nEDM @ PSI
Searching for the electric
dipole moment of the
neutron
Lepton Moments 2014
Craigville
07/22/2014
P. Schmidt-Wellenburg
The collaboration
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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•
•
•
•
14 Institutions
Eight countries / 14 institutions
48 Members
11 PhD students
PSI in Switzerland
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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Outline
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Introduction
• How do we search for the nEDM?
Status at PSI
• The UCN source
• Statistical sensitivity
Neutron/mercury gyromagnetic ratio
• Magnetometry
• Preliminary Results
The measurement technique
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
Measure the difference of precession frequencies in
parallel/anti-parallel fields:
 Δ   2 d n  E   E    2 μ n  B   B  
Best nedm limit: 2.9×10-26ecm
Baker et al., PRL97(2006)
for dn<10-26
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ωL≈30Hz for B0=1μT
Δω < 60 nHz
The Ramsey technique
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 nRF
Ramsey resonance curve
Spin “down”
neutron...
Apply /2 spin
flip pulse...
B0↑
t  2s
B0↑ + Brf
Free
precession
at ω L
Second /2
spin
flip pulse.
T  t
B0↑
Sensitivity:

T
B0↑ + BrfN
E
𝜎 𝑓 =
Visibility of resonancet 
Time of free precession
Number of neutrons
Electric field strength
2s
ℏ
2𝛼𝑇𝐸 𝑁
Ultracold neutrons (UCN)
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
σ dn


1
T
N
& systematic effects
Gravity
102 neV/m
V
V F  Nb  350 neV
Strong
VF
Storage properties are
material dependent
350 neV ↔ 8 m/s ↔ 500 Å ↔ 3 mK
7 E. Fermi & W.H. Zinn(1946),
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Storable neutrons
(UCN)
Y. B. Zeldovich, Sov. Phys. JETP (1959)389
Magnetic
∼60 neV/T
Overview
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Introduction
• How do we search for the nEDM?
Status at PSI
• The UCN source
• Statistical sensitivity
Neutron/mercury gyromagnetic ratio
• Magnetometry
• Preliminary Results
PSI UCN source
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Main shutter
Neutron guide to experiments
1m
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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UCN storage volume
Protons
590 MeV
2.2 mA
Golub, R. & Pendlebury, J. M
PLA (1975)133
Anghel, et. al
NIMA (2009) 272
UCN convertor (solid D2 @ 5K)
Spallation target
En~MeV
D2O moderator
Neutrons thermalized to 25 meV
UCN source performance
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Standard operating Pulse
Norm Pulse
UCN Counts /s
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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VAT
• 30 UCN/cm3 at beam exit
(~550 000 UCN/25 liter)
Cascade
Detector
VAT
1m glass
tube
UCN
- NiMo coating
- Stainless
steel flanges
- shutter DLC coated
The nEDM spectrometer
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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Polarization
• Optimize product 𝛼 𝑁
14000
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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N (after 30s storage)
Filling UCN
Simultaneous spin detection
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~20% increase in sensitivity
(for 2014)
Storage life time
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• Chamber made
of dPS insulator
ring and DLC
electrodes
• Two exp fit:
ts~30s
tf ~180s
• Max no. UCN
measured after
180s storage:
10 500
Transversal depolarization
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Best @ PSI
T2(B0↓) : 1158 ± 94s
T2(B0↑): 836 ± 63s
2013:
T2(B0↓/B0↑)~600s
→ Excellent magnetic field homogeneity
Neutron EDM sensitivity
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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σ
2α T E
N
RAL/Sussex/ILL*
PSI 2013
best
avg
best
avg
8.8
8.3
12
10.3
14 000
14 000
10 500
6 500
Tfree
130
130
200
180
Tduty
240
240
340
340
Α
0.68
0.51
0.62
0.57
~0.75
𝜎/d (10-25ecm)
2.0
2.8
1.5
2.8
<2
E-field
Neutrons
~7500
2013 data taking: 3266 cycles
25 days
Stopped by switch break down
2013 accumulated sensitivity 6×10-26 e.cm
* Best nedm limit:
Baker et al., PRL97(2006)
Overview
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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Introduction
• How to search for the nEDM?
Status at PSI
• The UCN source
• Statistical sensitivity
Neutron/mercury gyromagnetic ratio
• Magnetometry
• Preliminary Results
The measurement technique
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
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Measure the difference of precession frequencies in
parallel/anti-parallel fields:
 Δ   2 d n  E   E    2 μ n  B   B  
Statistical accuracy of a magnetometer correcting for a change in B should
be better than the neutron sensitivity per cycle:
δf n 
1
2π α T
N
 1 1μ H z
B  1μ T
0


δB  4 0 0fT
Mercury co-magnetometer
τ = 140s
B0 ≈ 1μT
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
PM
polarization cell
¼ wave plate
linear polarizer
Hg lamps
HgO source
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• Average magnetic field
(volume and cycle)
• σB ~ 400 fT
• τ > 100 s without HV
• s/n ~ 500 without HV
Mercury co-magnetometer
UCN precession chamber
PMT
polarization
chamber
circular polarizer unit
253.7 nm UV light,
circularly polarized
Hg source
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50m
•
•
•
Fourth harmonic generator (IR→VIS→UV)
Toptica TA FHG (20 mW @ 254 nm)
50 m away from the nEDM setup
( M. Fertl, PhD-Thesis 2013 )
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
Goal: Measure magnetic field drifts via optically detected
nuclear magnetic resonance (ODMR) with 199Hg atoms
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
Sensitivity improvement from ~400fT → <100fT
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( M. Fertl, PhD-Thesis 2013 )
Hg laser readout
Frequency ratio R = fn/fHg
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
• There are two drawbacks using the HgM
as field monitor:
• Center of mass offset
• Non-adiabaticity
𝛾Hg
≈ 8 Hz/μT
2𝜋
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UCN
𝛾n
≈ 30 Hz/μT
2𝜋
𝑣Hg ≈ 160 m/s vs. 𝑣UCN ≈ 3 m/s
CsM - array
+ further sys.
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199Hg
𝑅=
𝑓UCN
𝑓Hg
𝛾n
𝜕𝐵 Δℎ
𝐵2 ⊥
=
1∓
+
∓ 𝛿Earth + 𝛿Hg−lightshift
𝛾Hg
𝜕𝑧 𝐵0
𝐵0 2
Cesium gradiometer
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Monitoring of vertical magnetic gradients
•
•
•
•
Six HV CsM
Ten ground CsM
Stabilized laser
PID phase locked DAQ
±120kV
1
2
…
6
7
…
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Gradients from CsM
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• Field parameterized by 9 parameters (2nd order)
• The parameters are adjusted to 12
magnetometers readings using least squares
• Residuals are about 30 pT
(500 pT using only linear model)
• Jackknife procedure to estimate the error on
the gradient G
Extracting R
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𝑁up =𝑁0 1 − 𝛼 cos 𝐾
Run 6043
Χ2/dof = 1.3
𝑓
RF − 𝑅
𝑓
Hg
4𝑡
𝐾 = 2𝜋 𝑇 +
𝜋
𝑓Hg
R vs gradient
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B0 up
B0 down
𝑅=
𝑓UCN
𝑓Hg
𝛾n
𝜕𝐵 Δℎ
𝐵2 ⊥
=
1∓
+
∓ 𝛿Earth + 𝛿Hg−lightshift
𝛾Hg
𝜕𝑧 𝐵0
𝐵0 2
Frequency ratio R = fn/fHg
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
• There are two drawbacks using the HgM
as field monitor:
• Center of mass offset
• Adiabatic vs Non-adiabatic field sampling
𝛾Hg
≈ 8 Hz/μT
2𝜋
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𝛾n
≈ 30 Hz/μT
2𝜋
UCN
𝑣Hg ≈ 160 m/s vs. 𝑣UCN ≈ 3 m/s
CsM - array
+ further sys.
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199Hg
𝑅=
𝑓UCN
𝑓Hg
𝛾n
𝜕𝐵 Δℎ
𝐵2 ⊥
=
1∓
+
∓ 𝛿Earth + 𝛿Hg−lightshift
𝛾Hg
𝜕𝑧 𝐵0
𝐵0 2
Field maps
Transversal fields
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UCNs:
199Hg:
Adiabatic regime
Non-adiabatic regime
• Mapping with fluxgate and
vector cesium magnetometer
• -20<z<20, -10<r<30, Δφ = 5°
𝑅=
𝑓UCN
𝑓Hg
𝛾n
𝜕𝐵 Δℎ
𝐵2 ⊥
=
1∓
+
∓ 𝛿Earth + 𝛿Hg−lightshift
𝛾Hg
𝜕𝑧 𝐵0
𝐵0 2
Earth rotation correction
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δ E arth 

f E arth
γ n  f E arth



γ Hg  fn
fHg
5 .3  1 0

 sin  λ 


6
B0up
λ
𝑅=
𝑓UCN
𝑓Hg
𝛾n
𝜕𝐵 Δℎ
𝐵2 ⊥
=
1∓
+
∓ 𝛿Earth + 𝛿Hg−lightshift
𝛾Hg
𝜕𝑧 𝐵0
𝐵0 2
Vector light shift in 199Hg
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
Atomic energy levels are influenced by the interaction with light
Spin dependent part can be by modeled as effective magnetic field
(which only alters the Larmor frequency of the Hg atoms, not of the UCN)
Effective magnetic field depends on:
• Light intensity
• Light polarization
• Light frequency
• Light beam alignment
B0
Beff
BLS
All four parameters have to be under control for reliable calculations.
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Hg lightshift measurement
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ppm
1.5
-1.5
1.5
ppm
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• Change of light intensity 30% / 70%
• Change of relative angle B-field / light beam
𝑅=
𝑓UCN
𝑓Hg
𝛾n
𝜕𝐵 Δℎ
𝐵2 ⊥
=
1∓
+
∓ 𝛿Earth + 𝛿Hg−lightshift
-1.5
𝛾Hg
𝜕𝑧 𝐵0
𝐵0 2
Result (submitted to PLB)
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γn
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γHg
Error budget
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Conclusion
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• The UCN source delivers sufficient
statistics for data taking, potential
improvements are being identified
• The nEDM experiment is taking data,
operational reliability has been improved
during shutdown 2014
• As a test of magnetic field control we have
measured the most precise gyromagnetic
ratio of mercury-199 and neutron.
• We expect with 300 data-days until 2016 a
statistical sensitivity of σ ≲10-26 e⋅cm
Thank you for your
attention.
Ort, Datum
Philipp SchmidtWellenburg
Measurements on beam West-1
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UCN Counts /s
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Standard operating Pulse
Norm Pulse
detector
1
Cascade UCN
detector
Cs OPM
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Servo
Vector cesium magnetometer
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Cs vector magnetometer
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By
Bz
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75 fT
|B|
400 fT
Bx
10 – 20 pT
Analysis of a run
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The analysis:
Ramsey Fit
Cut on cycles
with residuals
larger than 3
(10 %)

N N
Typical χ2=1.5

A  a

N N
Mercury frequency
UCN depolarization
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Measurement of frequency ratio R as
function of vertical gradient revealed
unexpected polarization behavior and
non-linearity for large gradients.
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
Sensitivity improvement from ~400fT → ~100fT
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( R&D for n2EDM, tested in nEDM )
Hg laser readout
Gravitationally enhanced depolarization
Philipp Schmidt-Wellenburg Lepton Moments 2014, 0721-07/24/14
• Ultracold neutrons with
different energies average
differently over the storage
volume
• UCN with low energies, will
depolarize faster:
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Figures from:
P. G. Harris et al, PRD89, 016011(2014)
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→ contribute less to the average
field measurement,
→ frequency changes non-linear
B  E   B0 
B
z
hcm  E 
Side remark: nEDM sensitivity
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𝜎 𝑑n =
ℏ
2𝛼𝐸𝑇 𝑁
RAL/Sx/ILL*
E-field
Neutrons
PSI 2012
PSI 2013
best
avg
best
avg
best
avg
8.8
8.3
8.33
7.9
12
10.3
10 500
6 500
14 000
14 000 9 000 5 400
Tfree
130
130
200
200
200
180
Tduty
240
240
360
360
340
340
α
0.6
0.453
0.65
0.57
0.62
0.57
𝜎/d (10-25ecm)
2.3
3.0
2.3
3.5
1.5
2.8
2013 data taking: 3266 cycles
25 days
Stopped by switch break down
2013 accumulated sensitivity 6×10-26 e.cm
*Best nedm limit:
Baker et al., PRL97(2006)
Magnetic fields
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BT
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B0
Gradients from CsM
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• Field parameterized by 9 parameters (next-to-linear)
• The parameters are adjusted to 12
magnetometers readings using least squares
• Residuals are about 30 pT
(500 pT using linear model)
• Jackknife procedure to estimate the error on
the gradient G
fn/fHg vs gz
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Earth rotation correction
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λ
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B0up
The pseudo magnetic field
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𝑓UCN
𝑓Hg
𝛾n
𝜕𝐵 Δℎ
𝐵2 ⊥
𝑏5
=
1∓
+
∓ 𝛿Earth ±
𝛾Hg
𝜕𝑧 𝐵0
𝐵0 2
𝐵0
𝑅u = 3.8424642 21
𝑅d = 3.8424648 22
𝑅u − 𝑅d
𝑏5 = 𝐵0
= 0.08 ± 0.56 pT
𝑅u + 𝑅d
A new limit
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𝑔S 𝑔P 𝜆2 < 2.5 × 10−27
gSgP 2 4:8 1027
HV performance
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HV (kV)
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+160
-160
• Best HV performance 144 kV → 12.0 kV/cm
• Average E-Field while data taking: 10.3 kV/cm
Hg co-magnetometer
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Extract B field from Larmor frequency
and correct UCN frequency (lamp data)
1.5 pT
16 h