# Presentation: 832567

## Polarized Cold Neutron Capture on the Proton: 

*Precision Measurement of Parity Violation in *
*Polarized Cold Neutron Capture on the Proton*:
- the  **N P D ******  Experiment
- at the **L**os **A**lamos **N**eutron **S**cience **C**enter
**Bernhard Lauss**
**UC Berkeley**
- for the NPDGamma Collaboration

- Design

*PANIC’05 Conference*
*Santa Fe, NM*
*October 24-28, 2005*

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## What kind of physics do we study in NPD ?

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## The Process

- two-body interaction

- We study is the *hadronic weak interaction** *between
*spin-polarized neutrons and protons* in the
- reaction.       E = 2.2 MeV

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## The Process

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- flip n-spin

- Weak-Interaction violate parity
- If the up/down γ rates differ, parity is violated !

- NPDG measures *A****, *the parity-violating asymmetry in the distribution of emitted ’s.

- strength of strong / weak interaction ~ 10-8

- We study is the *hadronic weak interaction** *between
*spin-polarized neutrons and protons* in the
- reaction.       E = 2.2 MeV

## Simple Level Diagram of n-p System                       

- Simple Level Diagram of *n-p* System

- Low-energy
- continuum states

- Bound states

- M1 (PC)

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- capture

## Simple Level Diagram of n-p System                       

- Simple Level Diagram of *n-p* System

- Low-energy
- continuum states

- Bound states

- M1 (PC)

-   Weak interaction mixes in *P* waves to the singlet and triplet *S*-waves in initial and final states.
-   Parity conserving transition is *M*1.
-   Parity violation arises from mixing in *P* states and interference of the *E*1 transitions.
-   *A* is coming from 3*S*1 - 3*P*1 mixing and interference of *E*1-*M*1transitions - *I * = 1 channel.

- Mixing amplitudes:

- is primarily sensitive to the ∆*I *= 1 component of the weak interaction

-  exchange
-  exchange

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- capture

## The Hadronic Weak Interaction

- W and Z boson exchange

- Nucleon interaction takes place on a scale of 1 fm -- short range repulsion. Due to the heavy exchange particles, the range of W± and Z0 is 1/100 fm, weak interaction probes quark-quark interaction and correlations at small distances.
- At low energies  N-N weak interaction modeled as meson exchange with one strong PC vertex, one weak PV vertex.
- classical
- The weak PV couplings contribute in various mixtures and a variety of observables:

**DDH - Model**
- Desplanque, Donohue, Holstein 1980

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- W and Z boson exchange

## The Hadronic Weak Interaction

- W and Z boson exchange

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- new model independent  EFT approach by *Ramsey-Musolf, Holstein, van Kolck, Zhu and Maekawa*
- describes processes in terms of low-energy constants/amplitudes  describing short-range force and pion interaction (EFT: 5 low-energy PV amplitudes  without explicit pions , 8 with explicit pions) - calculate these from first principles

- Nucleon interaction takes place on a scale of 1 fm -- short range repulsion. Due to the heavy exchange particles, the range of W± and Z0 is 1/100 fm, weak interaction probes quark-quark interaction and correlations at small distances.
- At low energies  N-N weak interaction modeled as meson exchange with one strong PC vertex, one weak PV vertex.

## Constraints on Weak N-N Coupling

-   **Negligible (less than 1%)  **
**    ****contributions from **
**    ************ ****exchanges**
-   **No uncertainty from nuclear**
**    ****wave functions**

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- DDH
- EFT

## Constraints on Weak N-N Coupling

- anapole moment

- photon polarization

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- contradictory results

## Constraints on Weak N-N Coupling

- anapole moment

- photon polarization

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- contradictory results

## Constraints on Weak N-N Coupling

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-                                                                                   f - 0.12 h - 0.18 hx 107

## Typical Measurement

- Cl

**NPDG**
**GOAL = 10****-8   ****!!!**

## How do we perform the measurement of the npd  process ? 

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## Neutron Source 

**20 Hz pulsed **
**neutron beam**

**~ 6 x 10****8**** cold neutrons per 20 Hz pulse out of the end of the 21 m supermirror guide**

**FP12 views a cold hydrogen moderator in backscattering geometry**

- Liquid H2 coupled moderator:
- a) liquid H2      b) H2O partially coupled
- c) Be-reflector  d) Pb-reflector

**NPDG cave**

- LANSCE

- Neutron Source

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## Frame Definition Chopper

-   Pulsed beam: neutron time-of-flight determines neutron velocity, energy
-   PV asymmetry is independent of energy
-   Very slow neutrons can overlap with faster neutrons from later pulse
-   Chopper rotor coated with Gd2O3 absorbs slow neutrons up to 30 meV, opens window for faster ones
-  up to 1200 RPM
-  settings: opens with n-pulse onset             4 ms later open , closes after 30 ms, 4ms later totally closed

- 22m

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## Experiment

- Setup

- Cave

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- beam

## Experiment  Setup

- B

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## Experiment  Setup

- Guide Field

- whole setup contained in 10 Gauss
- guide field to prevent Stern-Gerlach
- steering of neutrons
- requires gradient  1 mGauss / cm
- or smaller

**B**

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## Neutrons are polarized by Optically-Polarized 3He Spin Filter

- ln(1/T) is a linear function of tof

- 3He neutron spin filter:
- In a 3He cell Rb atoms are polarized by laser light.    Through spin exchange, 3He gas is nuclear polarized.
- neutron capture cross section of the 3He singlet state is much larger than the triplet state.     (104 difference)
- Therefore, neutrons with spin antiparallel with 3He spins are absorbed and neutrons with spin parallel with 3He spins are transmitted  neutron spin filter

- Neutrons are polarized by Optically-Polarized 3He Spin Filter

- 12 cm

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## Spin Flipper

- In a DC magnetic field Bo, a resonant RF magnetic field (B1cos**t) is applied for a time =1/B1, to precess the neutron spin, around B1, by Hz pulse pattern
- B1(t) 1/TOF, for reversing neutron spin in wide energy range (~0.5-50 meV).
- RF spin flipper is the main control of systematic errors. Spin flip sequence is           “  ****** ******** ******** ******** ******** ******** ******** ******  ” .
- Grad.Bz/  z  < 1 mgauss/cm  no Stern-Gerlach steering force (.B)   no false asymmetry.
- High maximum spin reversal efficiency for 0 < En < 100 meV, ~ 95% for En = 4 meV

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- Flipper coil

## Beam Monitors

- 3 beam monitors used to measure
- 	i) neutron flux out from the guide
- 	ii) beam polarization                                                              	iii) ortho/para ratio in LH2 target
- n+ 3He      p + t + 765 keV    ionizes gas mixture [3He +4He(~.5 atm) +N2(~.5 atm)]
- Ratio of 3He to 4He  (~5% to 100%),
- abs(3He ) >> abs (4He )

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## 20-liter Liquid Para-Hydrogen Target

- To maintain neutron spin in scattering a para- hydrogen target is required.
- The 30 cm in diameter and 30 cm long target captures 60% of incident neutrons.
- At 17 K only 0.05% of LH2 is in ortho state  1% of incident neutrons will be depolarized.
- Target cryostat materials selected so that false asymmetries < 10-10.

- Neutron mean free paths at 4 meV in
- - ortho-hydrogen is = 2 cm,
- - para-hydrogen is = 20 cm
-  for a *n-p* capture is = 50 cm.

- useful range 1-15 meV

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## 20-liter Liquid Para-Hydrogen Target

- - presently being tested
- - has to be LANL safety commissioned
- - ready end of 2005

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## CsI detector array

- spin flipper

- beam

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- 5x107 ’s/pulse are expected into the detector array   Detection in current mode. --- Electrical noise kept significantly smaller than counting statistics / use sum + difference amplifier
- The 3 detector array employs 48 CsI (Tl) scintillator crystals (15x15x15cm3), each coupled with a 3-inch vacuum photo-diode.
- Gain provided by low noise solid-state preamplifiers. Gains are magnetic field insensitive.
- Interaction length of a 2.23 MeV ray in CsI ~ 5.5 cm. ~95% of  ’s stop in 15 cm.

## CsI detector array

- 5x107 ’s/pulse are expected into the detector array   Detection in current mode. --- Electrical noise kept significantly smaller than counting statistics / use sum + difference amplifier
- The 3 detector array employs 48 CsI (Tl) scintillator crystals (15x15x15cm3), each coupled with a 3-inch vacuum photo-diode.
- Gain provided by low noise solid-state preamplifiers. Gains are magnetic field insensitive.
- Interaction length of a 2.23 MeV ray in CsI ~ 5.5 cm. ~95% of  ’s stop in 15 cm.

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## First Results 

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-  engineering materials check
-  study of hadronic weak interaction in
-   atoms with A ~ 50 (experiment is running)

## First Results 

**thanks to PhD students **
**M.Dabagian & R.Mahurin**

-  engineering materials check
-  study of hadronic weak interaction in
-   atoms with A ~ 50 (experiment is running)

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## Summary

-  npd is ready end of this year for production data

-  2006 @ LANSCE   A < 10-7

-  move to SNS
- 	start data taking in 2008
-    	->    A <  1 ·10-8 at FNPB

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## The NPD Collaboration 

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-  J.David Bowman,1 Roger D. Carlini,2 Timothy E. Chupp,3 Wangchun Chen,4, Silviu Corvig,6 Mikayel Dabaghyan,6 Dharmin Desai,7 Stuart J. Freedman,8 Thomas R. Gentile,5 Michael T. Gericke,9 R. Chad Gillis,9 Geoffrey L. Greene,7,10
- F. William Hersman,6 Takashi Ino,11 Takeyasu Ito,7 Gordon L. Jones,12 Martin Kandes,3 Bernhard Lauss,8 Mark Leuschner,4 Bill Losowki,13 Rob Mahurin,7 Mike Mason,6 Yasuhiro Masuda,11 Jiawei Mei,4 Gregory S. Mitchell,1 Suguro Muto,11 Hermann Nann,4 Shelley Page,9 Seppo Pentilla,1 Des Ramsay,9,14 Satyaranjan Santra,15 Pil-Neyo Seo,16 Eduard Sharapov,17 Todd Smith,18 W.M. Snow,4 W.S. Wilburn,1 Vincent Yuan,1 Hongguo Zhu,6
-       1 Los Alamos National Laboratory, Los Alamos, NM 87545
-       2 Thomas Jefferson National Accelerator Facility, Newport News, VA 23606
-       3 Dept. of Physics, Univ. of Michigan, Ann Arbor, MI 48109
-       4 Dept. of Physics, Indiana University, Bloomington, IN 47408
-       5 National Institute of Standards and Technology, Gaithersburg, MD 20899
- 6 Dept. of Physics, Univ. of New Hampshire, Durham, NH 03824
-        7 Dept. of Physics, Univ. of Tennessee, Knoxville, TN 37996
-       8 Univ. of California at Berkeley, Berkeley, CA 94720
-       9 Dept. of Physics, Univ. of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canad
- 10 Oak Ridge National Laboratory, Oak Ridge, TN 37831
-      11  High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan
-       12 Dept. of Physics, Hamilton College, Clinton, NY 13323
-       13 Indiana University Cyclotron Facility, Bloomington, IN 47408
-        14 TRIUMF, Vancouver, British Columbia V6T2A3 Canada
-      15 Bhabha Atmoic Research Center, Mumbai, India
-       16 Dept. of Physics, North Carolina State University, Raleigh, NC 27695
- 17 Joint Institute of Nuclear Research, Dubna, Russia
- 18 Dept. of Physics, Univ. of Dayton, Dayton\\%, OH 45469-2314

## Thanks

**for your attention !**

*PANIC’05 BL*

## Systematic Issues 

- activated materials - emit s in -decay
- Stern-Gerlach steering
- L-R asymmetry
*n - p *elastic scattering
- n - p parity allowed asymmetry
- Mott-Schwinger scattering
- Instrumental sources
- electronics, stray magnetic fields, gain stability
- Monitoring:
- Null test at *E*n > 15 meV and at end of each pulse.

- Systematic Issues

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