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

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.