1. 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

2. 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!

3. 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, (2005)

4. 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.

5. 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

6. Determine efficiency and wavelength from simulations:

7. 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 !

8. 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.

9. 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)

10. 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:

11. 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.

12. 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)

13. 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.

14. 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)

15. 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

16. 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.

17. 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

18. 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

19. 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

20. 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.