The photon asymmetry in the reaction p(\vec{\gamma},\pi^{0})p close to threshold has been measured for the first time with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the S-wave and all three P-wave amplitudes. The low-energy theorems based on the parameter-free third-order calculations of heavy-baryon chiral perturbation theory for P1 and P2 agree with the experimental values.
Polarized photon beam.
The cross section and tensor analysing power t_20 of the d\vec{d}->eta 4He reaction have been measured at six c.m. momenta, 10 < p(eta) < 90 MeV/c. The threshold value of t_20 is consistent with 1/\sqrt{2}, which follows from parity conservation and Bose symmetry. The much slower momentum variation observed for the reaction amplitude, as compared to that for the analogous pd->eta 3He case, suggests strongly the existence of a quasi-bound state in the eta-4He system and optical model fits indicate that this probably also the case for eta-3He.
The spin-averaged amplitude squared is defined as follows: ABS(AMP)**2 = (P_deut/P_eta)*D(SIG)/D(OMEGA) and obtained by assuming the angular distributions to be isotropic. The errors in this quantity includes a contribution from Delta(P_eta). The statistical error of about 2% are added quadratically to the systemat ic error.
We report a new measurement of parity nonconserving (PNC) optical rotation on the 6p1/2- 6p3/2 transition in atomic thallium near 1283 nm. The result expressed in terms of the quantity R=Im{E1PNC/M1} is −(15.68±0.45)×10−8, and is consistent with current calculations based on the standard model. In addition, limits have been set on the much smaller nuclear spin-dependent rotation amplitude at RS=(0.04±0.20)×10−8; this is consistent with theoretical estimates which include a nuclear anapole contribution.
Spin of the Tl nucleus is 1/2.
The first measurement of incoherent η-photoproduction from the deuteron in the threshold region is reported. The experiment was carried out at the MAMI accelerator with the TAPS spectrometer. Total and differential inclusive cross sections have been obtained between 627 and 790 MeV. It is found that the reaction is completely dominated by the incoherent part. An upper limit for coherent η-photoproduction on the deuteron is deduced, which is substantially lower than the result from an earlier measurement. The incoherent cross section is reproduced in a participant-spectator approach under the assumption of an energy-independent ratio between the neutron and proton cross sections. Best agreement is found for the ratio σ n σ p ≈ 2 3 . The implications for the isospin components of the electromagnetic excitation of the S 11 (1535) resonance are discussed.
The helicity amplitudes A(1/2) = <S11|j(em)|nucleon> are measured.
We report a new measurement of parity nonconserving (PNC) optical rotation near the 1.28 μm, 6P1/2→6P3/2 magnetic dipole transition in thallium. We find the ratio of the PNC E1 amplitude to the M1 amplitude to be R=(−14.68±0.17)×10−8, which within the present uncertainty of atomic theory yields the thallium weak charge Qw(T205l)=−114.2±3.8 and the electroweak parameter S=−2.2±3.0. Separate measurements on the F=1 and F=0 ground-state hyperfine components of the transition yield R1−R0=(0.15±0.20)×10−8, which limits the size of nuclear spin-dependent PNC in Tl.
Spin of the Tl nucleus is 1/2.
New data on the ν ′ ν system produced in the π − p charge exchange at 38 GeV c momentum confirm the existence of the narrow ν ′ ν resonance X(1910). The hypothesis of the X(1910) quantum numbers being a J PC = 1 −+ , forbidden for the q[ovbar|q] meson, is proposed to explain the extremely low limits obtained for X(1910) decays through π 0 π 0 , νν and K 0 s K 0 s channels and the flat t -dependence of the X(1910) production cross section.
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Parity-violating optical rotation induced by the neutral weak-current interaction has been detected and measured for the first time in atomic thallium vapour. Accurate atomic calculations predicting the size of the rotation are available for this element; thallium also benefits from the Z3 enhancement of the effect. The magnetic-dipole transition 6p1/2-6p3/2 at 1.283 μm was excited using a single-mode semiconductor laser and the small optical rotation was measured using a sensitive polarimeter. The result, expressed in terms of the quantity R = Im E1p.v./M1, is - 12.5(19)10-8 and is consistent with recent calculations based on the standard model.
Spin of the Tl nucleus is 1/2.
A direct experimental reconstruction of the five complex pp elastic-scattering amplitudes has been performed at 447, 497, 517, 539, and 579 MeV. The reconstruction is done over the c.m. angles from 38° to 90° and is based on either 11 or 15 spin observables depending on the angular range. The reconstructed amplitudes are presented and compared to phase-shift analysis. A smooth energy behavior is observed for the amplitudes.
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The KS0KS0π0 system has been studied in the exclusive reaction π−p→KS0KS0π0n at 21.4 GeV/c. Evidence for the production of the f1(1285) and the η(1460) is presented. The η(1460) is produced away from minimum momentum transfer in the presence of nonresonant K*K (S-wave) production and phase-space background. The observed mass, width, and decay properties of the η(1460) are consistent with those attributed to the ι(1460) observed in radiative Jψ decay.
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We have made an improved measurement of the parity-nonconserving electric-dipole transition amplitude between the 6S and 7S states of atomic cesium. We obtain Im(EPNC)β=−1.576(34) mV/cm, which is in good agreement with the predictions of the standard model and earlier less precise measurements. This places more stringent constraints on alternatives to the standard model. We also see the first evidence of a nuclear-spin-dependent contribution to atomic parity nonconservation. The nuclear-spin dependence observed is in agreement with that predicted to arise from a nuclear anapole moment.
(7s)2S1/2:F=4 --> (6s)2S1/2:F=3 transition.
(7s)2S1/2:F=3 --> (6s)2S1/2:F=4 transition.
Average of (7s)2S1/2:F=4:MF=+-4 --> (6s)2S1/2:F=3:MF=+-3 and (7s)2S1/2:F=3:MF=+-3 --> (7s)2S1/2:F=4:MF=+-4 transitions.
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