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

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 trans- verse 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 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.

ALICE status and highlights

“…The charged particle density dN ch /dη as well as the multiplicity distributions were measured at 0.9, 2.36 and 7 TeV for both inelastic as well as non-single diffractive collisions [7,8,9]. The energy dependence of the multiplicity is well described by a power law in energy, s 0.1 , and increases significantly stronger than predicted by most event generators.…”

Underlying Event measurements in pp collisions at $ \sqrt {s} = 0.9 $ and 7 TeV with the ALICE experiment at the LHC

“…Its high granularity and particle identification capabilities can be exploited for precise measurements of global event properties [18][19][20][21][22][23][24]. The central barrel covers the polar angle range 45 • −135 • (|η| < 1) and full azimuth.…”

Soft QCD Measurements at LHC – Status and Outlook

“…Figure 1 shows examples of these measurements. Further measurements from LHC can be found in [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] which span a significant fraction of the phase space of particles produced in pp collisions at the LHC as also LHCb and TOTEM contribute with their forward capabilities. Comparisons of the results between the experiments show good agreement.…”