# Presentation: 422150

## Measuring the PNC Spin-Rotation of Polarized Neutrons Traversing Liquid Helium

- C.Bass,  D.Luo,  H.Nann,  M.Sarsour,  W.Snow
*Indiana University*
- P.Huffman
*NIST*
- C.Gould,  D.Haase,  D.Markoff
*North Carolina State University*
- E.Adelberger,  B.Heckel,  H.Swanson
*University of Washington*

## Seminar Overview

- Weak NN-Interaction and the Meson-Exchange Model
- Spin-Rotation Observable
- Experimental Apparatus
- Project Status

## Nuclear Force:The Meson Exchange Model

- separation distance  <  0.8 fm:
- repulsive core best described by spin-spin interaction between constituent quarks
- separation distance  >  2 fm:
- one-meson exchange model yields excellent fit to data
- intermediate separation distances:
- various parameters need to be fitted by hand to both types of models

**N**

**N**

**N**

**N**

**STRONG**

**STRONG**

- exchange
- mesons

## Weak NN Interaction

- Z, W are massive (effective range 10-3 fm)
- the low energy weak interaction is essentially point-like
- NN-interaction is strongly repulsive at short distances:
- essentially no direct weak interaction at low energies
- weak PNC potential characterized by weak meson exchange coupling constants
- essentially all of the weak interaction physics is contained within the values of these coupling constants

**N**

**N**

**N**

**N**

- exchange
- mesons

**STRONG**
**(PC)**

**WEAK**
**(PNC)**

## How Big is the Effect?

## Which Mesons?

- At low energies, light mesons should dominate the PNC potential because of their longer ranges
- possible scalar and pseudoscalar exchanges are limited by Barton’s theorem:
- CP invariance forbids coupling between J=0 neutral mesons and on-shell nucleons
-  , 0, , and  0 exchanges dominate the low energy PNC potential
- the weak meson exchange coupling constants:
*f*, *h*0, *h*1, *h*2, *h*1, *h*0, *h*1

## Meson Exchange Coupling Constants

- theoretical calculations of these coupling constants limited by uncertainties with quark model
- 6 independent coupling constants require 6 independent experiments
- the number of parameters can be reduced to 2 combinations of the couplings that dominate the observables:
- f, and ( *h*0   +  0.6 *h*0).
- experimental uncertainties are somewhat increased by allowing for variations of the four minor degrees of freedom:
*h*1, *h*2, *h*1 and residual in *h*0

## Weak meson-nucleon couplings constants

| h1 | h0 | h2 | h1 | h0 | f |  |  | Coupling |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| -1.9  0.8
 | -10.3  5.7
 | -11.0  -7.6
 | -0.38  0
 | -31  11.4 | 0  11.4 | (DDH) | range |  |
| -1.1 | -1.9 | -9.5 | -0.19 | -11.4 | 4.6 | (DDH) | “best value” |  |
| .-2.2 | .308 | -6.8 | 0.38 | -8.4 | 1.1 | (DZ) | value | Theoretical |
| -3.8  -1.1
 | -10.6  2.7
 | -9.5  -6.1
 | -1.1  0.4
 | -31  11
 | 0  6.5 | (FCDH) | range |  |
| -2.3 | -4.9 | -6.8 | -0.4 | -3.8 | 2.7 | (FCDH) | “best value” |  |
| -2.3 | -6.5 | -6.8 | -0.4 | -6.1 | 2.7 | (D) | value |  |
| -1.0 | -3.8 | -3.8 | -0.02 | -1.9 | 0.19 | (KM) | value |  |
| -0.6 | -4.9 | -7.6 | -0.2 | -5.7 | 2.3 |  | best fit | Experimental |
| -1.9  -0.8
 | -10  5.7
 | -11  -7.6
 | -0.4  0.0
 | -31  11
 | 0  11
 |  | range |  |

- _Weak meson-nucleon couplings constants_

## Experimental Constraints on Weak Meson Exchange Constants

- _Experimental Constraints on Weak Meson Exchange Constants_

## Optical Spin-Rotation

- polarized photons propagating through a “handed” medium undergo spin-rotation:

- cold neutrons propagating through spin-0 nuclei experience a similar rotation of the spin-polarization vector, but the “handedness” is the weak interaction

## Neutron Optics

## Neutrons Traveling Through Helium

## Spin Rotation Observable

## Experiment Concept

- cold neutrons are transversely polarized
- neutrons travel through a helium target
- PNC spin-rotation
- PC spin-rotation
- background B-field in target region
- need to maximize PNC signal and minimize PC signal
- Baxial = 0.5 Gauss  MAG ~ 10 rad/m,
- magnetic shielding  Baxial < 100 Gauss
- neutrons enter the analyzer
- transmitted neutron flux contains information about the PC and PNC spin-rotation
- goal of experiment:
**** 2 **** 10-7 rad/m  sensitivity

- 

-  pn

- pn

***l***

- PC + PNC

- 

-  B

## Experiment Overview

**neutron**
**flux**
**detector**

**analyzer**
**(SM)**

**polarizer**
**(SM)**

**guide**
**tube**

**input**
**coil**

**output**
**coil**

**pi-coil**

**front**
**target**

**rear**
**target**

**inner**
**mu-metal**
**shield**

**outer**
**mu-metal**
**shield**

**LHe**
**cryostat**

**neutron**
**beam**

- _Experiment Overview_

## Neutron Beam

- NG-6 beamline at NIST (Gaithersburg, MD)
- energies in the 10-3 eV range ( ~ 5A)
- beryllium filters provide high-energy cut-off
- essentially 0% transmission below 3.4A
- approx. 4% between 3.4A and 3.9A
- about 90% above 3.9A

## Supermirror Polarizer and Analyzer

- neutrons are polarized through spin-dependent scattering from magnetized mirrors
- one spin-state is preferentially reflected by the mirror surface while the other state is transmitted and absorbed
- designed to pass neutrons with the “up” spin state in the vertical direction
- typical polarization: 98%

## Input Coil

- spins precess about aligned vertical fields as the neutrons pass adiabatically through the input coil
- neutrons reach a current sheet at the back of the coil and pass non-adiabatically into the field-free region

## Magnetic Shielding

- mu-metal shielding surrounds the target region (including cryostat)
- solenoidal coils inside shielding further reduces any residual axial B-fields

## -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

- 

- -coil

- z

- y

- x

- x

- x

- y

- y

- -

- beam direction

## Helium Target and Operation

- TOP VIEW

- cold
- neutron
- beam

- cold
- neutron
- beam

## Output Coil

- neutron spins pass non-adiabatically through front of output coil
- transverse component of spin adiabatically rotated into a horizontal B-field (y-axis)
- the orientation of this (y-axis) B-field is flipped at a rate of ~ 1 Hz
- spins then adiabatically rotated into the vertical (x-axis) direction of the analyzer
- neutrons spins are now either parallel or antiparallel to the analyzer (depending on the target state and the orientation of the y-axis B-field)

## 3He Neutron Detector

- neutrons detected through the following reaction:
- n **** 3He  3H **** 1H
- charged reaction-products ionize the gas mixture
- high voltage and grounded charge-collecting plates produce a current proportional to the neutron flux

## Previous Version of Experiment (1996)

- reached a sensitivity of ~2.6x10-6 rad/day of accumulated data
- limited by statistics
- systematic limits of the apparatus not reached
-  PNC(n,)  (8.014[stat]  2.2[syst])  10-7 rad/m

## Redesign of Experiment

- increase available statistics by improving reliability and decreasing downtime
- increase the detected beam flux
- 	(NIST reactor upgrade: factor ~1.5)
- use of superfluid helium
- additional layer of mu-metal shielding
- want a factor of x10 higher sensitivity in order to obtain a non-zero / null result:
- 	  ~ 0.610-6 rad / day of accumulated data
- estimate  ~30 days of data for desired sensitivity

## New Target

- use of superfluid helium (~1.7K)
- lower temp requires additional refrigeration: 1K-Pot
- superfluid leaktight
- non-magnetic and non-superconducting materials
- stainless steel won’t work
- new electrical feedthroughs (epoxy resin based)
- liquid helium valve

- pi-coil

- front
- target

- back
- target

- 1K-pot
- (evaporation
- refrigerator)

- electrical
- feedthroughs

- LHe valve

- (surrounding canister
-  not shown for clarity)

## More Shielding

- installation of 3rd layer of magnetic shielding: Cryoperm-10
- preliminary B-field mapping inside all three nested shields:
- measured ~50 Gauss in target region without solenoidal coils
- previous version designed for 100 Gauss background
- want to further reduce this by 1/2 with trim coils

## Current Status

- field mapping of in/output coils and magnetic shielding
- analysis of systematic effects
- computer simulations
- new target ready for machining
- machining of target components
- run at NIST in fall 2003