Determination of the p 6 p ! p 6 p 1 n Cross Section Near Threshold

Though widely successful in particle physics, QCD is notoriously difficult to apply at low energies. However, with the development of chiral perturbation theory (ChPT) which exploits the chiral symmetries of QCD [1], a means is now available for addressing low energy questions such as the determination of the strength of the simplest of the strongly interacting systems, the p-p interaction. One of the most fruitful ways of investigating this interaction experimentally has involved the measurement of threshold pion-induced pion production cross sections. Such reactions have traditionally been analyzed using the model of Olsson and Turner [2] which involves representing the p-p interaction in terms of the chiral symmetry breaking parameter j. In a recent publication [3], Olsson and coworkers found that the inclusion of higher-order terms, which were neglected in the earlier work, complicated the extraction of p-p scattering lengths from threshold pion production data. Subsequently, Bernard, Kaiser, and Meisner [4] used heavy baryon chiral perturbation theory (HBChPT), which incorporates the effects of higher baryon resonances, to obtain new relationships between the threshold amplitudes for pion production and the p-p scattering lengths. The p 2 p ! p 2 p 1 n reaction involves both isospin 2 and isospin 0 p-p interaction amplitudes and the p 1 p ! p 1 p 1 n reaction involves only isospin 2. Consequently, cross-section measurements of both reactions near threshold can be used to determine both isospin amplitudes. As shown in Fig. 3(b), a consistent body of experimental data exists for the p 2 p ! p 2 p 1 n reaction at energies above 200 MeV. However, Bernard et al. [4] state that only data below 200 MeV can be used to extract p-p scattering lengths. The situation for the p 1 p ! p 1 p 1 n reaction, as shown in Fig. 3(a), is less satisfactory with a discrepancy of more than a factor of 2 between the data of the OMICRON collaboration [5] and that of Sevior et al. [6], the only two experiments for which data were obtained within 40 MeV of threshold. Given the importance of a precise determination of the p-p scattering lengths, it is essential that near-threshold cross-section data be measured for both charge channels by a group other than OMICRON in order to provide a consistent set of data for extracting both isospin reaction amplitudes. To this end, the “active target” system developed by Sevior et al., for the p 1 p ! p 1 p 1 n reaction [7] was adapted for the p 2 p ! p 2 p 1 n case. Although it involves a cross section which is about 10 times larger than that for the p 1 reaction, the method has to contend with the fact that only a single positive pion occurs in the final state. This paper presents the results of measurements of p 2 p ! p 2 p 1 n cross sections at incident pion energies of 180, 184, 190, and 200 MeV, together with measurements of the p 1 p ! p 1 p 1 n cross section at 184 and 200 MeV to check the