Net-charge fluctuations in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

We report the first measurement of the net-charge fluctuations in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV, measured with the ALICE detector at the CERN Large Hadron Collider. The dynamical fluctuations per unit entropy are observed to decrease when going from peripheral to central collisions. An additional reduction in the amount of fluctuations is seen in comparison to the results from lower energies. We examine the dependence of fluctuations on the pseudorapidity interval, which may account for the dilution of fluctuations during the evolution of the system. We find that the fluctuations at LHC are smaller compared to the measurements at the Relativistic heavy Ion Collider (RHIC), and as such, closer to what has been theoretically predicted for the formation of the Quark-Gluon Plasma (QGP).

Figures

Figure 1

 Dynamical net-charge fluctuations, $\nu_{(+-,dyn)}$ and their corrected values,$\nu^{corr}_{(+-,dyn)}$, for charged particles produced in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV as a function of centrality, expressed as the number of participating nucleons. $\nu^{corr}_{(+-,dyn)}$, points are shifted along $x$-axis for better representation. Superimposed are the results for pp collisions at $\sqrt{s}=2.76$ TeV. The statistical (bar) and systematic (box) errors are plotted.

Figure 2

 $< N_{\rm {ch}}>\nu^{corr}_{(+-,dyn)}$ (left axis) and $D$ (right axis) as a function of the number of participants for $\Delta \eta$=1 and 1.6 in Pb-Pb at $\sqrt{s_{\rm NN}}=2.76$ TeV and pp collisions at $\sqrt{s}=2.76$ TeV. Corresponding results from the HIJING and PYTHIA event generators are also presented. The data points are shifted minimally along $x$-axis for clear view. Both statistical (error bar) and systematic (box) errors are plotted.

Figure 3

 $< N_{\rm {ch}}>\nu^{corr}_{(+-,dyn)}$ (left axis) and $D$ (right axis) as a function of $\Delta\eta$ window for three different centrality bins in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The data points are fitted with the functional form, erf($\Delta\eta/\sqrt{8}\sigma_{\rm f}$). The dashed lines correspond to the extrapolation of the fitted curves. The points are shifted minimally along $x$-axis for clear view Both statistical (error bar) and systematic (box) errors are shown.

Figure 4

 Energy dependence of the net-charge fluctuations, measured in terms of $< N_{\rm {ch}}>\nu^{corr}_{(+-,dyn)}$ (left axis) and $D$ (right axis), for the top central collisions. The results from the STAR and ALICE experiments are presented for $\Delta\eta=1$ after the correction for charge conservation. The ALICE result for $\Delta\eta=1.6$ is also shown. Both statistical (error bar) and systematic (box) errors are plotted.