Proton-Proton Central Exclusive Pion Production at $\sqrt{13}$ TeV with the ALFA and ATLAS Detector

The cross section for proton-proton central exclusive dipion production, $pp\rightarrow p + \pi^+ \pi^- + p$, has been measured at the center of mass energy $\sqrt{s}=13\mathrm{TeV}$, where double Pomeron exchange is expected to dominate. The LHC was used to create the central diffractive events, and all the final state particles were measured using the ATLAS and ALFA detector. After correcting for detector inefficiencies and proton acceptance using the Donnachie-Landshoff model with $\alpha'=0.126\mathrm{GeV}^{-2}$ and $\epsilon=0.085$, the cross section was found to be: \begin{equation} \sigma_{pp\rightarrow p+\pi^+\pi^-+p}=18.754\pm0.048(\mathrm{stat}.)\pm0.770(\mathrm{syst}.)\mu\mathrm{b} , \quad |\eta_{\pi}| < 2.5, \quad p_{t,\pi} > 100\mathrm{MeV} \end{equation} The invariant mass spectrum of the pions has been analyzed in order to extract the widths and masses of the $f_0(500)$, $f_0(980)$, $f_2(1275)$ and $f_0(1500)$ mesons. A correlation between the two outgoing protons has been observed, which is not in agreement with Pomeron exchange models, where the exchange vertices are assumed to be uncorrelated. Different central diffractive models have been compared to data, and it appears that a Pomeron trajectory with low $\alpha'$ and $\epsilon$ is favored. The cross section for proton-proton central exclusive four pion production, has also been measured at the energy $\sqrt{s}=13\mathrm{TeV}$. \begin{equation} \sigma_{pp\rightarrow p+\pi^+\pi^-\pi^+\pi^-+p}=3.575\pm0.065(\mathrm{stat}.)\pm0.338(\mathrm{syst}.)\mu\mathrm{b} , \quad |\eta_{\pi}| < 2.5, \quad p_{t,\pi} > 100\mathrm{MeV} \end{equation} The mesons $f_1(1285)$ and $f_0(1500)$ are observed in the four pion invariant mass spectrum.