Neutron β-decay with Ultra-Cold Neutrons (UCN)

• Ultra-Cold Neutrons (UCN) provide unique system to study fundamental neutron properties

• UCN experiments have different systematics compared to cold neutron beams – Polarization process and background sources differ

significantly

• UCNA First experiment to measure neutron decay correlation - A - with UCN – First UCNA data: 12/07 (Actually 12/15-12/17/07)

– Plans for future data (6-12/08, 6-12/09)UW 10/9/2008

The Caltech UCN group

Justin ChenNick HutzlerGary ChengJenny HsiaoRiccardo SchmidKevin HickersonJunhua YuanBrad Plaster Bob CarrMichael MendenhallJianglai LiuBF

UCNA CollaborationCalifornia Institute of Technology

R. Carr, B. Filippone, K. Hickerson, J. Liu, J. Martin, M. Mendenhall, B. Plaster, R. Schmid, B. Tipton, J. Yuan

Institute Lau-LangevinP. Geltenbort

Los Alamos National LaboratoryJ. Anaya, T. J. Bowles, T. Brun, M. Fowler, R. Hill, G. Hogan, T. Ito, K. Kirch, S. Lamoreaux, C.-Y.

Liu, C. L. Morris, M. Makela, A. Pichlmaier, A. Saunders (co-spokesperson), S. Seestrom, P. Walstrom, J. Wilhelmy

North Carolina State University/TUNLH. O. Back, L. Broussard, A. T. Holley, R. K. Jain, R. W. Pattie, K. Sabourov, A. R. Young (co-

spokesperson), Y.-P. XuPetersburg Nuclear Physics Institute

A. Aldushenkov, A. Kharitonov, I. Krasnoshekova, M. Lasakov, A. P. Serebrov, A. VasilievTohoku University

S. KitagakiUniversity of Kyoto

M. Hino, T. Kawai, M. UtsuroUniversity of Washington

A. Garcia, S. Hoedl, D. Melconian, A. Sallaska, S. SjueVirginia Polytechnic Institute and State University

R. Mammei, M. Pitt, R. B. Vogelaar

Neutron Beta Decay

or in quark picture…

udd

udu

νe

e-

n

p

W-

Precision neutron decay measurements

• < 0.3% measurements can be sensitive to new physics (from loops in electroweak field theory)

Radiative Corrections

q

W

q

W

χ0

CKM Matrix and Unitarity

÷÷÷

÷÷÷

=÷÷÷

bsd

bsd

w

w

w

99.004.0005.004.097.022.0005.022.0975.0

Unitarity, , (or lack thereof) of CKM matrix tests existence of further quark generations and

possible new physics (eg. Supersymmetry)

÷÷÷

÷÷÷

=÷÷÷

bsd

VVVVVVVVV

bsd

tbtstd

cbcscd

ubusud

w

w

w

Weak eigenstates Mass eigenstates

eg. |Vud|2 + |Vus|2 + |Vub|2 = 1

• Vud in Standard Model

(from µ vs. β-decay)

Sensitivity to New Physics?Kurylov&Ramsey-Musolf

Phys. Rev. Lett. 88, 071804 (2002)

W- e-

GFVudµ- νµ

W- e-

GF d u

νe νe

µ- νµ

W- e-

νe

µ~ ν~χ0~ d

W- e-

νe

d~ u~χ0~ u

• Supersymmetric particles produce loop corrections

• a, A and B are correlations that depend on the axial and vector weak coupling constants GA and GV

What is Big A?÷÷

+Γ=Γ

ne

e

Ep.

Ad +ν

ν

Ep.nσBrrr

nσr

+νe

νe

EEp.p

arr

1 +νe

e

EExp

D νprrnσr.

Differential neutron decay rate With no e- polarization

• Γn = 1/τn is total decay rate

• Thus A and τn gives Vud

÷÷

+Γ=Γ

ne

e

Ep.

Ad +ν

ν

Ep.nσBrrr

nσr

+νe

νe

EEp.p

arr

1 +νe

e

EExp

D νprrnσr.

Uncertainties in Vud

Electroweak corrections(Z0 and hadron loops) Neutron has potential to

yield most precise resultRecently cut in half by Marciano & Sirlin

CKM Summary:

New τn !!

UC

NA

Particle Data Group recommended

Why UCNA?• For accurate measurement of A & GA/GV (and Vud

via neutron lifetime) need to characterize and minimize systematic uncertainties

• Different experimental approaches are critical to reducing systematic uncertainties

– PERKEO II/III • Supermirror polarizer • Cold neutron beam from CW reactor • Scintillator β detector

– UCNA • Superconducting magnet polarizer • Trapped UCN from pulsed proton beam • Scintillator & MWPC β detector

UCN Polarization via high B-field

“High field seekers”

“Low field seekers”

Reduced Background with pulsed Source of UCN

PERKEO II (99)A-correlation experiment(at Reactor)

UCNA data12/07

(pulsed source)

UCNA Proposal4/00

(pulsed source)

What are UCN ?– Very slow neutrons

(v < 8 m/s ’ λ > 500 Å )that cannot penetrate into

certain materials Neutrons can be trapped in bottles or by magnetic field

UCN Properties

g3m

UCN

The coherent attractive nuclear potential can lead to a repulsive pseudopotential (Fermi potential) if a > 0

Attractive potential can also lead to neutron absorption but often Lmfp >> λn (~10-5 probability per bounce)

For EUCN < VF, UCN are trapped

EUCN

Fermi Pseudo-potential

Typical Fermi PotentialsMaterial VF (neV)

Al 54

58Ni 350

Ti - 48

Cu 171

Stainless Steel 188

Diamond-Like Carbon (DLC)

282

neutron velocityvn ~ 8 m/s

How to make UCN?• Conventional Approach:

– Start with neutrons from nuclearreactor core

– Use collisions with nuclei to slow down neutron

– Record density at Institut Laue-Langevin (ILL) reactor in Grenoble (40 UCN/cm3)

Some of neutron’s energy lostto nuclear recoil in each collision Gives a Maxwell-

Boltzmann Distribution

Higher Density UCN Sources• Use non-equilibrium system

(aka Superthermal)– Superfluid 4He (T<1K)

Very few 11K phonons if T<1K ∴ minimal upscattering

11K (9Å ) incidentneutron produces a phonon & becomes a UCN

Used in on-goingNIST τn Experiment

(neutron)

& Future newNeutron EDM exps.

– Solid deuterium (SD2) Gollub & Boning(83)

– Small absorption probability– Faster UCN production– Small Upscattering if T < 6K

UCN

Phonon

Cold Neutron

Can be optimally used at a pulsed source (e.g. accelerator-based “spallation” neutron source)

Schematic of prototype SD2 source

Liquid N2

Be reflector

Solid D2

77 K polyethylene

Tungsten Target

58Ni coated stainless guide

UCN Detector

Flapper valve

LHe

Caltech, LANL, NCState, Princeton, VaTech, Russia, France, JapanCollaboration

New World Record UCN

Density

Measurements of Ultra Cold Neutron Lifetimes in Solid Deuterium [PRL 89,272501 (2002)]

Demonstration of a solid deuterium source of ultra-cold neutrons [Phys. Lett. B 593, 55 (2004)]

Previous record for bottled UCN = 41 UCN/cm3 (at ILL)

UCNA Experiment Design

UCN Decay Trap

(7.0 T & Adiabatic Fast Passage Spin Flipper in 1T)

From

Asymmetry Measurement with UCN

e-

n

θ

UCNA

Overview of UCNA experiment

• SD2 Superthermal UCN source

• UCN guides– Stainless Steel, Cu, Diamond-coated Quartz

• Polarizer and spin-flipper system

• Spectrometer & β-decay detectors

• First data...

UCNA Experiment Layout

Superconducting Spectrometer

Electron Detectors

Neutron Polarizing Magnets

UCN Source

Liquid N2

Be reflector

Solid D2

77 K poly

Tungsten Target

LHe

UCNA experiment

UCNA SD2 source

Solid Deuterium

Flapper Valve

Tungsten TargetProton bunch

UCN Guides

• From source through first polarizing magnet = Stainless Steel– “Dairy Guide”– May be depolarizing (mildly ferromagnetic)

• Through AFP spin-flipper = Diamond coated Quartz

• Into 1T spectrometer = Electro-polished Copper

mai

ntai

ns n

eutro

n po

lariz

aion

UCNA neutron polarization• Pre-polarizing 6T magnet allows good

UCN transport through vacuum window(isolates source and detector system for safety

• 2nd 7T magnet further filters UCN and allows for spin flip – Adiabatic Fast Passage (AFP) resonator

• UCNA Polarimetry = measuring depolarized fraction – When polarization is very high only modest

measurement of depolarization fraction needed

Polarizer/AFP Spin Flipper

e-7T

Depolarization Measurements

Crossed polarizer: Uses AFP to flip UCN to low field seekers

UCN inSample during bottle emptying: change state of AFP at end of run cycle and monitor depolarized UCN leaking back to detector

UCN in

7T

Polarizer/AFP7T

AFP

UCN detector

UCN transmission through “crossed polarizers”

vs AFP frequency

Spin-Flipper Tuning during β-decaySpectrometerAFP

Magnetized foil

UCN detector

Side View

Polarizer

Depolarization Trapping (in situ)PolSwitcher SpecAFP

LOAD ≅ 1hr CLEAN 150s UNLOAD 100s

AFP OFF AFP ONAFP OFFBACKGROUND

AFP ONAFP ON AFP ON AFP OFF AFP OFF

Flow-Through Leakage

α (UCN from B+C+D) + β (UCN from A+Switcher Leakage)

Draining of A+B+C+D

Superconducting Spectrometer

Neutron Decay Tube

Decay Electron Detectors

1 Tesla Central Fieldwith 0.6 T field expansion to

suppress backscattering

Solenoid Bore • 35 cm diameter SS warm bore• 12.5 cm diameter electropolished Cu

decay tube• Be-coated Mylar endcap windows

(2.5 µm in 07, 0.7 µm in 08)

• 110 cm diameter Lucite β collimator

Decay Tube

β-de

tect

or

β-de

tect

or

β-detector System• Requirements:

– Low Background, Reasonable Energy Resolution, Minimal e- Backscattering

• Design:

25 or 6 µm Entrance Window

Low Pressure MultiWire Proportional Chamber MWPC (100 torr)

3.5 mm Scintillator e-

25 or 6 µm Exit Window

Scintillator + MWPC reduces room background > factor of 25

Full Detector Schematic

MWPC Preamp Cards

e-PMT

PMT

Fe Magnetic Shields (also vacuum seal)

100 torr Neopentane100 torr N2

Assembled Detector

PMT

PMT

Detector Studies

• At Caltech with 130 keV electron gun

• At LANSCE with 113Sn (Eβ ~ 370 keV) and 207Bi (Eβ ~ 505 & 995 keV) sources

• At LANSCE with neutron β-decay

Detailed Backscattering studies completed at Caltech

(comparison with GEANT4 and PENELOPE Monte Carlo)

"New measurements and quantitative analysis of electron backscattering in the energy range of neutron beta-decay", J.W. Martin et al., Phys. Rev. C. 73, 015501 (2006).

"Measurement of electron backscattering in the energy range of neutron beta decay", J.W. Martin et al., Phys. Rev. C 68, 055503 (2003).

Spectrometer studies at LANSCE with 113Sn source in 1T field

Fiducial VolumeCut

σ ~ 1.5mm

Cosmic ray induced events (x,y)

80 mm diameter fiducial cut

UCN Decay Events (x,y)

80 mm diameter fiducial cut

110 mm diameter β collimator

LANSCE

UCN Source (Area B)

Proton Linac (½ mile long): 0.8 GeV, 1-2 mA

(Los Alamos Neutron Science CEnter)

Recent Pictures of LANSCE Area B (10/5/08)

Recent Pictures of LANSCE Area B

Liquid He “farm” (~ 2000 liters/day)

UCN Switcher

UCN “Accelerator”...

UCNA-07 results• First measurements of depolarized fraction

• “Crossed polarizers” & trapped depol. Neutrons

• Detailed measurements of UCN β-decay spectra

• First measurements of UCN asymmetry

Note: All 07 data acquired with• 2X25 mm b detector windows (ΔE ~ 20 keV)• 2.5 mm endcap windows • < 50 hours total acquisition time Fo

r rob

ustn

ess

& “s

afet

y”

Statitistical sensitivity of UCNA

• Experiment can only run on nights & weekends (Due to nuclear weapon stockpile stewardship

studies – proton radiography)• Modest downtime for polarization flips &

depolarization measurements• A sensitivity: σA/A ~ 3%/month/sqrt(Hz)

– < 0.2 Hz in 05– ~ 1.7 Hz in 06– ~ 7 Hz in 07– ~ 18 Hz in 08

PERKO II 02: 0.7%UCNA goal: <0.5%

Reduced Background with pulsed Source of UCN

PERKEO II (99)A-correlation experiment(at Reactor)

UCNA data12/07

UCNA Proposal4/00

(pulsed source)

Hz

Room Background

Scintillator rate increases during beam pulses

First results from UCNA

-0.1500

-0.1125

-0.0750

-0.0375

0

1,970 1,980 1,990 2,000 2,010

World Asymmetry data

TIME (year)

A

07 08 09 UCNA

UCNA(07) UCNA(08) PERKEOII(02)Systematic Corr. Error Corr. Error Corr. Error

Polarization <0.5% 1.0% <0.2% 0.2% 1.1% 0.3%

Spin-flip <0.2% 0.1% 0.3% 0.3%

Background <0.1% 0.2% <0.1% 0.1% 0.5% 0.3%

Detector

Linearity 1.5% <0.3% 0.2%

Width/Ped 0.1% <0.1% 0.1%

Drifts 0.2% <0.1% 0.006%

Edge Effects -0.24% 0.1%

Angle Effects -1.6% 0.5% <-0.4% 0.2%

e- Trajectories

Mirror Effect -0.06% 0.06% -0.06% 0.06% 0.09% 0.02%

Backscattering

1.1% 0.5% 0.5% 0.2% 0.2% 0.1%

Statistics 4.0% <0.8% 0.45%

Total 0.5 +/- 4.4% 0.5% +/- 0.9% 2.0 +/- 0.7 %

Preliminary Projected

Asymmetry Systematics

Improvements for 2008• Thinner endcap/detector windows

– 2.5 µm/25 µm 0.7 µm/6 µm– Reduces systematic corrections/uncertainties

• Improved energy calibrations– Three sources (113Sn, 85Sr, 207Bi, 114In?) and variable pulsed LED

system – Reduces systematic uncertainties

• Higher beam current & more hermetic UCN guides– Typically running at 15-18Hz compared to 7 Hz in 2007

• Plans for 2009 - DLC Coated decay tube– Raises spectrometer Fermi potential– Should ~ double β-decay rate

Total counts vs Time (08)

2007 Run

Beyond 2008 with UCN

• Big A measurement with <0.5% precision

• Neutron lifetime with “Halbach” magnetic array

• Correlation experiments with Si detectors

• a & B measurements possible with proton detection

Summary • High density UCN source developed at

LANL• First UCN correlation experiment

underway• Future high precision measurements

possible with UCN