Further results from charge-exchange photoproduction

Abstract

It is shown that the energy dependence of charge-exchange a: photoproduction (yp-nu:) agrees with a one-pion exchange mechanism. No evidence for the photoproduction of the a T(1260) is observed. If the dynamics of a , and a, photoproduction are assumed to be the same, the absence of evidence for the a , is shown to be consistent with either an extremely large a , hadronic width or with an a of mass somewhat less than 1260 MeV.PACS number(s): 13.60. Le, 12.40.Gg, 14.40.C~ Since the earliest photoproduction experiments, it has been known that the dominant reaction is the diffractive photoproduction of vector mesons. Arguments based on the vector meson dominance model have often been used to exclude anything but a minor amount of particle exchange as a mechanism for photoproduction [I]. Perhaps the most dramatic evidence of this is the unnatural parity-exchange contribution to w photoproduction [2]( yp -+wp ) where the cross section falls from 1 . 3 i 0 . 3 p b at 4.7 GeV to 0.1 i 0 . 2 p b at 9.3 GeV. Since the radiative width of the w is large [r(w-n-Oy )=850+50 keV], such a drop in the unnatural parity-exchange w production cross section leads to the conclusion that one-pion exchange (OPE) rapidly becomes an ineffectual production mechanism as E , increases. Complications in this interpretation do arisk, however. Some years ago the reaction Y P --+ -A + + was observed to persist to rather large energies. At 19.5 GeV this reaction was clearly evident with a production cross section [3] of 0.224i0.045 pb. While it was natural to assume that the reaction was mediated by OPE [r(p--+n--y ) = 7 1 f 12 keV], the decay angular distributions, the t' dependence of the production cross section, and its E y dependence all indicated difficulties in the interpretation of the reaction as evidence for OPE. Similar data and arguments had been noted previously at lower energies [4].Recently, in a study of the reaction [5] yp-+a2-A++-+n-+n--n--n-+p at 19.5 GeV; it was observed that the decay angular distributions of the A + + and a 2 as well as the t' distribution showed good agreement with OPE. Since this was the initial quantitative report of a , A t + associated photoproduction at any energy, we could obviously not study its E y dependence. The purpose of this paper is to present our evidence for a $ photoproduction from the reaction yp -nn-+rin--, which we will compare with lower-energy data. We shall also comment on the significance of this result as regards to the photoproduction of the a ,( 1260).Our data derive from a high-statistics hydrogen bubble chamber experiment which was performed at the Stanford Linear Accelerator Center with the primary purpose of studying open charm photoproduction. The experimental details regarding the data-taking phase of the experiment may be gleaned from prior publications [6].The photon beam, produced by backscattering laser light from SLAC 30-GeV electron beam had an average energy of 19.3 GeV with a full width at half maximum of 1.7GeV. The event sample for the nn-+n-+n--final state was...

Photoproduction ofa2(1320)in a Regge model

“…The estimation of the exclusive photoproduction cross section for a 2 has been performed according to the one-pion exchange (OPE) mechanism with absorption in [23]. The experimental results for the values and energy dependence of this cross section at relatively low (<20 GeV) energies are quite consistent with the prediction of this model [52]. Nevertheless the en- * xywang@impcas.ac.cn † avg@jinr.ru ergy range covered by the existing experimental data is not enough to distinguish between the OPE prediction and the Regge trajectory approach [10].…”

Light-meson spectroscopy

“…The most significant contributions to meson spectroscopy since that time are by the LAMP2 experiment at Daresbury (Barber 1978(Barber , 1980 with photon beam energies up to 5 GeV, and the Ω-Photon collaboration at CERN (Aston, 1982) using energies between 20 and 70 GeV. Spectroscopy in exclusive photoproduction has also been carried out by E401 at Fermilab (Busenitz, 1989), also an electronic detector with a relatively open trigger; E687 at Fermilab (Frabetti, 1992), an evolution of E401 which concentrated on heavy quark physics; and by the SLAC hybrid bubble chamber and a laser-backscattered photon beam (Condo, 1993).…”

A sum rule approach to the violation of Dashen's theorem

“…A closer look at the literature, however, reveals that a small value for the radiative width is not ruled out. Indeed, a recent photoproduction experiment has found no trace of a 1 production [33] ( while observing very clean evidence for a 2 (1320) production). In this experiment, the photon is on shell, which is not necessarily exactly satisfied in the experiment of ref.…”