Exclusive measurements of p 6 p ˜p 1 p 6 n near threshold M. Kermani, 1 P. A. Amaudruz, 4 F. Bonutti, 2,3 J. T. Brack, 4, * P. Camerini, 2,3 L. Felawka, 4 E. Fragiacomo, 2,3 E. F. Gibson, 5 N. Grion, 2 G. J. Hofman, 1, * R. R. Johnson, 1 E. L. Mathie, 6 S. McFarland, 1 R. Meier, 4,² D. Ottewell, 4 K. J. Raywood, 1 R. Rui, 2,3 M. E. Sevior, 7 G. R. Smith, 4,‡ and R. Tacik 6 ~The CHAOS Collaboration! 1 Physics Department, University of British Columbia, Vancouver, British Columbia, Canada V6T 2A6 2 Istituto Nazionale di Fisica Nucleare, 34127 Trieste, Italy 3 Dipartimento di Fisica dell’Universita’ di Trieste, 34127 Trieste, Italy 4 TRIUMF, Vancouver, British Columbia, Canada V6T 2A3 5 California State University, Sacramento, California 95819 6 University of Regina, Regina, Saskatchewan, Canada S4S 0A2 7 School of Physics, University of Melbourne, Parkville, Victoria, 3052, Australia ~Received 3 June 1998! The pion induced pion production reactions p 6 p →p 1 p 6 n were studied at projectile incident energies of 223, 243, 264, 284, and 305 MeV, using a cryogenic liquid hydrogen target. The Canadian High Acceptance Orbit Spectrometer was used to detect the two outgoing pions in coincidence. The experimental results are presented in the form of single differential cross sections. Total cross sections obtained by integrating the differential quantities are also reported. In addition, the invariant mass distributions from the ( p 1 p 2 ) channel were fitted to determine the parameters for an extended model based on that of Oset and Vicente-Vacas. We find the model parameters obtained from fitting the ( p 1 p 2 ) data do not describe the invariant mass distribu- tions in the ( p 1 p 1 ) channel. @S0556-2813~98!00112-5# PACS number~s!: 13.75.Lb, 13.60.Hb, 13.60.Le, 13.75.Gx I. INTRODUCTION It was Weinberg who first suggested @1# that the reaction p N →pp N may be used as a probe of pp scattering. Among the processes contributing to the reaction p N →pp N is the one pion exchange mechanism ~OPE!, which accounts for the interaction of an off-shell pion with the physical pion. Although the OPE mechanism may be used to study pp interactions, other processes such as resonance exchanges also contribute to the reaction amplitude. This presents theoretical difficulties as well as potential ambigu- ities in extracting pp scattering observables from p N →pp N data. Although work is in progress @2# to extend the predictive range of chiral perturbation theory ~ChPT! well above the p N →pp N threshold, presently the connection between theory and experiment is reliable only at threshold. On the other hand, a study of the background ~non-OPE! mecha- nisms, which also contribute at threshold, is best suited to the near threshold region. Constraints for the effective couplings of these background mechanisms can only be obtained by comparing a broad set of experimental results with the pre- dictions of theoretical models. In the present experiment, exclusive measurements of the p 6 p →p 1 p 6 n reactions are reported in the near threshold region, at T p 5223, 243, 263, 285, and 305 MeV. The threshold for these reactions is at T p 5172.4 MeV. Total and single differential cross sections are reported, and inter- preted within the framework of a microscopic model of the p N →pp N reaction first developed by Oset et al. @3# and later extended by Sossi et al. @4,5#. The measured invariant mass distributions for the p 1 p 2 channel were fit to deter- mine the model parameters ~coupling constants for isobar exchanges which contribute to the non-OPE background!.A separate publication @6,7# will deal with the double differen- tial cross sections, Chew-Low and dispersion analyses, and extraction of the pp scattering length obtained from these experimental results. The present work is organized as fol- lows. Section II summarizes previous experiments. Section III describes the experimental apparatus. The techniques used in the data reduction process are described in Sec. IV. The results of the experiment are presented in Sec. V. The model is introduced in Sec. VI, and the data are compared to the model results there. Conclusions are summarized in Sec. VII. II. PREVIOUS pN˜ppN EXPERIMENTS There are five different experimentally accessible chan- nels of elementary pion induced pion production on the pro- ton. These are p 6 p →p 1 p 6 n , p 6 p →p 6 p 0 p , and p 2 p →p 0 p 0 n . In the past, the above reactions have been studied over a wide range of incident pion energies. However, only the near threshold @8–18# measurements ( T p <400 MeV) are of interest in the current work. In 1974 Jones et al. measured angular distributions in the ( p 1 p 2 ) and ( p 2 p 0 ) channels at an incident pion momen- tum of 415 MeV/ c @8#. They employed a 180 liter hydrogen *Present address: University of Colorado, Boulder, CO 80309. ² Present address: Physikalisches Institut, Universita¨t Tu¨bingen, 72076 Tu¨bingen, Germany. ‡ Corresponding author. Electronic address: smith@triumf.ca PHYSICAL REVIEW C DECEMBER 1998 VOLUME 58, NUMBER 6 PRC 58 0556-2813/98/58~6!/3419~12!/$15.00 3419 ©1998 The American Physical Society