Measurement of the Charged-Hadron Multiplicity in Proton-Proton Collisions at LHC with the CMS Detector

Charged-hadron pseudorapidity densities and multiplicity distributions in protonproton collisions at s = 0.9, 2.36, 7.0 TeV were measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield was obtained by counting the number of hit-pairs (tracklets). The charged-particle multiplicity per unit ofpseudorapidity d N ch /d η ||η|<0.5 at s = 7.0 TeV is 5.78 ± 0.01(stat.) ± 0.23(syst.) for nonsingle-diffractive events, higher than predicted by commonly used models. The relativeincrease in charged-particle multiplicity from s = 0.9 to 7 TeV is 66.1% ± 1.0%(stat.) ±4.2%(syst.) and strong KNO violation is observed in the multiplicity distributions. Results are compared with low energy measurements.Thesis Supervisor: Wit BuszaTitle: Francis Friedman Professor of Physics