Measurement of thepd→3Heηcross section between 930 and 1100 MeV

Abstract

The differential cross section for the pd→ 3 He reaction has been measured at four beam energies 930, 965, 1037, and 1100 MeV. The total cross section is nearly constant throughout the whole energy region, despite the c.m. angular distribution becoming more anisotropic with increasing energy. The data join smoothly onto the results of near-threshold measurements, which are dominated by the S-wave 3 He final state interaction. At all the energies the differential cross section is maximal for cos Ϸϩ0.5 and this may provide a clue to the reaction mechanism and, in particular, to two-step contributions.

Measurements of branching ratios for η decays into charged particles

“…The reaction pd → 3 He η was used to produce η mesons at energies close to the production threshold, where the most favorable ratio between the η production cross section and background reactions is found. The cross section of this production reaction is 0.40(3) μb [ 3,4], meaning that up to eight events containing η mesons are produced per second at the peak luminosity of 2 × 10 31 cm −2 s −1 . The WASA detector is a fixed-target spectrometer, with a forward detector arranged to measure hadronic ejectiles and a central detector to detect light mesons or their decay products.…”

“…In the case of the pd → 3 He η reaction the 3 He stops in the first layer of FRH, so the trigger included a veto on the signals from the second layer. Since fusion to a 3 He represents only about 1% of the total cross section at this energy, the above conditions were sufficient to bring the trigger rate down to below a thousand events per second, which were recorded. 3 × 10 7 events containing η mesons were collected in total, with 1 × 10 7 being collected during the first period in 2008.…”

“…The solenoid field setting was 0.85 T. The trigger conditions were based on information from the forward detector only, meaning the trigger was unbiased with respect to a decay mode of the η meson. The trigger identified 3 He ions by demanding large energy deposits in overlapping azimuthal sectors of the scintillator layers. In the case of the pd → 3 He η reaction the 3 He stops in the first layer of FRH, so the trigger included a veto on the signals from the second layer.…”

“…The trigger identified 3 He ions by demanding large energy deposits in overlapping azimuthal sectors of the scintillator layers. In the case of the pd → 3 He η reaction the 3 He stops in the first layer of FRH, so the trigger included a veto on the signals from the second layer. Since fusion to a 3 He represents only about 1% of the total cross section at this energy, the above conditions were sufficient to bring the trigger rate down to below a thousand events per second, which were recorded.…”

“…Forward-scattered tracks are reconstructed by using hit patterns in the FPC and matching them to signals in the scintillator layers. To separate 3 He ions from protons, deuterons, and charged π mesons, the energy deposited in the FWC is correlated with the energy deposited in the stopping layer. Once the 3 He ion is identified, the missing mass, MM( 3 He), can be calculated by determining the invariant mass remaining when the measured 3 He four-vector is subtracted from the known initial conditions of the beam and target.…”