π⁰ π⁰,η and η' photoproduction off the deuteron, or, the search for missing resonances

Abstract

The main topic of this thesis are the cross sections, angular distributions and Dalitz plotmeasurements of the photoproduction of π⁰ π⁰, η and η′ -mesons off the deuteron. These measurements were done in order to study nucleon resonance, of mass locations between 1.2 GeV and 2.5 GeV, that couples strongly to the neutron. The study of the nucleon structure is one of the primary interests in strong interaction physics and has been the subject of experimental and theoretical studies for several decades. One of the primary manifestations of the complex internal structure of the nucleon is the existence of its excited states, i.e. baryon resonances (see Figure 1.1). These play an important role in intermediate energy phenomenon and understanding their nature is a necessarily step to reach a comprehansive picture of strong interaction physics. The excited states of the nucleon were first observed in πN scattering in which their contribution was clearly evident as bumps in the total cross section. These measurements allowed a first classification of the excitation spectrum of the nucleon, providing measurement of the masses, widths, quantum numbers, and branching ratios ofmany baryon resonances [1]. In spite of the large amount of information collected by these experiments, the number of states that were identified was less than that predicted by the standard quark model [2]. A possible explanation is that such “missing” statesmay decouple from the πN channel,making them undetectable in experiments with pion beams. Other explanations come from theoretical models that are able to predict a smaller number of states based on a reduced set of degrees of freedom [3]. Unraveling this problem requires measurements with probe other than pion beams. The construction of high intensity and high duty cycle electron and photon facilities, two decades ago, opened new possibilities for the study of baryon resonances using electromagnetic probes. These provide information on the resonance and nucleon wavefunctions through the measurement of the helicity amplitudes, i.e. the electromagnetic couplings between nucleon ground state and initial states. In addition electroproduction also allows us to explore baryon structure for different distance scales by varying the photon virtuality. Nowadays electroexcitation processes are a fundamental tool to pursue these studies. However due to the complexity of the baryon spectrum, the proximity and overlapping nature of the various excited states, the measurement of a single channel is not sufficient to complete this research program. On the contrary a thorough study of resonance properties requires the measurement of cross sections, angular distributions, Dalitz plots, as well as polarization observables for different final states. Figure 1.2 illustrates the problems, a possible and partial solution by looking at specific meson(s) photoproduction that can tag specific resonances! Abroad experimental programfor the study of nucleon resonanceswas setup in 2001 and is still in progress in Bonn by the CB-ELSA/TAPS collaboration. During the measurement campaign different settings were used for the electron beam (2.6 GeV, 2.8 GeV, 3.2 GeV and 3.5 GeV electron beam of the ELSA accelerator) and different target types (liquid hydrogen, liquid deuterium and solid targets : C, Ca, Nb and Pb). For the meson (s) photoproduction off the deuteron, a tagged photon beam has been produced by bremsstrahlung with two different settings: 2.6 GeV and 3.2 GeV electron beam of the ELSA accelerator. The target was a liquid deuterium target of 5 cm. An almost 4π detection system was used, centered around the target. It was composed of: Crystal Barrel (CB) and Travel Around Photon Spectrometer (TAPS) wall, and their respective Charge Particle Counters (CPC), the inner detector and the veto wall. The deuteriumdata was taken between January and August 2003.