Measurement of Permanent Electric Dipole
Moments of Proton, Deuteron and Light
Nuclei in Storage Rings
J. Pretz
RWTH Aachen/ FZ Jülich
on behalf of the JEDI collaboration
SSP2012, Groningen June, 2012
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Outline
Electric Dipole Moments (EDMs)
What is it?
Why is it interesting?
What do we know about it?
How to measure it?
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What is it?
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Electric Dipole Moments: What is it?
EDM: Permanent spatial separation of positive and
negative charges
Water molecule: d = 2 · 10−9 e· cm
Water molecule can have large dipole moment because
ground state has two degenerate states of different parity
This is not the case for proton.
Here the existence of a permanent EDM requires both T
and P violation, i.e. assuming CPT invariance this implies
CP violation:
That makes it interesting!
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T and P violation of EDM
~d: EDM
µ
~ : magnetic moment
–
d~
µ
~
P
+
+
T
+
–
CPT
T violation → CP violation
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Why is it interesting?
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Why is it interesting?
CP violation of Standard Model predicts Proton EDM
< 10−31 e·cm
This corresponds to a separation of two u−quarks from a
d−quark by ≈ 10−30 cm, i.e 10−17 of the proton radius!
1.6fm
10−17fm
d~
+ 23
+ 23
− 13
Not reachable experimentally in foreseeable future
Extensions of Standard Model predict EDM as large as
10−24 e·cm
Sources of CP outside the realm of SM are needed to
explain matter/anti-matter imbalance in universe
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What do we know about
(hadron) EDMs?
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What do we know about (hadron) EDMs?
Particle/Atom
Neutron
199 Hg
→ Proton
Current Limit/e·cm
< 3 · 10−26
< 3.1 · 10−29
< 7.9 · 10−25
Deuteron
?
3 He
?
direct measurement only for neutron
proton deduced from atomic EDM limit
no measurement for deuteron (or other nuclei)
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History of Neutron EDM
50 years of effort
Extensions of SM allow for large EDMs
from K. Kirch
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Sources of CP violation
CP can have different sources
Weak Interaction (unobservably small)
QCD θ term (limit set by neutron EDM measurement)
———– Part of Standard Model ———–
sources beyond SM
⇒ It is important to measure neutron and proton and
deuteron and . . . EDMs in order to disentangle various sources
of CP violation.
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How to measure it?
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How to measure it?
General Idea:
For all edm experiments (neutron, proton, atom, . . . ):
~
Interaction of ~d with electric field E
For charged particles: apply electric field in a storage ring:
d~s ~ ~
∝d ×E
dt
~s
~
E
φ
Wait for build-up of vertical polarization s⊥ ∝ |d|, then
determine s⊥ using polarimeter
d~s
~ × ~s
In general:
=Ω
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“Thomas-BMT” formula
~ =
Ω
e~
~
mc [GB+
G−
1
γ 2 −1
~
~ ~v × B)]
~ 1 η(E+
~v × E+
2
e~ ~
~
~d = η e~ S,
~µ = 2(G + 1)
S
2mc
2m
Several Options:
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“Thomas-BMT” formula
~ =
Ω
e~
~
mc [GB+
G−
1
γ 2 −1
~
~ ~v × B)]
~ 1 η(E+
~v × E+
2
e~ ~
~
~d = η e~ S,
~µ = 2(G + 1)
S
2mc
2m
Several Options:
1
Pureelectric ring
1
with G − 2
= 0 , works only for G > 0
γ −1
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“Thomas-BMT” formula
~ =
Ω
e~
~
mc [GB+
G−
1
γ 2 −1
~
~ ~v × B)]
~ 1 η(E+
~v × E+
2
e~ ~
~
~d = η e~ S,
~µ = 2(G + 1)
S
2mc
2m
Several Options:
1
2
Pureelectric ring
1
with G − 2
= 0 , works only for G > 0
γ −1
~ B
~ ring
Combined
E/
1
~
~ =0
~v × E
GB + G − 2
γ −1
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“Thomas-BMT” formula
~ =
Ω
e~
~
mc [GB+
G−
1
γ 2 −1
~
~ ~v × B)]
~ 1 η(E+
~v × E+
2
e~ ~
~
~d = η e~ S,
~µ = 2(G + 1)
S
2mc
2m
Several Options:
1
2
3
Pureelectric ring
1
with G − 2
= 0 , works only for G > 0
γ −1
~ B
~ ring
Combined
E/
1
~
~ =0
~v × E
GB + G − 2
γ −1
Pure magnetic ring
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Required field strength
G=
proton
g−2
2
1.79
p/GeV/c
ER /MV/m
BV /T
0.701
10
0
deuteron
−0.14
1.0
−4
0.16
3 He
−4.18
1.285
17
-0.05
Ring radius ≈ 40m
Smaller ring size possible if BV 6= 0 for proton
GBcβγ 2
E=
1 + Gβ 2 γ 2
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1. Pure Electric Ring
Brookhaven Proposal
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~ B
~ ring
2. Combined E/
Under discussion in Jülich
(design: R. Talman)
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3. Pure Magnetic Ring
Main advantage:
Experiment can be performed at the existing (upgraded) COSY
(COoler SYnchrotron) in Jülich on a shorter time scale!
COSY provides (polarized ) protons and deuterons with
p = 0.3 − 3.7GeV/c ⇒ Ideal starting point
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3. Pure Magnetic Ring
e~
~ + 1 η~v × B
~
Ω=
GB
mc
2
Problem:
Due to precession caused by magnetic moment, 50% of time
longitudinal polarization components is || to momentum, 50% of
the time it is anti-||.
~ ∗ = ~v × B
~
E
~s
p~
50% ~s˙ d = ⊗
J
~
B
50% ~s˙ d = J
~∗
E
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3. Pure Magnetic Ring
e~
~ + 1 η~v × B
~
Ω=
GB
mc
2
Problem:
Due to precession caused by magnetic moment, 50% of time
longitudinal polarization components is || to momentum, 50% of
the time it is anti-||.
~ ∗ = ~v × B
~
E
E ∗ field in the particle rest frame
tilts spin due to EDM up and down
~s
p~
⇒ no net EDM effect
50% ~s˙ d = ⊗
J
~
B
50% ~s˙ d = J
~∗
E
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3. Pure Magnetic Ring
e~
~ + 1 η~v × B
~
Ω=
GB
mc
2
Problem:
Due to precession caused by magnetic moment, 50% of time
longitudinal polarization components is || to momentum, 50% of
the time it is anti-||.
~ ∗ = ~v × B
~
E
E ∗ field in the particle rest frame
tilts spin due to EDM up and down
~s
p~
⇒ no net EDM effect
> 50% ~s˙ d = ⊗
Use resonant “magic Wien-Filter”
~ + ~v × B
~ = 0):
in ring (E
~
B
∗
E = 0 → part. trajectory is not
affected but
B ∗ 6= 0 → mag. mom. is influenced
⇒ net EDM effect can be observed!
J
<50%
50% ~s˙ d = J
~∗
E
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Summary of different options
1.) pure electric ring
(BNL)
2.) combined ring
(Jülich)
3.) pure magnetic ring
(Jülich)
,
~ field needed
no B
/
works only for p
~ and B
~
both E
required
existing (upgraded) COSY lower sensitivity
ring can be used ,
shorter time scale
works for p, d, 3 He, . . .
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Statistical Sensitivity
~
σ≈p
NfT τp PEA
P
beam polarization
0.8
τp
Spin coherence time/s
1000
E
Electric field/MV/m
10
A
Analyzing Power
0.6
N
nb. of stored particles/cycle
4 × 107
f
detection efficiency
0.005
T
running time per year/s
107
⇒ σ ≈ 10−29 e·cm/year (for magnetic ring ≈ 10−24 e·cm/year)
Expected signal ≈ 3nrad/s (for d = 10−29 e·cm)
(BNL proposal)
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Systematics
One major source:
Radial B field mimics an EDM effect:
Difficulty: even small radial magnetic field, Br can mimic
EDM effect if :µBr ≈ dEr
Suppose d = 10−29 e·cm in a field of E = 10MV/m
This corresponds to a magnetic field:
dEr
10−22 eV
Br =
=
≈ 3 · 10−17 T
µN
3.1 · 10−8 eV/T
Solution: Use two beams running clockwise and counter
clockwise, Separation of the two beams is sensitive to Br
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Main Challenges
Spin Coherence Time (SCT)≈ 1000s
Beam positioning ≈ 10nm (relative between CW-CCW)
Polarimetry on 1 ppm level
Field Gradients ≈ 10MV/m
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Polarimeter
Principle: Particles hit a target:
Left/Right asymmetry gives information on EDM
Up/Down asymmetry gives information on g-2
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Polarimeter
Cross Section &
Analyzing Power
for deuterons
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Polarimeter
Available at COSY for tests:
EDDA polarimeter
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Results on Spin Coherence Time (SCT)
Spins decohere during storage time
very preliminary results form Cosy run May 2012 using
correction sextupole
⇒ SCT of ≈ 200s already reached
(Ed. Stephenson)
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pEDM at Brookhaven
Time-lines:
2013-2014
2014
R&D preparation
final ring design
2015-2017
ring/beam-line construction
2017-2018
Installation
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Stepwise approach of JEDI project in Jülich
JEDI = Jülich Electric Dipole Moment Investigation
(Collaboration since March 2012, ≈ 70 members, still growing)
1 Spin coherence time studies
Systematic Error studies
2 COSY upgrade
first direct measurement
at 10−24 e· cm
3 Build dedicated ring for
p,d and 3 He
4 EDM measurement
at 10−29 e· cm
COSY
COSY
COSY
Dedicated
ring
Time scales: Steps 1 and 2 <5 years
Steps 3 and 4 >5 years
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Storage Ring EDM Efforts
Common R&D work
• Spin Coherence Time
• BPMs
• Spin Tracking
• Polarimetry
• ...
Jülich
BNL
• all electric ring (p)
• first direct measuremet
with upgraded COSY
• all-in-one ring (p,d,3He)
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Summary
EDM of (charged) hadrons are of high interest to
disentangle various sources of CP violation searched for to
explain matter - antimatter asymmetry in the Universe
Measurements of p,d and 3 He needed in addition to
neutron
efforts at Brookhaven and Jülich to perform such
measurements
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EDM Workshop at ECT∗ Trento
EDM Searches at Storage Rings
October 1-5, 2012
http://www.ectstar.eu
Organizers:
Frank Rathmann (Jülich)
Hans Ströher (Jülich)
Andreas Wirzba (Jülich)
Mei Bai (BNL)
William Marciano (BNL)
Yannis Semertzidis (BNL)
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