Phenomenology of strangeness-changing two-body currents in antineutrino scattering
Antineutrinos produce hyperons without kaons through Cabibbo-suppressed quasi-elastic scattering on a single nucleon. A two-body current acting on a correlated nucleon pair, nubar [NN] -> l+ Y N', gives the same hyperons plus a second nucleon. A companion paper constructs this current without fixing its size. StrangeMC, the only generator with the channel, set it to a flat 15% of the quasi-elastic hyperon cross section. We determine the ratio f = sigma_2p2h/sigma_QE of the two-body to the quasi-elastic hyperon cross section on carbon versus antineutrino energy, and the detector signatures of two-body events. The strange two-body current and its non-strange counterpart are computed on the same nucleon pair. Their ratio is converted into f with the Valencia non-strange 2p2h and quasi-elastic cross sections for carbon, assuming that nuclear effects missing from the pair are common to both. Under this assumption f rises from 0.14 at 0.6 GeV to 0.20-0.22 at 2-5 GeV. Averaged over the Booster antineutrino spectra it is 0.15-0.17, close to the 15% placeholder, which StrangeMC now replaces with f(E). The largest evaluated uncertainty comes from separating the two-body current from one-body production followed by rescattering: across the separation prescriptions explored, f at 1 GeV ranges from 0.12 to 0.22. Interference with one-body production is not calculated and could be as large as the two-body rate. In a simulated SBND antineutrino-mode exposure on argon with the carbon f, two-body events are about 11% of events that produce a strange hadron. They supply more than a quarter of the kaon-less Sigma+ events used to tag hyperon charge exchange, partly as Sigma+ made at the weak vertex. They reconstruct 12-30% low in energy under the quasi-elastic hypothesis. After final-state interactions half of them have a proton above 40 MeV at the vertex, against a fifth of quasi-elastic events.