Measuring Parity Violating Neutron Spin Rotation

Kangfei Gan

The George Washington Unversity

n

NNPSS ’07 20 Jul 2007

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

n

f h h NNg g g

Motivations

Mesons

N N

N N*Holstein. Weak Interactions in Nuclei

In Nucleon-Nucleon (NN) interactions, the weak interaction couplings are not well understood. DDHmodel (Desplanques et al, 1980)

Compared to strong interaction, the magnitude of weak interaction is very small.

710~

Direct exchange of short ranged weak vector bosons (W, Z) suppressed by repulsive strong interaction.

Theoretical descriptionn

Historically the PV nuclear interaction has been described by the DDH quark model => 6 weak meson exchange coupling constants: f , h

0 , h1 , h

2 , h0 , h

1

n-4He spin rotation in terms of weak couplings

*New “Hybrid” Effective Field Theory description is valid for ELab< 40MeV5 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

mradhhhhf /)(23.011.033.097.0 1010

PV Neutron Spin Rotationn

))(()()()()0( NnnnNnnNn SkEkSDkCSBAf

Forward scattering amplitude for low-energy neutrons:

)0(2

1n2

fk

NS

For 4He: = 0 )()()0( nnPNCPCnn kffkCAf

Index of Refraction of a medium:

Neutron’s phase as it passes through the medium:

zkfk n

)0(

21

2

PNCPNC

nPCPC

fz

zkfk

2

21

2

For a neutron polarized in the +y direction: zzy

2

1

z

ii

z

ii PNCPCPNCPC eeee

2

1

2

1

PV Neutron Spin Rotation

PVk

k

PC B

n

• PV rotation angle / unit length (dPV/dx) approaches a finite limit for zero neutron energy:

dPV/dx ~ 10-6 rad/m based on dimensional analysis

3x10-7 rad/m goal in n+4He

• dPC/dx (due to B field) can be much larger than dPV/dx, and is vn dependent

detectorASMPSM

z

y

x

PNCPNCPNC lf 42

Cross section of Spin Rotation Apparatusn

Side View

Polarizing Super Mirrorn

unpolarized beam

Polarized beam

Bmrad

• 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%

1.2 x 109

n/sec/cm2

3.1x 108

n/sec/cm2

Neutron flux

Input Coil

Beam Guide

Target Designn

LHe level sensor(1 per

chamber)

thermometry(1 per target

body)fluxgate

magnetometer(2 per target

body

LHe centrifugal

pump driveshaft

control strings:

drainpipes & magnetomet

ers

pi-coil

-Coil

• 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

-coilz

y

x

x

y

beam direction

n

x

y

-

• Moving LHe target about pi-coil isolates ϕPV from ϕPC signal that can be measured as an asymmetry in beam intensity between target states

BACK TARGET

PI-COIL

MAGNETIC SHIELDING

FLIPPING COIL

Polarimeter Designn

TOP VIEW

PSM ASM

Empty

FRONT TARGET

Empty

PV Material

PV Material

Flight of the Neutronn

L R

L R

polarizer passes neutrons with “up” vertical spin state into target

Flight of the Neutronn

L R

L & R neutrons experience Larmor precessionabout residual B-fields

R neutrons experience additional rotation due to NN weak interaction

L R

PC PV + PC

Flight of the Neutronn

L R

Pi-Coil precesses neutron spin 180o about vertical axis

L R

- PC - (PC + PV)

Flight of the Neutronn

L RL & R neutrons experience Larmor precession about residual B-fields

L neutrons experience additional rotation due to NN weak interaction with LHe

L R

-PC + (PC + PV)

= + PV

-(PC + PV) + PC= - PV

Flight of the Neutronn

• 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

FRONT TARGET BACK TARGET

PI-COIL

MAGNETIC SHIELDING

FLIPPING COIL

NN

NNsin

PSM ASM

Empty PV Material

PV Material Empty

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)

3 3

( 763.75 keV)

n He p H Q

Q

415

19

1415

1 2.5 103.84 10 /

1.6 10

( ) :

( / )2.6 10 ( / )

3.84 10 ( /

Ce C

x A

x C sx s

C

neutron ion pairsn;

So for a current of

n

n)

21 3 4

Layout (4 segmentations)

Graph by: Chris S. Blessinger