Study of central exclusive production in proton-proton collisions at $$\sqrt{s} = 5.02$$s=5.02 and 13TeV
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Regular Article - Experimental Physics
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Study of central √ exclusive π π production in proton-proton collisions at s = 5.02 and 13 TeV CMS Collaboration
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CERN, 1211 Geneva 23, Switzerland Received: 5 March 2020 / Accepted: 17 June 2020 © CERN for the benefit of the CMS collaboration 2020
Abstract Central exclusive and semiexclusive production + − of π π pairs is measured with the CMS detector in protonproton collisions at the LHC at center-of-mass energies of 5.02 and 13 TeV. 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 semiexclusive 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 pT (π ) > 0.2 GeV and pseudorapidity |η(π )| < 2.4. The production cross sections for the four 0 resonant channels f 0 (500), ρ (770), f 0 (980), and f 2 (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.
1 Introduction The central exclusive production (CEP) process has been studied for a long time from both theoretical [1–7] and experimental [8–18] perspectives. In this process, both protons remain intact in the collision and a central system is produced. The process is referred to as exclusive when no particles other than the central system are produced. If one or both protons dissociate into a forward diffractive system, the process is called semiexclusive production. Various central + − + − systems can be produced in this process, like π π , K K , + − and 4π . In this paper, the π π central system is measured. At the CERN LHC energies, the two dominant mechanisms + − of π π production via CEP are double pomeron exchange (DPE) and vector meson photoproduction (VMP), which are illustrated by the diagrams shown in Fig. 1. The pomeron (P) is a color singlet object introduced to explain the rise of
the inelastic cross section at high collision energies [19,20]. The quantum numbers of the pomeron constrain the possible central systems in DPE processes, whereas the photon exchange restricts the central system in VMP processes. By functioning as a quantum number filter, the CEP process is suitable to study low-mass resonances, which would be difficult to study otherwise. Furthermore, DPE processes are also suitable to search for glueballs (bound states of gluons without valence quarks), because they provide a gluon-rich environment [21,22]. Another process that could contribute + − to the same final state is the two-photon fusion γ γ → π π , which is expected to have a much smaller cross section than DPE and VMP processes and gives a negligible contri
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