Transverse-momentum and pseudorapidity distributions of charged hadrons in pp collisions at $\sqrt{s} = 0.9 $ and 2.36 TeV

Abstract

Measurements of inclusive charged-hadron transverse-momentum and pseudorapidity distributions are presented for proton-proton collisions at √ s = 0.9 and 2.36 TeV. The data were collected with the CMS detector during the LHC commissioning in December 2009. For non-single-diffractive interactions, the average charged-hadron transverse momentum is measured to be 0.46 ± 0.01 (stat.) ± 0.01 (syst.) GeV/c at 0.9 TeV and 0.50 ± 0.01 (stat.) ± 0.01 (syst.) GeV/c at 2.36 TeV, for pseudorapidities between −2.4 and +2.4. At these energies, the measured pseudorapidity densities in the central region, dN ch /dη| |η|<0.5 , are 3.48 ± 0.02 (stat.) ± 0.13 (syst.) and 4.47 ± 0.04 (stat.) ± 0.16 (syst.), respectively. The results at 0.9 TeV are in agreement with previous measurements and confirm the expectation of near equal hadron production in pp and pp collisions. The results at 2.36 TeV represent the highest-energy measurements at a particle collider to date.

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

“…The RHIC results are multiplied by numerical factors to match the N part -normalised multiplicity observed at the LHC for central collisions. 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].…”

“…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 efficiency of the pixel layers is larger than 99%, measured from pp data [17]. The pixel detector has low occupancy even in central heavy-ion collisions because of its fine segmentation.…”

“…the probability that a cluster is detected once a charged particle crosses a pixel-detector layer), as well as the fraction of large clusters split into two are important quantities for the measurement. The pixel-cluster efficiency has been extensively studied in pp collisions [4,17], indicating an efficiency of (99.5 ± 0.5)%. Despite larger particle multiplicities in PbPb collisions, the occupancy of the pixel detector is still smaller than 1% owing to its high granularity (whereas in the strip detector it reaches 20%), and the pixel detector exhibits the same excellent perfomance in PbPb as in pp collisions.…”

“…The hit-counting method gives three largely independent measurements for the three barrel layers. A similar method was used by the PHOBOS experiment at RHIC [18] and also by CMS for earlier pp analyses [4,17]. One of the disadvantages of the method is the strong reliance on detector simulation for correction factors.…”