the cryoedm experiment at ill - boston university

Philip Harris

The CryoEDM Experimentat ILL

1

P. HarrisLepton Moments 2010

OverviewMotivation/history (very brief)Measurement principleEDM to CryoEDMDetectors; HV; magnetic field; superfluid containmentSystematic errorsFuture plans


Electric Dipole Moments

EDMs are P, T oddComplementary study of CPv: must test elsewhere than K, BStrong CP problem

Constrains models of new physicsSUSY CP problem

Baryon asymmetrySM CPv inadequateNew models bring larger EDMs

Clean system – background free

E +

n n

γ

p

π −

×

q q

γ

gaugino

squark


History

Factor 10every 8 yearson average

CryoEDM


Measurement principle

ν(↑↑) – ν(↑↓) = – 4 E d/ hassuming B unchanged when E is reversed.

B0 E<Sz> = + h/2

<Sz> = - h/2

hν(0) hν(↑↑) hν(↑↓)

B0 B0 E

Use NMR on ultracold neutrons in B, E fields.

Sensitive to splitting ~10-21-10-23 eV


Ramsey method of Separated Oscillating Fields

4.

3.

2.

1.

Free precession...

Apply π/2 spin-flip pulse...

“Spin up”neutron...

Second π/2 spin-flip pulse

130 s

2 s

2 s


Ramsey resonance “2-slit” interference patternPhase gives freq offset from resonance

29.7 29.8 29.9 30.0 30.1

10000

12000

14000

16000

18000

20000

22000

24000

xx

x = working pointsResonant freq.

xx

Spin

-Up

Neu

tron

Cou

nts

Applied Frequency (Hz)


ILL, Grenoble


Reactor core

Cold source

Vertical guide tube

Neutron turbineA. Steyerl (TUM - 1986)

The ILL reactor


Current world limit

NS

Magnetic shielding

Storage cell

Magnet & polarizing foil Ultracold

neutrons(UCN)

UCN detector

Approx scale 1 m

Magnetic field coil

B

High voltage lead

E

/analysingfoil

dn < 2.9 x 10-26 ecm


UCN production in liquid helium

1.03 meV (11 K) neutrons downscatter by emission of phonon in liquid helium at 0.5 KUpscattering suppressed: Boltzmann factor e-E/kT

means not many 11 K phonons present

λn = 8.9 Å; E = 1.03 meV

Landau-Feynman dispersion curve for 4He excitations

Dispersion curve for free neutrons

R. Golub and J.M. Pendlebury Phys. Lett. 53A (1975), Phys. Lett. 62A (1977)


UCN production rate vs λn

1.19±0.18 UCN cm-3 s-1 expected, 0.91± 0.13 observedC.A.Baker et al., Phys.Lett. A308 67-74 (2002)

Multiphonon production

Single-phonon production


Statistical limits

Polarisation+detection: α = 0.75 x 1.2Electric field: E = 106 V/m x 4Precession period: T = 130 s x 2Neutrons counted: N = 6 x 106 /day x 4.5

(with new beamline) x 2.6

Parameter Room-tmpr. expt Sensitivity

Total increase approx factor 100

σd =

h/2αET N


CryoEDM overviewNeutron beam input

Transfer section

Cryogenic Ramsey chamber


Cryogenic Ramsey chamber

Superfluid He

HV electrode (Be)

n storage cells(4 eventually)

HV feed

SQUID loops(not shown)


UCN detection in liquid helium

Solid-state detectors developed for use in LHeThin surface film of 6LiF: n + 6Li → α + 3H


UCN detection in liquid helium

Triton peakAlpha peak

C.A.Baker et al., NIM A487 511-520 (2002)


Detector development

Noisy environment: gammas currently hide alpha peak – not long-term problemIdeally want large area, multi-channel detector (redundancy, rate): may not be possible with SiScintillator (doped?) with light pipe to LN2 layer, and optical detector


High voltage

Currently testing breakdownof LHe under HV: dependenceon pressure & temperatureCharging currents can confirm that HV is actually appliedLong term: consider using Kerr effect to measure E field – difficult.


Superfluid containment

Not trivial to find non-SC, non-magnetic materials to hold & seal superfluidCurrently using provisional SS vessel: mapping/correction underwayLong term: plastic? CuBe? Currently investigating sealing techniques


Magnetic fieldSC coil/shield will give improved field shape/shielding

trim coils to compensate gradients: same freq in different cells

At present, Pb shield too short: flux lines clip coil end, inducing current in whole coil

Introduces common-mode noise: sensitivity limit 1E-27 e.cmSQUIDs measure common-mode fluctuations

sensitivity ~10-13 T, similar to room-temp Hg co-magnetometer

Inner SC shield will increase total SF to ~ 107


Systematics: Geometric phase

rBzB

r ∝⇒∂∂

and, from Special Relativity, extra motion-induced field

2

1cEvBrr

×=′

γ

Combination oftwo effects:

J. Pendlebury et al., PRA 70 032102 (2004)


Systematics: Geometric Phase

Br

Br

Bnet

Bnet

Bv

Bv

Bv

Bnet

Bnet

Bottle(top view)

... so particlesees additionalrotating field

Frequency shift∝ E

Looks likean EDM, butscales withdB/dz


Systematics: Geometric phaseRoom-temp expt: <1 nT/m → <4E-26 ecmNeutrons 50x less sensitive than HgFor neutrons, scales as 1/B2; increase B 5x to obtain factor 25 protectionNet protection factor 50x25 = 1250, so 1 nT/m

→ 3E-29 e.cm-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

0 0.5 1 1.5 2

Orbit freq/Larmor freq

Fals

e ED

M e

ffect

(arb

. uni

ts)


Systematics: E x v

Translational:Vibrations may warm UCN, cause CM to rise ~1 mm in 300 s → 3E-6 m/sIf E, B misaligned 0.05 rad., gives 2E-29 e.cm

Rotational:Net rotation damped quickly: walls not completely smooth. Delay before NMR pulses allows rotation to die away.Neutrons enter E-field cells centred horizontally; no preferred rotation Below 1E-29 e.cm


Systematics: 2nd order E x v

Perpendicular component, adds in quadrature to B.Prop. to E2; gives signal if E reversal is asymmetricCancellations (double cell; B reversals) reduce effect to < 3E-29 e.cm


Systematics: μ metal hysterisis

Room-temp expt: Pickup in B coil from E field reversals; return flux causes hysterisis in μ metalCoil here is SC, not power-supply drivenInner shield is SC alsoSmall effect from trim coils, enhanced by any misalignmentsNet estimate < 1E-30 e.cm


Systematics: E induced cell movement

Electrostatic forces of order 20 N; ∝ E2

Radial gradients of order 3 nT/mMust keep displacement on E reversal to ~ 0.01 μm Cancellation with double cellSymmetric voltages to ~2%Net effect < 1E-28 e.cm


Systematics: Leakage currents

Azimuthal current components generate axial contributions to B Cancellation in adjacent cellsConservative estimate: 1 nA → 5E-29 e.cmIn reality LHe should keep surface currents significantly below this


Systematics: HV supply contamination

HV circuit isolated as far as possible to minimise earth contamination. Separate computer control.10 kHz ripple on HV line can “pull”resonant freq. Estimate 1E-30 e.cmLikewise 50 Hz ripple: estimate ~1E-29 e.cmDirectly generated AC B fields negligible


Systematics: SummaryEffect Size (e.cm)B fluctuations 1 x 10-30

Geometric phase 3 x 10-29

Exv translational 2 x 10-29

Exv rotational 1 x 10-29

Exv 2nd order 3 x 10-29

μ metal hysterisis 1 x 10-30

E-induced cell movement 1 x 10-28

Leakage currents 5 x 10-29

HV line contamination 1 x 10-29


Simulations

Found G4UCN awkward, so have developed our own ROOT-based neutron-transport simulationShortly to incorporate neutron-spin tracking, based on our well-established routine from room-temp. expt.Also have analytical models for some processes – relaxation, losses etc


Data blinding

Collaboration has agreed to blind dataAdjust neutron numbers in data to apply fairly large (~10-25 ecm) offset to EDM

sign & approx magnitude known, as sanity check on analysis

“Raw” numbers will be in data files but encoded“Blind but not dumb”: committed to publish whatever comes out, but justify any changes due to unforeseen circs.


Current status

Mag. scan of provisional storage vesselMaintenance/reassembly over summerNext cooldown starts in SeptFirst neutron resonance by DecemberEDM results ~2013 at ~3E-27 levelNew beamline: move 2013-14


ConclusionsSignificant delays, but CryoEDMnow commissioning provisional systemSystematics well understoodSensitivity ~ few 10-27 ecm in ~3 years, ~few 10-28 ecm ultimatelyWatch this space!


Spare slides


nEDM measurement

0 5 10 15 20 2529.9260

29.9265

29.9270

29.9275

29.9280

29.9285

29.9290

29.9295

ΔB = 10-10 T

Raw neutron frequencyCorrected frequency

Prec

essi

on fr

eque

ncy

(Hz)

Run duration (hours)


-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

0 0.5 1 1.5 2

Trajectory orbit freq/Larmor freq

Fals

e ED

M e

ffect

(arb

. uni

ts)

Low speed (neutroncase): goes like v2

High speed (mercury case): independent of velocity

Bottle orbital freq = Larmor freq

Geometric Phase Velocity dependence


Geometric Phase How to measure it

Consider

Should have value 1R is shifted by magnetic field gradientsPlot EDM vs measured R-1:

n

Hg

Hg

nRγγ

νν

⋅=


-100

-50

0

50

100

-5 0 5 10Magnetic field gradient (nT/m)

EDM

(10-2

6 ecm

)

Geometric Phase

Magnetic field down

→∂∂zB


Geometric Phase

Magnetic field up

-100

-50

0

50

100

-5 0 5 10Magnetic field gradient (nT/m)

EDM

(10-2

6 ecm

)

←∂∂zB


Results

R-1

EDM

0

B down

B up The answer?

Nearly...

-150

-100

-50

0

50

100

150

-40 -20 0 20 40

R-1 (ppm)

EDM

(10-2

5 e.c

m)

-150

-100

-50

0

50

100

150

-10 0 10 20 30 40

R-1 (ppm)

EDM

(10-2

6 e.c

m)


Results

R-1

EDM

0

Small dipole/quadrupole fields can pull lines apart

& add GP shifts

B down

B up


Results

R-1

EDM

0

Small dipole/quadrupole fields can pull lines apart

& add GP shiftsB up

B down• Use variable-height bottle to measure B field shape• Also look at depolarisation vs. R-1• From these, calculate zero-gradient values of R-1 to give true EDM

the cryoedm experiment at ill - boston university

Documents