Speaker
Description
To characterize the global features of proton-proton (pp) collisions, charged-particle production is analyzed in topological regions defined relative to the particle with the highest transverse momentum $(p^{\mathrm{leading}}_{\mathrm{T}})$. The production in regions orthogonal to (transverse) and along (toward and away) the $p^{\mathrm{leading}}_{\mathrm{T}}$ direction encodes information about the underlying event (UE) and hard-scattering fragmentation products, respectively. Characterizing the UE is essential for tuning Monte Carlo generators and understanding QGP-like signatures in small systems via multi-parton interactions.
This talk reports UE measurements at midrapidity in pp collisions at $\sqrt{s}=13.6$ TeV. Charged-particle number and summed transverse-momentum densities are measured as a function of $p^{\mathrm{leading}}_{\mathrm{T}}$ across the three regions. In the transverse region, particle densities increase steeply at low $p^{\mathrm{leading}}_{\mathrm{T}}$ before reaching a plateau. These results are compared with measurements at lower center-of-mass energies to illustrate the energy dependence of UE activity and are contrasted with predictions from PYTHIA 8 and EPOS 4.
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