Understanding the transverse-spherocity biased data from pp collisions at the LHC energies

Abstract

The ALICE collaboration recently reported the mean transverse momentum as a function of charged-particle multiplicity for different pp-collisions classes defined based on the "jettiness" of the event. The event "jettiness" is quantified using transverse spherocity that is measured at midpseudorapidity ($|\eta|<0.8$) considering charged particles with transverse momentum within $0.15<p_{\rm T}<10$ GeV/$c$. Comparisons to PYTHIA 8 (tune Monash) predictions show a notable disagreement between the event generator and data for jetty events that increases as a function of charged-particle multiplicity. This paper reports on the origin of such a disagreement using PYTHIA 8 event generator. Since at intermediate and high $p_{\rm T}$ ($2<p_{\rm T}<10$ GeV/$c$), the spectral shape is expected to be modified by color reconnection or jets, their effects on the average $p_{\rm T}$ are studied. The results indicate that the origin of the discrepancy is the overpredicted multijet yield by PYTHIA 8 which increases with the charged particle multiplicity. This finding is important to understand the way transverse spherocity and multiplicity bias the pp collisions, and how well models like PYTHIA 8 reproduce those biases. The studies are pertinent since transverse spherocity is currently used as an event classifier by experiments at the LHC.