Article

First proton–proton collisions at the LHC as observed with the ALICE detector: measurement of the charged-particle pseudorapidity density at GeV

European Physical Journal C (Impact Factor: 5.44). 01/2010; 65(1):111-125.

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 450GeV 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 dN
ch/dη=3.10±0.13(stat.)±0.22(syst.) for all inelastic interactions, and dN
ch/dη=3.51±0.15(stat.)±0.25(syst.) for non-single diffractive interactions. These results are consistent with previous measurements
in proton–antiproton interactions at the same centre-of-mass energy at the CERN Sp
[`(p)]\overline{\mathrm{p}}
S 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.

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