Transverse sphericity of primary charged particles in minimum bias proton–proton collisions at √s= 0.9, 2.76 and 7 TeV

Abstract

Measurements of the sphericity of primary charged particles in minimum bias proton–proton collisions at √ s = 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC are presented. The observable is measured in the plane perpendicular to the beam direction using primary charged tracks with pT > 0.5 GeV/c in |η| <0.8. The mean sphericity as a function of the charged particle multiplicity at mid-rapidity (Nch) is reported for events with different pT scales (“soft” and “hard”) defined by the transverse momentum of the leading particle. In addition, the mean charged particle transverse momentum versus multiplicity is presented for the different event classes, and the sphericity distributions in bins of multiplicity are presented. The goal of this analysis is to understand the interplay between the event shape, the charged particle multiplicity, and their transverse momentum distribution. Hence, the present paper is focused on the following aspects: – The evolution of the mean transverse sphericity with multiplicity, ST (Nch). This study was done for different subsets of events defined by the transverse momentum of the leading particle; – the behavior of the mean transverse momentum as a function of multiplicity, pT (Nch); – the normalized transverse sphericity distributions for various multiplicity ranges. The results of these analyses are compared with event generators and will serve for a better understanding of the underlying processes in proton–proton interactions at the LHC energies. The data are compared with calculations of standard Monte Carlo event generators. The transverse sphericity is found to grow with multiplicity at all collision energies, with a steeper rise at low Nch, whereas the event generators show an opposite tendency. The combined study of the sphericity and the mean pT with multiplicity indicates that most of the tested event generators produce events with higher multiplicity by generating more back-to-back jets resulting in decreased sphericity (and isotropy). The PYTHIA6 generator with tune PERUGIA-2011 exhibits a noticeable improvement in describing the data, compared to the other tested generators. 1 Introduction Minimum bias proton–proton collisions present an interesting, and theoretically challenging subject for detailed studies. Their understanding is important for the interpretation of measurements of heavy-ion collisions, and in the search for signatures of new physics at the Large Hadron Collider (LHC) and Fermilab. e-mail: aortizve@cern.ch http://aliceinfo.cern.ch/ However, the wealth of experimental information is currently poorly understood by theoretical models or Monte Carlo (MC) event generators, which are unable to explain with one set of parameters all the measured observables. Examples of measured observables which are not presently well described theoretically include the reported multiplicity distribution [1–3], the transverse momentum distribution [4] and the variation of the transverse momentum with multiplicity [5–7]. In this paper, we present measurements of the transverse sphericity for minimum bias pp events over a wide multiplicity range at several energies using the ALICE detector. Transverse sphericity is a momentum space variable, commonly classified as an event shape observable [8]. Event shape analyses, well known from lepton collisions [9–11], also offer interesting possibilities in hadronic collisions, such as the study of hadronization effects, underlying event characterization and comparison of pQCD computations with measurements in high ET jet events [12–14].