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3 (n)EDMs – so? I P- and T-violating CPV in SM not fully understood e.g. insufficient CPV for baryon asymmetry Strong CP problem θ CP < rad. Axions? James KaramathUniversity of Sussex27/02/ :33:11 n n p × S + - d S - + d
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The cryogenic neutron EDM experiment at ILL
Technical challenges and solutions
James Karamath
University of Sussex
2
In this talk…
(n)EDM motivation
Measurement principles and sensitivity
Brief (recent) nEDM history
The Cryo-EDM experiment Overview of apparatus Summary of my DPhil work
Summary/conclusions
James Karamath University of Sussex 08/05/23 03:10 PM
3
(n)EDMs – so? I
P- and T-violating
CPV in SM not fully understood e.g. insufficient CPV for baryon asymmetry
Strong CP problem θCP < 10-10 rad. Axions?
James Karamath University of Sussex 08/05/23 03:10 PM
n n
p
×
S+
-d
S
-
+
d
4
(n)EDMs – so? II
Estimated EDMs model dependent SM dn ~ 10-31 ecm Other models typically 105-6 times greater
e.g. SUSY: CP < 10-2
quark electric dipole moments: q q
gaugino
squark
5
nEDM measurement principle
B0 E<Sz> = + h/2
<Sz> = - h/2
h(0) = -2μ.B h()=2(-μ.B+dn.E)
h()=2(-μ.B-dn.E)
B0 B0 E
dn defined +ve
↑↑ - ↑↓= Δ = 4dn.E / h
Ramsey NMR performed on stored Ultra Cold Neutrons (UCN)
6
Ramsey’s method of separated fields
Start with spin polarised neutrons in uniform B-field (Bz)
Apply oscillating B-field pulse (Bxy) perpendicular to Bz. Precession axis rotates down to xy-plane
Apply large E-field and allow to precess freely for ~300s
Apply 2nd, phase coherent with the first, oscillating Bxy. Neutron precession axis rotates down to –z axis.
7
Ramsey’s method of separated fields
However if an EDM is present a phase difference builds up during the free precession
If 180 out of phase second pulse returns spin back to +z axis.
8
Ramsey’s method of separated fields(2n-1)π out of phase
Experimental runs taken at approx π/2 off resonance. Here dN/dν is a maximum.
9
nEDM statistical limit
Fundamental statistical limit
α = visibility [polarisation product]E = E-field strengthT = NMR coherence timeN = total # counted
NET
dn
2
James Karamath University of Sussex 08/05/23 03:10 PM
10
nEDM systematic limit
Main concern: changes in B-field accidentally correlated with E-field changes give false dn signal
h(ν↑↑–ν ↑↓) = 2|μn|(B↑↑–B↑↓) – 4dnE
True nEDM signal
False signal due to varying B
11
nEDM experiments: history
Co-magnetometer era
Cryogenic UCN era
RT stored UCN era
NET
hdn
2
Beam eraΔB ≈ v x E / c2 limited
12
RT nEDM experiment at ILL
Create UCN, can then be guided & stored
Polarise UCN UCN admitted into
cell with E and B-fields and stored…
Mercury polarised by Hg lamp and added to cell
N S
Storage cell
Magnet & polarizing foil / analysing foil
UCN
Approx scale 1 m
BE
Magnetic field coil
High voltage lead
James Karamath University of Sussex 08/05/23 03:10 PM
Magnetic shielding
13
RT nEDM experiment at ILL
Ramsey NMR performed
Released from cell Neutrons spin
analysed (# fn of precession)
Mercury spin analysed.
Repeat: E=↓or 0, B=↓
N S
Magnetic shielding
Storage cell
UCN detector
Approx scale 1 m
Magnetic field coil
B
High voltage lead
E
Magnet & polarizing foil / analysing foil
James Karamath University of Sussex 08/05/23 03:10 PM
14
Systematics I
Mercury fills cell uniformly, UCN sag under gravity, lower by ~3 mm.
Thus don’t sample EXACTLY the same B-field. Axial (z) gradients → problems…
Magnetometer problems
Hg nz
James Karamath University of Sussex 08/05/23 03:10 PM
15
Systematics II
Two conspiring effects v x E: motional particle in electric field
experiences B-field: ΔB ≈ v x E / c2
Axial field gradient dB/dz creates radial B-field (since .B=0) proportional to r, Br r
Let’s look at motion of a mercury atom across the storage cell
Geometric Phase Effect (GPE)
16
Systematics III Geometric Phase Effect (GPE)
dB/dz → B r
B v x E Scales with E
like EDM!!!
Scales with dB/dz
(GPEHg ~ 40GPEn)
Resultant
i.e. B0 field into page has gradient
Shifts resonance of particle
Using Mercury
introduces error
E and B0 into page
Rotating B field
17
Final result
Room temperature experiment gave the result;
dn = (+0.61.5(stat) 0.8(syst)) x 10-26) ecm
i.e. |dn| < 3.0 x 10-26 ecm (90% CL).
New cryogenic experiment will eventually be x100 more sensitive…
www.neutronedm.org
18
The Cryogenic nEDM experiment
Reminder: NET
dn
2
RT Cryo
N /day 6x106 ~6x108
T /s ~130 ~260 0.75 ~0.9E /kV/cm ~12 ~25(B0 /μT 1 5)
~10-28ecm
*
*with new beamline
x20 x5*
x2
x1.2
x2
19
Improved production of UCN (↑N) I
Crosses at 0.89 nm for free (cold) n. Neutron loses all energy by phonon emission → UCN.
Reverse suppressed by Boltzmann factor, He-II is at 0.5K, no 12K phonons.
Dispersion curves for He-II and free neutrons
James Karamath University of Sussex 08/05/23 03:10 PM
20
Improved production of UCN (↑N) II
Idea by Pendlebury and Golub in 1970’s, experimentally verified in 2002 (detected in He-II) for cold neutron beam at ILL (~1 UCN/cm3/sec).
Also better guides – smoother & better neutron holding surfaces, Be / BeO / DLC coated → more neutrons guided/stored. Allows longer T too.
James Karamath University of Sussex 08/05/23 03:10 PM
21
Polarisation and detection (α) I
Polarisation by Si-Fe multi-layer polarizer, 95±6% initial polarisation.
Can lose polarisation in 2 ways: “Wall losses” magnetic impurities in walls,
generally not aligned with neutron spin Gradients in B-field, if not smooth and steady
have similar effect
James Karamath University of Sussex 08/05/23 03:10 PM
22
Polarisation and detection (α) II
Detector: solid state, works in 0.5K He-II.
n (6Li, α) 3H reaction - alpha or triton detected
Thin, polarised Fe layer - spin analysis
James Karamath University of Sussex 08/05/23 03:10 PM
23
Magnetic field issues I
Target – need ~ 100 fT stability (NMR)Need ~ 1 nT/m spatial homogeneity (GPE)Perturbations ~ 0.1 μT (cranes!)Need (axial) shielding factor ~ 106
Mu-metal shielding ~ 50 Superconducting shielding ~ 8x105
Active shielding (feedback coils) ~ 15
Shielding factors
24
Magnetic field issues II
CRYOGENIC nEDM! Utilise superconducting shield and B0 solenoid. Major part of fluctuations across whole chamber
(common mode variations) Magnetometer (zero E-field) cell(s) see same Very stable B0(t) current
Holding field x5 to reduce GPE of the neutrons by factor of 25 (GPEn 1/B0
2)
Extra benefits
James Karamath University of Sussex 08/05/23 03:10 PM
25
Magnetic field issues III
~fT sensitivity 12 pickup loops will
sit behind grounded electrodes.
Will show temporal stability of B-field at this level.
Additional sensitivity from zero-field cell(s)
SQUIDS
26James Karamath University of Sussex 08/05/23 03:10 PM
Now have a 400 kV supply to connect to HV electrode.
Will sit in 3bar SF6. For 160 kV use N2:CO2 first.
Improving the E-field (↑E) I: The HV
27
Improving the E-field (↑E) II: HV line 1
50 kV ~1 GOhm resistors
Superfluid containment vessel (SCV)
HV electrode Ground electrodes
400 kV bipolar stack
N.B. Diamond-like-carbon coated titanium electrodesBeO spacers
28
Improving the E-field (↑E) II: HV line 2
Spellman +130 kV
Spellman -130 kV
Thick walled PTFE tube and thin-walled SS tube HV “cryo-cable”.
Standard 150 kV cable
HV connection
29
The dielectric strength of LHe
Has been tested in the past, mostly at 4.2 K (760 torr), at small electrode gaps (sub-mm) and with small electrodes. Superfluid data is limited and generally at low voltages (sub-40 kV, often sub-20 kV).
Usually the breakdown strength as a function of gap is studied. We’d like to know the strength as the pressure/temperature falls – esp. in the superfluid state.James Karamath University of Sussex 08/05/23 03:10 PM
30
The dielectric strength of LHe II Past literature
He-I data
4.2 < T(K) < 2.2Nope – put in the final versions from thesis
Past literature representative Vbd (d ) data, T = 4.2 K.
1
10
100
1000
0.001 0.01 0.1 1 10
Electrode Separation d /cm
Bre
akdo
wn
Vol
tage
V /k
V
31
The dielectric strength of LHe III Past literature
He-II data
2.2 < T(K) < 1.4
Past literature representative Vbd (d ) data, T < 2.3 K.
1
10
100
1000
0.001 0.01 0.1 1 10Electrode Separation d /cm
Bre
akdo
wn
Vol
tage
V /k
V
A
32
The dielectric strength of LHe IV
Test electrodes submerged in He-II in bath cryostat.
Studying Vbd and Ileak as function of d, T, dielectric spacers, purity… up to 130 kV. Also electrode damage.
E
±HV
cryostat
He-II (T, purity…)
gap (d, V, spacers)
Sussex HV tests
33
The dielectric strength of LHe IV Sussex HV results
Vbd (P) at d = 0.50 cm (many data runs)
0102030405060708090
100110120
1 10 100 1000Pressure /torr
Bre
akdo
wn
Vol
tage
V /k
V
34
The dielectric strength of LHe V
Vbd (T) (past literature and present data)
0
20
40
60
80
100
120
140
160
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Temperature /K
Bre
akdo
wn
Vol
tage
V /k
V
Long d=5 cm scaled Long d=0.5 cmKaramath d=0.7 cm scaled Karamath d=0.5 cm rawWu d=0.127 cm scaled (1 bar) Blank d=0.1 cm scaledWu d=0.0254 cm scaled (1 bar) Mathes d=0.019 cm scaled
l
35
The dielectric strength of LHe VI
Histogram of all 0.7 cm sub-1.8 K Vbd data
0
5
10
15
20
25
30
35
40
35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-80
Breakdown Voltage V /kV
Freq
uenc
y
35 45 55 65 75
All sub-1.8 K d=0.7cm breakdown dataGaussian
Extremal Type I
Extremal Type II
Statistics
36
The dielectric strength of LHe VII
Size effects: Weak but important dependence on electrode area or stressed fluid volume may decrease dielectric strength.
Leakage currents never found to be >0.1 nA (sensitivity limited) even immediately below breakdown.
~0.3 mm craters in electrodes when breakdown occurs at >80 kV. Bad news for DLC coated electrodes.
37
The dielectric strength of LHe VI
Breakdown strength reduced by insulating BeO spacers by a factor of ~1.4. Due to surface tracking along the BeO.
38
The dielectric strength of LHe summary
At 0.7 cm gap the breakdown field strength was approx 80 ± 10 kV/cm. i.e. ~50 kV/cm for 1 in 1000 chance of breakdown. May have to half this if size effects indeed exist.
What controls breakdown – pressure or temperature?! May hold key to improving Vbd.
39
And so, the CryoEDM experiment I
n guide tubes + spin analyser
E ~ 25kV/cm
E = 0kV/cmSpin flipper coil (measure other spin)
40
And so, the CryoEDM experiment IIHV electrode
Ground electrodes
HV in
z
Carbon fibre
support
BeO spacers
41
And so, the CryoEDM experiment IIIHV electrode
Ground electrodes
G10 Superfluid
containment vessel
HV in
z Neutrons in/out
Guides not shown
250l He-II 0.5K
**
* BeO spacers/guides
42
And so, the CryoEDM experiment IV
1m
Dynamic shielding coils
Magnetic (mu-metal) shields
Superconducting shield and solenoid
The shielded region
43
Schedule / Future
Finish construction THIS YEARStart data taking THIS YEARFirst results ~2009Upgrade neutron guide to ↑N ~2009 ?
James Karamath University of Sussex 08/05/23 03:10 PM
44
Summary(n)EDMs help study T-violation and are
constraining new physics.Final RT result: |dn| < 3.0 x 10-26 ecm.Aim to push well into 10-28 ecm.Further work needed to understand
dielectric properties of He-II. Only 20 kV/cm? (Paper in preparation.) Can pressure/purity/electrode material make a difference?
45
Done!
Thanks for listening