Antiproton-nuclei annihilation cross section measurement at 125 keV

The experimental research with slow antiprotons and antineutrons to study hadron and nuclear physics has been widely performed in the 80’s and in the first half of the 90’s at LEAR (CERN). After the stop of LEAR in 1996, the activity in the nuclear physics field was continued and is currently carried out at at the Antiproton Decelerator (AD, CERN), the successor of LEAR, by the ASACUSA Collaboration who, alongside its main program devoted to the hyperfine spectroscopy of the ground state of antihydrogen and to the laser spectroscopy of antiprotonic helium, is also involved in performing measurements of the annihilation cross section $\sigma$$_{ann}$ of low energy antiprotons on nuclei. The motivations for these measurements include the possibility of gaining information on the strong interaction both for the quark model and for the potential model by fixing the parameters of the antinucleon-nucleon interaction. In addition, both quark and potential models predict the existence of a resonant or bound antinucleon-nucleon state which could be revealed studying the behaviour of $\sigma_{ann}$ at different energies and with different targets. Fundamental cosmology could benefit from a better knowledge of the antiproton $\sigma$$_{ann}$ as well. The models that assume the existence of 'islands' of antimatter to explain the antimatter-matter asymmetry, need to investigate the importance of the annihilation mechanism to describe the evolution of the process in the spatial region of overlap between matter and antimatter. The existing experimental data show behaviours that cannot be described in a simple way. For instance for low antiproton momenta p$_{lab}$ below 60 MeV/c, the annihilation cross sections measured at LEAR do not increase with the mass number but have similar values for H, D and He, while in the case of heavy nuclei at 100 MeV/c, the $\sigma$$_{ann}$ values measured by our group at AD seem to be larger than the one expected from a simple geometrical description. A 'modified black-disk model', which takes into account the focusing effect of the Coulomb attraction between the antiproton and the nuclei, seems to be in a quite good agreement with our experimental data. Also an extension of the high-energy Glauber model down to the MeV region, if the Coulomb and nuclear interaction and the change of the antiproton momentum inside a nucleus are introduced, reproduces quite well the experimental data for both high and low energy, although a mismatch between the model and some data needs more investigation. Recently, emphasis has been put also on a discrepancy between the results obtained at LEAR with antineutrons and those expected for antiprotons, since an optical potential that fits well all the available data on antiproton interactions with nuclei is not in agreement with the $\sigma$$_{ann}$ for antineutrons on nuclei. Our group in the ASACUSA Collaboration, after having measured the antiproton annihilation $\sigma$$_{ann}$ on Mylar, Sn, Ni and Pt at 5.3 MeV of kinetic energy, has addressed its experimental investigation to an energy below the MeV region where no data exist at all. This thesis work deals with the experimental apparatus, the technique to perform the measurements, the data analysis and the experiment results. Chapter 1 is devoted to the physical motivations of the experiment: after a brief historical introduction on the antiproton annihilation field, the open problems with the existing data will be presented. In Chapter 2 an introduction on the AD facility where the experiment has been performed will be given. The apparatus installed on the ASACUSA beam line will be described in all its parts, from the delivery of the antiprotons from the decelerator ring to the dump of the antiproton bunch on the end wall. The scintillators used for the detection of the antiproton annihilations in the target region, their acquisition system and the validation tests performed with cosmic rays and a dedicated beamtest, will be treated in Chapter 3. In Chapter 4 a description of the measurement technique which has been invented and developed for the experiment will be given. In the same Chapter, the data analysis and the needed Montecarlo simulations will be presented. Finally, the results of the antiproton $\sigma$$_{ann}$ measurements will be given together with a short discussion on the comparison with the existing data and a brief outlook on future possibilities.