n
Measuring Parity Violating Neutron
Spin Rotation
Kangfei Gan
The George Washington Unversity
NNPSS ’07
20 Jul 2007
n
Collaboration
C.D. Bass1, B.E. Crawford2, J.M. Dawkins1, T.D. Findley1, K. Gan3,
B.R. Heckel4, J.C. Horton1, C.R. Huffer1, P.R. Huffman5, D. Luo1, D.M. Markoff6,
A.M. Micherdzinska1, H.P. Mumm7, J.S. Nico7, A.K. Opper3, E. Sharapov8,
M.G. Sarsour1, W.M. Snow1, H.E. Swanson4, V. Zhumabekova9
Indiana University / IUCF 1
Gettysburg College 2
The George Washington University 3
University of Washington 4
North Carolina State University / TUNL 5
North Carolina Central University 6
National Institute of Standards and Technology (NIST) 7
Joint Institute for Nuclear Research, Dubna, Russia 8
Al-Farabi Khazakh National University 9
NSF PHY-0100348
Motivations
Compared to strong interaction, the
magnitude of weak interaction
is very small.
~ 10 7
Direct exchange of short ranged weak vector
bosons (W, Z) suppressed by repulsive
strong interaction.
N
N
Mesons
f  h  h



gNN g  g 
N
N
*Holstein. Weak Interactions in Nuclei
In Nucleon-Nucleon (NN)
interactions, the weak
interaction couplings are not
well understood. DDH
model (Desplanques et al, 1980)
Theoretical description
n
Historically the PV nuclear interaction has been described by the DDH
quark model
=> 6 weak meson exchange coupling constants: f , h0 , h1 , h2 , h0 , h1
n-4He spin rotation in terms of weak couplings


 0.97 f  0.33h0  0.11h1  0.23(h0  h1 ) rad / m
*New “Hybrid” Effective Field Theory description is valid for ELab< 40MeV
5 dimensionless Danilov parameters (related to the S-P scattering
amplitudes: 1S0-3P0, 3S1-1P1, 3S1-3P1 transitions)
and a long-range one-pion exchange parameter (proportional to the PV
pion-nucleon coupling constant h1)
*Liu, C.P., 2006, Parity Violating Observables of Two-Nucleon Systems in Effective Field
Theory, arXiv: nucl-th/0609078 v1 28 Sep 2006
Low Energy, Few Body Interaction measurements are useful
 Neutron Spin Rotation project
PV Neutron Spin Rotation
n
Forward scattering amplitude for low-energy neutrons:

 
 
  
f (0)  A  B( n  S N )  C ( n  k n )  D( S N  k n )  E ( n  (k n  S N ))

 
 
4
f (0)  A  C ( n  k n )  f PC  f PNC ( n  k n )
For He: S N = 0

Index of Refraction of a medium:
 2 
n  1   2   f (0)
k 
Neutron’s phase as it passes through the medium:


  1 
2

f
(
0
)
k n z
2
k

PNC
For a neutron polarized in the +y direction:

 2

f
PC  k n z
2
k


 2  z f PNC
PC  1 
)
1 iPC iPNC
e e

2
z


y
1

2

)
z
1 iPC iPNC

e e

2
 
z
z
)
PV Neutron Spin Rotation
n
 
  B  PC
y
z
x

k


 
  k  PV
PSM


ASM
detector

 PNC      2PNC  4   lf PNC
•
•
PV rotation angle / unit length (dPV/dx) approaches a finite limit for zero
neutron energy:
dPV/dx ~ 10-6 rad/m based on dimensional analysis
3x10-7 rad/m goal in n+4He
dPC/dx (due to B field) can be much larger than dPV/dx, and is vn dependent
n
Cross section of Spin Rotation Apparatus
Side View
Polarizing Super Mirror
n
unpolarized
beam
B
Polarized
beam
3.1x 108
n/sec/cm2
14 mrad
1.2 x 109
n/sec/cm2
Neutron flux
•
spin-dependent scattering from
magnetized mirrors
•
Alternating layers of magnetic surface
(cobalt) and absorptive layer (titanium
and gadolinium); 1mm separation;
Placed in 300 G permanent box.
•
Typical polarization: 98%;
transmission: 25%
Input Coil
Beam Guide
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Target Design
fluxgate magnetometer
(2 per target body
LHe centrifugal
pump driveshaft
LHe level sensor
(1 per chamber)
pi-coil
thermometry
(1 per target body)
control strings:
drainpipes &
magnetometers
-Coil
n
y
beam direction
z
-coil
x
y
y

x
 - 
x
•
a rectangular coil that produces a vertical
magnetic field in the path of the beam
•
wound to prevent field leakage beyond the coil
•
designed so that the spin of a typical cold neutron
will precess a total of  radians over the path of the
coil
Polarimeter Design
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MAGNETIC SHIELDING
PI-COIL
PSM
Empty
PV Material
PV Material
Empty
FRONT TARGET
BACK TARGET
FLIPPING COIL
TOP VIEW
•
Moving LHe target about pi-coil isolates ϕPV from ϕPC signal that can be
measured as an asymmetry in beam intensity between target states
ASM
Flight of the Neutron
n
polarizer passes neutrons with
“up” vertical spin state into target
L
R
L
R
Flight of the Neutron
n
L & R neutrons experience Larmor precession
about residual B-fields
R neutrons experience additional rotation due
to NN weak interaction
PC
L
PV + PC
R
L
R
Flight of the Neutron
n
- PC
L
R
L
- (PC + PV)
R
Pi-Coil precesses neutron spin 180o about vertical axis
Flight of the Neutron
n
-PC + (PC + PV)
= + PV
L
L
-(PC + PV) + PC
= - PV
R
R
L & R neutrons experience Larmor precession about residual Bfields
L neutrons experience additional rotation due to NN weak
interaction with LHe
Flight of the Neutron
n
MAGNETIC SHIELDING
PI-COIL
PSM
Empty
PV Material
PV Material
Empty
FRONT TARGET
BACK TARGET
FLIPPING COIL
•
Neutron spins are either parallel or antiparallel to the ASM
•
Parallel spins pass through ASM and enter 3He Ion Chamber detector
•
Asymmetry of count rate for flipping coil states & target states yields
spin rotation
sin  
N  N
N  N
ASM
Looking from downstream End.
Top view.
Segmented Ion Chamber Detector
•
•
•
•
High flux n beam, current mode detector
Not sensitive to gammas
High efficiency, low noise
Separate PV rotation (v independent) from NPV rotation (v dependent)
n  3 He  p  3 H  Q
(Q  763.75 keV)
1 neutron  2.5 104 ion pairs 
15

3.84

10
C / n;

19
e  1.6 10 C

So for a current of x( A) :
x (C / s )
14

2.6

10
 x (n / s)
15
3.84 10 (C / n)
Layout (4 segmentations)
1
2
3
4
Graph by: Chris S. Blessinger