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Register a new userElectro-magnetic field fluctuation and its correlation with the participant plane in Au+Au and isobaric collisions at $sqrt{s_{NN}}=200$ GeV
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Intense transient electric ({bf E}) and magnetic ({bf B}) fields are produced in the high energy heavy-ion collisions. The electromagnetic fields produced in such high-energy heavy-ion collisions are proposed to give rise to a multitude of exciting phenomenon including the Chiral Magnetic Effect. We use a Monte Carlo (MC) Glauber model to calculate the electric and magnetic fields, more specifically their scalar product $bf{E}cdotbf{B}$, as a function of space-time on an event-by-event basis for the Au+Au collisions at $sqrt{s_{NN}}=200$ GeV for different centrality classes. We also calculate the same for the isobars Ruthenium and Zirconium at $sqrt{s_{NN}}=200$ GeV. In the QED sector $bf{E}cdotbf{B}$ acts as a source of Chiral Separation Effect, Chiral Magnetic Wave, etc., which are associated phenomena to the Chiral Magnetic Effect. We also study the relationships between the electromagnetic symmetry plane angle defined by $bf{E}cdotbf{B}$ ($psi_{E.B}$) and the participant plane angle $psi_{P}$ defined from the participating nucleons for the second-fifth order harmonics.
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With a Yang-Mills field, stratified shear flow initial state and a high resolution (3+1)D Particle-in-Cell Relativistic (PICR) hydrodynamic model, we calculate the $Lambda$ polarization for peripheral Au+Au collisions at RHIC energy of $sqrt{S_{NN}}=200$ GeV. The obtained longitudinal polarization in our model agrees with the experimental signature and the quadrupole structure on transverse momentum plane. It is found that the relativistic correction (2nd term), arising from expansion and from the time component of the thermal vorticity, plays a crucial role in our results. This term is changing the signature and exceeds the first term, arising from the classical vorticity. Finally, the global polarization in our model shows no significant dependence on rapidity, which agrees with the experimental data. It is also found that the second term flattens the sharp peak arising from the classical vorticity (1st term).
Measurements of three-dimensional correlation functions of like-sign low transverse momentum kaon pairs from Au+Au collisions at top RHIC energy $sqrt s_{NN}$=200 GeV are presented. The extracted kaon source function is narrower than the pion one and does not have the long tail along the pair transverse momentum direction. This indicates a much smaller role of long-lived resonance decays and/or of the emission duration on kaon emission. Three-dimensional Gaussian shape of the kaon source function can be adequately reproduced by Therminator simulations with resonance contributions taken into account. Comparison to pion data at the same energy reveals that the kaon Gaussian radii in the outward and sideward directions scale with the transverse mass $m_T$. In the longitudinal direction, unlike at lower SPS energies, the Gaussian radii do not seem to follow the exact $m_T$ scaling between kaons and pions.
We report the STAR measurements of dielectron ($e^+e^-$) production at midrapidity ($|y_{ee}|<$1) in Au+Au collisions at $sqrt{s_{rm NN}}$ = 200,GeV. The measurements are evaluated in different invariant mass regions with a focus on 0.30-0.76 ($rho$-like), 0.76-0.80 ($omega$-like), and 0.98-1.05 ($phi$-like) GeV/$c^{2}$. The spectrum in the $omega$-like and $phi$-like regions can be well described by the hadronic cocktail simulation. In the $rho$-like region, however, the vacuum $rho$ spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77$pm$0.11(stat.)$pm$0.24(sys.)$pm$0.33(cocktail) is determined with respect to the hadronic cocktail simulation that excludes the $rho$ meson. The excess yield in the $rho$-like region increases with the number of collision participants faster than the $omega$ and $phi$ yields. Theoretical models with broadened $rho$ contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies.
We report first measurements of $e^{+}e^{-}$ pair production in the mass region 0.4 $<M_{ee}<$ 2.6 GeV/$c^{2}$ at low transverse momentum ($p_T<$ 0.15 GeV/$c$) in non-central Au$+$Au collisions at $sqrt{s_{NN}}$ = 200 GeV and U$+$U collisions at $sqrt{s_{NN}}$ = 193 GeV. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40-80% centrality of these collisions. The excess yields peak distinctly at low-$p_T$ with a width ($sqrt{langle p^2_Trangle}$) between 40 to 60 MeV/$c$. The absolute cross section of the excess depends weakly on centrality while those from a theoretical model calculation incorporating an in-medium broadened $rho$ spectral function and radiation from a Quark Gluon Plasma or hadronic cocktail contributions increase dramatically with increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the $p^{2}_{T}$ distributions.
Dihadron azimuthal correlations containing a high transverse momentum ($p_T$) trigger particle are sensitive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium, i.e. jet-quenching. Previous measurements revealed a strong modification to dihadron azimuthal correlations in Au+Au collisions with respect to p+p and d+Au collisions. The modification increases with the collision centrality, suggesting a path-length or energy density dependence to the jet-quenching effect. This paper reports STAR measurements of dihadron azimuthal correlations in mid-central (20-60%) Au+Au collisions at $sqrt{s_{_{rm NN}}}=200$ GeV as a function of the trigger particles azimuthal angle relative to the event plane, $phi_s=|phi_t-psi_{rm EP}|$. The azimuthal correlation is studied as a function of both the trigger and associated particle $p_T$. The subtractions of the combinatorial background and anisotropic flow, assuming Zero Yield At Minimum (ZYAM), are described. The correlation results are first discussed with subtraction of the even harmonic (elliptic and quadrangular) flow backgrounds. The away-side correlation is strongly modified, and the modification varies with $phi_s$, with a double-peak structure for out-of-plane trigger particles. The near-side ridge (long range pseudo-rapidity $Deltaeta$ correlation) appears to drop with increasing $phi_s$ while the jet-like component remains approximately constant. The correlation functions are further studied with subtraction of odd harmonic triangular flow background arising from fluctuations. It is found that the triangular flow, while responsible for the majority of the amplitudes, is not sufficient to explain the $phi_s$-dependence of the ridge or the away-side double-peak structure. ...
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