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Global baryon number conservation encoded in net-proton fluctuations measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

Experimental results are presented on event-by-event net-proton fluctuation measurements in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV, recorded by the ALICE detector at the CERN LHC. These measurements have as their ultimate goal an experimental test of Lattice QCD (LQCD) predictions on second and higher order cumulants of net-baryon distributions to search for critical behavior near the QCD phase boundary. Before confronting them with LQCD predictions, account has to be taken of correlations stemming from baryon number conservation as well as fluctuations of participating nucleons. Both effects influence the experimental measurements and are usually not considered in theoretical calculations. For the first time, it is shown that event-by-event baryon number conservation leads to subtle long-range correlations arising from very early interactions in the collisions.

 

Submitted to: PRL
e-Print: arXiv:1910.14396 | PDF | inSPIRE
CERN-EP-2019-253

Figures

Figure 1

Measured second cumulants of net-proton distributions (red solid boxes) compared with the sum of the mean multiplicities (open squares). The second cumulants of single proton and antiproton distributions are presented with the filled and open circles, respectively. The first cumulants of protons and antiprotons are hardly distinguishable because of the nearly equal mean numbers of protons and antiprotons at LHC energies. In the middle and bottom panels the normalized cumulants R$_{1}$and R$_{2}$ are presented. The band visible in the bottom panel is the prediction for R$_{2}$ in the presence of volume fluctuations~
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Figure 2

Pseudorapidity dependence of the normalized second cumulants of net-protons R$_1$. Global baryon number conservation is depicted as the pink band. The dashed lines represent the predictions from the model with local baryon number conservation~. The blue solid line, represents the prediction using the HIJING generator.
[png]   [pdf]   [eps]