Apparent strangeness enhancement from multiplicity selection in high energy proton-proton collisions

The increase of strange-particle yields relative to pions as a function of the event charged-particle multiplicity in proton–proton (pp) collisions at the LHC is argued to indicate the relevance of the final system produced in the collision. The rise with multiplicity (referred to as strangeness enhancement) is usually described by microscopic or hydrodynamical models as a result of the increasing density of produced partons or strings and their interactions. Instead, in this paper, we consider the multiple partonic interaction (MPI) picture originally developed in the context of the PYTHIA event generator to describe the rich structure of the underlying event in pp collisions. We find that strangeness enhancement in PYTHIA is hidden by a large excess of low-pT multi-strange baryons, which mainly results from the hadronization of u-quark, d-quark and gluon (udg) strings. Strange baryons produced in strings formed from parton showers initiated by strange quarks (s-fragmentation), however, describe well the spectral shapes of Ξ and Ω baryons and their multiplicity dependence. Since the total particle yield contains contributions from soft and hard particle production, which cannot be experimentally separated, we argue that the correct description of the pT-spectra is a minimum requirement for meaningful comparisons of multiplicity dependent yield measurements to MPI based calculations. We demonstrate that the s-fragmentation component describes the increase of average pT and yields with multiplicity seen in the data, including the approximate multiplicity scaling for different collision energies. When restricted to processes that reproduce the measured pT-spectra, the MPI framework exhibits a smooth evolution from strictly proportional multiplicity scaling (K0 S, Λ, where the udg-hadronization component dominates) to linearity (sfragmentation) and on to increasingly non-linear behavior (c-, b-quark and high-pT jet fragmentation), hence providing a unified approach for particle production across all particle species in pp collisions. ar X iv :2 10 9. 05 18 1v 1 [ he pph ] 1 1 Se p 20 21 One of the pillars of the high-energy ultra-relativistic heavy-ion physics program is to study the evolution and onset of potential quark-gluon plasma (QGP) phenomena in smaller collision systems like pp and pA collisions at RHIC and LHC collision energies [1]. In high-energy pp collisions at the LHC, the integrated yields of strange and multi-strange particles relative to pions were reported to increase significantly with the event multiplicity, reaching values similar to those in pPb and PbPb collisions [2]. Strangeness enhancement was originally proposed as a signature of QGP formation in nuclear collisions [3]. In the picture of statistical hadronization, the increase with multiplicity in smaller systems, which is rather independent of collision species and energy [4, 5], is a result of decreasing canonical suppression [6] towards the grand-canonical ensemble realized in more central PbPb collisions [7, 8]. The strangeness data together with other measurements in high-multiplicity pp collisions, in particular the ridge [9, 10], resemble collective properties as found in heavy-ion collisions and provoke speculation about the formation of a QGP in these collisions [1, 11]. Indeed, models incorporating final-state interactions originally developed for heavy-ion collisions, are also able to describe the pp data [12–15]. Refined models based on the “core/corona” approach, where the core of the collisions is assumed to “hydrodynamize”, while the corona is treated as a superposition of independent nucleon–nucleon collision, achieve a good description of data across all systems with a rather universal approach [16,17]. In this paper, we focus on an alternative description of the pp data [2,5] based on the MPI model, as implemented in the PYTHIA event generator [18]. Unlike other works [19–21], we rely on the original string fragmentation, but include coherence effects between multiple scatterings (e.g. color reconnection [22, 23]). We discuss the qualitative expectations of the model for multiplicity dependent measurements as well as the possible effects that can explain strangeness enhancement in pp collisions. An important ingredient for the MPI model is the pp impact parameter (b) dependence of the number of MPI (Nmpi), which was originally introduced for an accurate description of the dispersion of multiplicity distributions and the pedestal effect in underlying event measurements [24]. More recently it has been shown that the pp impact parameter dependence is also important for understanding the centrality dependence of hard processes in pPb [25,26] and peripheral PbPb collisions [27]. In both cases a detailed study of possible nucleon–nucleon impact parameter biases was triggered by unexpected deviations from the number of binary collisions (Ncoll) scaling. In pp collisions, an equivalent scaling factor would be Nmpi, however at present it cannot be extracted from data. Hence, in the absence of a standard scaling expectation, one has to be particularly careful with the interpretation of multiplicity-dependent mea-