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Register a new userComparative studies of the interaction in the low energy ppeta and ppeta systems
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Authors
Pawel Klaja
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The COSY-11 collaboration measured the pp -> ppeta and pp -> ppeta reactions in order to perform comparative studies of the interactions within the proton-proton-meson system. This thesis presents in detail the analysis of the pp -> ppeta reaction which was measured at the proton beam momentum of 3.260 GeV/c. The elaboration results in differential distributions of squared invariant proton-proton (s_pp) and proton-eta (s_peta) masses, as well as in angular distributions and the total cross section at an excess energy of 16.4 MeV. The differential distributions s_pp and s_peta are compared to theoretical predictions and to the analogous spectra determined for the pp -> ppeta reaction. The comparison of the results for the eta and eta meson production rather excludes the hypothesis that the enhancement observed in the invariant mass distributions is due to the meson-proton interaction. Further, the shapes of the distributions do not favour any of the postulated theoretical models.
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We present a comparison of the two-body invariant mass distributions for the pp -> ppeta and pp -> ppeta-prime reactions, both measured at a nominal excess energy value of Q = 15.5 MeV. For the pp -> ppeta reaction, in addition, the differential cross sections were extracted for an excess energy of Q = 10 MeV. The comparison of the results for the eta and eta-prime meson production rather excludes the hypothesis that the enhancement observed in the invariant mass distributions is due to the interaction of the meson and the proton.
A high statistics measurement of the pp --> ppeta reaction at an excess energy of Q = 15.5 MeV has been performed at the internal beam facility COSY-11. The stochastically cooled proton beam and the used detection system allowed to determine the momenta of the outgoing protons with a precision of 4 MeV/c (sigma) in the center-of-mass frame. The determination of the four-momentum vectors of both outgoing protons allowed to derive the complete kinematical information of the ppeta-system. An unexpectedly large enhancement of the occupation density in the kinematical regions of low proton-eta relative momenta is observed. A description taking the proton-proton and the eta-proton interaction into account and assuming an on-shell incoherent pairwise interaction among the produced particles fails to explain this strong effect. Its understanding will require a rigorous three-body approach to the ppeta system and the precise determination of contributions from higher partial waves. We also present an invariant mass spectrum of the proton-proton system determined at Q = 4.5 MeV. Interestingly, the enhancement at large relative momenta between protons is visible also at such a small excess energy. In contrast to all other determined angular distributions, the orientation of the emission plane with respect to the beam direction is extracted to be anisotropic.
The proton-proton and proton-eta invariant mass distributions have been determined for the pp -> ppeta reaction at an excess energy of Q = 16.4 MeV. The measurement was carried out using the COSY-11 detector setup and the proton beam of the cooler synchrotron COSY. The shapes of the determined invariant mass distributions are similar to those of the pp -> ppeta reaction and reveal an enhancement for large relative proton-proton momenta. This result, together with the fact that the proton-eta interaction is much stronger that the proton-eta interaction, excludes the hypothesis that the observed enhancement is caused by the interaction between the proton and the meson.
Polarisation observables constitute a powerful tool for establishing the production mechanism of the eta meson and for infering the presence of higher partial waves in the final system. Measurements of the proton analysing power for the p(pol)p-->ppeta reaction have been performed by the COSY-11 group at three different excess energies: Q=10, 37 and 40 MeV. Data at Q=40 MeV indicate that the eta meson is probably produced in partial waves higher than s wave.
Two-particle Hanbury-Brown-Twiss (HBT) interferometry is an important probe for understanding the space-time structure of particle emission sources in high energy heavy ion collisions. We present the comparative studies of HBT radii in Pb+Pb collisions at $sqrt{s_{rm{NN}}}$ = 17.3 GeV with Au+Au collisions at $sqrt{s_{rm{NN}}}$ = 19.6 GeV. To further our understanding for this specific energy regime we also compare the HBT radii for Au+Au collisions at $sqrt{s_{rm{NN}}}$ = 19.6 GeV with Cu+Cu collisions at $sqrt{s_{rm{NN}}}$ = 22.4 GeV. We have found interesting similarity in the $R_{rm out}/R_{rm side}$ ratio with $m_{rm T}$ across the collision systems while comparing the data for this specific energy zone which is interesting as it acts as a bridge from SPS energy regime to the RHIC energy domain.
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