The Parity Violating Longitudinal Asymmetry in Polarized Cold Neutron Capture on Helium 3 J.D. Bowman, S.I. Penttil¨a Oak Ridge National Laboratory R. Carlini Jefferson National Laboratory M. Gericke, S.A. Page University of Manitoba C. Crawford University of Kentucky V. Gudkov University of South Carolina J. Martin, A. Micherdzinska University of Winnipeg C. Gillis Indiana University C .Gould NC State University M. Viviani INFN, Sezione di Pisa Anna Hayes, Gerry Hale  Los Alamos National Laboratory

What are the possible Observables Spatial asymmetry in capture locus: Really small Gamma asymmetry: Very small branching ratio essentially the same as Directional asymmetry in the number of tritons Nominally the same Directional asymmetry in the number of protons But protons have a factor of 3 larger range!

Initial Considerations For the Chamber Chamber mostly filled with Helium 3 Want to let protons range out Proton range rp~5.5 cm Neutron mfp should be < rp / 2 Optimal wavelength range > 4.7 A Will have 6.5 x 1010 n/s Optimal P.R. Huffman et al. NIST J. Res. 110, 161-168 (2005)

Principle of Measurement Measure the number of forward going protons in a 3He wire chamber: wires run vertical or horizontal signal wire may be summed to reduce electronics (use NPDGamma ADCs) rotate chamber around vertical axis to make transverse asymmetry tests no crossed wire: keep the field simple to avoid electron multiplication (non-linearities) Largest sensitivity at forward angles and for captures toward the back of the target.

majority of neutrons capture in the first few wire planes, where the sensitivity is small -- this would be a self-normalizing detector chamber large enough let protons range out further into the target only forward going protons are detected (longitudinal asymmetry) need at least two pulses to form an asymmetry

Determine efficiency and wavelength from simulations:

Statistics We see 9 x 1016 neutrons/sec in a 1 x 107 sec run A 7% measurement in the best case ! A 40% measurement in the worst case scenario !

Systematics The measurement will be done by reversing the neutron spin direction in fixed detector geometry and observing the change in the detector yield. Cartesian invariants that do not involve the spin, do not produce detector asymmetries.

Estimated Size Of The Asymmetry Asymmetry from mixing in the intermediate bound states (RMS width estimate) has a resonance at Parity mixing can occur with the resonance at G.E. Mitchell et al. Phys. Rep. 354, 157 (2001)

Asymmetry contribution from the the scattering states How big is this? Bunakov and Gudkov, Nuclear Physics A401 (1983) Asymmetry contribution from the the scattering states Scale down from p-p scattering results at 22.5 MeV:

It sure would be nice to have real calculation and make a connection with the NN theory Weak Pion-Nucleon Coupling 1) Michele Viviani (PISA) : Calculation of the scattering WF and S matrix elements 2) Anna Hayes: No-core shell model calculation with AV8 potential, etc.

Relation to the theory couplings We establish the dominant contribution to the asymmetry, from the underlying coupling constant as follows: Spin and Iso-spin S(I) of the system: n = 1/2(1/2) 3He = 1/2(1/2) p = 1/2(1/2) T = 1/2(1/2) But each couples to intermediate 0+(0) 4He states, which leads to the possible transitions 1S0 (I = 0) <-> 3P0 (I = 0) Within pion-less effective field theory: Iso-spin mixing adds a ~10% I=1 contribution (Gerry Hale - qualitative)

The DDH Meson Coupling Model Pion-Nucleon is just one of a number of couplings involved. The measured asymmetries can be expanded in terms of 6 weak meson-nucleon coupling constants: One way to study the weak N-N interaction is with experiments using low energy neutrons.

Pion (Chiral-PT) and Pion-less EFT Models Calculations produce new potentials with various couplings Short range O(q) (Pion-less  ~ 140 MeV: , ) Long range O(1/q) (Chiral PT based) DDH Yukawa analog Next to leading order terms (Chiral PT based) Short range O(q) (With Pions  ~ 1 GeV , ) Medium range O(q) (Two Pion exchange: , ) Long range O(q) (Loop correction: ) Two body current operators (PV processes with photons ( ): ) Specific sensitivity of observables to many of these couplings has yet to be calculated. and contribute to all processes: SR O(q) < < LR O(1/q)

Motivation This experiment constitutes a measurement in a few body system It largely isolates the I=0 component which has never been measured in isolation The asymmetry is large It could produce the most accurate measurement in a calculable system

Backgrounds gamma background from neutron capture on other materials: wire chamber very insensitive to gamma background unless the gammas produce Compton scattered eletrons in the walls and wires need to simulate beta decay in activated aluminum walls: the effect was estimated to be very small for the NPDGamma experiment -- of most concern is the front (entrance) window and the beta electrons won't travel beyond the first wireplane.

More on Systematics 3He polarization and strong spin dependence: 3He may become polarized and you get more protons for one spin state than the other – if you only look for forward going protons this looks like a proton asymmetry need to counter with holding field reversal and possibly by cycling the 3He to depolarize it. Fractional change of neutron kinetic energy in the spin flipper Treated in NPDGamma spin flipper paper (Pil-Neyo Seyo et al. Unpublished) One can get rid of this by properly designing the spin flipper N+he reaction polarizes he3

More on Systematics Stern Gerlach Over the path of the neutron, between the RFSF and the wire chamber, we need: or: less severe than for NPDGamma The effect causes a change in neutron velocity along the beam. Transverse Polarization transverse polarization from super mirror polarizer if B is not parallel to the magnetization use the same holding field coil to magnetize the SM (in transverse mode) transverse polarization will precess en-route to the chamber and de-phase

Wire Plane Alignment Scheme Transverse polarization is a problem if the wire planes are misaligned with the holding field in the chamber, as this introduces contributions from PA left-right asymmetries, Stern-Gerlach, and Mott-Schwinger. 1) measure the average at two different places along the beam using the wire chamber 2) align the B field parallel to 3) align the wire planes to be perpendicular to the holding field (same as ) to 2 degrees by dead reckoning 4) rotate the chamber by 180 degrees about the holding field and measure again to cancel small residuals Use a magnetic compass which can measure the field direction to 0.1 deg

Budget For a neutron beam that is initially polarized in the transverse direction the holding field budget is probably an over estimate. More money may be needed for the DAQ, depending on the wire summing scheme.