The Search for an Electric Dipole Moment of the Neutron

CryoEDM – A CryogenicNeutron-EDM ExperimentCollaboration:Sussex University, RAL, ILL, Kure University,Oxford UniversityHans Kraus

nEDM Overview• Theoretical Background• The Method of Ramsey Resonance• Room-temperature Experiment• Ultra-Cold Neutrons• The Cryogenic Experiment

Fundamental InversionsC particle antiparticle (Q → −Q)P position inverted (r →−r)T time reversed (t → −t)

SymmetriesWeak interactions are not P or C symmetric.Normal matter - n, μ, π - readily breaks C / P symmetry.The combination CP is different:Normal matter respects CP symmetry to a very highdegree.Time reversal is equivalent to CP (with CPT ok):Normal matter respects T symmetry to a very highdegree.

EDM breaks P and T Symmetryneutron EDM:spindn=d sˆnP odd T odd+−P−++−T−+(P no big deal) (CP big deal)(assuming CPT invariance)

Theoretical Overview• SM contribution is immeasurably small, but...• most models beyond SM predict values about10 6 greater than SM. Very low background!• EDM measurements already constrainedSUSY models.• “Strong CP Problem”: Why does QCD notviolate CP? Nobody knows.• Finally, there is the question of baryon – antibaryonasymmetry in the universe.

Two Motivations to Measure EDMEDM violates T symmetryDeeply connected to CP violation and thematter-antimatter asymmetry of theUniverseStandard model EDM is effectively zero, butbig enough to measure in non-standardmodelsDirect test of physics beyond the standardmodel

A Bit of HistoryA factor 10 improvementon average every 8years.Step change throughnew techniques.NowCryoEDM

The ILL in Grenoble

The Neutron SourceNeutron turbineA. Steyerl (TUM - 1986)Vertical guide tubeCold sourceReactor core

Room Temperature ExperimentFour-layer mu-metal shieldQuartz insulatingcylinderMain storage cellHigh voltage leadCoil for 10 mGmagnetic fieldUpper electrodeHg u.v.lampPMT to detectHg u.v. lightVacuum wallMercuryprepolarising cellRF coil to flip spinsHg u.v. lampMagnetSNUCN guidechangeoverUCN polarising foilUltracoldneutrons(UCN)UCN detector

Measurement PrincipleMeasure Larmor spin precession frequencyin parallel & antiparallel B and E fields:BEd n makes precessionfaster...μ B... or slower.

Measurement PrincipleApply const magnetic field, reverse electric field:Δ W = 2μ ⋅ B + 2d⋅E↑↑Δ W = 2μ ⋅B − 2d⋅E↑↓nnnnν↑↑ν− = 4 d ⋅E/↑↓nhMeasure difference in precession frequencyand find Electric Dipole Moment

Ramsey’s Technique1.2.3.4.“Spin up”neutron...Apply π/2 spinflip pulse...Freeprecession...Second π/2 spinflip pulse.2 s130 s2 s

2400022000200001800016000140001200010000Ramsey ResonancePhase gives frequency offset from resonance.Spin-Up Neutron Countsxx xxResonant freq.29.7 29.8 29.9 30.0 30.1Applied Frequency (Hz)x = working points

The Measurement

Mercury MagnetometryPolarized mercury atomsPMT

With Magnetometer

False EDM Signals∂ B1 vBrr=× E⇒ ∝B′2∂zγ cfrom div B = 0 from spec. rel.Different for 199 Hg and neutronssee: Phys. Rev. A 70, 032102 (2004)Need for precise magnetometry

Room Temperature ResultsRoom temperature neutron EDM result:C.A. Baker et al., Phys. Rev. Lett. 97, 131801 (2006) or hep-ex/0602020|d n |< 2.9×10 -26 e.cm (90% C.L.)

CryoEDM – the new generationNew technology:• More neutrons• Higher E field• Better polarisation• Longer NMR coherence time100-fold improvement in sensitivity

Ultra-cold Neutron Productioncold neutronphononultra-cold neutronλ n = 8.9Å; E = 1.03meV1.03 meV (11K) neutrons down-scatter by emission ofphonons in superfluid helium at 0.5KUp-scattering suppressed: hardly any 11K phonons

Ultra-cold Neutron Productioncold neutronphononSingle-phononproductionultra-cold neutronMulti-phonon production1.19 ± 0.18 UCN cm −3 s −1 expected0.91 ± 0.13 UCN cm −3 s −1 observedC A Baker et al., Phys. Lett. A 308 (2002) 67

Ultra-cold Neutron Detection• ORTEC ULTRA at 430mK temperature.• Equipped with thin surface layer of 6 Li.• Using:n + 6 Li → α+ 3 H2.73 MeV Triton peakAlpha peakC.A.Baker et al., NIM A487 (2002) 511

The Cryogenic SetupNeutron beam inputCryogenicRamsey chamberSuperconducting shieldTransfer section

Superconducting Shield

Improvements on Statisticsσ =Dh /2αET NParameter RT Expt Sensitivity• Polarisation+detection α = 0.75 x 1.2• Electric field: E = 10 6 V/m x 4• Precession period: T = 130 s x 2• Neutrons counted: N = 6 x 10 6 /day x 4.5(with new beamline) x 2.6Total improvement: appr. x 100

SQUIDS from CRESSTSQUIDs for Magnetometry

Magnetometry Tests6Squid OP [V]54321SQUID 4SQUID 5Pickup LoopSQUIDs00 4 8 12 164 5Time [s]0.020.015SQUID 4 + SQUID 5Squid 4 + Squid 5 [V]0.010.0050-0.005-0.01Resolution 0.16 pT(1.6nG) @ 1Hz-0.015-0.020 4 8 12 16Time [s]

Ramsey Cell and HV Stack

Reality CheckIf neutrons were the size of the Earth …+e-eΔx… then current EDM limit meanscharge separation of Δx ≈ 1.5μm

Summary• After half a century, no sign of an EDM …• SUSY being squeezed. Ever-tighter limitscontinue to constrain physics beyond SM.• Room-temperature experiment finished.• CryoEDM: under way with aim of ~100improvement in sensitivity.