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Register a new userEnergy and Centrality dependence of $dN_{rm ch}/deta$ and $dE_{rm T}/deta$ in Heavy-Ion Collisions from $sqrt{s_{rm NN}}$ =7.7 GeV to 5.02 TeV
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The centrality dependence of pseudorapidity density of charged particles and transverse energy is studied for a wide range of collision energies for heavy-ion collisions at midrapidity from 7.7 GeV to 5.02 TeV. A two-component model approach has been adopted to quantify the soft and hard components of particle production, coming from nucleon participants and binary nucleon-nucleon collisions, respectively. Within experimental uncertainties, the hard component contributing to the particle production has been found not to show any clear collision energy dependence from RHIC to LHC. The effect of centrality and collision energy in particle production seem to factor out with some degree of dependency on the collision species. The collision of Uranium-like deformed nuclei opens up new challenges in understanding the energy-centrality factorization, which is evident from the centrality dependence of transverse energy density, when compared to collision of symmetric nuclei.
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Transverse spherocity is an event shape observable having a very unique capability to separate the events based on their geometrical shapes. Recent results from experiments at the LHC suggest that transverse spherocity is an important event classifier in small collision systems. In this work, we use transverse spherocity for the first time in heavy-ion collisions and perform an extensive study on azimuthal anisotropy of charged particles produced in Pb-Pb collisions at $sqrt{s_{rm{NN}}} = 5.02$ TeV using A Multi-Phase Transport Model (AMPT). The azimuthal anisotropy is estimated using the 2-particle correlation method, which suppresses the non-flow effects significantly with an appropriate pseudorapidity gap of particle pairs. The results from AMPT are compared with estimations from PYTHIA8 (Angantyr) model and it is found that with the chosen pseudorapidity gap the residual non-flow effects become negligible. We found that the high spherocity events have nearly zero elliptic flow while low spherocity events contribute significantly to elliptic flow of spherocity-integrated events. Our studies indicate that using transverse spherocity in heavy-ion collisions, one can enhance and/or suppress the collective effects.
Centrality definition in A$+$A collisions at colliders such as RHIC and LHC suffers from a correlated systematic uncertainty caused by the efficiency of detecting a p$+$p collision ($50pm 5%$ for PHENIX at RHIC). In A$+$A collisions where centrality is measured by the number of nucleon collisions, $N_{rm coll}$, or the number of nucleon participants, $N_{rm part}$, or the number of constituent quark participants, $N_{rm qp}$, the error in the efficiency of the primary interaction trigger (Beam-Beam Counters) for a p$+$p collision leads to a correlated systematic uncertainty in $N_{rm part}$, $N_{rm coll}$ or $N_{rm qp}$ which reduces binomially as the A$+$A collisions become more central. If this is not correctly accounted for in projections of A$+$A to p$+$p collisions, then mistaken conclusions can result. A recent example is presented in whether the mid-rapidity charged multiplicity per constituent quark participant $({dN_{rm ch}/deta})/{N_{rm qp}}$ in Au$+$Au at RHIC was the same as the value in p$+$p collisions.
A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy-ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical $Kpi$, $ppi$, and $Kp$ fluctuations as measured by the STAR experiment in central 0-5% Au+Au collisions from center-of-mass collision energies $rm sqrt{s_{NN}}$ = 7.7 to 200 GeV are presented. The observable $rm u_{dyn}$ was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the $Kpi$, $ppi$, and $Kp$ pairs. The energy dependences of these fluctuations from central 0-5% Au+Au collisions all demonstrate a smooth evolution with collision energy.
We report a new measurement of $D^0$-meson production at mid-rapidity ($|y|$,$<$,1) in Au+Au collisions at ${sqrt{s_{rm NN}} = rm{200,GeV}}$ utilizing the Heavy Flavor Tracker, a high resolution silicon detector at the STAR experiment. Invariant yields of $D^0$-mesons with transverse momentum $p_{T}$ $lesssim 9$,GeV/$c$ are reported in various centrality bins (0--10%, 10--20%, 20--40%, 40--60% and 60--80%). Blast-Wave thermal models are used to fit the $D^0$-meson $p_{T}$ spectra to study $D^0$ hadron kinetic freeze-out properties. The average radial flow velocity extracted from the fit is considerably smaller than that of light hadrons ($pi,K$ and $p$), but comparable to that of hadrons containing multiple strange quarks ($phi,Xi^-$), indicating that $D^0$ mesons kinetically decouple from the system earlier than light hadrons. The calculated $D^0$ nuclear modification factors re-affirm that charm quarks suffer large amount of energy loss in the medium, similar to those of light quarks for $p_{T}$,$>$,4,GeV/$c$ in central 0--10% Au+Au collisions. At low $p_{T}$, the nuclear modification factors show a characteristic structure qualitatively consistent with the expectation from model predictions that charm quarks gain sizable collective motion during the medium evolution. The improved measurements are expected to offer new constraints to model calculations and help gain further insights into the hot and dense medium created in these collisions.
In this work a study of the fractional momentum loss ($S_{rm loss}$) as a function of the characteristic path-length ($L$) and the Bjorken energy density times the equilibration time ($epsilon_{rm Bj}tau_{0}$) for heavy-ion collisions at different $sqrt{s_{rm NN}}$ is presented. The study has been conducted using inclusive charged particles from intermediate to large transverse momentum ($5<p_{rm T}<20$ GeV/$c$). Within uncertainties and for all the transverse momentum values which were explored, the fractional momentum loss linearly increases with $({epsilon_{rm Bj}tau_{0}})^{3/8}$$L$. The functional form of $S_{rm loss}$ vs. $({epsilon_{rm Bj}tau_{0}})^{3/8}$$L$ seems to be universal. Moreover, for identified charged hadrons a linear relationship between $S_{rm loss}$ and $L$ is also observed. The behaviour of data could provide important information aimed to understand the parton energy loss mechanism in heavy-ion collisions and some insight into the expected effect for small systems.
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