Determination of the η mass from the production threshold for the γp → pη reaction

Abstract

This thesis is dedicated to a new precise determination of the η meson mass based on a measurement of the threshold for the γp → pη reaction. This experiment was performed in the years 2004/2005 using the Crystal Ball/TAPS detector setup and the recently developed tagger focal-plane microscope detector at the MAMI-B facility in the Institut für Kernphysik of the Johannes Gutenberg-Universität Mainz. The real photon beam was produced by Bremsstrahlung of the 883 MeV electrons from MAMI-B on a thin diamond radiator. The absolute electron energy of the incident beam was precisely determined in the 3rd race-track microtron of MAMI-B. The tagged photon energies were determined using the Glasgow photon tagging spectrometer (tagger). The tagger focal-plane microscope detector was used for the first time in this experiment. It was placed in front of the main focal-plane detector array and improved the tagged photon energy resolution, covering the energy region around the η production threshold (<i>E</i>_thr = 707 MeV) from <i>E</i>_γ = 674 MeV to <i>E</i>_γ = 730 MeV at an electron beam energy <i>E</i>₀ = 883 MeV. Made of 96 scintillator strips overlapping to one third, the tagger microscope provided 191 tagging channels with an energy resolution of about 290 keV per channel compared to approximately 2 MeV available from the main focal-plane detector. The liquid hydrogen target was located at the center of the Crystal Ball detector. The Crystal Ball, covering polar angles between <i>θ</i> = 20^° and <i>θ</i> = 160^°, consisted of 672 NaI crystals. In order to distinguish between neutral and charged particles detected by the Crystal Ball, the system was equipped with a particle identification detector (PID) made of plastic scintillator. The forward wall detector, TAPS, had 510 BaF₂ hexagonally shaped crystals, each equipped with a 5 mm thick plastic scintillator for identifying charged particles. The TAPS detector, intended for detecting particles in the forward direction (<i>θ</i> = 4^°-20^°), was located at a distance of 173 cm from the Crystal Ball center, making it possible to use time-of-flight analysis for particle identification.<br /> Special care was taken of the energy calibration of the tagger microscope with electrons of different known energies from MAMI. The calibration of the tagger microscope was performed by varying the magnetic field <i>B</i>_cal in the tagging spectrometer around the value <i>B</i>_exp used in the experiment. This was done with three different MAMI energies to scan across the tagger microscope by increasing the value of <i>B</i>_cal in small steps and plotting the measured hit position of the beam in the microscope versus the equivalent energy. The fit was performed by a least squares minimization with the aid of the MINUIT package, supposing a linear dependence between tagging electron energy and microscope channel number.<br /> The η mesons were identified via their two main decay modes, η → 2γ and η → 3π⁰, with the Crystal Ball/TAPS setup, which measured energies and emission angles of particles. The identification of the η → 2γ decay was performed using events with two clusters detected as photons, ignoring all other particles, and the standard invariant mass analysis. Cuts were applied on the invariant and missing mass distributions. The identification of the η → 3π⁰ → 6γ decay concentrated on events with six clusters detected as photons. Among fifteen possible combinations of six photons to be arranged in three pairs, the combination with the smallest χ²-value for the three pion masses was assumed to be correct. Cuts were applied on the χ²-distribution and on the invariant and missing mass distributions. The normalization of the total cross section was obtained from the target thickness, the intensity of the photon flux, the simulated acceptance of the Crystal Ball, and the branching ratios of the η decays. The determination of the η mass required a very precise measurement of the production threshold. This was obtained by fitting the measured cross section as a function of photon energy and gave the result for the η mass <i>m</i>_η = (547.851 ± 0.031_stat. ± 0.062_syst.) MeV. This result agrees very well with the precise values of the NA48, KLOE, and CLEO collaborations and deviates by about 5s from the smaller, but also very precise value obtained by the GEM collaboration at COSY.