The cryogenic neutron EDM experiment at ILL

1. The cryogenic neutron EDM experiment at ILL Technical challenges and solutions James Karamath University of Sussex

2. In this talk… • (n)EDM motivation • Measurement principles and sensitivity • Brief (recent) nEDM history • The Cryo-EDM experiment • Overview of apparatus • Summary of my DPhil work • Summary/conclusions James Karamath University of Sussex 09/09/2014 07:23:15

3. (n)EDMs – so? I S S + + d d - - • P- and T-violating • CPV in SM not fully understood e.g. insufficient CPV for baryon asymmetry • Strong CP problem • θCP < 10-10 rad. Axions? g p  × p n n James Karamath University of Sussex 09/09/2014 07:23:15

4. g squark quark electric dipole moments: q q gaugino (n)EDMs – so? II • Estimated EDMs model dependent • SM dn ~ 10-31ecm • Other models typically 105-6 times greater • e.g. SUSY: CP< 10-2

5. nEDM measurement principle B0 B0 B0 E E <Sz> = + h/2 h()= 2(-μ.B-dn.E) h()= 2(-μ.B+dn.E) h(0) = -2μ.B <Sz> = - h/2 dn defined +ve ↑↑ - ↑↓= Δ = 4dn.E / h Ramsey NMR performed on stored Ultra Cold Neutrons (UCN)

6. Ramsey’s method of separated fields Start with spin polarised neutrons in uniform B-field (Bz) Apply oscillating B-field pulse (Bxy) perpendicular to Bz. Precession axis rotates down to xy-plane Apply large E-field and allow to precess freely for ~300s Apply 2nd, phase coherent with the first, oscillating Bxy. Neutron precession axis rotates down to –z axis.

7. Ramsey’s method of separated fields However if an EDM is present a phase difference builds up during the free precession If 180 out of phase second pulse returns spin back to +z axis.

8. Ramsey’s method of separated fields (2n-1)π out of phase Experimental runs taken at approx π/2 off resonance. Here dN/dν is a maximum.

9. nEDM statistical limit • Fundamental statistical limit α = visibility [polarisation product] E = E-field strength T = NMR coherence time N = total # counted James Karamath University of Sussex 09/09/2014 07:23:15

10. nEDM systematic limit • Main concern: changes in B-field accidentally correlated with E-field changes give false dn signal h(ν↑↑–ν ↑↓) = 2|μn|(B↑↑–B↑↓) – 4dnE True nEDM signal  False signal due to varying B 

11. Beam era ΔB ≈ v x E / c2 limited RT stored UCN era nEDM experiments: history Co-magnetometer era Cryogenic UCN era

12. B E RT nEDM experiment at ILL Magnetic shielding • Create UCN, can then be guided & stored • Polarise UCN • UCN admitted into cell with E and B-fields and stored… • Mercury polarised by Hg lamp and added to cell High voltage lead Magnetic field coil Storage cell Approx scale 1 m Magnet &polarizing foil /analysing foil S N UCN James Karamath University of Sussex 09/09/2014 07:23:15

13. B E RT nEDM experiment at ILL Magnetic shielding • Ramsey NMR performed • Released from cell • Neutrons spin analysed (# fn of precession) • Mercury spin analysed. • Repeat: E=↓or 0, B=↓ High voltage lead Magnetic field coil Storage cell Approx scale 1 m Magnet & polarizing foil/ analysing foil S N UCN detector James Karamath University of Sussex 09/09/2014 07:23:15

14. Magnetometer problems Systematics I • Mercury fills cell uniformly, UCN sag under gravity, lower by ~3 mm. • Thus don’t sample EXACTLY the same B-field. Axial (z) gradients → problems… z n Hg James Karamath University of Sussex 09/09/2014 07:23:15

15. Geometric Phase Effect (GPE) Systematics II • Two conspiring effects • v x E: motional particle in electric field experiences B-field: ΔB ≈v x E / c2 • Axial field gradient dB/dz creates radial B-field (since .B=0) proportional to r, Br r • Let’s look at motion of a mercury atom across the storage cell

16. Geometric Phase Effect (GPE) Systematics III Scales with E like EDM!!! B  v x E Scales with dB/dz dB/dz → B  r i.e. B0 field into page has gradient (GPEHg ~ 40GPEn) Resultant Rotating B field Using Mercury introduces error Shifts resonance  of particle E and B0 into page

17. www.neutronedm.org Final result • Room temperature experiment gave the result; dn = (+0.61.5(stat) 0.8(syst)) x 10-26) ecm i.e. |dn| < 3.0 x 10-26ecm (90% CL). • New cryogenic experiment will eventually be x100 more sensitive…

18. The Cryogenic nEDM experiment • Reminder: * x20 x5* x2 ~10-28ecm x1.2 x2 *with new beamline

19. Improved production of UCN (↑N) I • Crosses at 0.89 nm for free (cold) n. Neutron loses all energy by phonon emission → UCN. • Reverse suppressed by Boltzmann factor, He-II is at 0.5K, no 12K phonons. Dispersion curves for He-II and free neutrons James Karamath University of Sussex 09/09/2014 07:23:15

20. Improved production of UCN (↑N) II • Idea by Pendlebury and Golub in 1970’s, experimentally verified in 2002 (detected in He-II) for cold neutron beam at ILL (~1 UCN/cm3/sec). • Also better guides – smoother & better neutron holding surfaces, Be / BeO / DLC coated → more neutrons guided/stored. Allows longer T too. James Karamath University of Sussex 09/09/2014 07:23:15

21. Polarisation and detection (α) I • Polarisation by Si-Fe multi-layer polarizer, 95±6% initial polarisation. • Can lose polarisation in 2 ways: • “Wall losses” magnetic impurities in walls, generally not aligned with neutron spin • Gradients in B-field, if not smooth and steady have similar effect James Karamath University of Sussex 09/09/2014 07:23:15

22. Polarisation and detection (α) II • Detector: solid state, works in 0.5K He-II. • n (6Li, α) 3H reaction - alpha or triton detected • Thin, polarised Fe layer - spin analysis James Karamath University of Sussex 09/09/2014 07:23:15

23. Magnetic field issues I Shielding factors • Target – need ~ 100 fT stability (NMR) • Need ~ 1 nT/m spatial homogeneity (GPE) • Perturbations ~ 0.1 μT (cranes!) • Need (axial) shielding factor ~ 106 • Mu-metal shielding ~ 50 • Superconducting shielding ~ 8x105 • Active shielding (feedback coils) ~ 15

24. Magnetic field issues II Extra benefits • CRYOGENIC nEDM! Utilise superconducting shield and B0 solenoid. • Major part of fluctuations across whole chamber (common mode variations) • Magnetometer (zero E-field) cell(s) see same • Very stable B0(t) current • Holding field x5 to reduce GPE of the neutrons by factor of 25 (GPEn 1/B02) James Karamath University of Sussex 09/09/2014 07:23:15

25. Magnetic field issues III SQUIDS • ~fT sensitivity • 12 pickup loops will sit behind grounded electrodes. • Will show temporal stability of B-field at this level. • Additional sensitivity from zero-field cell(s)

26. Improving the E-field (↑E) I: The HV • Now have a 400 kV supply to connect to HV electrode. • Will sit in 3bar SF6. For 160 kV use N2:CO2 first. James Karamath University of Sussex 09/09/2014 07:23:15

27. Improving the E-field (↑E) II: HV line 1 400 kV bipolar stack 50 kV ~1 GOhm resistors Superfluid containment vessel (SCV) HV electrode Ground electrodes N.B. Diamond-like-carbon coated titanium electrodes BeO spacers

28. Improving the E-field (↑E) II: HV line 2 Thick walled PTFE tube and thin-walled SS tube HV “cryo-cable”. Spellman +130 kV Spellman -130 kV Standard 150 kV cable HV connection

29. The dielectric strength of LHe • Has been tested in the past, mostly at 4.2 K (760 torr), at small electrode gaps (sub-mm) and with small electrodes. Superfluid data is limited and generally at low voltages (sub-40 kV, often sub-20 kV). • Usually the breakdown strength as a function of gap is studied. We’d like to know the strength as the pressure/temperature falls – esp. in the superfluid state. James Karamath University of Sussex 09/09/2014 07:23:15

30. Past literature The dielectric strength of LHe II He-I data 4.2 < T(K) < 2.2 Nope – put in the final versions from thesis

31. Past literature The dielectric strength of LHe III He-II data 2.2 < T(K) < 1.4

32. Sussex HV tests The dielectric strength of LHe IV ±HV • Test electrodes submerged in He-II in bath cryostat. • Studying Vbd and Ileak as function of d, T, dielectric spacers, purity… up to 130 kV. Also electrode damage. E cryostat gap (d, V, spacers) He-II (T, purity…)

33. Sussex HV results The dielectric strength of LHe IV

34. The dielectric strength of LHe V

35. Statistics The dielectric strength of LHe VI

36. The dielectric strength of LHe VII • Size effects: Weak but important dependence on electrode area or stressed fluid volume may decrease dielectric strength. • Leakage currents never found to be >0.1 nA (sensitivity limited) even immediately below breakdown. • ~0.3 mm craters in electrodes when breakdown occurs at >80 kV. Bad news for DLC coated electrodes.

37. The dielectric strength of LHe VI • Breakdown strength reduced by insulating BeO spacers by a factor of ~1.4. Due to surface tracking along the BeO.

38. The dielectric strength of LHe summary • At 0.7 cm gap the breakdown field strength was approx 80 ± 10 kV/cm. i.e. ~50 kV/cm for 1 in 1000 chance of breakdown. May have to half this if size effects indeed exist. • What controls breakdown – pressure or temperature?! May hold key to improving Vbd.

39. And so, the CryoEDM experiment I E ~ 25kV/cm n guide tubes + spin analyser E = 0kV/cm Spin flipper coil (measure other spin)

40. HV electrode HV in Carbon fibre support BeO spacers Ground electrodes And so, the CryoEDM experiment II z

41. G10 Superfluid containment vessel HV in HV electrode * * Neutrons in/out Guides not shown Ground electrodes *BeO spacers/guides And so, the CryoEDM experiment III z 250l He-II 0.5K

42. The shielded region 1m And so, the CryoEDM experiment IV Dynamic shielding coils Magnetic (mu-metal) shields Superconducting shield and solenoid

43. Schedule / Future • Finish construction THIS YEAR • Start data taking THIS YEAR • First results ~2009 • Upgrade neutron guide to ↑N ~2009 ? James Karamath University of Sussex 09/09/2014 07:23:15

44. Summary • (n)EDMs help study T-violation and are constraining new physics. • Final RT result: |dn| < 3.0 x 10-26ecm. • Aim to push well into 10-28ecm. • Further work needed to understand dielectric properties of He-II. Only 20 kV/cm? (Paper in preparation.) Can pressure/purity/electrode material make a difference?

45. Done! • Thanks for listening