Underlying event properties in pp collisions at $$ \sqrt{s} $$ = 13 TeV

Abstract

This article reports measurements characterizing the Underlying Event (UE) associated with hard scatterings at midrapidity (|η| < 0.8) in pp collisions at √ s = 13 TeV. The hard scatterings are identified by the leading particle, the charged particle with the highest transverse momentum (p leading T) in the event. Charged-particle number-densities and summed transverse-momentum densities are measured in different azimuthal regions defined with respect to the leading particle direction: Toward, Transverse, and Away. The Toward and Away regions contain the fragmentation products of the hard scatterings in addition to the UE contribution, whereas particles in the Transverse region are expected to originate predominantly from the UE. The study is performed as a function of p leading T with three different p T thresholds for the associated particles, p track T > 0.15, 0.5, and 1.0 GeV/c. The charged-particle density in the Transverse region rises steeply for low values of p leading T and reaches a plateau. The results confirm the trend that the charged-particle density in the Transverse region shows a stronger increase with √ s than the inclusive charged-particle density at midrapidity. The UE activity is increased by approximately 20% when going from 7 TeV to 13 TeV pp collisions. The plateau in the Transverse region (5 < p leading T < 40 GeV/c) is further characterized by the probability distribution of its charged-particle multiplicity normalized to its average value (relative transverse activity, R T) and the mean transverse momentum as a function of R T. Experimental results are compared to model calculations using PYTHIA 8 and EPOS LHC. The overall agreement between models and data is within 30%. These measurements provide new insights on the interplay between hard scatterings and the associated UE in pp collisions.

Probing the interaction of semi-hard quarks and gluons with the underlying event in light- and heavy-flavor triggered proton-proton collisions

“…This remarkable flavor hierarchy represents the flavor (mass) dependent fragmentation. Similarly to what has been observed for light flavor [21], in the transverse region there is a near-saturation towards high p trigger T . We see that the saturation value does not depend significantly on the flavor of the trigger.…”

“…These distributions are shown in Fig. 1 for both the toward and the transverse regions, and they are compared to recent experimental data (open circles) for charged particles measured by the ALICE Collaboration [21].…”

“…The azimuthal angular difference between the trigger particles and the associated particles is used to define the following azimuthal regions considered in this study: |Δϕ| < π 3 for the toward region and π 3 < |Δϕ| < 2π 3 for the transverse region, dominated by the UE. PYTHIA 8 is known to reproduce the chargedhadron multiplicities in the jet as well as in the underlyingevent regions [21]. Besides our reference simulations we also generated samples with non-physical settings, to understand the individual effect of model components: without FSR and ISR, disabled and enhanced CR.…”

Soft QCD results from ALICE

“…√ s NN = 5.02 TeV The underlying event is the collection of all particles not originating from the primary scattering or the related fragmentation. Conventionally, UE analyses include the charged-particle density measurement in the toward side (or near side, NS), transverse side (TS), and away side (AS) with respect to the leading trigger particle -the highest transverse momentum (p > 5 GeV/c, charged-particle density is nearly insensitive to the hard component as also observed in pp collisions [11]. The figure also shows the comparison with PYTHIA8 (Angantyr) [7] and EPOS LHC [8] models.…”

“…The very forward energy measurement by the ZDC is a powerful tool to characterise the proton breakup, providing unique insights into the initial stages of the collision. Figure 2 shows the self-normalised ZN signal (measured with the neutron calorimeter) at forward rapidity [10] and the charged-particle density in the transverse region at midrapidity [11] . This indicates that the forward energy is correlated with midrapidity UE activity and that small forward energy detection selects events with high charged-particle multiplicity and high p leading T particle at midrapidity.…”