Charged-particle multiplicities in proton–proton collisions at $$\sqrt{s} = 0.9$$ s = 0.9 to 8 TeV

Abstract

A detailed study of pseudorapidity densities and multiplicity distributions of primary charged particles produced in proton-proton collisions, at √ s = 0.9, 2.36, 2.76, 7 and 8 TeV, in the pseudorapidity range |η| < 2, was carried out using the ALICE detector. Measurements were obtained for three event classes: inelastic, non-single diffractive and events with at least one charged particle in the pseudorapidity interval |η| < 1. The use of an improved track-counting algorithm combined with ALICE's measurements of diffractive processes allows a higher precision compared to our previous publications. A KNO scaling study was performed in the pseudorapidity intervals |η| < 0.5, 1.0 and 1.5. The data are compared to other experimental results and to models as implemented in Monte Carlo event generators PHOJET and recent tunes of PYTHIA6, PYTHIA8 and EPOS.

Charged-particle multiplicity distributions over a wide pseudorapidity range in proton-proton collisions at $$\sqrt{s}=$$ s = 0.9, 7, and 8 TeV

“…In the tails, the CMS data are systematically lower, due to normalization. This behavior was also observed when comparing CMS distributions to those obtained in a narrower pseudorapidity range where only SPD tracklets are used [4] as shown in the distribution for |η| < 1.0 in Fig. 6.…”

“…This analysis uses only clusters from the inner layer of the SPD to provide the largest pseudorapidity coverage for particle detection. Alternatively, clusters from the two SPD layers together with the primary vertex can be combined to form tracklets [4], allowing to select primary particles with very high efficiency. The charged-particle multiplicity is then estimated by counting the number of tracklets.…”

“…5.2. Extending the pseudorapidity coverage with respect to the previous ALICE publications [2][3][4] allows to extend the high-multiplicity reach.…”

“…The results are determined using both the Silicon Pixel Detector (SPD) and the Forward Multiplicity Detector (FMD) in ALICE to widen the pseudorapidity coverage with respect to previous ALICE results [2][3][4], which made exclusive use of the SPD. The extension of the pseudorapidity coverage allows us to increase the high-multiplicity reach of the distributions by around 70-90% with respect to the previous ALICE publication [4], exploring a wider phase space.…”

“…The systematic uncertainties are described in Sect. 4 and the results along with comparisons to models are presented in Sect. 5, which contains also the analysis of the NBD fits.…”