Master thesis defense by Emil Sørensen Bols – University of Copenhagen

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Master thesis defense by Emil Sørensen Bols

Title: Proton-Proton Central Exclusive Pion Production at √s = 13 TeV with the ALFA and ATLAS detector.

Abstract:
The cross section for proton-proton central exclusive dipion production, pp → p +  π+ + π-  + p, has been measured at the center of mass energy sqrt(s) = 13TeV, where double Pomeron ex-change 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 α* = 0.126GeV-2 and α(0)= 1.085, the cross section was found to be: σ(pp → p +  π+ + π-  + p) = 18.754 ± 0.048(stat.) ± 0.770(syst.)μb, |η π | < 2.5, p t,π > 100MeV
The invariant mass spectrum of the pions has been analyzed in order to extract the widths and masses of the f0(500), f0(980), f2(1275) and f0(1500) mesons. A correlation between the two outgoing protons has been observed, which is not in agreement with Pomeron ex-change 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 α* and α(0) is favored.
The cross section for proton-proton central exclusive four pion production, has also been measured at the energy sqrt(s) = 13TeV.
σ(pp → p +  π+ + π- + π+ + π-  + p) = 3.575 ± 0.065(stat.) ± 0.338(syst.)μb, |η π | < 2.5, p t,π > 100MeV
The mesons f1(1285) and f0(1500) are observed in the four pion invariant mass spectrum