| We completed the first measurement of the parity non-conserving neutron spin-rotation in helium, {dollar}varphisb{lcub}rm PNC{rcub}.{dollar} The PNC spin-rotation is sensitive to the isovector pion exchange amplitude, {dollar}fsb{lcub}pi{rcub},{dollar} in the meson exchange model that describes the weak NN interaction. The strength of the pion coupling is of particular interest because it probes the neutral current contribution to the weak NN interaction, and it is not well known.; We measured the PNC spin-rotation of cold neutrons transmitted through a liquid helium target located between a crossed polarizer and analyzer. We obtained the value {dollar}varphisb{lcub}rm PNC{rcub}{dollar} = (3.7 {dollar}pm{dollar} 6.6(stat) {dollar}pm{dollar} 1.0(syst)) {dollar}times{dollar} 10{dollar}sp{lcub}-7{rcub}{dollar} radians in a 46 cm target. We combined our results for {dollar}varphisb{lcub}rm PNC{rcub}(veceta + alpha){dollar} with the longitudinal analyzing power measured in the isospin mirror system, {dollar}Asb{lcub}L{rcub}(vec p + alpha){dollar} (LMM{dollar}sp+{dollar}86) to obtain a value for {dollar}fsb{lcub}pi{rcub}{dollar} that is insensitive to the contribution from the isoscalar couplings. We obtain a range, {dollar}-34 < fsb{lcub}pi{rcub} < 23{dollar} in units of the sum rule value, 3.8 {dollar}times{dollar} 10{dollar}sp{lcub}-8{rcub},{dollar} which is consistent with the predicted range of {dollar}0 < fsb{lcub}pi{rcub} < 30{dollar} and with previous experimental values. Although our results are limited by statistics, this is currently the most sensitive measurement of any PNC neutron spin-rotation. |
