Nonresonant central exclusive production of charged hadron pairs in proton-proton collisions at $\sqrt{s} = 13~\mathrm{TeV}$

The central exclusive production of charged hadron pairs in pp collisions at a centre-of-mass energy of 13 TeV is examined, based on data collected in a special high-$\beta^*$ run of the LHC. Events are selected by requiring both scattered protons detected in the TOTEM Roman pots, exactly two oppositely charged identified particles in the CMS silicon tracker, and the energy-momentum balance of these four particles. The nonresonant continuum processes are studied with the invariant mass of the centrally produced two-pion system in the resonance-free region, $m < 0.7~\mathrm{GeV}$ or $m > 1.8~\mathrm{GeV}$. Differential cross sections as functions of the azimuthal angle between the surviving protons, squared four-momenta, and two-hadron invariant mass are measured in a wide region of scattered proton transverse momenta $0.2~\mathrm{GeV} < p_\text{1,T}, p_\text{2,T} < 0.8~\mathrm{GeV}$ and for hadron rapidities $|y| < 2$. A rich structure of interactions related to double pomeron exchange emerges. The parabolic minimum in the distribution of the two-proton azimuthal angle is observed for the first time. It can be understood as an effect of additional pomeron exchanges between the protons from the interference between the bare and the rescattered amplitudes. After model tuning, various physical quantities related to the pomeron cross section, proton-pomeron and hadron-pomeron form factors, trajectory slopes and intercepts, as well as coefficients of diffractive eigenstates of the proton are determined.