particles and the global polarization of hyperons in 200 AGeV Au+Au collisions at RHIC

In non-central relativistic heavy-ion collisions, the non-colliding nucleons drag the colliding nucleons along the longitudinal direction asymmetrically, producing a longitudinally tilted quark-gluon plasma (QGP) fireball. Meanwhile, these colliding nuclei deposit a huge initial orbital angular momentum into the system, leading to the polarization of partons inside the QGP along the direction of the total angular momentum. Based on the optical Glauber model, we develop a 3-dimensional initial condition of the tilted QGP. By combining it with the (3+1)-dimensional viscous hydrodynamic model CLVisc, we investigate the directed flow of charged hadrons and the global polarization of <inline-formula><tex-math id="M2">\begin{document}$ \Lambda/\bar{\Lambda} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20222391_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20222391_M2.png"/></alternatives></inline-formula> hyperons in heavy-ion collisions. Our calculation indicates that the combination of a tilted initial condition of the QGP and the hydrodynamic model can provide a satisfactory description of the directed flow and global polarization observed at RHIC-STAR. This offers a theoretical baseline for using these observables to further constrain the initial geometry and kinematic properties of the nuclear matter created in heavy-ion collisions.

Signature of intermittency in hybrid UrQMD-hydro data at 10 AGeV Au$$+$$Au collisions

An attempt has been made, in the light of scaled factorial moment (SFM) analysis, to investigate hybrid UrQMD-hydro generated events of Au+Au collisions at 10 AGeV to realize the role of hydrodynamic evolution on observed intermittency, if any. $$ln\langle F_{q}\rangle $$ values for $$q=2$$ –6 are found to increase with increasing values of $$lnM^{2}$$ indicating unambiguously the presence of intermittency in our data sample generated with both chiral and hadronic equations of state (EoS). Although various late processes like meson-meson (MM) and meson-baryon (MB) hadronic re-scattering and/or resonance decays are found to influence the intermittency index significantly, these process could not be held responsible for the observed intermittency in hybrid UrQMD-hydro data. Moreover, the signature of intermittency is also found to exists in different sets of data sample generated with a change in initial conditions such as the start time ( $$t_{start}$$ ) and transition energy density (TED) of the UrQMD-hydro model confirming the robustness of the observed power law behavior $$F_{q} \propto (M^{2})^{\alpha _{q}}$$ in our various generated sets of hydro data.

Heavy quark energy loss and D-mesons in RHIC and LHC energies

We obtain the radiative energy loss of a heavy quark in a deconfined medium due to radiation of gluons off them using a recently derived generalised gluon emission spectrum. We find that the heavy flavour loses energy almost in a similar fashion like light quarks through this process. With this, we further analyse the nuclear modification factor for D-meson at LHC and RHIC energies. In particular, the obtained result is found to be in close agreement with the most recent data from ALICE collaboration at 2.76 ATeV Pb–Pb collisions. We also discuss the nuclear modification factor due to the collisional energy loss. Furthermore, the result of non-photonic single electron from the decay of both D- and B-mesons is compared with the RHIC data at 200 AGeV Au–Au collisions, which is also in close agreement.

Collision centrality andτ0dependence of the emission of thermal photons from a fluctuating initial state in an ideal hydrodynamic calculation

Fluctuations in the initial QCD matter density distribution are found to enhance the production of thermal photons significantly in the range 2 \leq pT \leq 4 GeV/c compared to a smooth initial state averaged profile in ideal hydrodynamic calculation for 200 AGeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and 2.76 ATeV Pb+Pb collisions at the Large Hadron Collider (LHC). The thermal emission of photons is strongly dependent on the initial temperature of the system where the presence of 'hotspots' in the initial state translates into enhanced production of photons compared to a smooth profile. The effect of fluctuations in the initial state is found to be stronger for peripheral collisions and for lower beam energies. The pT spectra are found to be quite sensitive to the value of the initial formation time of the plasma which is not known unambiguously and which may vary with collision centralities at a particular beam energy. Increase in the value of the formation time lowers the production of thermal photons compared to the results from a shorter formation time. However, the relative enhancement from fluctuating initial tates (compared to a smooth initial state) is found to be stronger for the larger values of formation time. The pT spectra alone are found to be insufficient to quantify the fluctuations in the initial density distribution due to the uncertainties in the initial conditions. A suitably normalized ratio of central-to-peripheral yield as a function of collision centrality and pT can be a useful measure of the fluctuation size scale.

Space-momentum correlations in 8 AGeV Au+Au collisions

Within the RQMD model, space-momentum correlations, i.e. the correlations between final momentum anisotropy and initial eccentricity, are studied for 8 AGeV Au+Au events classified according to the multi-particle azimuthal correlations. The results show that the final elliptic flow fluctuations depend on the initial collision geometry. There are clear space-momentum correlations for nucleons during the whole dynamical evolution of the collisions.

Non-Identical Particle Correlations in STAR

Non-identical particle correlations offer new methods of probing the dynamics of heavy-ion collision. In STAR we have performed a correlation analysis of pion-kaon and pion-proton systems for √sNN = 130 AGeV Au+Au collisions. The results show that average emission space-time points of pions, kaons and protons are not the same. These results are studied with the help of heavy-ion collision models. The asymmetries appear as a consequence of space-momentum correlations produced by transverse radial expansion of the system. The effects of emission time differences, coming from the decay of resonances are also investigated, and found to explain only part of the asymmetry.

Ξ production at AGS energies

A relativistic transport model is used to study Ξ− production in 2–11 AGeV Au+Au collisions. Introducing the strangeness-exchange reactions between antikaons and hyperons as the sources for Ξ−, we find that the cascade yield in these collisions is in reasonable agreement with the data. Although the Ξ− abundance does not reach chemical equilibrium unless the cross section for strangeness-exchange reactions is enhanced by six times, it exhibits the strongest enhancement with increasing centrality of collision and with increasing beam energy.

System, centrality, and transverse mass dependence of two-pion correlation radii in heavy ion collisions at11.6Aand14.6AGeV/c

Two-pion correlation functions are analyzed at mid-rapidity for three systems (14.6 A-GeV Si+Al, Si+Au, and 11.6 A-GeV Au+Au), seven distinct centrality conditions, and different kT bins in the range 0.1--0.5 GeV/c. Source reference frames are determined from fits to the Yano-Koonin source parameterization. Bertsch-Pratt radius parameters are shown to scale linearly with both number of projectile and total participants as obtained from a Glauber model calculation. A finite emission duration that increases linearly with system/centrality is also reported. The mT dependence of the Bertsch-Pratt radii is measured for the central Si+Au and Au+Au systems. The system/centrality dependence is investigated separately for both high and low mT regions.

Centrality and momentum-selected elliptic flow: Tighter constraints for the nuclear equation of state

Proton elliptic flow is studied as a function of impact-parameter $b$, for two transverse momentum cuts in 2 - 6 AGeV Au + Au collisions. The elliptic flow shows an essentially linear dependence on b (for $1.5 < b < 8$ fm) with a negative slope at 2 AGeV, a positive slope at 6 AGeV and a near zero slope at 4 AGeV. These dependencies serve as an important constraint for discriminating between various equations of state (EOS) for high density nuclear matter, and they provide important insights on the interplay between collision geometry and the expansion dynamics. Extensive comparisons of the measured and calculated differential flows provide further evidence for a softening of the EOS between 2 and 6 GeV/nucleon.

Analysis of one-and two-particle spectra at RHIC based on a hydrodynamical model

We calculate the one-particle hadronic spectra and correlation functions of pions based on a hydrodynamical model. Parameters in the model are so chosen that the one-particle spectra reproduce experimental results of √s= 130 AGeV Au + Au collisions at RHIC. Based on the numerical solution, we discuss the space-time evolution of the fluid. Two-pion correlation functions are also discussed. Our numerical solution suggests the formation of the quark-gluon plasma with large volume and low net baryon density.

The data collection modules and ATM-based event builder for the PHENIX experiment at RHIC

The PHENIX experiment at RHIC will study 200 AGeV Au+Au collisions as well as pp and pA collisions at higher energies. In order to study hard processes or high-mass vector mesons, which have small cross sections, PHENIX emphasizes measuring charged hadrons, leptons, and photons at a high rate in a limited aperture. At design RHIC luminosity, the minimum-bias interaction rate will be approximately 1000 Hz with an average zero-suppressed event size of 200 kB. We will discuss two major parts of the PHENIX online system: the data collection modules (DCM) and the event builder (EvB). The DCMs use a combination of FPGAs and DSPs to perform online pedestal subtraction, zero-suppression, and data formatting. They have been fully constructed and tested and are able to deliver data at a maximum rate of 160 MB/s. The event builder consists of commercially available PCs and a 10 Gb/s ATM switch. The software for it has been developed using Windows NT, Winsock-2 and Visual C++, allowing a fully object-oriented design. Currently, we are running configurations with up to 4 sources and 3 destinations that can deliver data at /spl sim/10 MB/sec/node and performance is improving steadily.