Paper Title :   pt Correlations in Au + Au Collisions in the High Baryon Density Region

  Target Journal : PRL

  PAs : Rutik Manikandhan, Rene Bellwied, Anthony Timmins, Caleb Broodo, Aranya Giri, Chun-Jian Zhang, Jinhui Chen

  PA representative :  Rutik Manikandhan

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 Abstract:
In this letter, we present the first detailed measurements of two-particle transverse momentum correlations for all charged particles within mid-rapidity at √sNN = 3.0, 3.2, 3.5, 3.9, 4.5, and 7.7 GeV from the STAR experiment. The results are compared with previous STAR measurements from the Beam Energy Scan Phase I (BES-I), as well as with model predictions from UrQMD and AMPT.

In central collisions, the measured pT correlations show a decreasing trend from √sNN = 3.0 GeV up to 7.7 GeV, followed by an increase toward 200 GeV. This non-monotonic behavior, observed over a broad range of collision energies, may reflect changes in the underlying dynamics of the system and could serve as a potential signal for the existence of a QCD critical point.Dynamical model calculations including a critical point are called for in order to understand these measurements.

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 Paper Figures:

 Fig 1: The efficiency uncorrected density distribution in transverse momentum ($p_{T}$) and particle pseudorapidity ($\eta_{lab}$) in lab frame within 0-5% centrality Au+Au
 collisions measured with STAR TPC in Au+Au collisions at $\sqrt{s_{NN}}$ = 3.0,3.2,3.5,3.9,4.5 & 7.7 (Collider) GeV. The dashed blue lines are midrapidity at each energy
 and the black window is the analysis window.Note that the density distribution is self-normalized in each panel.

Fig 2 : Histograms of the uncorrected average transverse momentum per event for Au+Au collisions at √sNN = 3.0, 3.2, 3.5, 3.9, 4.5 and 7.7 GeV (COL) for the 5% most central collisions at each energy. The dashed line represents gamma distribution fits. Accompanied by the extracted mean and sigma plotted as a function of collision energy compared to previous STAR publication.

  Fig. 3 : The two particle $p_{T}$ correlator as a function of collision energy $sqrt{s_{NN}}$ in Au+Au 0-5% central collisions. Statistical and systematic uncertainties are shown as
  bars and gray bands, respectively. Note that for all the measurements the charged particles are selected within a $p_{T}$ acceptance of [0.2,2.0] GeV/c and pseudorapidity ($\eta$)
  acceptance of $|\eta_{cm}|$ < 0.5, where  \eta_{cm} = \eta_{lab} - \eta_{mid}. UrQMD calculations are shown in black and red hollow markers, modified AMPT calculations are shown in pink
  squares.

Fig 4 : The relative dynamical correlation, $\sqrt{\langle \Delta p_{t,i} \Delta p_{t,j} \rangle} / \langle\langle p_{t} \rangle\rangle$, is presented as a function of $N_{\text{part}}$ for six collision energies, using centrality bins of 10%. Statistical and systematic errors are shown. The solid straight lines represent a power law. All of these measurements are done within the same $p_{T}$ acceptance of [0.2,2.0] GeV/c and pseudorapidity ($\eta$) acceptance of $|\eta_{cm}|$ < 0.5, where  \eta_{cm} = \eta_{lab} - \eta_{mid}.

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Conclusions:
We report first measurements of transverse momentum (pT ) correlations in Au+Au collisions at √sN N = 3.0–7.7 GeV, covering a baryon chemical potential range of approximately 760–400 MeV. Mean pT distributions were constructed, and the corresponding mean and width were extracted and studied as functions of collision energy. Two-particle pT correlations were measured as a function of centrality, revealing a consistent increase in correlation strength toward peripheral collisions across all energies. These observations deviate systematically from expected power-law scaling behavior. The scaled correlator exhibits signs of thermalization in the most central (0–5%) collisions, supporting the validity of a thermal description in the search for the QCD critical point. Notably, the two-particle correlator displays significant non-monotonic behavior as a function of collision energy in the 0–5% centrality bin, which may be indicative of critical phenomena. We make comparisons to different versions of UrQMD and AMPT transport code and observe deviations from both the models. Extensive cross-checks using the UrQMD transport model were performed to examine contributions from identified particles, frame-dependence of measurements, and efficiency robustness.

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Presentations:

Presentations at the BES-II Workshop 2024:
https://drupal.star.bnl.gov/STAR/system/files/Rutik_BES-II_WorkshopClosed.pdf
https://drupal.star.bnl.gov/STAR/system/files/Rutik_BES-II_workshop.pdf

Oral Talk at FAIRNess 2024:
https://indico.gsi.de/event/18746/contributions/82724/attachments/48699/70721/FAIRNess%202024%20%284%29.pdf

Oral Talk at CPOD 2024:
https://conferences.lbl.gov/event/1376/contributions/8802/attachments/5149/4966/Rutik_CPOD_Final.pdf

Talks at STAR Collaboration Meetings:

October 2024 : https://drupal.star.bnl.gov/STAR/system/files/STARCollaborationMeeting_0.pdf

March 2024    : https://drupal.star.bnl.gov/STAR/system/files/CollaborationMeeting32923.pdf

Presentations for Preliminary request:

3.0 GeV :
https://drupal.star.bnl.gov/STAR/system/files/Correlationsat3GeV.pdf

3.2 GeV:
https://drupal.star.bnl.gov/STAR/system/files/Preliminary.pdf

3.5,3.9 & 7.7 GeV:
https://drupal.star.bnl.gov/STAR/system/files/Prelim_all_energies.pdf