Energy Dependence of Net-λ Fluctuations Measured by the Star Detector at RHIC



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The measurement of conserved charge distributions has generated considerable interest in understanding the cumulants of conserved quantum numbers in the quantum chromo-dynamics (QCD) phase diagram, in particular the behavior near a possible critical end point and hadronization near the chemical freeze-out line. Net-protons and net-kaons have been used as proxies for net-baryons and net-strangeness, respectively. In this work, the measurement of efficiency corrected and feed-down corrected cumulant ratios (C2/C1, C3/C2) of net-Λ are presented. Net-Λ fluctuations were subjected to strangeness and baryon number conservation. Results are presented for five beam energies (√sNN = 19.6, 27, 39, 62.4, and 200 GeV Au + Au collisions) as a function of centrality and rapidity. We compared the net-Λ results to the published net-proton and net-kaon results at STAR. The results are presented with the comparisons to Poisson baseline, the negative binomial distribution (NBD) expectations, the ultra relativistic quantum molecular dynamics model and the hadron resonance gas (HRG) model. The data were corrected for efficiency in two ways, a transverse momentum (pT ) - dependent and a pT - independent method. Both methods yielded similar results. Any feed-down contribution from multi-strange baryons did not affect the net-Λ cumulant ratios. A non-monotonic behavior of the net-Λ cumulant ratios indicating QCD critical point was not observed as a function of collision centrality or energy. The energy dependence of the measured net-Λ C2/C1 stayed closer to HRG calculations assuming kaon freeze-out conditions than the HRG calculations based on proton/charge freeze-out conditions. This could potentially be an indication of the predicted sequential hadronization. The net-Λ cumulant ratios showed weak dependence on the selected rapidity window. The deviation of NBD expectation for net-Λ C2 from the measured C2 increased as a function of increasing rapidity window, which could be attributed to baryon-number conservation.



Fluctuations, Net-Λ, Strangeness, Chemical freeze-out, STAR