1. Electric Dipole Moments: Searches at Storage Rings
September | Hans Ströher (Forschungszentrum Jülich)

2. New physics: empirical evidence (n-oscillations, dark
matter, baryon asymmetry) doesn´t a priori point to a
specific mass scale
Direct searches („energy frontier“)
Indirect searches („precision frontier“)
Precision observables that vanish (or are suppressed)
by symmetry in the SM allow for new physics searches
with indirect reach in both
Energy scale
Strength of coupling
Example: new CP-odd sources  EDMs
Required for baryogenesis (Sakharov conditions)
Strong CP problem (suppression of q QCD) 2

3. Permanent Electric Dipole Moments (EDM) of non-
degenerate fundamental systems (particles):
A vector (d), connecting the centroids of positive and
negative charge distributions, in the direction of the
spin vector (µ). 3

4. Permanent Electric Dipole Moments (EDM) of non-
degenerate fundamental systems (particles):
violate P- and T-, and (via CPT) also CP-symmetry 4

5. Permanent Electric Dipole Moments (EDM) of non-
degenerate fundamental systems (particles):
F. Wilczek, Jan. 2014 5

6. Multiple experimental input is required to disentangle
the fundamental source(s) of EDMs:
Jordy de Vries, 2012 6

7. A huge ongoing experimental effort …
Multiple experimental input is required to disentangle
the fundamental source(s) of EDMs:
A huge
ongoing
experimental
effort … 7

8. Experimental status of EDMs: best limits for bare
nucleon (n, p), lepton (e, m), diamagnetic atom (199Hg),
paramagnetic atom (205Tl) and molecules (YbF, ThO):
Aim: few times ecm
Direct measurements:
 srEDM
ThO |de| < 8.7 x ecm 90% C.L. factor 10 in next 10 yrs
P. Harris, K. Kirch, July 2012 8

9. Charged particle (p, d) EDMs - principle of experiment:
Inject polarized particles into a storage ring
Apply radial electric field E:
Non-zero EDM  spin rotation out of the plane

10. Charged particle (p, d) EDMs - principal challenges:
Spin motion („Thomas-BMT equation“)
 effect due to magnetic moment
Way out:
(i) B = 0 („all-electric ring“)
(ii) „magic momentum“, i.e.:
(only for G > 0, proton)
„frozen spin“ method
(NB: for deuteron: combined E-B fields)

11. + - Charged particle (p, d) EDMs - principal challenges
EDMs are very (!) small:
 systematics, fake EDM effects +
<10 µm -
Neutron
1 fm
1022 fm

12. Charged particle (p, d) EDMs: technological challenges
Current solution:
 dedicated precision storage ring with two beams
(CW, CCW)
 goal: sensitivity of e cm
(stepwise approach: R&D, precursor, srEDM-ring)
pEDM: (all-electric)
one ring
dEDM: (combined E-B)
two separate rings

13. Charged particle (p, d) EDMs – Step-1: key technologies
Spin tune measurement:
Measurement at COSY:
970 MeV/c: ns = 120 kHz
u/d asymmetry dC scattering
precision ~ 10-8 (Dt = 2.6 s)
precision ~ (Dt = 100 s)
“COSY-Jülich”
PRL 115, (2015)
PR STAB 20, (2017)

14. Charged particle (p, d) EDMs – Step-1: key technologies
Spin coherence time (SCT):
Optimization of SCT at COSY:
Beam bunching
Electron cooling
Sextupole field corrections
Limit in beam current
SCT ~ 1000s
“COSY-Jülich”
PRL 117, (2016)
CERN Courier, Aug. 2016

15. Charged particle (p, d) EDMs – Step-1: key technologies
Phase locking of spin precession:
 Prerequisite for precursor
experiment …
Control/feedback at COSY:
970 MeV/c: ns = 120 kHz
Real-time synchronisation
w/ an RF-solenoid
- Control of phase s = 0.21 rad
“COSY-Jülich”
PRL 119, (2017)

16. Charged particle (p, d) EDMs – Step 2: dEDM at COSY
Precursor experiment  (w/ „RF Wien-filter“):
1st commissioning done
1st measurement in 2018
sensitivity e.cm (stat.)
NIM A 828 (2016) 116
AdG „srEDM“, No

17. Charged particle (p, d) EDMs – Step 3: final ring design
CDR (or TDR) of CPEDM-ring:
Design parameters (p):
Momentum: GeV/c
Energy: GeV
Circumference: ~ 500 m
Bending sections: ~ 10 m
Straight sections: ~ 21 m
E-Field: MV/m

18. Charged particle (p, d) EDMs – Summary:
Science case (CP-V)
New option srEDM
Stepwise approach 
JEDI at COSY:
PBC at CERN

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