Exclusive vector meson production in pp collisions at the COMPASS experiment

Mechanisms for particle production in proton-proton collisions at intermediate energies are studied within the COMPASS collaboration using the COMPASS spectrometer at the M2 beam line of the SPS at CERN. The possible production mechanisms are investigated using the production of the vector mesons ω and φ and include resonant diffractive excitation of the beam proton with a subsequent decay of the resonance, central production and the related “shake-off” mechanism. The data which were used for this thesis were collected in the years 2008 and 2009 with a 190 GeV / $c$ proton beam impinging on a liquid hydrogen target that was surrounded by a recoil proton detector (RPD). The RPD is an integral part of the newly developed hadron trigger system for which in addition several new detectors have been build. The performances of both RPD and hadron trigger system are scrutinised and efficiency parameters are extracted. A method for reconstruction of recoil protons and for calibration is developed and described. The production of ω mesons is studied with the reaction $pp$ → $pωp$, ω → π+π−π$^{0}$ and the production of φ mesons with the reaction $pp$ → $pφp$, φ → K$^{+}$K$^{−}$ for momentum transfers squared between 0.1 (GeV/c)$^{2}$ and 1 (GeV/c)$^{2}$. The production ratio σ ($pp$ → $pφp$)/σ($pp$ → $pωp$) is determined as a function of the longitudinal momentum fraction $\chi$$_{F}$ and compared to the OZI rule prediction. A significant violation of the OZI rule dependent on $\chi$$_{F}$ is found and discussed with respect to resonant structures in the pω mass spectrum. Removing the low pω/pφ mass region which includes these structures eliminates the $\chi$$_{F}$ dependence. In addition, the spin density matrix element ρ$_{00}$ ,i.e. the spin alignment, for both ω and φ mesons is studied. One study is performed in the helicity frame that allows to discriminate resonant diffractive excitation. In a second study, a reference frame with respect to the direction of the momentum transfer is chosen which allows to single out central mechanisms like central production or shake-off. Dependences of the spin alignment on the invariant mass of the proton-vector meson system and $\chi$$_{F}$ are found. These dependences can be related to resonant contributions to the ω meson production. The discussion of all results with respect to the expected production mechanisms concludes the thesis.