A framework for hadronic rescattering in pp collisions

Abstract

In this article, a framework for hadronic rescattering in the general-purpose Pythia event generator is introduced. The starting point is the recently presented space–time picture of the hadronization process. It is now extended with a tracing of the subsequent motion of the primary hadrons, including both subsequent scattering processes among them and decays of them. The major new component is cross-section parameterizations for a range of possible hadron–hadron combinations, applicable from threshold energies upwards. The production dynamics in these collisions has also been extended to cope with different kinds of low-energy processes. The properties of the model are studied, and some first comparisons with LHC $$\mathrm {p}\mathrm {p}$$ p p data are presented. Whereas it turns out that approximately half of all final particles participated in rescatterings, the net effects in $$\mathrm {p}\mathrm {p}$$ p p events are still rather limited, and only striking in a few distributions. The new code opens up for several future studies, however, such as effects in $$\mathrm {p}$$ p A and AA collisions.

On nuclear coalescence in small interacting systems

“…Such an increase of the average p T with multiplicity has been observed by all experiments at LHC, being reasonably reproduced by Pythia (see, e.g., Ref. [64]) and leading to a gradual decrease ofσ ⊥ up to the rather high values of dN ch /dη. On the other hand, this effect is not properly described by QGSJET-II, in which case the decrease ofσ ⊥ is saturated already for relatively small values of dN ch /dη.…”

Hadronic rescattering in pA and AA collisions

“…[41]. A preliminary study in [34] showed that azimuthal anisotropies in the individual HH collision give unambiguous, but miniscule flow effects, and furthermore the many HH event planes of an AA collision point in random directions, further diluting any such effects. In the end, it is the asymmetries related to the AA geometry that matter for our studies.…”

“…A related issue arises if the two programs represent event records differently, so that it might be impossible to trace the full particle history. A desire for convenience is one of the main motivations behind a recently developed framework for hadronic rescattering, implemented natively in Pythia [34]. This framework was inspired by UrQMD in the way it handles some of the processes and cross, sections.…”

“…In [34], initial studies using the framework were limited to implications for pp collisions, which not unexpectedly were found to be of moderate size. That is, while visible enough in model studies, generally they are less easy to pin down experimentally, given all other uncertainties that also exist.…”

Setting the string shoving picture in a new frame

“…Also, after the hadrons are fully formed we expect final state interactions between them. In this article we will not investigate final-state hadron interactions, although it is able to produce a sizable flow signal in PbPb collisions with Angantyr initial conditions [43], and has recently been implemented in PYTHIA8 [42]. As indicated by the discussion about the inherent ambiguity in defining a "hadronization time" in section 3, defining a transition between a string-dominated final state at early times, and a hadron dominated final state at late times, will require scrutiny.…”