Centrality determination of Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with ALICE

TL;DR

The paper presents a rigorous program to determine centrality in Pb--Pb collisions at $ oot ull orac{s}{NN}=2.76$ TeV with ALICE by anchoring centrality to 90% of the hadronic cross section (Anchor Point) and mapping experimental observables to Glauber-model geometry. It employs two complementary methods: correcting the measured multiplicity distribution and fitting it with a Glauber–NBD model, yielding consistent centrality anchors and allowing precise extraction of $ig<N_{part}ig>$, $ig<N_{coll}ig>$, and $ig<T_{AA}ig>$ across centrality classes. The analysis carefully addresses backgrounds (machine-induced and electromagnetic) and quantifies systematic uncertainties from trigger efficiency, sample purity, and model assumptions, achieving centrality resolutions as good as ~0.5%–2% depending on the observable. The resulting centrality framework enables robust cross-experiment comparisons and provides a reliable linkage between measured observables and the initial collision geometry relevant for QCD matter studies. Overall, the work establishes a dependable, multi-faceted centrality calibration for ALICE heavy-ion analyses at LHC energies.

Abstract

This publication describes the methods used to measure the centrality of inelastic Pb-Pb collisions at a center-of-mass energy of 2.76 TeV per colliding nucleon pair with ALICE. The centrality is a key parameter in the study of the properties of QCD matter at extreme temperature and energy density, because it is directly related to the initial overlap region of the colliding nuclei. Geometrical properties of the collision, such as the number of participating nucleons and number of binary nucleon-nucleon collisions, are deduced from a Glauber model with a sharp impact parameter selection, and shown to be consistent with those extracted from the data. The centrality determination provides a tool to compare ALICE measurements with those of other experiments and with theoretical calculations.

Figures (16)

• Figure 1: (Color online) Compilation of total $\sigma_{\rm NN}^{tot}$, elastic $\sigma_{\rm NN}^{el}$, and inelastic $\sigma_{\rm NN}^{inel}$ cross sections of pp and p$\bar{\mathrm {p}}$ collisions Denterria2011Nakamura:2010zziBlock. The $\sigma_{\rm NN}^{el}$ curve is a fit performed by the COMPETE Collaboration also available at Nakamura:2010zziCOMPETE. The pp data from ATLAS ATLAScrossec, CMS CMScrossec, TOTEM, TOTEMcrossec and ALICE Poghosyan:2011 agree well with the interpolation for $\sigma^{\rm inel}_{\rm NN}$.
• Figure 2: Geometric properties of Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV obtained from a Glauber Monte Carlo calculation: Impact parameter distribution (left), sliced for percentiles of the hadronic cross section, and distributions of the number of participants (right) for the corresponding centrality classes.
• Figure 3: Sensitivity of $N_\mathrm{part}$ (left) and $N_\mathrm{coll}$ (right) to variations of parameters in the Glauber Monte Carlo model of Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV. The gray band represents the RMS of $N_\mathrm{part}$ and $N_\mathrm{coll}$ respectively. It is scaled by a factor 0.1 for visibility.
• Figure 4: Time distribution of signals in the VZERO detector on the A side. The peaks corresponding to beam-beam, beam-gas and satellite collision events are clearly visible.
• Figure 5: (Color online) Correlation between the sum and the difference of times recorded by the neutron ZDC on either side of the interaction region. The large cluster in the middle corresponds to collisions between ions in the nominal RF buckets of each beam, while the small clusters along the diagonals (spaced by 2.5 ns in the time difference) correspond to collisions in which one of the ions is displaced by one or more RF buckets.