Measurement of Λ(1405) and ^0(1385) photoproduction on the proton at Eγ=1.5-3.0 GeV

Abstract

The (1405) is assigned as a p-wave q3 baryon in a quark model. However, it is also suggested that (1405) has a non-q3 structure such as a meson-baryon molecular state. Some theoretical works using chiral Lagrangian and coupled unitary model predict the line shape of (1405) as 0 and KN. Its 2-pole structure is also suggested. On the other hand, 0(1385) is firmly established as a q3 state baryon. The difference of the internal structure of (1405) and 0(1385) may appear in the photoproduction cross sections and/or photon beam asymmetries of these two hyperons. Recently, differential cross sections for p → K+(1405) and p → K+0(1385) reactions were measured by LEPS collaboration. However, the statistics were limited. A new experiment was carried out at SPring-8/LEPS with liquid hydrogen target and linearly polarized photon beam. In order to detect decay products of hadrons, a time projection chamber(TPC) surrounding the target was used together with the LEPS spectrometer. The spectrum of hyperons are obtained from a missing mass of p → K+X, but (1405) and 0(1385) can not be separeted due to their intrinsic widths. In order to obtain the yeilds of (1405) and 0(1385) separately, we selected following two reactions and required some cut conditions: (1) p → K+0(1385) →K+0 →K+p− 0 , (2) p → K+(1405) →K+ ±∓ →K+n+−. The spectrum of 0(1385) was extracted from reaction (1) using following cut conditions: (i) K+ was detected in the forward spectrometer, (ii) a proton and a − were detected in the TPC, (iii) a (1116) was identified using the invariant mass of p−. Because (1405) is prohibited from decaying into 0 by isospin conservation, the yield of 0(1385) can be estimated by its decay branching ratios. The spectrum of (1405) was obtained from reaction (2) requiring following cut conditions: (iv) K+ in the forward spectrometer, (v) a + and a − were detected in the TPC, (vi) a neutron was identified using the missing mass of p → K++−X . It is pointed out that because of a strong interference of the isospin 0 and 1 terms of the scattering amplitudes, the line shape of (1405) could be different in −+ and +− decay modes. In order to separate both decay modes, a kinematic fit with two constraints, MM(K++−) = M(n) and MM(K+±) = M(∓), was applied. The isospin interference term is canceled by summing the spectra of the −+ and +− modes. The summed spectrum was obtained after correcting for the decay branch of + → p0. The differential cross sections of (1405) and 0(1385) were measured at three photon energy regions at 0.8 < cosK+CM < 1.0, where K+CM is the polar angle of K+ in the center of mass system. In the reference [24], the production ratio of (1405) to 0(1385) decreases with increasing photon energy and that may suggest the difference of the production mechanism of (1405) to 0(1385). The maximum photon energy of this data set is higher than that of thre previous experiment at LEPS. The differential cross sections of (1405) production were measured at three photon energy regions to be 0.236 ± 0.017(stat.) ± 0.011(syst.) μb for 1.5 < E < 2.0 GeV, 0.153 ± 0.015(stat.) ± 0.016(syst.) μb for 2.0 < E < 2.4 GeV and 0.642 ± 0.035(stat.) ± 0.017(syst.) μb for 2.4 < E < 3.0 GeV. The differential cross section at the highest photon energy region was found to be larger than at lower energy region. This indicates t-channel contribution which can not be explained by tree-level K− exchange, such as Regge trajectory of Vector Meson Dominance. In order to investigate exchanged particle in t-channel, the photon beam asymmetry of (1405) photoproduction should be measured at forward angle of K+.