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

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 pseudo- for non-single-diffractive collisions. The relative increase in charged-particle multiplicity from the lower to higher energy is 24.7% ± 0.5%(stat.) +5.7 −2.8 %(syst.) for inelastic and 23.7% ± 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 protonantiproton data and to model predictions.

Dependence on pseudorapidity and on centrality of charged hadron production in PbPb collisions at $ \sqrt {{{s_{\text{NN}}}}} = 2.76 $ TeV

“…The pp results shown in the figure are obtained from the measured non-single-diffractive (NSD) dN ch /dη| η=0 = 4.47 ± 0.2 (CMS) [17] and the inelastic dN ch /dη| η=0 = 3.77 +0. 26 −0.13 (ALICE) [22] values at 2.36 TeV, using the √ s depen--14 - [20], from RHIC [21] at 200 GeV and 19.6 GeV, and from extrapolated pp results from CMS [17] and ALICE [22]. Systematic uncertainties affecting the scale of the measurements from this analysis are shown as inner green error bands and the total systematic uncertainties as an outer grey band, while the error bars indicate statistical uncertainties.…”

“…The dpmjet-III model is capable of describing the charged hadron multiplicity in the most-central collisions, but shows a stronger rise with centrality than observed in the data. The measured particle densities provide basic constraints on the initial conditions of the quark-gluon plasma in any hydrodynamical approach employed to study PbPb interactions at the LHC [32].The collision-energy dependence of the measured (dN ch /dη| η=0 )/(N part /2) for 0-5% and 0-70% centrality from this analysis and from ALICE, PHENIX, and PHOBOS can be [4,17,22,34,35], respectively. The N part values at different collision energies are different for a fixed centrality bin.…”

“…The NSD pp results of CMS, ALICE, UA5, and UA1 are from refs. [4,17,22,34,35], respectively. The N part values at different collision energies are different for a fixed centrality bin.…”

Underlying Event measurements in pp collisions at $ \sqrt {s} = 0.9 $ and 7 TeV with the ALICE experiment at the LHC

“…Its high granularity and particle identification capabilities can be exploited for precise measurements of global event properties [18][19][20][21][22][23][24]. The central barrel covers the polar angle range 45 • −135 • (|η| < 1) and full azimuth.…”

Measurement of the underlying event activity in pp collisions at $ \sqrt {s} = 0.9 $ and 7 TeV with the novel jet-area/median approach

“…As a consequence of the higher particle multiplicities observed in LHC collision data at 0.9 TeV and 7 TeV [24][25][26][27][28] compared to existing model predictions, the new tunes Z1 [29] and Z2 were developed. Both tunes employ a new model for MPIs and a fragmentation function derived with the professor [30] tool, as well as p T -ordered parton showers.…”