Photoproduction of High-Mass Di-Pion Pairs at 15 BeV

We have measured the cross section for the photoproduction of dipions in the mass range (0.9-2.2) BeV/c2. The experiment was performed with a 16 BeV bremsstrahlung beam on a Be target. Upper limits on the cross section for photoproduction of high mass vector mesons are determined. (Submitted to Phys. Rev. Letters.) Work supported by the U. S. Atomic Energy Commission. ‘Now at Physics Department, MIT, Cambridge, Massachusetts and DESY, Hamburg, Germany. Most of the previous studies of dipion photoproduction have been confined to the mass range Mrr < 1 BeV/c’ where rho production dominates. Two studies of high mass production at forward angles and at photon energies less than 9 BeV have been reported in the literature. 192 The work reported here covers the range of momentum transfer It-t,mi < 0.3 (BeV/c) 2 and dipion decay angle t& lcos Onn 1 I 0.6. The experimental technique, described in more detail elsewhere, 3 consisted of a 16 BeV bremsstrahlung beam incident on a Be target which was followed by an analyzing magnet and a system of scintillator hodoscopes and wire spark chambers D The geometry of the system was arranged so that the dipion mass acceptance (for energies 2 14 BeV) extended from 0,7 to 2.5 BeV/c’ and was very insensitive to mass from 1 to 2 BeV/c’. As the incident photon and target recoil were not observed, each event was analyzed assuming the reaction was of the form y + A A + 7r+ + ?rIT-. A comparison of the energy spectra of the dipions and the incident photon beam reveals that for Enr _ > 14 BeV, the upper limit on the inelastic contamination is 10% for M mr 5 1.0 BeVjc2 and 25% for 1.0 < Mnn 5. 2.0 BeV/c’, The spectrometer acceptance was unfolded from the observed distributions with the assumption that the cross section was azimuthally uniform in the helicity system D We observed a total of 620 events in the intervals Enr 1 14 BeV, I t-tmin( I 0.3 (BeV/c)2, 0,9 5 M,, 5 2.2 BeV/c2, and )cos 0~~1 5 0,6. The distribution of weighted events versus polar decay angle 8rr evaluated in the helicity system is shown in Fig. 1. The cutoff at cos 8?nr = 0.6 is imposed by the act eptanc e 0 The data are consistent with p-wave dominated dipions following a sin2 8 ~~ distribution indicated by the smooth curve.