Initial-state geometry and multiplicity distributions in pp and pPb collisions

TL;DR

The paper investigates whether the initial-state geometry of the proton, particularly a baryon-junction (BJ) Y-shape, leaves observable imprints in charged-particle multiplicity distributions in $pp$ and $p$Pb collisions at the LHC. It uses a MC-KLN implementation of the CGC with KLN unintegrated gluon distributions to propagate four nucleon geometries (hard-sphere, Gaussian, BJ1, BJ2) through $k_T$-factorization to $P(N_{ch})$, incorporating intrinsic fluctuations of the saturation scale. Comparison to ALICE data shows that hard-sphere and Gaussian proton shapes are inconsistent with $pp$ data, BJ1 is incompatible with $p$Pb, while BJ2, when intrinsic fluctuations are included, describes the data and preserves KNO scaling in the studied ranges. The results suggest the BJ configuration could survive high-energy evolution and influence initial-state geometry, motivating further confirmation at forthcoming electron-ion collider experiments.

Abstract

This work investigates the possibility of accessing the initial geometric shape of the proton in proton-proton and proton-nucleus collisions at the LHC, in particular the configuration in which the proton is made of three quarks linked by a Y-shape gluon string, called baryon junction. This initial state spatial configuration has been used in the past to describe data on baryon rapidity distributions, diffractive $J/ψ$ production and multiplicity distributions in pp collisions. In spite of its success in explaining the data, the evidence of the baryon junction still needs confirmation. Further studies will be undertaken at the electron-ion collider. In this work we study multiplicity distributions measured in pp and pPb collisions. Different initial state geometries are used as input in a Monte Carlo event generator which implements the $k_T$-factorization formalism of the CGC with KLN unintegrated gluon distributions. The results indicate that the hard-sphere and Gaussian proton configurations are incompatible with the data. In contrast, the baryon junction configuration can describe the data provided that intrinsic fluctuations of the saturation scale are included.

Figures (4)

• Figure 1: (Left) Proton thickness. (Middle) Lead participant thickness. (Right) $dN/dy \, d^2 \mathbf{r}$ of the produced gluons at midrapidity. The rows represent different density distributions of the nucleons. From top to bottom: hard-sphere, Gaussian, BJ1, BJ2.
• Figure 2: KNO plot for pp multiplicity distribution at a) $\sqrt{s}=2.76 \text{ TeV}$, b) $\sqrt{s}=5.02 \text{ TeV}$, c) $\sqrt{s}=7 \text{ TeV}$, d) $\sqrt{s}=8 \text{ TeV}$ and e) $\sqrt{s}=13 \text{ TeV}$. The four different lines correspond to the four different initial geometries. The experimental data (black points) are from alice24.
• Figure 3: KNO plot for pPb multiplicity distribution at a) $\sqrt{s}=5.02 \text{ TeV}$ and c) $\sqrt{s}=8.16 \text{ TeV}$. The four different lines correspond to the four different initial geometries. Comparison of BJ2 with and without intrinsic fluctuations at b) $\sqrt{s}=5.02 \text{ TeV}$ and d) $\sqrt{s}=8.16 \text{ TeV}$. The experimental data (black points) are from alice24.
• Figure 4: a) KNO plot for $\sqrt{s}=5.02,8.16 \text{ TeV}$. The experimental data are from alice24; b) Predictions for pPb multiplicity distributions at $\sqrt{s}=5.02 \text{ TeV}$ for two rapidity intervals $|\eta|<0.5$ and $|\eta|<1.0$ presented in the KNO form.