Speaker
Description
Ultraperipheral collisions (UPCs), where the impact parameter is larger than the sum of radii of two nuclei, provide a clean environment for rare multi-hadron resonances studies. The excited states of $\rho$ meson are predicted by theory, however purely known. The ALICE Collaboration has measured exclusive four-pion photoproduction cross section times branching ratio in UPCs of Pb-Pb at $\sqrt{s_{\rm NN}} = 5.02$~TeV, where single and double resonance structure were considered. The ALICE data suggest two resonance structure which stays in contrast with the recent H1 data results supporting the single resonance hypothesis. The upgraded ALICE detector has a new possibility of observing four- and six-pion states, including decay products of charmonia in Run 3. The data allow for more detailed study of the interplay of resonant and non-resonant contributions in hadronic systems. The invariant mass of these systems is large enough to be considered as a hard probe, while keeping the mass small, hence giving a better sensitivity to phenomenon such as saturation than for example charmonia probes. In this talk, the newly published result of exclusive four-pion final state from Run 2 will be shown, as well as the ongoing studies of multi-hadron final states from Run 3.
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