The Search for Exotic Baryons at the HERMES Experiment.

Abstract

One of the interesting questions of Quantum Chromodynamics, the theory that governs the interactions between quarks and gluons, has been whether it is possible to observe hadrons which can not be explained as a combination of only two or three valence quarks. In numerous searches the existence of these exotic hadrons could not be confirmed. Recently, calculations based on the quark soliton model predicted the narrow exotic baryons Theta+ and Xi--. A narrow resonance identified as the Theta+ was observed by several experiments at the predicted mass of 1540 MeV, but later followed by several dedicated experiments that could not confirm these positive results.

At the HERMES experiment a search for the quasi-real photoproduction of the exotic baryon Theta+ on a deuterium target and the subsequent decay through p K0(S) to p pi+ pi- revealed a narrow resonance in the p K0(S) invariant mass distribution at 1528 MeV. In the search for the corresponding antiparticle Theta- the result is consistent with zero events.

In this thesis we present the search for the exotic baryon Xi-- on a deuterium target in the data sample used for the observation of the Theta+. An upper limit on the cross section of the exotic baryon Xi-- is determined. The search for the exotic baryon Theta+ on hydrogen and deuterium targets at the HERMES experiment is extensively discussed. The event mixing method can be used to estimate the distribution of background events. Several difficulties with this method were addressed, but the background description in the case of the exotic baryon Theta+ remains unconvincing. Between the years 2002 and 2005 the HERMES experiment operated with a magnetic holding field around the hydrogen target. A method for the reconstruction of displaced vertices in this field was developed. The data collected during the years 2006 and 2007 offer an integrated luminosity that is several times higher than in previous data sets. After investigating all data sets collected with the HERMES experiment on hydrogen and deuterium targets, we are not able to observe a resonance peak at an invariant mass of 1540 MeV consistent with the exotic baryon Theta+.