The production of $\pi^\pm$, ${ \rm K}^\pm$, and $(\overline{ \rm p})$p is measured in pp collisions at $\sqrt{ s}=13$ TeV in different topological regions. Particle transverse momentum ($p_{ \rm T}$) spectra are measured in the ``toward'', ``transverse'', and ``away'' angular regions defined with respect to the direction of the leading particle in the event. While the toward and away regions contain the fragmentation products of the near-side and away-side jets, respectively, the transverse region is dominated by particles from the Underlying Event (UE). The relative transverse activity classifier, $R_{ \rm T}=N_{ \rm T}/\langle N_{ \rm T}\rangle$, is used to group events according to their UE activity, where $N_{ \rm T}$ is the measured charged-particle multiplicity per event in the transverse region and $\langle N_{ \rm T}\rangle$ is the mean value over all the analysed events. The first measurements of identified particle $p_{ \rm T}$ spectra as a function of $R_{ \rm T}$ in the three topological regions are reported. The yield of high transverse momentum particles relative to the $R_{ \rm T}$-integrated measurement decreases with increasing $R_{ \rm T}$ in both the toward and away regions, indicating that the softer UE dominates particle production as $R_{ \rm T}$ increases and validating that $R_{ \rm T}$ can be used to control the magnitude of the UE. Conversely, the spectral shapes in the transverse region harden significantly with increasing $R_{ \rm T}$. This hardening follows a mass ordering, being more significant for heavier particles. The $p_{ \rm T}$-differential particle ratios $({ \rm p+\overline{ p}})/(\pi^+ +\pi^-)$ and $({ \rm K^+ +K^-})/(\pi^+ +\pi^-)$ in the low UE limit $(R_{ \rm T}\rightarrow 0)$ approach expectations from Monte Carlo generators such as PYTHIA 8 with Monash 2013 tune and EPOS LHC, where the jet-fragmentation models have been tuned to reproduce ${ \rm e^+ e^-}$ results.
Categories: nucl-ex hep-ex