Charged-particle multiplicity measurement in proton-proton collisions at $\sqrt{s}$ = 0.9 and 2.36 TeV with ALICE at LHC

TL;DR

ALICE measures charged-particle production in pp collisions at √s = 0.9 and 2.36 TeV, providing dNch/dη in |η|<0.5 and detailed multiplicity distributions via SPD tracklets in |η|<1.4. Using an unfolding procedure with a detector-response matrix, the study corrects for acceptance, efficiency, and diffractive contributions, and cross-checks with TPC data. The results show a ~24% increase in multiplicity from 0.9 to 2.36 TeV, exceed several model predictions, and are broadly consistent with CMS for NSD events; KNO scaling is largely maintained with mild deviations. These high-precision measurements constrain soft QCD modeling and guide tuning of event generators for LHC minimum-bias and underlying-event studies.

Abstract

Charged-particle production was studied in proton-proton collisions collected at the LHC with the ALICE detector at centre-of-mass energies 0.9 TeV and 2.36 TeV in the pseudorapidity range |$η$| < 1.4. In the central region (|$η$| < 0.5), at 0.9 TeV, we measure charged-particle pseudorapidity density dNch/deta = 3.02 $\pm$ 0.01 (stat.) $^{+0.08}_{-0.05}$ (syst.) for inelastic interactions, and dNch/deta = 3.58 $\pm$ 0.01 (stat.) $^{+0.12}_{-0.12}$ (syst.) for non-single-diffractive interactions. At 2.36 TeV, we find dNch/deta = 3.77 $\pm$ 0.01 (stat.) $^{+0.25}_{-0.12}$ (syst.) for inelastic, and dNch/deta = 4.43 $\pm$ 0.01 (stat.) $^{+0.17}_{-0.12}$ (syst.) for non-single-diffractive collisions. The relative increase in charged-particle multiplicity from the lower to higher energy is 24.7% $\pm$ 0.5% (stat.) $^{+5.7}_{-2.8}$% (syst.) for inelastic and 23.7% $\pm$ 0.5% (stat.) $^{+4.6}_{-1.1}$% (syst.) for non-single-diffractive interactions. This increase is consistent with that reported by the CMS collaboration for non-single-diffractive events and larger than that found by a number of commonly used models. The multiplicity distribution was measured in different pseudorapidity intervals and studied in terms of KNO variables at both energies. The results are compared to proton-antiproton data and to model predictions.

Figures (12)

• Figure 1: Distributions of reconstructed event vertices along the beam direction ($z$) obtained from hit correlations in the two pixel layers of the ALICE inner tracking system for the event samples used in the analysis (see text): $\sqrt{s} = \unit[0.9]{TeV}$ (full symbols) and $\sqrt{s} = \unit[2.36]{TeV}$ (open symbols). The lines are from Monte Carlo simulations. Vertical dashed lines delimit the region $|z|<10$ cm, where the events for the present analysis were selected.
• Figure 2: Charged-particle multiplicity dependence of the selection efficiency, for INEL events (triangles) with MB$_{\mathrm{OR}}$ selection, and for NSD (circles) and SD (squares) events with MB$_{\mathrm{AND}}$ selection obtained with a simulation of the $0.9$ TeV data.
• Figure 3: Measured raw multiplicity distributions in three pseudorapidity ranges for both energy samples ($0.9$ TeV full lines, $2.36$ TeV dashed lines). Note that for $|\eta|<1.0$ and $|\eta|<1.3$ the distributions have been scaled for clarity by the factor indicated.
• Figure 4: Graphical representation of the detector response matrix: number of tracklets found in the SPD ($m$) vs the number of generated primary particles in $|\eta| < 1.0$ ($t$) for $\sqrt{s} = 0.9$ TeV. The distribution of the measured tracklet multiplicity for a given generated multiplicity shown with its most probable value (dots), r.m.s. (shaded areas), and full spread (squares).
• Figure 5: Measured raw multiplicity distribution (elements of vector $M$, histogram), superimposed on the convolution $RU$ of the unfolded distribution with the response matrix (crosses), at $\sqrt{s} = 0.9$ TeV for $|\eta| < 1.0$ (upper plot). The error bars are omitted for visibility. Normalized residuals, i.e. the difference between the measured raw distribution and the corrected distribution folded with the response matrix divided by the measurement error (lower plot). The inset shows the distribution of these normalized residuals fitted with a Gaussian ($\sigma \approx 1.06$).