# 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

On 23^rd 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 d$N_{\rm ch}$/d$\eta$ = 3.10 +- 0.13 (stat.) +- 0.22 (syst.) for all inelastic interactions, and dNch/deta = 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 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.

Figures

## Figure 1

 The first pp collision candidate shown by the event display in the ALICE counting room (3D view, $r$-$\phi$ and $r$-$z$ projections), the dimensions are shown in cm The dots correspond to hits in the silicon vertex detectors (SPD, SDD and SSD), the lines correspond to tracks reconstructed using loose quality cuts. The ellipse drawn in the middle of the detector surrounds the reconstructed event vertex.

## Figure 2

 Online display of the vertex positions reconstructed by the High-Level Trigger (HLT). The figure shows, counter-clockwise from top left, the position in the transverse plane for all events with a reconstructed vertex, the projections along the transverse coordinates $x$ and $y$, and the distribution along the beam line ($z$-axis).

## Figure 3

 Arrival time of particles in the VZERO detectors relative to the beam crossing time (time zero). A number of beam-halo or beam--gas events are visible as secondary peaks in VZERO-A (left panel) and VZERO-C (right panel). This is because particles produced in background interactions arrive at earlier times in one or the other of the two counters The majority of the signals have the correct arrival time expected for collisions around the nominal vertex.

## Figure 7

 Charged-particle pseudorapidity density in the central rapidity regionin proton--proton and proton--antiproton interactions as a function ofthe centre-of-mass energy. The dashed and solid lines (for INEL and NSD interactions respectively) indicate thefit using a power-law dependence on energy.