Study of Parity Nonconservation inpαScattering

Abstract

Parity nonconservation in^a scattering has been studied by comparing the cross sections a + and a" for longitudinally polarized 46-MeV protons of positive and negative helicity. The longitudinal analyzing power is found to be A M = (0.3 ± 1.3) x 10" 7 . This result, together with earlier measurements on parity nonconservation in££ scattering, gives new limits for the weak pion-nucleon coupling constant.PACS numbers: ll.30. Er, 25.40.Cm, 13.75.Gx, 24.70,+s According to present theory, the interaction between nucleons (jsfN interaction) contains a small contribution from weak interactions (hadronic weak currents) which can be described by weakcoupling constants associated with the exchange of various mesons (rr, p, a>, etc.). Experimentally, the weak NN potential can be detected because it leads to a small parity nonconservation in nuclear interactions. The presence of such effects has been clearly demonstrated, e.g., through the detection of circular polarization P y in the y decay of various nuclei, but there is insufficient information at present to determine the individual weak meson-nucleon coupling constants (see Haeberli 1 and Desplanques 2 for reviews).The work reported here is the first measurement of parity nonconservation in pa scattering. The experiment consists of comparison of cross sections a + and a" for logitudinally polarized proton beams of positive and negative helicity. Recent calculations 3 * 4 of the longitudinal analyzing power A z = (a + -a" )/(a + +a") predict values of the order of a few times 10" 7 and suggest that the contribution from the weak pion-nucleon coupling constant,/fr, should dominate A z . The value of f v is of particular interest because this coupling constant is strongly affected by the weak neutral currents in the theory of Weinberg and Salam. Study of the pot system is attractive because it is the simplest one available (apart from the pp and and np systems) and because its scattering states are relatively well known.The experimental technique was essentially the same as in our earlier investigation on pp scattering. 5 * 6 The arrangement is shown schematically in Fig. 1. Protons are scattered in a 100-bar He target whose walls are sufficiently thick (2-mm Al alloy) that charged particles from breakup reactions are stopped. Protons scattered by 23°-97° are detected in a hydrogen-filled (1-bar) cyclindrical ionization chamber which surrounds the target. The proton beam was provided by the Swiss Institute for Nuclear Research cyclotron which is equipped with an atomic-beam-type polarized-ion source. The arrangement in Fig. 1 is preceded by a spin-precession solenoid and a 47.6° deflection magnet, which, together, precess the vertical polarization of the beam from the cyclotron (± P y ) into a beam of longitudinal polarization ± P z or =F P Z , depending on the sign of the solenoid field. The helicity of the beam is reversed every 30 msec by switching rf transitions in the ion source. Every few hours the overall phase of P z is reversed by reversal of

Low-energy weak interactions of quarks

“…A second class of experiments deals with low-A targets, which are presumably amenable to reliable calculations. Three experiments in this class are measurements of the asymmetry in scattering of longitudinally polarized protons from deuterium [173]and 4He targets [174],and the resonant capture of a particles on deuterium leading to the 1-excited state of 6Li, a process which is forbidden if parity is conserved [175]. Current experimental results are compared with theoretical predictions in table 9.4, where it is seen that agreement is again quite reasonable.…”

A New Paradigm for Hadronic Parity Nonconservation and Its Experimental Implications

“…2. The longitudinal analyzing power for scattering 46 MeV polarized protons on a 4 He target was measured at PSI [49,50] A L ( pα…”

Neutron spin rotation measurements

“…, which need to be fixed by measurement. Several measurements from NN system to heavy nuclei were carried out but only some place constraints on HWI [11], which include p+p [12][13][14][15][16][17], p+ 4 He [18,19], P γ ( 18 F) [20][21][22][23], and A γ ( 19 F) [24][25][26]. In heavy nuclei measurements, an asymmetry enhancement is observed but the theory calculations are complicated [5,27].…”