Study of Anomalous Pion Production in Proton-Nucleus Collisions

Several recent measurements of proton induced pion production on medium mass tar-get nuclei revealed a considerable increase of pion yields at about 350 MeV proton energy with a relatively narrow width of about 5 MeV[1,2]. This resonance-like structure could be accounted for by a new phenomena in connection with non-nucleon degrees of freedom in nuclei. Normal nuclear excitations would be extremely broad at such a high excitation energy. Since the more definite justification of the resonance-like structure comes from low energy (below 70MeV) pion production, it was proposed by A.Kurepin and K.Oganesyan[1,3] that the resonance could be due to the quasi-bound state of two ∆(1232) isobars in nuclear matter, and predominantly decayed in the two pion channel. Then the maximum pion energy is Tp(∼350MeV) − 2 ×mπ+ 70MeV. There are still discussions on the experimental results[4]. In order to confirm the struc-ture experimentally, we performed precise measurements of excitation functions for positive pion production in proton-nucleus collisions. The LAS spectrometer was used to analyze pion momentum. After rejection of backgrounds by dE-dE and time-of-flight mesurements by two scintillators, the position at focal plane was determined by two VDCs. The pio