Measuring the Neutron Electric Dipole Moment - A Tiny Number with Big Implications

Measuring the Neutron Electric Dipole Moment - A Tiny Number with Big Implications Imperial/RAL Kyoto University nd, 2005

The Universe passed through a period of the very high energy density early in the Big Bang Matter was produced via reactions like γ+γ p+p This should have produced equal quantities of matter and anti-matter.

It didn t.

Kyoto Seminar

Diffuse γ-ray flux expected from annihilation d 0 = 20 Mpc d 0 = 1000 Mpc See Cohen, De Rujula, Glashow; astro-ph/9707087

Sakharov Conditions: (A.D. Sakharov, JETP Lett. 5, 24-27, 1967) To produce a matter anti-matter asymmetry requires: Baryon number violation Conserved at tree level in the Standard Model More complex SM processes lead to B violation C violation

CP violation in weak decays K 0 L 2π

CP violation in weak decays BELLE and BaBar CPLear (amongst others)

So are we done now? No (you don t get out of this talk that easy) The CP violation in the Standard Model is too small by many orders of magnitude to explain the observed matter-anti-matter asymmetry (also called the baryon asymmetry) of the Universe (hep-ph/0303065) There must be CPV in laws of physics we don t know yet! We have to keep looking

Neutron Electric Dipole Moment

Neutron Electric Dipole Moment _ +

Neutron Electric Dipole Moment _ + s

Neutron Electric Dipole Moment _ + s Would lead to a non-zero value for d n, either parallel or anti-parallel to s d n would be: P odd T odd CP odd!

Particle EDMs are a particularly promising laboratory for CP violation The Standard Model contribution is very small Contributions from new physics tend not to be

10-20 10-22 10-24 10-26 10-28 10-30 10-32 neutron Range of d (e cm) in various models 10-34 10-36 Standard Model 10-38 electron

10-20 10-22 10-24 Multi Higgs 10-30 10-32 Left-Right Electromagnetic SUSY φ 1 φ α/π neutron Range of d (e cm) in various models 10-34 10-36 Standard Model 10-38 electron

Basic Idea of the Measurement B µ B

Basic Idea of the Measurement B E µ B d n??

Basic Idea of the Measurement B E µ B d n?? Look for a shift in the Larmor frequency of 2 E d n as E is flipped relative to B

The Ramsey Separated Oscillator Method 1. 2. 3. Spin up neutron... Apply π/2 spin flip pulse... Free precession... N.F. Ramsey, Phys.Rev.76 996 (1949) 4. Second π/2 spin flip pulse.

Kyoto Seminar

λ>> interatomic spacing; neutrons see Fermi potential V F Critical velocity for reflection: ½mv c2 = V F Ultracold neutrons (UCN): v 6 m/s: total internal reflection possible. n

λ>> interatomic spacing; neutrons see Fermi potential V F Critical velocity for reflection: ½mv c2 = V F Ultracold neutrons (UCN): v 6 m/s: total internal reflection possible. n v c depends on orientation of neutron spin, so can polarise by transmission.

Prepare neutrons in polarisation state 1, execute Ramsey cycle and measure the number left in states 1 and 2, repeat with B and E fields parallel ( ) and anti-parallel ( ), then: ( N N N 2αETN N + 1 2 1 2 dn = ) h Where α = the product of the neutron polarisation and the analyzing power, E is the applied field strength, T is the storage time, and N is the number of neutrons detected The resulting statistical sensitivity is: σ ( d ) n = 2 α h ET N Must add any systematics to this to determine the sensitivity of the experiment.

The Institute Laue-Langevin

Neutron Turbine Vertical guide Reactor Core Liquid D 2 moderator

Neutron Energy Annoyances Cold neutrons have: E 0.1 1 mev v 100 m/s λ 1-10 Å T 10 K Ultra-cold neutrons have: E 1 10 nev v 0-15 m/s λ 1000 Å T real small (normally not thermal)

Four-layer mu-metal shield Quartz insulating cylinder High voltage lead Coil for 10 mg magnetic field Main storage cell Hg u.v. lamp Vacuum wall Upper electrode PMT to detect Hg u.v. light Current Room- Temperature nedm Experiment RF coil to flip spins Magnet UCN polarising foil S N Mercury prepolarising cell Hg u.v. lamp UCN guide changeover Ultracold neutrons (UCN) UCN detector

Top view: PMT Hg Co-magnetometer 30000 20000 T' Digitised voltage (bits) 10000 0-10000 -20000-30000 -40000-50000 0 2000 4000 6000 8000 10000 12000 ADC reading no.

Kyoto Seminar

1999 Results (PG Harris et al, PRL 82, 904 (1999)) d n = (1.9 ± 5.4) 10 26 e cm d n 6.3 10 26 e cm (90% c.l.)

If neutron were the size of the earth... +e -e x... current EDM limit would correspond to charge separation of x 10 µ.

10-20 10-22 10-24 Multi Higgs 10-30 10-32 Left-Right Electromagnetic SUSY φ 1 φ α/π neutron Range of d (e cm) in various models 10-34 10-36 Standard Model 10-38 electron

Experimental Limit on d (e cm) 10-20 10-30 neutron: 1960 1970 1980 1990 Cited ~250 times already 2000 10-20 10-22 10-24 Multi Higgs 10-30 10-32 10-34 Left-Right Electromagnetic SUSY φ 1 φ α/π neutron Range of d (e cm) in various models Standard Model 10-36 10-38 electron

Recall: σ Published Data ( dn ) = 2 α ET Current Room-Temp h N Cryogenic Experiment E T N

Recall: σ Published Data ( dn ) = 2 α ET Current Room-Temp h N Cryogenic Experiment 0.5 E T N 4.5 kv/cm 130 s 13000

Recall: σ Published Data ( dn ) = 2 α ET Current Room-Temp h N Cryogenic Experiment 0.5 0.7 New polarisers E on Si wafers T N 4.5 kv/cm 130 s 13000

Recall: σ Published Data ( dn ) = 2 α ET Current Room-Temp h N Cryogenic Experiment 0.5 0.7 E 4.5 kv/cm 12 kv/cm Shorter, wider T bottle130 s 4 He buffer gas Better cleaning N methods 13000

Recall: σ Published Data ( dn ) = 2 α ET Current Room-Temp h N Cryogenic Experiment 0.5 0.7 E 4.5 kv/cm 12 kv/cm T N 130 s 13000 130 s

Recall: σ Published Data ( dn Larger volume bottle 0.5 Better neutron guides Loss of 50% from source E 4.5 kv/cm ) = 2 α ET Current Room-Temp 0.7 12 kv/cm h N Cryogenic Experiment T N 130 s 13000 130 s 14000

Recall: σ Published Data ( dn ) = 2 α ET Current Room-Temp h N Cryogenic Experiment 0.5 0.7 E 4.5 kv/cm 12 kv/cm T N 130 s 13000 130 s 14000

Overall factor of ~3 improvement Published Data Current Room-Temp Cryogenic Experiment 0.5 0.7 E 4.5 kv/cm 12 kv/cm T N 130 s 13000 130 s 14000

1999 Results (PG Harris et al, PRL 82, 904 (1999)) 10 8 6 4 1999 limit set here EDM (10-25 e.cm) 2 0 500 700 900 1100 1300 1500 1700 1900 2100-2 -4-6 -8-10 Stat. limit now 1.54 x 10-26 e.cm d n = (1.9 ± 5.4) 10 26 e cm d n 6.3 10 26 e cm (90% c.l.)

50 40 30 20 10 0 EDM (10-25 e.cm) 1 18 35 52 69 86 103 120 137 154 171 188 205 222 239 256 273 290 307 324 341 358 375 392 409 426 443 460 477 494 511 528 545 562 579 596 613 630 647 664 681 698-10 -20-30 -40-50 New nedm data χ 2 /ν = 1.13; CL = 0.7%! Smells fishy... Kyoto Seminar What s going on here?

Conspiracy Theory Two effects: B Br r z and, from Special Relativity, extra motion-induced field B = r r 1 v E 2 γ c

B net Geometric Phase B r B net B v... so particle sees additional rotating field B v Frequency shift E Looks like an EDM B v B net B r B net

Field Gradient Effect Verified in data: Measured EDM depends upon applied magnetic field gradient, with exactly predicted dependence Mercury magnetometer now provides our largest systematic! More precise field measurements will allow us to compensate this effect to ~4x10-27 e.cm Magnetic Field Down Magnetic Field Up 30 25 20 False EDM (1E-25 e.cm) 20 10 0-12 -7-2 3 8 13-10 -20 15 10 5 0-12 -10-8 -6-4 -2 0 2 4 6 8-5 -10-15 -30 db/dz (nt/m) -20 db/dz (nt/m)

Statistical Dipole & quadrupole shifts Enhanced GP dipole shifts 1.54E-26 e.cm 6E-27 e.cm (E x v)/c 2 from translation 1E-27 e.cm (E x v)/c 2 from rotation 1E-27 e.cm Light shift: direct B fluctuations E forces distortion of bottle Tangential leakage currents AC B fields from HV ripple Light shift: GP effects Error Budget for Total Data Set 4E-27 e.cm 8E-28 e.cm 7E-28 e.cm 4E-28 e.cm 1E-28 e.cm <1E-28 e.cm included

d n = (-0.31 ± 1.54 ± 1.00) x 10-26 e.cm My colleagues announced new results at SUSY05 (preliminary) New (preliminary) limit: d n < 3.1 x 10-26 e.cm (90% CL) Preprint expected soon

How will we do better?

Do we want to do better? Existing limits are already a challenge for theorists for instance, the strong interaction CP phase θ s must be 10-10 Many other models are also challenged, for instance the natural scale for nedm caused by CP violation in SUSY models is 10-23 to 10-24 e cm. We have therefore already learned something significant about SUSY if SUSY exists, there is some structure to the theory to suppress CP violation.

Do we want to do better? If we can push our sensitivity to ~ 10-28 e cm, then either: We will observe an nedm SUSY is not a property of nature (see below) CP violation is an approximate symmetry of nature CP violation has an off-diagonal structure, or there are large cancellations, or some as-of-yet unknown other mechanism strongly suppresses EDMs If SUSY does not exist, we still must explain the baryon asymmetry so investigations sensitive to new sources of CP violation are critical.

Cryogenic nedm Collaboration University of Sussex - K. Green, M.G.D. van der Grinten, P.G.Harris, J.M.Pendlebury, D.B.Shiers, K.Zuber RAL - S.N.Balashov, M.A.H.Tucker, D.L.Wark University of Oxford - H.Kraus, S. Henry Kure University H. Yoshiki Visiting Scientists P. Iaydjiev and S. Ivanov.

How will we do better? Need a new source of UCN. 11 K; 2π/k = 8.9 Å Landau-Feynman dispersion curve for 4 He excitations To use this we need. Dispersion curve for free neutrons Golub and Pendlebury, Phys. Lett. A53 133(1975)

Entire Cryostat and pumps provided by Hajime Yoshiki from Kure

Next Generation Experiment

Detectors that will work in 0.5K Development funded by PPARC Blue Skies Detector Fund LHe ORTEC ULTRA silicon detectors Coated with a thin layer of 6 LiF Observe n+ 6 Li α+t 7 6 5 They work, although the efficiency 4 3 2 1 could be further improved by lowering 0 noise 0 100 200 300 400 500 600 700 800 900 1000 See C.A.Baker et al., NIM A487 511-520 (2002) Counts/sec 14 13 12 11 10 9 8 Channel F 7%

Must demonstrate that the production A velocity-selected beam of cold neutrons passes through LHe, the UCN are bottled and detected mechanism really works 1.19±0.18 UCN cm -3 s -1 expected, 0.91± 0.13 observed See C.A.Baker et al., Phys.Lett. A308 67-74 (2002)

Show that polarisation is retained Inferred polarisation during downscattering No Fe polariser Polariser aligned Polariser anti-aligned UCN polarisation is (0.93 ± 0.10) of the polarisation of the incoming beam

Kyoto Seminar

SCV Ramsey Cell

All New Magnetometry System SQUID Magnetometers Developed at Oxford for CRESST Sensitivity sufficient to monitor field Measure field coupling a loop rather than volume average, therefore need quite a few

Ramsey Cell and SF Vessel

Kyoto Seminar

Recall: σ Published Data ( dn ) = 2 α ET Current Room-Temp h N Cryogenic Experiment 0.5 0.7 0.9 E 4.5 kv/cm 12 kv/cm 40 kv/cm T N 130 s 13000 130 s 14000 300 s 700000

Kyoto Seminar

The Competition A group at KEK/Osaka is building an experiment to be used at JPARC A group at PSI based around the group from PNPI are proceeding with an experiment. A very large American collaboration is proposing an experiment at the SNS their proposal lists 36 names, requests $11M, with a target date for first data of 2007. There is a German group at the new reactor in Munich. There are also experiments to measure the eedm, atomic EDMs, and even the muon EDM. Competition is good, but uncomfortable when you are the target of it! However any observation would need to be confirmed extended, so many experiments are needed.

Conclusions Strong evidence exists from astrophysical measurements that CP violation exists in some asof-yet undiscovered properties of fundamental interactions. Particle EDMs offer a sensitive probe of such new physics (at very modest cost) the final nedm limit from the existing experiment extends the sensitivity by another factor of 2. We have not yet reached any fundamental limits to increased sensitivity. The Japanese have made a very significant contribution to our new cryogenic experiment, and are welcome to help exploit it. Other EDM experiments are just as important.

Kyoto Seminar

Experimental Limit on d (e cm) 10-20 10-30 neutron: 1960 1970 1980 1990 2000 10-20 10-22 10-24 Multi Higgs 10-30 10-32 Left-Right Electromagnetic SUSY φ 1 φ α/π neutron Range of d (e cm) in various models 10-34 10-36 Standard Model 10-38 electron

Kyoto Seminar

Experimental Limit on d (e cm) 10-20 10-30 neutron: electron: 1960 1970 1980 1990 2000 10-20 10-22 10-24 Multi Higgs 10-30 10-32 Left-Right Electromagnetic SUSY φ 1 φ α/π neutron Range of d (e cm) in various models 10-34 10-36 Standard Model 10-38 electron

199 Hg Electric Dipole Moment Optically pumped 199 Hg atoms precess in B, E fields, modulating absorption signal hep-ex/0012001 Dual cells remove effect of drifts in B Result: d( 199 Hg) < 2.1 x 10-28 e cm Provides good limit on CPv effects in nuclear forces, inc. θ QCD If from valence neutron, corresponds to d n <2x10-25 ecm, because of electrostatic shielding.

Experimental Limit on d (e cm) 10-20 10-30 neutron: electron: 199 Hg atomic EDM 1960 1970 1980 1990 2000 Relative value of neutron, electron, atomic (and µ and τ) EDM is modeldependent, must pursue all 10-20 10-22 10-24 Multi Higgs 10-30 10-32 10-34 10-36 10-38 Left-Right Electromagnetic SUSY φ 1 φ α/π neutron Range of d (e cm) in various models Standard Model electron