We calculate the cross section and transverse-momentum (P⊥) distribution of the Breit-Wheeler process in relativistic heavy-ion collisions and their dependence on collision impact parameter (b). To ...accomplish this, the Equivalent Photon Approximation (EPA) was generalized in a more differential way compared to the approach traditionally used for inclusive collisions. In addition, a lowest-order QED calculation with straightline assumption was performed as a standard baseline for comparison. The cross section as a function of b is consistent with previous calculations using the equivalent one-photon distribution function. Most importantly, the P⊥ shape from this model is strongly dependent on impact parameter and can quantitatively explain the P⊥ broadening observed recently by RHIC and LHC experiments. This broadening effect from the initial QED field strength should be considered in studying possible trapped magnetic field and multiple scattering in a Quark-Gluon Plasma (QGP). The impact-parameter sensitive observable also provides a controllable tool for studying extreme electromagnetic fields.
Based on the coalescence model for light nuclei production, we show that the yield ratio Op-d-t=NH3Np/Nd2 of p, d, and 3H in heavy-ion collisions is sensitive to the neutron relative density ...fluctuation Δn=〈(δn)2〉/〈n〉2 at kinetic freeze-out. From recent experimental data in central Pb+Pb collisions at sNN=6.3 GeV, 7.6 GeV, 8.8 GeV, 12.3 GeV and 17.3 GeV measured by the NA49 Collaboration at the CERN Super Proton Synchrotron (SPS), we find a possible non-monotonic behavior of Δn as a function of the collision energy with a peak at sNN=8.8 GeV, indicating that the density fluctuations become the largest in collisions at this energy. With the known chemical freeze-out conditions determined from the statistical model fit to experimental data, we obtain a chemical freeze-out temperature of ∼144 MeV and baryon chemical potential of ∼385 MeV at this collision energy, which are close to the critical endpoint in the QCD phase diagram predicted by various theoretical studies. Our results thus suggest the potential usefulness of the yield ratio of light nuclei in relativistic heavy-ion collisions as a direct probe of the large density fluctuations associated with the QCD critical phenomena.
It is generally believed that the quark-hadron transition at small values of baryon chemical potentials μB is a crossover but changes to a first-order phase transition with an associated critical ...endpoint (CEP) as μB increases. Such a μB-dependent quark-hadron transition is expected to result in a double-peak structure in the collision energy dependence of the baryon density fluctuation in heavy-ion collisions with one at lower energy due to the spinodal instability during the first-order phase transition and another at higher energy due to the critical fluctuations in the vicinity of the CEP. By analyzing the data on the p, d and 3H yields in central heavy-ion collisions within the coalescence model for light nuclei production, we find that the relative neutron density fluctuation Δρn=〈(δρn)2〉/〈ρn〉2 at kinetic freeze-out indeed displays a clear peak at sNN=8.8GeV and a possible strong re-enhancement at sNN=4.86GeV. Our findings thus provide a strong support for the existence of a first-order phase transition at large μB and its critical endpoint at a smaller μB in the temperature versus baryon chemical potential plane of the QCD phase diagram.
STAR Highlights Xu, Zhangbu
Nuclear physics. A,
January 2021, 2021-01-00, 2021-01-01, Letnik:
1005, Številka:
C
Journal Article
Recenzirano
Odprti dostop
In this overview talk, I have represented STAR to highlight the results of 19 parallels talks and 38 posters at this Quark Matter Conference. The contents have 8 major categories: 1. experimental ...measurements related to the Breit-Wheeler process and vacuum birefringence; 2. probes of initial states of heavy-ion collisions (parton distribution function and azimuthal correlations in small systems); 3. constraints on temperature-dependent viscosity (η/s(T)) with multiple flow harmonics and rapidity correlations; 4. hard probes using jet structures and heavy flavors; 5. exploration of the origin of global polarization and vorticity; 6. chiral and thermal properties of the QGP; 7. Beam Energy Scan (BES) and the critical point search; 8. upgrades and summary.
During the early development of quantum chromodynamics, it was proposed that baryon number could be carried by a non-perturbative Y-shaped topology of gluon fields, called the gluon junction, rather ...than by the valence quarks as in the QCD standard model. A puzzling feature of ultra-relativistic nucleus-nucleus collisions is the apparent substantial baryon excess in the mid-rapidity region that could not be adequately accounted for in most conventional models of quark and diquark transport. The transport of baryonic gluon junctions is predicted to lead to a characteristic exponential distribution of net-baryon density with rapidity and could resolve the puzzle. In this context we point out that the rapidity density of net-baryons near mid-rapidity indeed follows an exponential distribution with a slope of
-
0.61
±
0.03
as a function of beam rapidity in the existing global data from A+A collisions at AGS, SPS and RHIC energies. To further test if quarks or gluon junctions carry the baryon quantum number, we propose to study the absolute magnitude of the baryon vs. charge stopping in isobar collisions at RHIC. We also argue that semi-inclusive photon-induced processes (
γ
+
p
/A) at RHIC kinematics provide an opportunity to search for the signatures of the baryon junction and to shed light onto the mechanisms of observed baryon excess in the mid-rapidity region in ultra-relativistic nucleus-nucleus collisions. Such measurements can be further validated in A+A collisions at the LHC and
e
+
p
/A collisions at the EIC.
STAR Highlights Xu, Zhangbu
Nuclear physics. A,
12/2020, Letnik:
1005
Journal Article
Recenzirano
Odprti dostop
In this overview talk, I have represented STAR to highlight the results of 19 parallels talks and 38 posters at this Quark Matter Conference. Overall, the contents have 8 major categories: 1. ...experimental measurements related to the BreitWheeler process and vacuum birefringence; 2. probes of initial states of heavy-ion collisions (parton distribution function and azimuthal correlations in small systems); 3. constraints on temperature-dependent viscosity (η/s(T)) with multiple flow harmonics and rapidity correlations; 4. hard probes using jet structures and heavy flavors; 5. exploration of the origin of global polarization and vorticity; 6. chiral and thermal properties of the QGP; 7. Beam Energy Scan (BES) and the critical point search; 8. upgrades and summary.
The linear and mode-coupled contributions to higher-order anisotropic flow are presented for Au+Au collisions at = 27, 39, 54.4, and 200 GeV and compared to similar measurements for Pb+Pb collisions ...at the Large Hadron Collider (LHC). The coefficients and the flow harmonics' correlations, which characterize the linear and mode-coupled response to the lower-order anisotropies, indicate a beam energy dependence consistent with an influence from the specific shear viscosity ( ). In contrast, the dimensionless coefficients, mode-coupled response coefficients, and normalized symmetric cumulants are approximately beam-energy independent, consistent with a significant role from initial-state effects. These measurements could provide unique supplemental constraints to (i) distinguish between different initial-state models and (ii) delineate the temperature (T) and baryon chemical potential ( ) dependence of the specific shear viscosity .
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