Recently, Chatterjee et al. Phys. Rev. C 101, 034902 (2020) used a hadronic transport model to estimate the resolution with which various experimental quantities select the impact parameter of ...relativistic heavy ion collisions at collision energies relevant to the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). Measures based on particle multiplicity at forward rapidity were found to be significantly worse than those based on midrapidity multiplicity. Using the same model, we show that a slightly more sophisticated measure greatly improves the resolution based on forward rapidity particles; this improvement persists even when the model is filtered through a realistic simulation of a recent upgrade detector to the STAR experiment. Overall, these results highlight the importance of optimizing centrality measures based on particles detected at forward rapidity, especially for experimental studies that search for a critical point in the QCD phase diagram. Such measurements usually focus on proton multiplicity fluctuations at midrapidity, hence selecting events based on multiplicity at midrapidity raises the possibility of nontrivial autocorrelations.
The strong nuclear force, as modeled by Quantum Chromodynamics (QCD), has been extensively studied in recent years in order to learn its thermodynamic properties. Modern collider experiments have ...been able to access the thermodynamic regime where QCD matter transitions from ordinary (hadronic) matter to a Quark-Gluon Plasma (QGP), which is characterized by colour parton degrees of freedom. This change in phase suggests a phase diagram; as such, many experiments have been aimed at finding the Critical Point (CP) in this phase diagram at the terminus of the 1st order phase transition line between hadronic matter and QGP. Among the observables theorized to be sensitive to the QCD CP are the net-proton distribution moment (μr) based cumulant ratios, C3/C2 and C4/C2 (Sσ and κσ2, respectively), which are believed to vary monotonically with center-of-momentum collision energy (√sNN ) in the absence of critical phenomena, but which are also correlated to the correlation/interaction length (ξ) and thus may show non-monotonic correlation with √sNN near the CP as ξ diverges at the CP.The sensitivity of Sσ and κσ2 to volume fluctuations implies they are sensitive to centrality fluctuations (CF); thus the method of centrality selection and determination is highly important to μr based analyses. In current analyses of the Beam Energy Scan (BES) program at the Solenoidal Tracker at RHIC (STAR), both μr analysis and centrality determination are carried out using the Time Projection Chamber (TPC); a mid-rapidity detector with a pseudorapidity (η) acceptance range of |η| ≤ 1. Using the same η range to determine both net-proton μr based values and centrality introduces the potential for autocorrelation errors (ACE). The acceptance range of the STAR Event Plane Detector (EPD) is 2.1 ≤ |η| ≤ 5.1, thus using the EPD to determine centrality while using the TPC for particle identification would avoid the potential for ACE.This thesis analyses net-proton distribution σ2/μ and Sσ values at √sNN = 14.6 GeV using both mid and forward(backward) η determined centrality. Mid-η centrality (XRM3) is determined with the STAR TPC, which is also a main detector used in particle identification by which net-proton distribution (∆Np) arrays are generated, and forward(backward)-η centrality (XEPD) is determined with the STAR EPD. In simulation, it is found that ∆Np σ2/μ values differ between distribution cuts made using XRM3 and XEPD, but that both are in range of σ2/μ values found using centrality determined via the impact parameter (b) in the most central bin (0-5%). In experiment, the σ2/μ and Sσ values determined using XRM3 and XEPD based centrality selection differ as the simulation values, with the σ2/μ values being within error of the simulation values. For both σ2/μ and Sσ, values found using XEP D as the centrality determination observable are higher across the centrality range than those found using XRM 3 as a centrality selector. This finding is congruent with XRM 3 centrality selection based analyses of ∆Np μr based observables suffering from ACE at √sNN = 14.6 GeV in the STAR experiment.This work will aid in vetting the results of any ∆Np μr based analyses in the STAR Collaboration, particularly net-proton kurtosis (κσ2) studies. The techniques evaluated herein can be readily applied to other BES collision energies, making this a valuable addition to current analyses across the STAR BES range without adding much in the way of additional work for researchers searching for the QCD CP.
Recently, Chatterjee et al used a hadronic transport model to estimate the resolution with which various experimental quantities select the impact parameter of relativistic heavy ion collisions at ...collision energies relevant to the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). Measures based on particle multiplicity at forward rapidity were found to be significantly worse than those based on midrapidity multiplicity. Using the same model, we show that a slightly more sophisticated measure greatly improves the resolution based on forward rapidity particles; this improvement persists even when the model is filtered through a realistic simulation of a recent upgrade detector to the STAR experiment. These results highlight the importance of optimizing centrality measures based on particles detected at forward rapidity, especially for experimental studies that search for a critical point in the QCD phase diagram. Such measurements usually focus on proton multiplicity fluctuations at midrapidity, hence selecting events based on multiplicity at midrapidity raises the possibility of nontrivial autocorrelations.
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