Motivated by the potential reach to high mass scales, many new searches for CP-violating electric dipole moments (EDM) of leptons, nucleons, atoms and molecules are underway. In many cases novel techniques are being applied to greatly increase the sensitivity to new physics (NP). The importance of improved searches in complimentary systems and the potential sensitivities of these new searches are discussed

Filippone, B. W.

Caltech Authors - Main

P
oS(FPCP2016)030
 
 Copyright owned by the author(s) under the terms of the Creative Commons 
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).  
Status of electric dipole moment (EDM) searches 
B. W. Filippone*1 
Kellogg Radiation Laboratory  
California Institute of Technology 
Pasadena, CA, USA 
E-mail: bradf@caltech.edu 
Motivated by the potential reach to high mass scales, many new searches for 
CP-violating electric dipole moments (EDM) of leptons, nucleons, atoms and 
molecules are underway. In many cases novel techniques are being applied to 
greatly increase the sensitivity to new physics (NP). The importance of 
improved searches in complimentary systems and the potential sensitivities of 
these new searches are discussed.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Flavor Physics and CP Violation 
6-9 June 2016 
Caltech, Pasadena CA, USA 
                                                         
* Speaker 
1 This work is supported by the U.S. National Science Foundation under grant 1506459 
P
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Status of electric dipole moment (EDM) searches B. W. Filippone 
2 
1. Introduction 
 
 Because electric dipole moments (EDM) are predicted to be so small in the Stand-
ard Model (SM) their sensitivity to new physics is dramatic, reaching to mass scales of 
multi-TeV [1] and in some models up to PeV [2-4].  Present experimental limits on the 
neutron [5] and electron [6] EDM are approximately 6 and 10 orders of magnitude larger 
than the SM predictions [7-8] respectively. With this significant discovery potential, a 
large number of experimental searches are underway worldwide.  
 EDM expeirments have been underway for more than 60 years, motivated by their 
unique senstitvity to new sources of CP violation. Such CP violation, beyond that present 
in the Cabbibo-Kobayashi-Maskawa quark weak mixing matrix, could signal the presence 
of new physics while also providing a resolution to the puzzle of the matter-antimatter 
asymmetry of the universe as suggested many years ago by Sakharov [9].  
2.  Complementarity of EDM probes  
 
 In order to characterize the possible sources of particle EDMs, effective 
Lagrangians are usually constructed to include both the short distance (e.g. fermion 
EDMs) and longer distance sources (e.g. hadronic contributions parametrized via pion 
exchange) of CP-violation. These are often discussed in terms of a number of parameters 
such as: 
• 𝑑𝑑𝑞𝑞 ,𝑑𝑑𝑙𝑙 - the elementary fermion EDMs for the quarks and leptons  
• ?̃?𝑑𝑞𝑞 - the so-called quark chromo-electric EDMs that include gluonic interactions 
• 𝜃𝜃𝑄𝑄𝑄𝑄𝑄𝑄 - the phase associated with the 𝐺𝐺𝜇𝜇𝜇𝜇𝐺𝐺�𝜇𝜇𝜇𝜇 term in the QCD Lagrangian  
• Other CP-violating e-N and 3-gluon parameters 
 To access these possible sources of CP-violation a number of probes and 
techniques is required. At present there are not enough observables to completely 
constrain these parameters (see eg. [10-11]).  
 
3.  Present status of EDM searches 
 
 Among the EDM searches with the greatest the published sensitivity, three of them 
provide the most competitive constraints on new physics. These are the neutron, 199Hg 
and ThO polar molecule EDM experiments. Each of these has recently provided new 
results. The neutron has a unique sensitivity to the quark EDM 𝑑𝑑𝑞𝑞 and 𝜃𝜃𝑄𝑄𝑄𝑄𝑄𝑄, while 199Hg 
is primarily sensitive to the chromo-EDM ?̃?𝑑𝑞𝑞 and ThO to the electron EDM. The quark 
EDM sensitivity for 199Hg results because it is a diamagnetic atom with the electron spins 
paired to up, greatly reducing sensitivity to the electron EDM. In contrast ThO is a polar 
molecule that can be optically pumped into a state that is highly sensitive to the electron 
EDM.  
 For the neutron, the Institut Laue-Langevin (ILL) experiment has used a more 
detailed analysis and improved calculations of systematic uncertainties as well as new 
data on internal field gradients to produce a new limit [5]:  
 
   |dn| < 3.0 x 10-26 e-cm (90% confidence limit)  
P
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Status of electric dipole moment (EDM) searches B. W. Filippone 
3 
 
This value is slightly larger than their previously published limit of 2.9 x 10-26 e-cm [12].  
  
 The 199Hg experiment, using an improved apparatus, has recently published [13] 
an improved limit of  
 
   |dHg| < 7.4 x 10-30 e-cm (95% confidence limit)  
 
which is a factor of 4 more sensitive than their previous result [14].  
 A new experiment by the ACME collaboration [6] has improved the limit on the 
electron EDM using the polar molecule ThO. In this experiment a cold beam of ThO is 
prepared via lasers in a number of initial states, passes through a region of uniform electric 
and magnetic fields and is then measured via laser beams and photodetectors to search 
for the electric-field-dependent frequency shift of the spin precession in the magnetic 
field. A schematic of the apparatus (taken from their paper) is shown in Fig. 1. A 
particularly unique handle in this experiment is the ability to reverse the effective electric 
field seen by the electron, by flipping the orientation of the polar molecule, while keeping 
the direction of the electron spin and the applied electric and magnetic field fixed (see 
Fig. 1). This allows precise studies of systematic effects associated with changing the 
external electric field. The published limit from ACME is 
 
   |de| < 8.7 x 10-29 e-cm (90% confidence limit) 
 
This is a factor of ten improvement over the previous best limits [15-16]. 
 
 
 
Figure 1: A. Schematic of the ACME apparatus for measuring the electron EDM. The beam of 
ThO is prepared and read out with lasers to measure the electric-field-dependent frequency shift 
that is characteristic of an EDM. B. Summary of state preparation of the ThO. The level diagram 
on the left shows the two states that can be selected where the effective electric field is flipped 
while the applied electric field is held fixed. Figure from ref [6].  
 
 
 While many other systems have been studied, their sensitivity to new physics is 
usually less than those discussed above, although this is a model-dependent statement. 
We will discuss many of these additional systems in the next section.  
 
P
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Status of electric dipole moment (EDM) searches B. W. Filippone 
4 
4. Future EDM sensitivity 
 
 A very active experimental program is underway to both improve the sensitivity 
of existing EDM searches and to explore additional systems.  
 For paramagnetic atoms and polar molecules, which are primarily sensitive to the 
electron EDM and the CP-violating e-N coupling, improvements in both the YbF [16] 
and ThO [6] experiments could eventually increase their sensitivity by a factor of 10. 
There is also the potential to trap ThO molecules allowing a longer measurement time 
compared to the cold molecular beam with a potential factor of 100 improvement [17]. 
New experiments using Cs, which has a different sensitivity to the effective theory 
parameters than the polar molecules, could reach sensitivities 10 – 50 times better than 
the published ThO limit using a Cs atomic fountain [18] or Cs confined in an optical 
lattice [19] . 
 New high sensitivity results from diamagnetic atoms are anticipated especially for 
highly-deformed, heavy, radioactive atoms. The high deformation and high proton 
number of these nuclei provide enhancement factors > 1000 in some cases. Programs are 
under development for 211Fr [20] and 222Rn/223Rn [21], while a first result has been 
reported for 225Ra [22] with a potential sensitivity of a factor of 100 better than the existing 
limit for 199Hg. 
 For charged particles there is the extra challenge of confining the particles in the 
presence of an electric field. The muon EDM has been studied parasitically in magnetic 
storage rings nominally used to measure (g-2) for the muon. An improved value for the 
muon EDM is expected from the latest (g-2) running at Fermilab [23]. An electrostatic 
storage ring is also being discussed [24] for measuring the proton EDM with statistical 
sensitivity that could be 1000 time better than the present neutron sensitivity. For the 
electron EDM a group at JILA is developing an ion trap for HfF+ [25] with potential 
sensitivity of a factor of 10 better than the present ThO limit.   
 The EDM of the tau can be accessed in e+-e- colliders via e+ + e-  τ+ + τ- by 
measuring the final state decay distributions. The published limit from Belle [26] will 
likely be improved in the near future using new data from both Belle and BaBar [27]. 
 Recent developments in producing new sources of higher density trapped neutrons 
(so-called ultra-cold neutrons) have led to the development of a large number of new 
experiments at existing facilities. These experiments are summarized in Table 1. Of these 
new experiments, most use trapped ultra-cold neutrons as do all of the high sensitivity 
searches – i.e. those with sensitivity < 5 x 10-28 e-cm which is nearly a factor of 100 
improvement over the existing limit. These higher sensitivity experiments are generally 
performed with measurement cells at room temperature and in vacuum using the 
techniques of the best published limit. This is not the case for one of the new techniques 
– the SNS nEDM experiment – which we will briefly discuss here.  
 The concept for the SNS nEDM experiment, as first discussed in ref. [28], is to 
use superfluid 4He as both the neutron moderator and the measurement medium. The 
moderation is done using a beam of cold neutrons scattering from the phonons in the 
superfluid to produce a relatively high density of trapped ultra-cold neutrons. In addition, 
by using the liquid 4He as an insulating dielectric much higher electric fields (by a factor 
of 5-8) can, in principle, be attained.  
 
  
 
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Status of electric dipole moment (EDM) searches B. W. Filippone 
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Experiment UCN source Cell Measurement 
Techniques 
σd  Goal 
(10-28 e-cm) 
ILL-PNPI 
           
ILL turbine 
PNPI/Solid D2  
Vacuum Ramsey technique with 
E=0 cell for magnetometer 
Phase1<100  
< 10  
ILL Crystal Cold n Beam solid Crystal Diffraction 
Non-Centrosymmetric crystal 
< 100 
PSI EDM Solid D2  Vacuum Ramsey for w, external Cs & 
3He,  Hg co-magnetometer & 
Xe or Hg comagnetometer 
Phase1 ~ 50 
Phase 2 < 5 
Munich 
FRMII 
Solid D2 Vacuum Room Temp. , Hg Co-mag., 
also external Cs magnetome-
ter 
< 5 
RCNP/TRI-
UMF 
Superfluid 
4He 
Vacuum Small vol., Xe co-mag. @ 
RCNP 
Then move to TRIUMF  
< 50 
< 5 
SNS nEDM Superfluid 
4He 
Liquid 4He Cryo-HV, 3He capture for w, 
3He co-mag. with SQUIDS & 
dressed spins, supercond. 
< 5 
JPARC  Solid D2 Vacuum Under Development < 5 
JPARC  Solid D2 Solid Crystal Diffraction 
Non-Centrosymmetric crystal 
< 10? 
LANL Solid D2 Vacuum Ramsey technique with Hg  ~ 30 
 
Table 1: Summary of the ongoing searches for a neutron EDM. The third colum refers to the 
medium used in measuring the EDM, the fourth column refers to the measurement technique and 
the last column is the potential EDM sensitivity after several years of running.  
 
    
 Co-magnetometry, where a separate species cohabitating the neutron storage 
volume is used to correct for local magnetic field fluctuations, is performed with a small 
density of polarized 3He that is added to the liquid 4He. Because of the electron cloud 
around the 3He atom, any contribution from the EDM of the 3He nucleus is dramatically 
suppressed, allowing it to be used to monitor variations of the ambient magnetic field 
with SQUID magnetometers to measure the 3He magnetization. In addition the polarized 
3He can be used as a neutron frequency monitor via the spin-dependent cross section for 
neutron capture by the 3He which produces an energetic proton and triton that generates 
UV scintillation in the liquid He which can be down-converted to visible light and 
detected with photodetectors. A schematic of this apparatus is shown in Fig. 2 and is 
described in ref. [29]. The collaboration is presently completing R&D on the most 
P
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Status of electric dipole moment (EDM) searches B. W. Filippone 
6 
challenging components and anticipates beginning integration of the components and 
conventional construction in less than 2 years. It is expected that new results for the 
neutron EDM will be forthcoming in the next several years from a number of the 
experiments listed in Table 1, with the full factor of 100 improvement in sensitivity 
obtained at least 5 years from now. 
  
 
 
 
 
 
 
 
 
 
 
Figure 2: Schematic of the nEDM apparatus for measuring the neutron EDM at the Spallation 
Neutron Source at Oak Ridge National Laboratory.  
5. Summary  
 
 Searches for EDMs continue to provide severe constraints on new sources of CP-
violation in physics beyond the Standard Model. Present limits can provide constraints 
on only a subset of possible sources of EDMs in effective theories. Future highly sensitive 
searches in new systems and greatly improved searches in existing systems would allow 
an improved determination of the parameters in the effective theory and potentially 
discover a new source of CP-violation. A number of novel technologies are being brought 
to bear to improve the sensitivity to new physics from one to three orders-of-magnitude 
depending on the system.   
 
References 
[1] M. Pospelov and A. Ritz, Electric dipole moments as probes of new physics, Annals Phys., 318: 
119-169, 2005. 
[2] D. McKeen, M. Pospelov, and A. Ritz, Electric dipole moment signatures of PeV-scale 
superpartners, Phys. Rev. D 87: 113002, 2013 . 
3He ATOMIC 
BEAM SOURCE 
3He DILUTION  
REFRIGERATOR 
CENTRAL DETEC-
TOR SYSTEM  
MAGNETIC SHIELD HOUSE 
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Status of electric dipole moment (EDM) searches B. W. Filippone 
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[3] A, Aboubrahim, T. Ibrahim, and P. Nath, Neutron electric dipole moment and probe of PeV scale 
physics Phys. Rev. D 91: 095017, 2015. 
[4] T. Ibrahim, A. Itani, and P. Nath, Electron electric dipole moment as a sensitive probe of PeV scale 
physics Phys. Rev. D 90: 055006, 2014. 
[5] J. M. Pendlebury et al., Revised experimental upper limit on the electric dipole moment of the 
neutron, Phys. Rev. D 92: 092003, 2015. 
[6] J. Baron et al.  Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron, 
Science, 343 (6168): 269-272, 2014. 
[7] I.B. Khriplovich and A.R. Zhitnitsky, What is the Value of the Neutron Electric Dipole Moment in 
the Kobayashi-Maskawa Model?  Phys. Lett., B109: 490, 1982. 
[8] M.E. Pospelov and I.B. Khriplovich, Electric dipole moment of the W boson and the electron in the 
Koyabyashi-Maskawa model, Sov. J. Nucl. Phys., 53: 638, 1991,[Yad. Fiz. 53, 1030, 1991]. 
[9] A.D. Sakharov, Violation of CP Invariance, c Asymmetry, and Baryon Asymmetry of the Universe, 
Pisma Zh. Eksp. Teor. Fiz., 5: 32, 1967. 
[10] T. Chupp and M. Ramsey-Musolf, Electric Dipole Moments:  A Global Analysis, Phys. Rev., C91: 
035502, 2015. 
[11] M. Jung, Proceedings of FPCP 2016, PoS(FPCP2015_026).  
[12] C.A. Baker et al., An improved experimental limit on the electric dipole moment of the neutron.  
Phys. Rev. Lett., 97: 131801, 2006. 
[13] B. Graner, Y. Chen ((陳宜)), E. G. Lindahl, and B. R. Heckel, Reduced Limit on the Permanent 
Electric Dipole Moment of Hg199, Phys. Rev. Lett. 116: 161601, 2016. 
[14] W.C. Griffith, et al., Improved Limit on the Permanent Electric Dipole Moment of Hg-199, Phys. 
Rev. Lett., 102: 101601, 2009. 
[15] B.C. Regan, E.D. Commins, C.J. Schmidt, and D. DeMille, New limit on the electron electric dipole 
moment,  Phys. Rev. Lett., 88: 071805, 2002. 
[16] J.J. Hudson, D.M. Kara, I.J. Smallman, B.E. Sauer, M.R. Tarbutt, et al., Improved measurement of 
the shape of the electron, Nature, 473: 493, 2011. 
[17] N. Hutzler, Private Communication. 
[18] B.J. Wundt, C.T. Munger, and U.D. Jentschura, Quantum dynamics in atomic-fountain experiments 
for measuring the electric dipole moment of the electron with improved sensitivity, Phys. Rev. X., 2:  
041009, 2012. 
[19] D.S. Weiss, Progress Toward a  de  Measurement Using Cs and Rb Atoms in Optical Lattices, 5th 
International Symposium on Lepton Moments, 2014. 
[20] C. Munger, et al., Development of a Francium Electron Electric Dipole Moment Experiment, APS 
Division of Nuclear Physics Hawaii Meeting, abstract #DD.001, 2014. 
[21] E. R. Tardiff, et al., The radon EDM apparatus, Hyperfine Interactions, 225: 197, 2014. 
[22] R.  H. Parker, et al., First Measurement of the Atomic Electric Dipole Moment of 225Ra, Phys. Rev. 
Lett. 114: 233002, 2015. 
[23] T. Bowcock, The g-2 Experiment, FPCP 2016. 
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Status of electric dipole moment (EDM) searches B. W. Filippone 
8 
[24] V. Anastassopoulos, et al., A Storage Ring Experiment to Detect a Proton Electric Dipole Moment, 
arXiv:1502.04317, 2015. 
[25] High-resolution spectroscopy on trapped molecular ions in rotating electric fields: A new approach 
for measuring the electron electric dipole moment 
A. E. Leanhardt, J. L. Bohn, H. Loh, P. Maletinsky, E. R. Meyer, L. C. Sinclair, R. P. Stutz, E. A. 
Cornell, J. Mol. Spectrosc. 270: 1, 2011. 
[26] K. Inami et al. (Belle Collaboration), Search for the electric dipole moment of the tau lepton, 
Phys.Lett. B 551: 16, 2003. 
[27] B. Echenard, Private Communication. 
[28] R Golub, SK Lamoreaux, Neutron electric-dipole moment, ultracold neutrons and polarized 3He, 
Physics Reports 237: 1, 1994. 
[29] S. Clayton, Search for an EDM of the neutron at the SNS, 5th International Symposium on Lepton 
Moments, 2014. 
 


Status of Electric Dipole Moment (EDM) searches

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 Copyright owned by the author(s) under the terms of the Creative Commons 
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).  
Status of electric dipole moment (EDM) searches 
B. W. Filippone*1 
Kellogg Radiation Laboratory  
California Institute of Technology 
Pasadena, CA, USA 
E-mail: bradf@caltech.edu 
Motivated by the potential reach to high mass scales, many new searches for 
CP-violating electric dipole moments (EDM) of leptons, nucleons, atoms and 
molecules are underway. In many cases novel techniques are being applied to 
greatly increase the sensitivity to new physics (NP). The importance of 
improved searches in complimentary systems and the potential sensitivities of 
these new searches are discussed.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Flavor Physics and CP Violation 
6-9 June 2016 
Caltech, Pasadena CA, USA 
                                                         
* Speaker 
1 This work is supported by the U.S. National Science Foundation under grant 1506459 
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Status of electric dipole moment (EDM) searches B. W. Filippone 
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1. Introduction 
 
 Because electric dipole moments (EDM) are predicted to be so small in the Stand-
ard Model (SM) their sensitivity to new physics is dramatic, reaching to mass scales of 
multi-TeV [1] and in some models up to PeV [2-4].  Present experimental limits on the 
neutron [5] and electron [6] EDM are approximately 6 and 10 orders of magnitude larger 
than the SM predictions [7-8] respectively. With this significant discovery potential, a 
large number of experimental searches are underway worldwide.  
 EDM expeirments have been underway for more than 60 years, motivated by their 
unique senstitvity to new sources of CP violation. Such CP violation, beyond that present 
in the Cabbibo-Kobayashi-Maskawa quark weak mixing matrix, could signal the presence 
of new physics while also providing a resolution to the puzzle of the matter-antimatter 
asymmetry of the universe as suggested many years ago by Sakharov [9].  
2.  Complementarity of EDM probes  
 
 In order to characterize the possible sources of particle EDMs, effective 
Lagrangians are usually constructed to include both the short distance (e.g. fermion 
EDMs) and longer distance sources (e.g. hadronic contributions parametrized via pion 
exchange) of CP-violation. These are often discussed in terms of a number of parameters 
such as: 
• 𝑑𝑑𝑞𝑞 ,𝑑𝑑𝑙𝑙 - the elementary fermion EDMs for the quarks and leptons  
• ?̃?𝑑𝑞𝑞 - the so-called quark chromo-electric EDMs that include gluonic interactions 
• 𝜃𝜃𝑄𝑄𝑄𝑄𝑄𝑄 - the phase associated with the 𝐺𝐺𝜇𝜇𝜇𝜇𝐺𝐺�𝜇𝜇𝜇𝜇 term in the QCD Lagrangian  
• Other CP-violating e-N and 3-gluon parameters 
 To access these possible sources of CP-violation a number of probes and 
techniques is required. At present there are not enough observables to completely 
constrain these parameters (see eg. [10-11]).  
 
3.  Present status of EDM searches 
 
 Among the EDM searches with the greatest the published sensitivity, three of them 
provide the most competitive constraints on new physics. These are the neutron, 199Hg 
and ThO polar molecule EDM experiments. Each of these has recently provided new 
results. The neutron has a unique sensitivity to the quark EDM 𝑑𝑑𝑞𝑞 and 𝜃𝜃𝑄𝑄𝑄𝑄𝑄𝑄, while 199Hg 
is primarily sensitive to the chromo-EDM ?̃?𝑑𝑞𝑞 and ThO to the electron EDM. The quark 
EDM sensitivity for 199Hg results because it is a diamagnetic atom with the electron spins 
paired to up, greatly reducing sensitivity to the electron EDM. In contrast ThO is a polar 
molecule that can be optically pumped into a state that is highly sensitive to the electron 
EDM.  
 For the neutron, the Institut Laue-Langevin (ILL) experiment has used a more 
detailed analysis and improved calculations of systematic uncertainties as well as new 
data on internal field gradients to produce a new limit [5]:  
 
   |dn| < 3.0 x 10-26 e-cm (90% confidence limit)  
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Status of electric dipole moment (EDM) searches B. W. Filippone 
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This value is slightly larger than their previously published limit of 2.9 x 10-26 e-cm [12].  
  
 The 199Hg experiment, using an improved apparatus, has recently published [13] 
an improved limit of  
 
   |dHg| < 7.4 x 10-30 e-cm (95% confidence limit)  
 
which is a factor of 4 more sensitive than their previous result [14].  
 A new experiment by the ACME collaboration [6] has improved the limit on the 
electron EDM using the polar molecule ThO. In this experiment a cold beam of ThO is 
prepared via lasers in a number of initial states, passes through a region of uniform electric 
and magnetic fields and is then measured via laser beams and photodetectors to search 
for the electric-field-dependent frequency shift of the spin precession in the magnetic 
field. A schematic of the apparatus (taken from their paper) is shown in Fig. 1. A 
particularly unique handle in this experiment is the ability to reverse the effective electric 
field seen by the electron, by flipping the orientation of the polar molecule, while keeping 
the direction of the electron spin and the applied electric and magnetic field fixed (see 
Fig. 1). This allows precise studies of systematic effects associated with changing the 
external electric field. The published limit from ACME is 
 
   |de| < 8.7 x 10-29 e-cm (90% confidence limit) 
 
This is a factor of ten improvement over the previous best limits [15-16]. 
 
 
 
Figure 1: A. Schematic of the ACME apparatus for measuring the electron EDM. The beam of 
ThO is prepared and read out with lasers to measure the electric-field-dependent frequency shift 
that is characteristic of an EDM. B. Summary of state preparation of the ThO. The level diagram 
on the left shows the two states that can be selected where the effective electric field is flipped 
while the applied electric field is held fixed. Figure from ref [6].  
 
 
 While many other systems have been studied, their sensitivity to new physics is 
usually less than those discussed above, although this is a model-dependent statement. 
We will discuss many of these additional systems in the next section.  
 
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4. Future EDM sensitivity 
 
 A very active experimental program is underway to both improve the sensitivity 
of existing EDM searches and to explore additional systems.  
 For paramagnetic atoms and polar molecules, which are primarily sensitive to the 
electron EDM and the CP-violating e-N coupling, improvements in both the YbF [16] 
and ThO [6] experiments could eventually increase their sensitivity by a factor of 10. 
There is also the potential to trap ThO molecules allowing a longer measurement time 
compared to the cold molecular beam with a potential factor of 100 improvement [17]. 
New experiments using Cs, which has a different sensitivity to the effective theory 
parameters than the polar molecules, could reach sensitivities 10 – 50 times better than 
the published ThO limit using a Cs atomic fountain [18] or Cs confined in an optical 
lattice [19] . 
 New high sensitivity results from diamagnetic atoms are anticipated especially for 
highly-deformed, heavy, radioactive atoms. The high deformation and high proton 
number of these nuclei provide enhancement factors > 1000 in some cases. Programs are 
under development for 211Fr [20] and 222Rn/223Rn [21], while a first result has been 
reported for 225Ra [22] with a potential sensitivity of a factor of 100 better than the existing 
limit for 199Hg. 
 For charged particles there is the extra challenge of confining the particles in the 
presence of an electric field. The muon EDM has been studied parasitically in magnetic 
storage rings nominally used to measure (g-2) for the muon. An improved value for the 
muon EDM is expected from the latest (g-2) running at Fermilab [23]. An electrostatic 
storage ring is also being discussed [24] for measuring the proton EDM with statistical 
sensitivity that could be 1000 time better than the present neutron sensitivity. For the 
electron EDM a group at JILA is developing an ion trap for HfF+ [25] with potential 
sensitivity of a factor of 10 better than the present ThO limit.   
 The EDM of the tau can be accessed in e+-e- colliders via e+ + e-  τ+ + τ- by 
measuring the final state decay distributions. The published limit from Belle [26] will 
likely be improved in the near future using new data from both Belle and BaBar [27]. 
 Recent developments in producing new sources of higher density trapped neutrons 
(so-called ultra-cold neutrons) have led to the development of a large number of new 
experiments at existing facilities. These experiments are summarized in Table 1. Of these 
new experiments, most use trapped ultra-cold neutrons as do all of the high sensitivity 
searches – i.e. those with sensitivity < 5 x 10-28 e-cm which is nearly a factor of 100 
improvement over the existing limit. These higher sensitivity experiments are generally 
performed with measurement cells at room temperature and in vacuum using the 
techniques of the best published limit. This is not the case for one of the new techniques 
– the SNS nEDM experiment – which we will briefly discuss here.  
 The concept for the SNS nEDM experiment, as first discussed in ref. [28], is to 
use superfluid 4He as both the neutron moderator and the measurement medium. The 
moderation is done using a beam of cold neutrons scattering from the phonons in the 
superfluid to produce a relatively high density of trapped ultra-cold neutrons. In addition, 
by using the liquid 4He as an insulating dielectric much higher electric fields (by a factor 
of 5-8) can, in principle, be attained.  
 
  
 
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Status of electric dipole moment (EDM) searches B. W. Filippone 
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Experiment UCN source Cell Measurement 
Techniques 
σd  Goal 
(10-28 e-cm) 
ILL-PNPI 
           
ILL turbine 
PNPI/Solid D2  
Vacuum Ramsey technique with 
E=0 cell for magnetometer 
Phase1<100  
< 10  
ILL Crystal Cold n Beam solid Crystal Diffraction 
Non-Centrosymmetric crystal 
< 100 
PSI EDM Solid D2  Vacuum Ramsey for w, external Cs & 
3He,  Hg co-magnetometer & 
Xe or Hg comagnetometer 
Phase1 ~ 50 
Phase 2 < 5 
Munich 
FRMII 
Solid D2 Vacuum Room Temp. , Hg Co-mag., 
also external Cs magnetome-
ter 
< 5 
RCNP/TRI-
UMF 
Superfluid 
4He 
Vacuum Small vol., Xe co-mag. @ 
RCNP 
Then move to TRIUMF  
< 50 
< 5 
SNS nEDM Superfluid 
4He 
Liquid 4He Cryo-HV, 3He capture for w, 
3He co-mag. with SQUIDS & 
dressed spins, supercond. 
< 5 
JPARC  Solid D2 Vacuum Under Development < 5 
JPARC  Solid D2 Solid Crystal Diffraction 
Non-Centrosymmetric crystal 
< 10? 
LANL Solid D2 Vacuum Ramsey technique with Hg  ~ 30 
 
Table 1: Summary of the ongoing searches for a neutron EDM. The third colum refers to the 
medium used in measuring the EDM, the fourth column refers to the measurement technique and 
the last column is the potential EDM sensitivity after several years of running.  
 
    
 Co-magnetometry, where a separate species cohabitating the neutron storage 
volume is used to correct for local magnetic field fluctuations, is performed with a small 
density of polarized 3He that is added to the liquid 4He. Because of the electron cloud 
around the 3He atom, any contribution from the EDM of the 3He nucleus is dramatically 
suppressed, allowing it to be used to monitor variations of the ambient magnetic field 
with SQUID magnetometers to measure the 3He magnetization. In addition the polarized 
3He can be used as a neutron frequency monitor via the spin-dependent cross section for 
neutron capture by the 3He which produces an energetic proton and triton that generates 
UV scintillation in the liquid He which can be down-converted to visible light and 
detected with photodetectors. A schematic of this apparatus is shown in Fig. 2 and is 
described in ref. [29]. The collaboration is presently completing R&D on the most 
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Status of electric dipole moment (EDM) searches B. W. Filippone 
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challenging components and anticipates beginning integration of the components and 
conventional construction in less than 2 years. It is expected that new results for the 
neutron EDM will be forthcoming in the next several years from a number of the 
experiments listed in Table 1, with the full factor of 100 improvement in sensitivity 
obtained at least 5 years from now. 
  
 
 
 
 
 
 
 
 
 
 
Figure 2: Schematic of the nEDM apparatus for measuring the neutron EDM at the Spallation 
Neutron Source at Oak Ridge National Laboratory.  
5. Summary  
 
 Searches for EDMs continue to provide severe constraints on new sources of CP-
violation in physics beyond the Standard Model. Present limits can provide constraints 
on only a subset of possible sources of EDMs in effective theories. Future highly sensitive 
searches in new systems and greatly improved searches in existing systems would allow 
an improved determination of the parameters in the effective theory and potentially 
discover a new source of CP-violation. A number of novel technologies are being brought 
to bear to improve the sensitivity to new physics from one to three orders-of-magnitude 
depending on the system.   
 
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Status of Electric Dipole Moment (EDM) searches

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