EDM Measurements using Polar Molecules

Transcription

1 EDM Measurements using Polar Molecules B. E. Sauer Imperial College London J. J. Hudson, M. R. Tarbutt, Paul Condylis, E. A. Hinds Support from: EPSRC, PPARC, the EU

2 Two motivations to measure EDMs EDM violates T symmetry Deeply connected to CP violation and the matter-antimatter asymmetry of the universe EDM is effectively zero in standard model but big enough to measure in non-standard models direct test of physics beyond the standard model

3 CP from particles to atoms (main connections) field theory CP model electron/quark level nucleon level nuclear level atom/molecule level Higgs SUSY Left/Right d e d q d n d para d c q Strong CP GG% NNNN Schiff moment d dia

4 Status of theory and experiment of electron EDM Predicted values for the electron edm d e (e.cm) Multi Higgs Left - Right MSSM φ ~ 1 Standard Model MSSM φ ~ α/π Thallium experiment (2002) d e < 1.6 x e.cm Limited by stray and motional magnetic fields New approach required

5 Using Dirac theory, the first order edm energy is H 1 = Ψ 0 -d e β Σ Ε Ψ 0. As a matrix equation this becomes 0 0 H 1 = f 0 f 0 g 0 0 2d g 0 e σ E g 0 is only appreciable at small r, where E Zr/r ^ 2, so H 1 g 0 2d e σ Zr g 0. r 2 ^ only acts on relativistic part of the wavefunction

6 expanding the wavefunction Ψ 0 in angular eigenstates: which to leading order gives Ψ 0 = a s s + a p1/2 p + Η 1 8a s a p1/2 Z(Zα) 2 ^ d e σ λ Z from electric field near nucleus (Zα) 2 from the small (relativistic) wavefunction ^ λ is the axis defined by the s-p mixing In atoms, a s a p1/2 ~ E ext, and for Z=70 Η 1 atom ~ d e 100 E ext. (Sandars, 1965)

7 In heavy polar molecules... the wavefunction is already mixed along the internuclear axis λ: a s a p ~ 0.1 a very modest external field can polarize λ along σ Η 1 molecule ~ d e 10 (in atomic units of field!) BaF YbF PbO* PbF HgF Effective field (GV/cm)

8 Polarization of YbF 20 Field ηe (GV/cm) Applied field E (kv/cm) for Tl atoms η is only ~ -600 E= 130kV/cm ηe = 0.08GV/cm

9 The basic idea of the experiment amplification E ηd e σ Interaction energy -d e ηe σ electric field system containing electron odd under P and T

10 The lowest two levels of YbF in an electric field E X 2 Σ + (N = 0,v = 0) +d e ηe F=1 -d e ηe 0 F=0 Goal: to measure the splitting 2d e ηe

11 Interferometer to measure 2d e ηe E +1 B 0 0? Pump A-X Q(0) F=1 Split MHz π pulse Recombine 170 MHz π pulse Phase difference = 2 (µ B B+d e ηe)t/h Probe A-X Q(0) F=0

12 Part of the optical setup

13 Interferometer results Scan a small magnetic field, measure the 0 signal. Fluorescence signal Phase difference = 2(µ B B+d e ηe ext )T/h

14 Measuring the edm -E E Detector count rate 4d e ηet/h δφ = 4d e ηet/h -B 0 B 0 Applied magnetic field

15 Histogram of measured d e /σ Phys. Rev. Lett. 89, (2002) Mean value: (-0.2±3.2) e.cm 90 mhz pure shot noise Extremely robust against systematic errors from B stray

16 Magnetic systematics? No coupling µ v E to motional magnetic field electron spin is coupled to internuclear axis and internuclear axis is coupled to E Yb + µ F - E < µ E > = 0 no motional systematic error

17 YbF is practically immune to B : = kV/cm F=1 F=0 Extra splitting is suppressed by the factor µ B2 B z B, 10-10! 2 Not unique to molecules, e.g. Xe*

18 bandhead at nm Why isn t the result better? P(8) 174 Q(0) 174 P(9) 174 Q(1) GHz GHz 174 Q(0) 172 P(8) 176 P(9) GHz

19 Signal to Noise ratio S N = d eηe ext T/h I 0 t 1/2 I B + I 0 /2 Cold molecules might help with: I o more molecules in ground rotational state I B less background from overlapping transitions T coherence time could be much longer for trapped molecules (1s vs. 1ms)

20 Supersonic YbF beam delay generator YAG laser 1064 or 532 nm 20 mj in 10 ns PMT scan 10 bar Ar dye laser 550 nm solenoid valve target skimmer

21 Cooling the rotational temperature 176 P(9) 172 P(8) T=1500K Fluorescence Signal 174 Q(0) T=10K T=5K Laser frequency offset (GHz)

22 Our approach: Cold slow molecules supersonic expansion gives low rotational temperature, narrow velocity distribution Xe carrier gas gives slow center-of-mass energy for Stark decelerator laser ablation is well suited to YbF, BaF etc. trap τ ~ 1s supersonic source decelerator pump E B split recombine probe interferometer

23 YbF 300 m/s Proposed decelerator for YbF Potential Energy off Potential Energy with switched fields off off off off 100 stages of 150 kv/cm can bring the YbF to rest

24 Prototype YbF decelerator Short alternating gradient decelerator built by Rijnhuizen/Berlin group of G. Meijer and R. Bethlem Test result using CO metastable CO signal time of flight (ms)

25 Signal:noise figures 2002 result supersonic beam cold cloud background 150kHz 640kHz 40kHz fringe height 1.5 khz 160 khz 10 khz coherence time 1.5 ms 1 ms 1 s d e in 1 day e cm e cm e cm long time = narrow fringes

26 Current status of EDMs neutron: electron: YbF expt d e.cm Multi Higgs Left-Right Electromagnetic SUSY φ 1 φ α/π d(muon) < d(proton) < d(neutron) < d(electron) < cold molecules