Study of energy dependence of pion production by proton on copper target near 350 MeV
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NUCLEI Experiment
Study of Energy Dependence of Pion Production by Proton on Copper Target near 350 MeV* E. V. Karpechev** , A. B. Kurepin, A. I. Reshetin, T. L. Karavicheva, K. Fujita1), K. Hatanaka1), J. Kamiya1), Y. Sakemi1), T. Wakasa1), H. P. Yoshida1), Y. Shimizu1), T. Fukuda2), P. K. Saha2), K. Tamura3), and T. Ishida4) Institute for Nuclear Research, Russian Academy of Sciences, pr. Shestidesyatiletiya Oktyabrya 7a, Moscow, 117312 Russia Received April 23, 2007
Abstract—New measurement of the energy dependence of pion production on a copper target near 350MeV proton energy has been performed. The E-170 experiment has been carried out at the Ring Cyclotron Facility of the Research Center for Nuclear Physics, Osaka University, Japan. A small enhancement of the low-energy pion production cross section was observed near the incident proton energy of 352 MeV. PACS numbers: 25.40.Qa, 25.45.-z, 13.85.Ni DOI: 10.1134/S1063778808010018
INTRODUCTION
ever, the statistical significance was very low because of a long flight path of produced pions.
The search for nonnucleon degrees of freedom in nuclei is one of the most important problems to be solved for understanding the nuclear structure. Assuming the possibility for pions and isobars to live in nuclei for a sufficiently long time, some new very high excited states of nuclei could be discovered. The excitation energies of these states could be comparable to the pion and isobar masses. The first indication of the existence of an even higher excited state in nuclei was obtained in the study of pion spectra by proton interaction with a copper target [1]. At the JINR Dubna phasotron, it was found that, at 350-MeV energy, the slope of a pion spectrum at the production angle of 90◦ was smaller than was expected from measurements at adjoining energies and the number of low-energy pions was increased. Different investigations of the protoninduced π + production on copper were carried out in order to confirm or reject that result [2–4]. Two experiments with a proton beam at TRIUMF [5] and with neutrons at PSI [6] revealed some structure near 350-MeV incident nucleon energy; how-
A detailed experiment on pion production was performed at the Moscow Meson Factory [7]. To detect pions, the magnetic spectrometer CLAMSUD of INFN, Catania [8], was used. There was only a 2-m flight path for pions and a large acceptance. Pion spectra at several proton energies near 350 MeV were measured and an anomalous dependence was observed. The pion spectrum at 348 MeV was more enhanced than at 350 and even at 356 MeV. The proton energy dependence for different parts of the pion spectra also showed a clear resonance-like structure at 348 MeV. This structure was found in different production runs shifted in time by several months. The ratios of the low-to-high-energy pion production showed a strong enhancement near the proton energy of 350 MeV with small statistical errors. However, there are some inaccuracies in the proton-energy determination of the order of 1 MeV, as well as the errors in the normaliz
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