First proton–proton collisions at the LHC as observed with the ALICE detector: measurement of the charged-particle pseudorapidity density at $\sqrt{s}=900$ GeV
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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 |$\eta$| < 0.5, we obtain dNch/deta = 3.10 $\pm$
0.13 (stat.) $\pm$ 0.22 (syst.) for all inelastic interactions, and dNch/deta =
3.51 $\pm$ 0.15 (stat.) $\pm$ 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
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.
