Central exclusive production with the CMS experiment

We published a paper on the central exclusive and semi-exclusive production of π⁺π⁻ pairs measured with the CMS detector in pp collisions at center-of-mass energies of √s=5.02 and 13 TeV [1]. The theoretical description of these nonperturbative processes, which have not yet been measured in detail at the LHC, poses a significant challenge to models. The two pions are measured and identified in the CMS silicon tracker based on specific energy loss, whereas the absence of other particles is ensured by calorimeter information. The total and differential cross sections of exclusive and semi-exclusive central π⁺π⁻ production are measured as functions of invariant mass, transverse momentum, and rapidity of the π⁺π⁻ system in the fiducial region defined as transverse momentum p_T,π > 0.2 GeV and pseudorapidity |η_π|<2.4 (Fig. 1). The production cross sections for the four resonant channels f₀(500), ρ⁰(770), f₀(980), and f₂(1270) are extracted using a simple model. These results represent the first measurement of this process at the LHC collision energies of 5.02 and 13 TeV [2].

We have completed the analysis of a large amount of high-quality pp collision data taken in 2018, at √s=13 TeV centre-of-mass energy. Our aim was to study and understand exclusive production, trying to uncover the spin-structure of the pomeron. Events are selected by requiring scattered protons detected in the very forward roman pot detectors, exactly two centrally produced oppositely charged particles identified in the silicon tracker, and a momentum balance of the four particles. A fully differential, detailed partial wave (spin-parity) analysis of the angular distributions of the decay daughters reveals several f₀ and f₂ resonances. Their helicity amplitudes as functions of invariants are precisely measured and compared to vector- and tensor- pomeron models. We have measured the effective meson-pomeron form factors, an essential input for theoretical models. The mass pole and couplings of the f₀(980) are measured, along with branching ratios of scalar and tensor resonances. The analysis of the four-hadron final state is promising, where we collaborate with other groups with the final goal to elucidate the case for some potential glueball candidates near 2 GeV mass. We are in the process of writing an initial paper on the nonresonant continuum and preparing another on resonance production.

[1] CMS Collaboration, Eur. Phys. J. C 80 (2020) 718.
[2] Surányi O (CMS), PoS ICHEP2020, 508 (2020).