1. Searching for the electron’s EDM
Is the electron round?
Searching for the electron’s EDM
E.A. Hinds
Centre for Cold Matter
Imperial College London
Les Houches, 24 September 2014

2. How a point electron gets structure+
How a point electron gets structure
point electron -
polarisable vacuum with increasingly
rich structure at shorter distances:
(anti)leptons, (anti)quarks, Higgs (standard model)
beyond that: supersymmetric particles ………?

3. T - - + + Electric dipole moment (EDM) If the electron has an EDM,
spin + -
edm
If the electron has an EDM,
nature has chosen one of these,
breaking T symmetry
. . . CP

4. Theoretical estimates of eEDM
eEDM (e.cm)
10-22 e g
selectron
10-24
Left -Right
other
SUSY
Multi Higgs
MSSM
The interesting
region of sensitivity
10-26
10-28
10-30
10-32
Insufficient CP
to make universe
of matter
10-34
10-36
Standard Model 4

5. The magnetic moment problem
Suppose de = 5 x e.cm
(the region to explore)
= 1 x e.a0
In a field of 10kV/cm de   E _ 1 nHz ~ E
When does mB.B equal this ?
B _ 1 fG ~
de
This is very small
electron 5

6. Structure-dependent relativistic factor  Z3
A clever solution
de 
Sandars
amplification
atom/molecule
containing electron E
electric field
energy
-de E•
F P
Polarization factor
Structure-dependent relativistic factor  Z3
For more details, see E. A. H. Physica Scripta T70, 34 (1997) 6

7. Our experiment uses a molecule – YbF
Amplification in YbF
15 GV/cm
EDM interaction energy is a million times larger (mHz)
needs “only” nG stray B field control 7

8. E The lowest two levels of YbF
X2S+ (N = 0,v = 0)
-dehE
+dehE + - E
| -1 >
| +1 >
F=1
170 MHz
| 0 >
F=0
Goal: measure the splitting 2dehE to <1mHz 8

9. How it is done rf pulse sin2f cos2f F=1 F=0 PMT HV+ 3K beam Pulsed YbF
Probe
A-X Q(0) F=0
PMT
Ch 15 Cold Molecules, eds. Krems,
Stwalley and Friedrich, (CRC Press 2009) B
HV+
rf pulse
3K beam
Pulsed YbF
beam
HV-
Pump
A-X Q(0) F=1
1.0
0.0
Fluorescence
cos2f
sin2f
F=1
F=0 1

10. Measuring the edm Interferometer phase f = 2(mB + dehE)t/h - -E E -B
Detector count rate B
Applied magnetic field 10

11. Modulate everything spin interferometer ±E ±B ±B ±rf1f ±rf2f ±rf1a
signal
±rf2a
±rf
±laser f
Generalisation of phase-sensitive detection
Measure all 512 correlations.
E·B correlation gives EDM signal
9 switches:
512 possible correlations 11

12. ** Don’t look at the mean edm **
We don’t know what result to expect.
Still, to avoid inadvertent bias we hide the mean edm.
An offset is added that only the computer knows.
More important than you might think.
e.g. Jeng, Am. J. Phys. 74 (7), 2006. 12

13. Measuring the other 511 correlations
mean s
mean/s
fringe slope calibration
beam intensity
f-switch changes rf amplitude
E drift
E asymmetry
inexact π pulse
The rest are zero (as they should be) !
Only now remove blind from EDM 13

14. 2011 Result: Hudson et al. Nature 473, 493 (2011)
6194 measurements (~6 min each) at 10 kV/cm. 2 1 -1 -2
EDM (10-25 e.cm)
25 million beam shots
de = (-2.4  5.7  1.5) ×10-28 e.cm
systematic - limited by statistical noise
68% statistical
Details in
Kara et al. NJP 14, (2012) 14

15. de = (−2.1 ± 3.7stat ± 2.5syst) × 10−29 (ten x better)
arXiv: (October 2013)
3D1 metastable state
see
Additional reversal
(-doublet)
helps some systmatics
But, signal limited by
2 ms decay lifetime
de = (−2.1 ± 3.7stat ± 2.5syst) × 10−29 (ten x better)

16. eEDM (e.cm) New excluded region MSSM Multi Higgs
10-22
de < 9 x e.cm
New excluded region
10-24
Left -Right
other
SUSY
Multi Higgs
MSSM
10-26
YbF
10-28
ThO
We plan to
explore this region
10-30
10-32
10-34
10-36
Standard Model 16

17. How we are improving Phase 1 Small upgrades: 30 x improvement
Phase 3 Laser-cooled molecular fountain
– being developed
How we are improving
Phase 1 Small upgrades: 30 x improvement
– in progress
Phase 2 Cryogenic source: 10 x more again
– almost ready 17

18. Phase 1: Longer interferometer - finer fringes
Better magnetic shielding – lower B noise
Better field plate structure – higher E and rf control
More molecules
Better detection efficiency

19. Getting more molecules to start in N=0, F=0-
A2P1/2
F=0, 1
F=1
F=2+
F=2-
F=3
N=2 +
F=0
F=1+
F=1-
F=2
N=1 -
X2S
rf mixing (~100 MHz)
Microwave mixing (14 GHz) +
F=1
N=0
F=0
gives 7 × more molecules

20. gives 14 × more photons per molecule and 2 × molecules
getting better detection by optical cycling -
A2P1/2
F=0, 1
F=1
F=2+
F=2-
F=3
N=2 +
F=0
F=1+
F=1-
F=2
N=1 -
X2S
Move cos2f population up here
bring cos2f population back
and and detect that too
cos2f +
N=0
sin2f
detect sin2f with closed system
gives 14 × more photons per molecule and 2 × molecules

21. => access to mid 10-30 e.cm range
Phase 2 - cryogenic buffer gas source of YbF
YbF beam
Yb+AlF3
target
YAG
ablation
laser
3K He gas cell
10  more molecules/pulse
3  slower beam
=> 10  better EDM signal:noise ratio
=> access to mid e.cm range 21

22. Phase 3 – a molecular fountain
HV+
HV-
Laser cooling
rf pulse
1/2 sec flight time (instead of 1/2 ms)
=> 610-31 e.cm statistical (in 8 hrs)
detection
laser cooling
3K beam
source
guide He
Tarbutt et al. arXiv: 22

23. Current status of EDMs d(muon) < 1.9×10-19 d(proton) < 5×10-24
d e.cm
d(muon) < 1.9×10-19
neutron:
10-20
electron:
10-22
Left-Right
MSSM
Multi
Higgs
d(proton) < 5×10-24
10-24
d(neutron) < 3×10-26
YbF
10-26
10-28
d(electron) < 9×10-29
ThO
10-29
1960
1970
1980
1990
2000
2010
2020
2030 23

24. Summary Atto-eV molecular spectroscopy
e- EDM is a direct probe of physics beyond SM
Left -Right
other
SUSY
Multi Higgs
MSSM
10-24
10-22
10-26
10-28
10-30
specifically probes CP violation
(how come we’re here?)
we see a way to reach 10-31
Atto-eV molecular spectroscopy
tells us about TeV particle physics:
the electron is too round for MSSM! 24

25. Thanks to my colleagues...
Jony Hudson
Mike Tarbutt
Ben Sauer
Laser cooling:
Valentina Zhelyazkova
Anne Cournol
James Almond
Jongseok Lim
Noah Fitch
EDM measurement:
Joe Smallman
Jack Devlin
Dhiren Kara
Isabel Rabey
Buffer gas cooling:
Sarah Skoff
Nick Bulleid
Rich Hendricks
James Bumby