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Pion induced pion production on deuterium

University of British Columbia

This thesis describes measurements of the pion induced pion production reaction π⁺d → π⁺π⁻p p performed with a 280 MeV incident π⁺ beam at TRIUMF. The data are compared with an improved version of the Oset and Vicente-Vacas theoretical model [12]. The goal of the experiment and of the analysis was to provide a larger body of data for the free reaction and to test the validity of theoretical models. In the process, the ability to determine the values of the coupling constants C, f∆ , gN*∆π within such a model framework would be explored. The knowledge of the precise value of these coupling constants would constrain N* decay branching ratios and other pion induced reaction mechanisms like Double Charge Exchange. A previous experiment [23] had indicated that the pion induced pion production on deuterium is essentially a quasifree process with the reaction occurring on the neutron leaving the proton merely a spectator. The main difference with respect to the free reaction is the effect of Fermi motion of the neutron. Although we were interested in studying the free reaction (π⁻p → π⁺π⁻n), we chose a deuterium target so that the experiment could be run with a π⁺beam, since the π⁻ beam flux is about 6 times lower than the flux of the positive pion beam at 280 MeV, the energy at which our experiment was performed. Such a flux would have required a much longer running time for the experiment in order to achieve the same statistical accuracy. The quasifree nature of the process was also confirmed in our experiment. This experiment involved a coincidence measurement of the quasifree process and as such provided four-fold differential cross section spectra of the reaction thus allowing for a microscopic comparison between data and theoretical models. In the theoretical description we incorporated additional amplitudes for the N* → N(ππ)p-wave diagrams required to describe the reaction cross section at Tπ = 280 MeV. We also added the Fermi motion of the nucleon to the model to account for the deuterium environment. The 'free' parameters of the model are the largely unknown coupling constants listed above. We fixed C to be -2.08 by requiring the energy dependence of the model to be that of the measurement of [22] and compared the energy and angular distributions of the model to our data for several values of the f∆ and gN*∆π coupling constants ranging between 0 and 2 (where the units are 4/5 fNNπ) and between 1.08 and 1.53 respectively. We found reasonable sensitivity of the model to the f∆ variation, but only limited sensitivity to the value of the gN*∆π coupling constant. Overall we achieved a very good agreement between data and the theoretical predictions for f∆ values smaller than 0.5 and gN*∆π values closer to its lower limit. Improved statistical accuracy of the data would however be needed to better constrain the values of the coupling constants. On the basis of our results we feel that this model is a useful tool for planning future experiments and that a more extensive (π, 2π) experimental program, where differential cross sections are measured for differing isospin channels, would provide a further, more stringent test on the model allowing for a more precise determination of the coupling constants.

Text

2011-02-18

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http://hdl.handle.net/2429/31516

Physics

University of British Columbia

Graduate