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Register a new userA measurement of parity-violating gamma-ray asymmetries in polarized cold neutron capture on 35Cl, 113Cd, and 139La
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An apparatus for measuring parity-violating asymmetries in gamma-ray emission following polarized cold neutron capture was constructed as a 1/10th scale test of the design for the forthcoming n+p->d+gamma experiment at LANSCE. The elements of the polarized neutron beam, including a polarized 3He neutron spin filter and a radio frequency neutron spin rotator, are described. Using CsI(Tl) detectors and photodiode current mode readout, measurements were made of asymmetries in gamma-ray emission following neutron capture on 35Cl, 113Cd, and 139La targets. Upper limits on the parity-allowed asymmetry $s_n cdot (k_{gamma} times k_n)$ were set at the level of 7 x 10^-6 for all three targets. Parity-violating asymmetries $s_n cdot k_{gamma}$ were observed in 35Cl, A_gamma = (-29.1 +- 6.7) x 10^-6, and 139La, A_gamma = (-15.5 +- 7.1) x 10^-6, values consistent with previous measurements.
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Parity-odd asymmetries in the electromagnetic decays of compound nuclei can sometimes be amplified above values expected from simple dimensional estimates by the complexity of compound nuclear states. In this work we use a statistical approach to estimate the root mean square (RMS) of the distribution of expected parity-odd correlations $vec{s_{n}} cdot vec{k_{gamma}}$, where $vec {s_{n}}$ is the neutron spin and $vec{k_{gamma}}$ is the momentum of the gamma, in the integrated gamma spectrum from the capture of cold polarized neutrons on Al, Cu, and In and we present measurements of the asymmetries in these and other nuclei. Based on our calculations, large enhancements of asymmetries were not predicted for the studied nuclei and the statistical estimates are consistent with our measured upper bounds on the asymmetries.
We report the first precision measurement of the parity-violating asymmetry in the direction of proton emission with respect to the neutron spin, in the reaction $^{3}mathrm{He}(mathrm{n},mathrm{p})^{3}mathrm{H}$, using the capture of polarized cold neutrons in an unpolarized active $^3rm{He}$ target. The asymmetry is a result of the weak interaction between nucleons, which remains one of the most poorly understood aspects of electro-weak theory. The measurement provides an important benchmark for modern effective field theory (EFT) calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is $A_{PV} = left( 1.58 pm 0.97 ~mathrm{(stat)} pm 0.24~mathrm{(sys)}right)times10^{-8}$, which has the smallest uncertainty of any parity-violating asymmetry measurement so far.
Significant progress has been made to experimentally determine a complete set of the parity-violating (PV) weak-interaction amplitudes between nucleons. In this paper we describe the design, construction and operation of the n$^3$He experiment that was used to measure the PV asymmetry $A_{mathrm{PV}}$ in the direction of proton emission in the reaction $vec{mathrm{n}} + {^3}mathrm{He} rightarrow {^3}mathrm{H} + mathrm{p}$, using the capture of polarized cold neutrons in an unpolarized gaseous $^3mathrm{He}$ target. This asymmetry has was recently calculated cite{Viviani,Viviani2}, both in the traditional style meson exchange picture, and in effective field theory (EFT), including two-pion exchange. The high precision result (published separately) obtained with the experiment described herein forms an important benchmark for hadronic PV (HPV) theory in few-body systems, where precise calculations are possible. To this day, HPV is still one of the most poorly understood aspects of the electro-weak theory. The calculations estimate the size of the asymmetry to be in the range of $(-9.4 rightarrow 3.5)times 10^{-8}$, depending on the framework or model. The small size of the asymmetry and the small overall goal uncertainty of the experiment of $delta A_{mathrm{PV}} simeq 1times10^{-8}$ places strict requirements on the experiment, especially on the design of the target-detector chamber. In this paper we describe the experimental setup and the measurement methodology as well as the detailed design of the chamber, including results of Garfield++ and Geant4 simulations that form the basis of the chamber design and analysis. We also show data from commissioning and production and define the systematic errors that the chamber contributes to the measured $A_{mathrm{PV}}$. We give the final uncertainty on the measurement.
We report the first observation of the parity-violating 2.2 MeV gamma-ray asymmetry $A^{np}_gamma$ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. $A^{np}_gamma$ isolates the $Delta I=1$, mbox{$^{3}S_{1}rightarrow {^{3}P_{1}}$} component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless EFT. We measured $A^{np}_gamma = [-3.0 pm 1.4 (stat) pm 0.2 (sys)]times 10^{-8}$, which implies a DDH weak $pi NN$ coupling of $h_{pi}^{1} = [2.6 pm 1.2(stat) pm 0.2(sys)] times 10^{-7}$ and a pionless EFT constant of $C^{^{3}S_{1}rightarrow ^{3}P_{1}}/C_{0}=[-7.4 pm 3.5 (stat) pm 0.5 (sys)] times 10^{-11}$ MeV$^{-1}$. We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.
We present measurements from the PHENIX experiment of large parity-violating single spin asymmetries of high transverse momentum electrons and positrons from $W^pm/Z$ decays, produced in longitudinally polarized $p$$+$$p$ collisions at center of mass energies of $sqrt{s}$=500 and 510~GeV. These asymmetries allow direct access to the anti-quark polarized parton distribution functions due to the parity-violating nature of the $W$-boson coupling to quarks and anti-quarks. The results presented are based on data collected in 2011, 2012, and 2013 with an integrated luminosity of 240 pb$^{-1}$, which exceeds previous PHENIX published results by a factor of more than 27. These high $Q^2$ data provide an important addition to our understanding of anti-quark parton helicity distribution functions.
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