Closepacking effects on strangeness and baryon production at the LHC

TL;DR

The paper tackles the observed rise of strange-hadron production with multiplicity in pp collisions at the LHC and the shortfalls of the default Monash tune in reproducing this feature. It advances a momentum-space implementation of three mechanisms—string closepacking, strange junctions, and popcorn destructive interference—that modify the Lund String Model by increasing an effective string tension $\\kappa_{\\text{eff}}$ and altering flavor production probabilities via relations such as $P'(s:u/d) = P(s:u/d)^{\\kappa_0/\\kappa_{\\text{eff}}}$ and $P'(qq:q) = P(qq:q)(1-\\mathcal{P}_q)^{n_q/9}(1-\\mathcal{P}_p)^{n_p/9}$. The authors perform a multi-stage tuning against ALICE strange-to-pion ratios and the $p/\\pi$ ratio using Professor, comparing to Monash, QCD CR, Rope hadronization, and earlier Closepacking tunes. While several observables are qualitatively well described, no configuration simultaneously reproduces all measured ratios, with persistent challenges in $\\phi/\\pi$, $p/\\pi$, and especially the heavy-flavour sector (e.g., $\\Xi_c/D^0$). The work highlights the potential of nonperturbative, density-dependent string effects to improve hadrochemistry in small systems, while underscoring the need for further development and generalization to other collision environments and heavy-flavour dynamics.

Abstract

Data from the LHC show a rise in strange-hadron production with charged-particle multiplicity in pp collisions. The Monte-Carlo event generator PYTHIA, using its default Monash tune, instead predicts constant strangeness. We investigate a mechanism invoked during hadronization called string closepacking, where overlapping strings generate a background field, here assumed to be predominantly aligned with the beam axis. This increases the effective string tension with rapidity, reducing strangeness suppression and thus enhancing strangeness production. We tune this model to LHC data and contrast it with several alternatives. We comment specifically on the challenge of simultaneously describing the strangeness data and the non-strange p/pi ratio, and introduce a mechanism which may act to suppress the latter. Many of the salient particle-production ratios can be qualitatively described by this model, although the XiC/D ratio and the shape of pT spectra remain challenging to account for.

Figures (19)

• Figure 1: Schematic illustration of hadronization given a string representation of the colour confinement field. (i) The initial red-antired quark-antiquark pair, $q_0\bar{q}_0$, moving apart at high energies. (ii) The string forms between the $q_0\bar{q}_0$ pair. (iii) The initial string breaks and a red-antired $q\bar{q}$ pair is created at the site of the string break. (iv) Further string breaking occurs. (v) Final jet of hadrons formed.
• Figure 2: An illustration of the stepwise process of the popcorn mechanism given a red-antired string. (a) the unbroken red-antired string. (b) a blue-antiblue virtual colour fluctuation on the red-antired string, which results in a short segment of "green-coloured" string. (c) a green-antigreen quark pair creation that combined with the blue-antiblue quark fluctuations forms a red-antired diquark-antidiquark pair, breaking the string.
• Figure 3: Minkowski diagram of the popcorn mechanism showing diquark-antidiquark formation and a single meson produced between the baryon and antibaryon i.e., $BM\bar{B}$ configuration. The flux direction of the strings are indicated by arrows, with colour flow from the triplet to antitriplet colour charge. In this picture, we assume the green-antigreen fluctuation forms with zero energy and momentum.
• Figure 4: Junction configuration before (left) and after (right) fragmentation, with quarks $q_2q_9q_5$ together forming the so-called junction baryon.
• Figure 5: Examples of QCD multiplets in overall colour-singlet states and their colour factors according to Casimir scaling. The total number of parallel and antiparallel strings, labelled by $p$ and $q$ respectively, correspond to the number of free indices on the left-hand "side" of the multi-string systems that are contracted with those on the right-hand side, with $p>q$. See also appendix \ref{['app:casimirs']}.