New results on the multiplicity and centre-of-mass energy dependence of identified particle production in pp collisions with ALICE

Abstract

The study of identified particle production in proton-proton (pp) collisions as a function of centerof-mass energy ( √ s) and event charged-particle multiplicity is a key tool for understanding similarities and differences between small and large collision systems. We report on new measurements of the production of identified particles and their dependence on multiplicity and √ s.Latest results for light flavor hadrons, comprising π ± , K ± , p(p), single-strange (K 0 S ,Λ, and Λ),and Ω + ) particles as well as resonances (K * 0 , K * 0 , φ ), are presented for √ s = 5.02, 7, and 13 TeV -measurements for √ s = 5.02 TeV pp collisions are reported here for the first time. The measured minimum bias p T spectra and yields were complemented with multiplicity-dependent measurements for single-and multi-strange hadrons at √ s = 7 and √ s = 13 TeV. Results are compared to measurements at lower collision energies as well as to those in proton-lead (p-Pb) and lead-lead (Pb-Pb) collisions, respectively at √ s NN = 5.02 TeV and √ s NN = 2.76 TeV.The results unveil intriguing similarities among collision systems at different center-of-mass energies. The production rates of strange hadrons are found to increase more than those of non-strange particles, showing an enhancement pattern with multiplicity which does not depend on the collision energy. These yield ratios take values which are alike for small systems at comparable multiplicities, and show smooth evolution with multiplicity across all collision systems; they tend to approach those measured in Pb-Pb collisions. Although, the multiplicity dependence of spectral shapes can be qualitatively described by general-purpose Monte Carlo (MC) event generators, the evolution of integrated yield ratios is barely (or not) captured at all by MC model predictions.

Highlights from the ALICE Experiment

“…In the generally accepted interpretation, this is a consequence of the radial flow, which pushes the heavier baryons in a more pronounced way than the lighter mesons [18]. A similar increase of the baryon-over-meson ratios as a function of event activity (charged particle multiplicity) has also been reported for pp and p-Pb collisions [5]. Even if the magnitude of the effect is less pronounced in the smaller colliding systems (possibly as a consequence of the smaller multiplicity range probed), this observation raises the question whether the same interpretation in terms of collective expansion is applicable.…”

“…New results on identified light-flavour hadron production in Pb-Pb collisions at √ s NN = 5.02 TeV [6] and in minimum-bias pp collisions at √ s = 5 and 13 TeV were also presented [5]. These new measurements represent the baseline for a precision study of hadronisation across all systems.…”

“…The ALICE experiment recently released a set of measurements on the production of strange hadrons as a function of multiplicity in the three colliding systems (pp, p-Pb, Pb-Pb) [4,5,6]. Figure 1 shows the ratio of several strange hadrons to pions.…”

“…Even if the magnitude of the effect is less pronounced in the smaller colliding systems (possibly as a consequence of the smaller multiplicity range probed), this observation raises the question whether the same interpretation in terms of collective expansion is applicable. The high statistics data collected in pp collisions at √ s = 5 TeV and, in particular, √ s = 13 TeV, enhanced by dedicated high-multiplicity triggers, will allow the ALICE collaboration to address this question in detail [5].…”