The n 3 He Experiment Probing the Hadronic Weak Interaction Abstract: Although QCD has had tremendous success in describing the strong interaction at high energy, the structure of nuclear matter remains elusive due to the difficulty of QCD calculations in the low energy frontier. Thus nuclear structure has typically been explored through electromagnetic interactions, like electron scattering. The hadronic weak interaction (HWI) is an attractive alternative because it involves only nucleons, but the weak component is short-range and precisely calculable at low energies. While the HWI is dominated by the strong force by a factor of 10 7, it can be isolated due to its unique property of parity violation (PV). N 3 He is a precision experiment designed to measure the proton asymmetry through the reaction n + 3 He  p + t. G.L. Greene, S. Kucuker University of Tennessee V. Gudkov, Y. Song Unviersity of South Carolina A. Barzilov, I. Novikov Western Kentucky University S. Baessler University of Virginia M. Viviani Istituto Nazionale di Fisica Nucleare, Sezione di Pisa Calarco University of New Hampshire M.A. Brown, C.B. Crawford, E. Martin University of Kentucky J.D. Bowman, S. Penttila Oak Ridge National Laboratory P.N. Seo Triangle Universities Nuclear Laboratory L. Baron Universidad Nacional Autónoma de México M.T. Gericke, S. Page, WTH. Van Oers, R. Mahurin, V. Tvaskis, M. McCrea, D. Harrison University of Manitoba Parity Violation The weak interaction is 10 7 times smaller than its strong counterpart. However, experiments can probe this small component of the hadronic interaction by observing a unique property of it, parity violation (PV). Weak interactions look different under spatial inversion (looking at them in a mirror.) The goal of the n 3 He is to determine the single-spin proton asymmetry in the reaction. The asymmetry is evident in the direction of the proton emission with respect to the polarity of the incoming neutron. Studying such a PV circumstance will shed light on the Hadronic Weak Interaction. Hadronic Weak Couplings In the DDH meson exchange model, the strength of the HWI is specified by coupling constants at the vertex where (when) an exchange meson is emitted or absorbed. The fundamental weak interaction occurs at the vertex. There are six unique couplings characterized by the type of meson exchanged and details of the vertex. By investigating the many different hadronic nuclear reactions with varying sensitivities to these couplings, experimental values can be obtained to test the DDH theory and eventually the EFT theories once the calculations have been completed in that context. The n 3 He experiment is one of the experiments which will allow for values of the coupling constants to be obtained. Once a number of the HWI experiments have been completed, you can develop a system of equations involving the asymmetries along with the coupling constants and their theoretically calculated counterparts (Ex. Below). Spallation Neutron Source The SNS, located at Oak Ridge National Laboratory, is an intense neutron beam produced by pulsing a high energy proton beam on a mercury target. The velocity, energy, and wavelength can all be extrapolated from the TOF of the neutrons in the 60 Hz pulse structure. The energy spectrum of the neutrons is nearly thermal, slightly higher than the temperature of the LH 2 moderator, located prior to the guide for the FnPB where the n 3 He experiment will be conducted. Experimental Setup RF Spin Rotator N 3 He is using a spin flipper with transverse windings which allows for both longitudinal and transverse spin rotation. It is being developed at UK based on calculations with the magnetic scalar potential. For this experiment, longitudinal polarized neutrons are required which called for the change from the NPDGamma RFSF. The spin rotator is based on NMR, where the neutron spin precesses at the Larmor frequency around a magnetic field. The spin rotator creates an RF field B RF which rotates in resonance with the Larmor frequency making it appear static, thus causing the neutron spin to also precess around this field, rotating the spin 180 o. The RFSF is ramped inversely proportional to the time of flight of the neutrons within the flipper to ensure that all the neutrons within a pulse are rotated efficiently. 3 He Target / Ion Chamber The target/ion chamber will serve both as an unpolarized 3 He target and an in situ detector of the proton current as a function of emission direction. As neutrons capture on the 3 He they create an excited 4 He nucleus which then decays into a proton and triton. Both particles ionize the gas as they travel. The negative ions collect on the sense wires at ground, while the positive ions travel to the high voltage field wire. The asymmetry is detected in current mode by looking at the where the ion current is greater. If the current is higher downstream, then the proton was emitted in the forward direction. Uncertainties Systematics Beam Fluctuations RFSF Efficiency Polarization Alignment (beam, field, chamber) PV was discovered by C.S. Wu in 1957, by observing a correlation between the polarization of Co nuclei and the direction of beta emission. TOF Spectrum Ionization distribution for a single capture event due to the proton carrying 3 times as much energy as the triton and depositing its energy at the end of its track Statistical The statistical uncertainty is dependent on the detector efficiency, the neutron flux, and the polarization. N = 2.2x10 10 n/s x 10 7 s P = 96.2% σ d = 6 10 Gauss solenoid RF spin rotator 3 He target / ion chamber supermirror bender polarizer (transverse) FnPB cold neutron guide 3 He Beam Monitor transition field (not shown) shim coils (not shown)