Quantifying the Underlying Event | Hadron Physics Research Group

Particle production as a function of the underlying event

The event-activity differential investigation of particle production reveals the connection between the leading process and the underlying event. It allows for the study of semi-soft vacuum-QCD effects potentially responsible for collectivity in events with higher activity, such as multiple-parton interactions (MPI). We investigated the production of charged particles associated with high-p_T trigger particles at midrapidity in proton-proton collisions at √s=5.02 TeV in simulations, in function of R_T, which is sensitive to multiple-parton interactions (MPI). We point out a correlation between RT and the leading p_T, caused by triggered samples being biased toward events with gluon-radiation. We also propose a refined event classifier, R_T,min, defined in the ‘transverse-min’ region (Fig. 1) that reduces contributions from initial- and final-state radiations of the main partonic scattering [1].

yield vs dphi in the transverse, trans-min and trans-max regions — **Figure 1.** Charged particle yield as a function of Δφ, in the transverse, trans-max and trans-min regions for low and high transverse activity.

Apparent modification of the jet-like yield in proton-proton collisions with large underlying event

We preformed a study of particle production in function of the relative transverse activity classifier R_T, which is sensitive to the MPI. We observe that the jet-like yields are independent of R_T in simulations which do not include initial- and final state radiations (ISR,FSR), however they increase with R_T with physical settings in the simulations. We point out that there is a correlation between the activity in the transverse region and the leading pT, which is partly responsible for the apparent modification of the yield increase in the toward region. This is caused by a selection bias originated from the sensitivity of the transverse region to ISR and FSR, and from the fact that the high-p_T trigger selection biases the sample towards events with a large amount of gluon radiation. We emphasize that this selection bias has to be taken into account in any analysis which uses R_T as an event classifier [2].

Quantifying the underlying event with angular dependence and multiplicity

We developed a method to quantify the underlying event based on the angular dependence of multiplicity classes and transverse-momentum spectra in high-energy pp collisions at LHC energies [3].

[1] Bencédi Gy., Ortiz A, Paz A, PoS LHCP2021 (2021) 234
[2] Bencédi Gy., Ortiz A., Tripathy S.,J.Phys.G 48 (2020) 1, 015007
[3] Mishra A N, Barnaföldi G G, Paić G, arXiv:2108.13938 [hep-ph] (submitted to PRC)