Measurement of the Reaction pp→ppπ+π-π0 with CELSIUS/WASA at 1.36 GeV

Sammanfattning: A versatile detector setup prepared by the CELSIUS/WASA collaboration for exclusive studies of rare processes produced in proton and deuteron collisions with hydrogen and deuterium pellet targets is presented. For the near-threshold production of light mesons, π0 and η, the scattered beam and target particles are measured in the forward detector, while the products of the decaying mesons are registered in a central detector covering close-to-4π sr.The central detector is composed of a mini-drift chamber consisting of 1738 straw tubes for tracking of the charged particles and a plastic scintillator barrel consisting of 146 scintillation counters providing fast signals of the passing particles. These two detector parts are placed inside of a superconducting solenoid providing an axial magnetic field.Outside of the solenoid is placed an electromagnetic calorimeter consisting of 1012 CsI crystals. The construction, calibration and performance of the plastic scintillator barrel is presented in detail. Its main purpose is the discrimination of charged and neutral particles on the trigger level. It is also used for the identification of charged particles by ΔE-p (ΔE-E) technique in the off-line analysis.The mini-drift chamber is a detector for determination of particles momenta and reaction vertex. It is built with minimum structural material and designed to withstand high particle fluxes at a luminosity 1032cm-2s-1. The design of the mini-drift chamber, a description of the calibration procedure, its performance and integration in the data analysis are presented.The overall performance of the WASA detector is demonstrated by the analysis of the pp→ppπ+π-π0 reaction at 1.36 GeV incident proton energy. The total cross section for the non-resonant part of the reaction is estimated for the first time in this energy region to be 4.6 ±1.2 -0.9 +0.7 μb. The pp→ppη part of the total reaction cross section is estimated to be 4.8 μb which is consistent with the existing experimental information.