First proton-proton collisions at the LHC as observed with the ALICE detector: Measurement of the charged particle pseudorapidity at √s = 900 GeV

ALICE Collaboration, Orlando Villalobos Baillie, Peter Jones, Lee Stuart Barnby, David Evans, Roman Lietava, Anton Jusko, Arvinder Palaha, Plamen Petrov, Marian Krivda, Cristina Lazzeroni, Zoe Matthews, Ravjeet Kour, Sparsh Navin, Goronwy Tudor Jones, Predrag Jovanovic, Richard Platt

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170 Citations (Scopus)
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Abstract

On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region was centred very well in both the longitudinal and transverse directions and 284 events were recorded in coincidence with the two passing proton bunches. The events were immediately reconstructed and analyzed both online and offline. We have used these events to measure the pseudorapidity density of charged primary particles in the central region. In the range |η| < 0.5, we obtain dNch/dη = 3.10±0.13(stat.)±0.22(syst.) for all inelastic interactions, and dNch/dη = 3.51 ± 0.15(stat.) ± 0.25(syst.) for nonsingle diffractive interactions. These results are consistent with previous measurements in proton–antiproton interactions at the same centre-of-mass energy at the CERN SppS collider. They also illustrate the excellent functioning and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment, in this early start-up phase.
Original languageEnglish
Pages (from-to)111-125
Number of pages15
JournalEuropean Physical Journal C
Volume65
Early online date11 Dec 2009
DOIs
Publication statusPublished - Jan 2010

ASJC Scopus subject areas

  • Nuclear and High Energy Physics

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