Study of Transverse-Spherocity Biased pp Collisions at the LHC Energies Using the PYTHIA 8 Event Generator

Abstract

The ALICE collaboration recently reported the mean transverse momentum as a function of charged-particle multiplicity for different pp-collision classes defined based on the “jettiness” of the event. The event “jettiness” is quantified using transverse spherocity that is measured at midpseudorapidity (|η|<0.8) considering charged particles with transverse momentum within 0.15<pT<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. Since at intermediate and high pT (2<pT<10 GeV/c), the spectral shape is expected to be modified by color reconnection or jets, their effects on the average pT 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.