First Study of Exclusive Production of Multi-Hadron Final States in Ultraperipheral Collisions

TL;DR

This work tackles the challenge of understanding resonances arising from gluonic interactions in exclusive multi-hadron production during ultra-peripheral collisions. It combines ALICE Run 2 analyses of exclusive four-pion photoproduction with Run 3 detector upgrades that enable higher-statistics, multi-prong final states and charmonium decays to four-hadron systems. The study provides evidence for two excited rho states, $\rho(1450)$ and $\rho(1700)$, in the four-pion channel and demonstrates azimuthal interference effects with a clear dependence on impact parameter. The Run 3 program extends these measurements to new channels, including $J/psi$ decays to four- and six-hadron final states, highlighting the potential to map dense gluonic matter through multi-differential analyses and resonant/non-resonant dynamics.

Abstract

The study of exclusive photoproduction of multi-hadron final states in ultra-peripheral collisions (UPCs) provides a unique avenue to explore quantum chromodynamics (QCD) and the nature of resonances emerging from gluonic interactions. The ALICE Collaboration has recently performed measurements of exclusive four-pion photoproduction using Run 2 data, favoring the presence of two resonances. However, the underlying nature of these resonances remains poorly understood. The enhanced capabilities of the ALICE detector during Run 3 open new opportunities for the study of multi-pion final states, including four and six-pion systems, and processes involving charmonium decays into four-hadron final states. These studies provide an invaluable way to probe the dynamics of highly dense gluonic matter and the interplay between resonant and non-resonant contributions in hadronic systems. In this paper, we outline the ALICE program focused on these analyses.

Figures (8)

• Figure 1: Lorentz contracted nuclei moving near the speed of light. The interaction is dominated by the exchange of a virtual photon when b $>$ 2R
• Figure 2: Exclusive production of a vector meson in UPCs, which then decays further.
• Figure 3: The measured $\varphi$ distribution for three different neutron classes used to extract the angular anisotropy. The leftmost plot represents the 0n0n class (large impact parameter); the middle plot represents 0nXn (medium impact parameter), and the rightmost plot represents XnXn (small impact parameter). The red curve shows the simultaneous fit performed to extract the anisotropy as a component of $\cos~2\varphi$.
• Figure 4: Measured coefficient $\it{cos~}$$2\varphi$ compared with multiple models and previous measurements.
• Figure 5: Invariant mass of four pions in UPC Pb--Pb collisions at LHC at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV.