Negative-pion trapping by a metastable state in liquid helium

Abstract

We have found a long-lived metastable state of stopped ${\mathrm{\pi }}^{\mathrm{-}}$ in liquid helium by measuring the time spectra of two different delayed products: (i) protons emitted after ${\mathrm{\pi }}^{\mathrm{-}}$ absorption by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ nuclei and (ii) 70-MeV electrons originating from free ${\mathrm{\pi }}^{\mathrm{-}}$→${\mathit{e}}^{\mathrm{-}}$ ${\nu }_{\mathit{e}}$¯ decay. The lifetime and fraction of delayed ${\mathrm{\pi }}^{\mathrm{-}}$ absorption obtained by using the emitted protons are 7.26±0.12 nsec and 1.66±0.05%, respectively. The free-decay fraction of a pion in liquid helium was obtained to be 0.64±0.03% from this result, which is consistent with the observed free-${\mathrm{\pi }}^{\mathrm{-}}$ decay fraction. These results imply that 2.30±0.07% of stopped ${\mathrm{\pi }}^{\mathrm{-}}$ are trapped in the metastable state which have an overall lifetime of 10.1±0.2 nsec. The same experimental and analysis were performed for stopped ${\mathrm{\pi }}^{\mathrm{-}}$ in liquid neon, where no evidence for trapping was found.