In-medium properties of the omega-meson studied in photonuclear reactions near the production threshold

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ThielMichaela_2010_12_03.pdf (9.21 MB)

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2010

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DOI:
http://dx.doi.org/10.22029/jlupub-9639

Abstract

Hadrons can be described with their fundamental properties like their mass or lifetime. Nowadays, these values seem to be well known in vacuum. But what happens, if hadrons are embedded in a strongly interacting environment? To answer this question, theoretical physicists are working on model calculations which (up to now) can not give a consistent conclusion: the results vary from predictions assuming a lowering of the vector meson mass in a strongly interacting environment up to predictions assuming a higher mass at high densities. Other models predict structures in the hadron spectral function due to a coupling of the hadron to nucleon resonances. Hence, experiments are needed to check the theoretical predictions.Light vector mesons seem to be the ideal probe to study in-medium modifications. Their lifetimes are short enough, so that their decay length is comparable to nuclear dimensions. Therefore the probability is high, that the vector mesons decay inside the nuclear medium. The measured decay products than carry the in-medium informations of the vector meson. In this thesis, possible modifications of the omega meson were studied in photonuclear reactions. Most of the former experiments on this topic studied the leptonic decay into e+e-. This is a clean and ideal method to study in-medium modifications since leptons do not underly the strong force. Nevertheless, for this thesis the hadronic decay omega -> pi0 gamma was chosen. This decay mode allows to measure the undistorted omega invariant mass spectrum since contributions from the rho meson are suppressed by two orders of magnitude. Having the pion as a strongly interacting particle in the final state is a disadvantage of this decay channel: due to final state interactions the reconstructed four-momenta can be distorted. Applying a cut on the kinetic energy of the pion T > 150 MeV reduces the amount of distorted events.The experiments for this thesis were performed at the Mainzer Mikrotron MAMI during three beamtimes in 2008. After an upgrade in the year 2006, a continuous electron beam with energies up to 1600 MeV is available. Real photons are produced due to the Bremsstrahlung process when the electron beam impinges on a radiator. The energy of the photons is determined using the Mainz-Glasgow tagging system.For this thesis the reactiongamma A -> (A-1) omega p -> (A-1)pi0 gamma p -> (A-1) gamma gamma gamma pwas investigated for the two different target materials carbon and niobium. To detect the decay products, the detector systems Crystal Ball and TAPS were used. Together they cover roughly the full solid angle of 4pi.Within this thesis, the lineshape of the pi0 gamma invariant mass was studied for different nuclear targets. As a reference the signal is compared to a signal obtained from a liquid hydrogen target. The omega meson was reconstructed using two different analysis branches. In an exclusive analysis three photons and one proton were requested, in a semi-exclusive analysis only three photons were requested. A sideband subtraction technique was used to remove chance coincidences and background contributions from pi0 eta decays from the invariant mass spectrum. The exact determination of the background was necessary, since it has a direct impact on the lineshape of the omega signal. Therefore the background contribution was identified using two different method. In the first method the background was determined by a fit, while in the second the background was directly obtained from the data, selecting four photon events and omitting one of them randomly.Both methods were compared and the result showed the systematic uncertainties in the determination of the omega lineshape. A comparison of the invariant mass distributions for the three different target materials (liquid hydrogen, carbon, niobium) showed good agreement for C and Nb and a slight broadening compared to the omega signal from the hydrogen target after correcting for the 5 cm target length. In addition niobium data was compared to GiBUU calculations. A deviation from the scenario assuming a mass shift of 16% was observed. Therefore, this scenario could be excluded. This was supported by the results obtained from the analysis of the omega meson momentum distributions, which was also part of this thesis. Here, a deviation of the experimental data from the theoretical scenarios including a mass shift could clearly be seen. The scenarios without mass shift were in good agreement with the experimental data.

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