Measurement of the sixth-order cumulant of net-proton multiplicity distributions in Au+Au collisions at √s_NN = 27, 54.4 and 200 GeV at RHIC

PAs (alphabetic) : Roli Esha, ShinIchi Esumi, Shu He, Huan-Zhong Huang, Xiaofeng Luo, Bedangadas Mohanty, Toshihiro Nonaka, Ashish Pandav, Nu Xu, Yu Zhang

Target Journal : Phys. Rev. Lett.

 

Abstract:

Fig. 1 : Net-proton distributions

Figure 1 : Event by event net-proton multiplicity distributions, ∆Np, for Au+Au collisions at √s_NN = 27, 54.4, and 200 GeV in 0-10% and 30-40% centralities at midrapidity (|y| < 0.5) for transverse momentum range of 0.4 < pT (GeV/c) < 2.0. These distributions are normalized by corresponding events and not corrected for detector efficiencies. Statistical uncertainties are shown as vertical lines. The dashed lines show the Skellam distributions for each collision energy and centrality. Bottom panel shows the ratio of the data to the Skellam expectations. 


Fig. 2 : Acceptance dependence

                    

Figure 2 : Net-proton C₆/C₂ as a function of rapidity acceptance (left) and transverse momentum acceptance (right) from √s_NN = 27 (crosses), 54.4 (open squares), and 200 GeV (filled circles) Au+Au collisions. The upper and lower plots are for 0-10% and 30-40% centralities, respectively. UrQMD transport model results are shown as shaded bands. Points for different beam energies are staggered horizontally to improve clarity. The Skellam expectation (C₆/C₂ = 1) and C₆/C₂ = 0 are shown as dashed and dotted lines, respectively, to guide 


Fig. 3 : Centrality dependence of C₆/C₂

                      

Figure 3 : Collision centrality dependence of net-proton C₆/C₂ in Au+Au collisions for √s_NN = 27, 54.4, and 200 GeV at |y| < 0.5 with transverse momentum 0.4 < p_T (GeV/c) < 2.0. The filled cross and square, and the open circle at ⟨Npart⟩ ≈ 200 represent the averaged results for 0-40% cen- trality at 27, 54.4, and 200 GeV, respectively. Shaded and hatched bands show the results from UrQMD, where the ratios for the 10% centrality bin width are averaged for 0-30%, 30-60%, and 60-80% to reduce statistical uncertainties. The lattice QCD calculations [16, 17] are shown in the red solid band at ⟨Npart⟩ ≈ 340 taking T = 158 − 160 MeV with μB < 110 MeV. Please note that an infinite volume is as- sumed in the lattice calculations. They are thus shown in the larger edge of ⟨Npart⟩. The Skellam expectation (C6/C2 = 1) and C6/C2 = 0 are shown in dashed and dotted lines, respectively to guide the eyes. Points for different beam energies are staggered horizontally to improve clarity. 

Summary:

 In summary, we report the first measurements of the net-proton higher-order 

cumulant ratio $C_{6}/C_{2}$ from 27, 54.4 and 200~GeV Au+Au collisions measured 

by the STAR detector at RHIC. The data is taken from the kinematic region ($|y|<0.5$ and $0.4<p_{T}\;({\rm GeV/c})<2.0$). 

Most of the ratios from 54.4~GeV collisions are found to be close to or around zero. 

Data from 27 and 200~GeV collisions, on the other hand, are found to be negative 

in more central collisions within the maximum acceptance. 

Without critical dynamics, the transport model UrQMD calculations predict the ratio $C_{6}/C_{2}$ 

around a statistical baseline in all cases. 

Lattice QCD calculations, with $T=160$~MeV and $\mu_{\rm B}=0$-$110$~MeV, 

predict the negative value of $C_{6}/C_{2}\sim-1.5$, which is qualitatively consistent with the experimental 

results of central Au+Au collisions at RHIC. 

These new measurements are statistics limited and seem to favor a smooth crossover for the QGP-hadronic matter transition. 

Future measurements with high statistics will provide more detailed information 

about the phase structure at the low baryon density region.

 

Important references

1. PQM model

• Analysis note
• Analysis note v2
• Analysis note v3
• Analysis note v4
•  

1. Centrality definition (200 GeV, Run11, RefMult3)
2. First look at efficiency corrected C₆ 
3. Comparison with preliminary results
4. Trigger by trigger check
5. Plan for QM17
6. Short update before QM17
7. Run14 data without HFT tracking
8. Centrality definition at 54.4 GeV
9.  C₆ at 54.4 GeV
10. Acceptance dependence of C₆ at 200 GeV
11. Systematic study 1
12. Systematic study 2
13. Systematic study 3
14. Track-by-track efficiency correction
    
15. Paper proposal
    Run18 27GeV