On the Λp enhancement near ΣN threshold

Abstract

New high statistics bubble chamber data on K −d → Λp π − at incident K − momenta between 680 and 840 MeV/ c were used to study the enhancement in the effective Λp mass near the ΣN threshold (∼2130 MeV). An analysis of the t-dependence of this effect shows that for minimum 4-momentum transfer | t| to the recoiling dibaryon system a narrow, symmetric peak with M = 2129.0 ± 0.4 MeV and Λ = 5.9 ± 1.6 MeV is observed. At larger negative t-values a broad asymmetric shoulder is seen with a sharp rise at ΣN threshold and extending to ∼2180 MeV. The broad structure is attributed to ΣN → Λp conversion. The dynamical origin of the peripherally produced narrow enhancement is discussed. The data favour strongly a 3S 1 state like the deuteron. In a simple coupled channel formalism it is shown that its interpretation as a Λp resonance is acceptable, but the mass and width parameters in this formalism need not correspond to the mass and width of the observed peak. Since the enhancement is found to be very close to ∑ +n threshold, an alternative explanation might be a threshold cusp in the elastic Λp scattering.