# Presentation: 307274

## First Results from the

**UCNA Experiment**

- Brad Plaster, University of Kentucky

- LANL P-25 — November 5, 2008

- Leah Broussard (Duke/TUNL)
- Kevin Hickerson (Caltech)
- Adam Holley (NCSU)
- Russ Mammei (Virginia Tech)
- Michael Mendenhall (Caltech)
- Robert Pattie (NCSU)

- Raymond Rios (Idaho/LANL)
- Anne Sallaska (U Washington)
- Riccardo Schmid (Caltech)
- Sky Sjue (U Washington)
- Junhua Yuan (Caltech)
- Yanping Xu (NCSU)

- Several New Collaborators as of 2008 Run

## This talk

_*Not*__ _ talk about

## Neutron β-decay

## Correlation coefficients

## Neutron lifetime

- “Master Formula”

- Marciano and Sirlin (2006)

- 1.0389 ± 0.0004

- (0.0008)

- δ(Vud) = ± 0.0002

## Status of A and 

**Published**

- 0.28%

## Status of n and neutron Vud

- Serebrov et al. (2005)

*Still no new (published) results since*

## Status of Vud

- 0+  0+

- neutron PDG

- nuclear mirror transitions 19Ne, 21Na, 35Ar

- pion

- Naviliat-Cuncic and Severijns, arXiv:0809.0994

## Why measurement of A via UCN ?

- PERKEO I (1986)

- Systematic _*Corrections*__ _  [ % ]

- ILL (1997)

- PNPI (1997)

- PERKEO II (2002)

- Polarization / Spin-Flip

- 2.6

- Backgrounds

- ~ 3

- 2.9

- Other

- ~13

- ~15

- magnetic mirroring

-  cos θ 

- 23

- 1.1

- 0.5

- ~ 3

*small*

*small*

- ~3

-  cos θ 

## Why measurement of A via UCN ?

- Advantages of UCNA experiment (A via UCN)

- 1) Polarization

- Expect to achieve ~100% polarizations via transport of UCN through 7-Tesla magnetic fields

- Spin-state selected via  μ • B  interaction, ± 60 neV/Tesla

- T < 335 nano-eV,  v < 8 m/s, stored in material bottles

## Why measurement of A via UCN ?

- Advantages of UCNA experiment (A via UCN)

- 2) Beam-related backgrounds

- Greatly reduced by operating “pulsed mode” spallation source coupled to superthermal UCN source

- T < 335 nano-eV,  v < 8 m/s, stored in material bottles

## Why measurement of A via UCN ?

- Advantages of UCNA experiment (A via UCN)

- 3) New approach to electron detection

- T < 335 nano-eV,  v < 8 m/s, stored in material bottles

- Low-threshold MWPC for detection of low-energy-deposition Coulomb backscattering events

- MWPC + plastic scintillator

- Low sensitivity to gammas (primary bkg’d in cold neutron expt’s)

## Why measurement of A via UCN ?

- Advantages of UCNA experiment (A via UCN)

- 4) Reduced neutron-generated backgrounds

- T < 335 nano-eV,  v < 8 m/s, stored in material bottles

- Long storage times for UCN in apparatus

- For given rate, need relatively smaller number of stored UCN as compared to cold neutron beam

## UCNA experiment

- 0.6-T field expansion

_*Angular Distribution*_

## UCNA experiment

*photograph 12/2007*

- 7-Tesla Polarizers

- UCN

- 1-Tesla Electron Spectrometer

- UCN Beamline

- muon vetoes

- spin flipper

## UCN spin polarization

- 7-Tesla Polarizer/Spin-Flipper Field Profile

- Spin-flipping via _A_diabatic _F_ast _P_assage  [AFP]

- 420 neV μ•B  barrier

- UCN flux

- 1-Tesla AFP spin-flip region

- Resonator with nominal frequency of ~29 MHz

- Spin-flipping cancels systematic errors

## Spin-flip efficiency

- UCN flux

- g

- UCN detector

- AFP resonator

- + Cu foil

## Spin-flip efficiency

- Transmission measured to be 0.40 ± 0.05 %

- Spin-Flip Efficiency > 99.6%

## In-situ depolarized fraction

- 1-T electron spectrometer

- B

- UCN from SD2 source

- 7-T Polarizer

- AFP

- UCN detector

- β-decay running

- drain “right spin”

- look for “wrong spin”

## In-situ depolarized fraction

- Spin Flipper State Change

- t for switcher leakage signal

- Upper Limit of 0.65% on Depolarized Fraction Present During Any Run

- Quote 1.3% Systematic

## Electron detection

- Baseline requirements

- Low-sensitivity to backgrounds

- Minimal electron backscattering

- Reasonable energy resolution

- _2007 Geometry_

- e

- 25-μm entrance and exit windows

- 3.5-mm plastic scintillator: energy, trigger

- low-pressure MWPC with low-Z fill gas

- 2.5-μm mylar foil

- Cu decay trap

## Electron spectrometer

- 4.5-m long superconducting solenoid

- 1.0-Tesla field

- 0.6-Tesla field-expansion region

- [ BP et al., NIMA 595, 587 (2008) ]

## Electron detection

- MWPC

- 100-Torr neopentane

- 2.54-mm wire spacing on anode and 2 cathode planes

- (163 х 163) mm2 active area

- Scintillator

- PMTs in 100-Torr N2

- Axial fields ~300 Gauss

- (x,y) reconstruction

- [ T.M. Ito et al., NIMA 571, 676 (2007) ]

- Fiducial volume definition

## Spectrometer performance

- suppression of gamma background     [ PERKEO II dominant background ]

- 113Sn 368 keV

- MWPC: gamma suppression

- 2007 calibrations:  113Sn (368 keV),  207Bi (503 keV, 995 keV)

- ~310 p.e. / MeV,  5.6% at 1 MeV      [ ~ 100 p.e./MeV in PERKEO II ]

## Spectrometer performance

- fiducial volume radius cut [decay trap walls]

- reconstruction of the *center* of Larmor spiral

- 113Sn 368 keV

## Event reconstruction

- MWPC

- MWPC

- scintillator

- scintillator

- decay trap foil

- decay trap foil

## Monte Carlo: GEANT4 / PENELOPE

- 1) Reconstruct β-decay energy on _event-by-event basis_ from measured “visible energy” deposition in scintillators

- “Invisible energy” loss in decay trap foils, MWPC windows/interior, and scintillator dead layer

- 2) Form experimental asymmetry,  Aexp(Erecon)  =  P **** β cos θ ****** **A

## Monte Carlo: GEANT4 / PENELOPE

- 3) Correct experimental asymmetry for subtle effects

- β cos θ  “acceptance”

- Unobservable backscattering

**** β cos θ **** for all triggering events

## Systematic corrections

- analysis window

**** β cos θ ****

- Backscattering

- 2007 Correction

- −1.6%

- +1.1%

## Uncertainties in corrections

- Uncertainty in correction for **** β cos θ ****** **acceptance ?

- How well is dE/dx energy loss modeled ?

- Assign (conservative) 25% uncertainty for geometry considerations

## Uncertainties in corrections

- Uncertainty in backscattering correction ?

- GEANT4

- PENELOPE

- Simulations tend to under-predict backscattering

- Assign 30% uncertainty to correction

## β-decay energy spectra

- UCNA 2007

- S/N 21:1

- PERKEO II

- S/N 7:1

## Asymmetry extraction

- Recoil-order corrections were applied to asymmetries (weak magnetism, gAgV interference, nucleon recoil)
- Standard calculational procedure of Wilkinson (1982)
- Included Fermi Function

## UCNA 2007 result

- UCNA 2007

- A = −0.1138 ± 0.0046 ± 0.0021

## UCNA 2008 running

- 2007 Run

- 25-μm MWPC windows, 2.5-μm decay trap foils

- 0.781M β-decay events

- 0.396M passed cuts (tight fiducial cut), 4.1% statistical error

- Need ~8.5-10M events for 1% result (depending on fiducial cut)

## Reduction in 2008 systematics

- Decay Trap Foils

- 0.7 μm

- 13 μm

**** β cos θ ****

- 1.5%

- 3.4%

- Backscattering

- −0.5%

- −2.7%

- Bias to Asymmetry

## UCNA 2008 running

- Run I

- Run II

## Projected impact of 2008/09 results

## Summary

- UCNA experiment has reported first-ever measurement of any

- neutron β-decay correlation parameter with UCN

- 2008 data collection proceeding well

- Poised to produce very interesting and competitive result at 1% level

- 4.5% “proof-of-principle” result

- Sub-1% level in 2009 running

*Higher rate: improved Fermi potential, beam current, SD2*

## Backgrounds vs. Spin State

- AFP off

- AFP on

*0 – 800 keV background rates (all event types)*

- East

- West

- 0.184 ± 0.004

- 0.188 ± 0.004

- 0.274 ± 0.005

- 0.268 ± 0.005

- no evident correlation (AFP-induced noise)

## Decay Trap (x,y) Spectra

- 40 mm cut

## Background Muons

## GMS Gain Corrections

- LED box

- optical fibers

- β-scintillator

- light guides to β-PMTs

- NaI

- GMS reference PMT

- 60Co

- gain correction factor for ith        β-PMT

- position of LED peak in GMS reference PMT

- position of 60Co peak(s) in GMS reference PMT

- position of LED peak in ith β-PMT

## GMS Gain Corrections

- 1.173 MeV

- 1.333 MeV

- East 60Co

- West 60Co