Transverse sphericity of primary charged particles in minimum bias proton-proton collisions at sqrts=0.9, 2.76 and 7 TeV

Submission Date: 
Article Information
Submission Form
900 GeV
2.76 TeV
7 TeV
Abstract Plain Text: 

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 linearized  to be collinear safe and 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 scales (“soft” and “hard”) defined by   the transverse momentum  of the leading particle.  In  addition,  the  mean charged-particle transverse momentum versus multiplicity is reported for the different  event  classes, and the sphericity spectra in bins of multiplicity are presented. The data are compared with calculations of standard Monte Carlo event generators. The transverse sphericity is found to grow with multiplicity for all collision energies, with  a steeper rise at low Nch . The tendency of the event  generators is opposite at high multiplicity. The combined study of the sphericity and the mean pT  with multiplicity indicates that the current event generators produce events with higher multiplicity by generating more jets resulting in decreased sphericity (and isotropy), while the data show the opposite with increased isotropy in events with higher multiplicity.