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Laboratoire National Champs Magnétiques Intenses, Toulouse, France M. Fouché, Agathe Cadène, Paul Berceau, Rémy Battesti, Carlo Rizzo
The QEDVacuum Magnetic Birefringence (BMV)
Experiment
E
Elliptic polarizationLinear polarization
Magnetic Birefringence
Epar
Eperp
x
y
E B
Linear polarization
Induced magnetic birefringence :
n = ( n// – n┴ ) α B2
This effect exists in any medium
n measurement to a few ppm
Pure QED test
Vacuum Magnetic BirefringenceQED theory predicts that this effect also exists in vacuum
• Due to the creation of virtual positron-electron pairs
nonlinear interaction between electromagnetic fields
• Calculated in the 70s :
0
2
54
32
4
251
15
2
B
cmn
e
At the lower orders in
2
24
T 1 10 00000500316994
B
n ..
O(3)
O(4)
O(5)
?
?
Fondamental constants : Codata 2010
Z. Bialynicka-Birula and I. Bialynicki-Birula, Phys. Rev. D 2, 2.34 (1970)V. I. Ritus, Sov.Phys. JETP 42, 774 (1975)
Experimental challenge
• BMV project
Development of a very sensitive ellipsometer in order to be able to observe this fundamental
prediction for the first time
• Extremely small Not yet experimentally observed
2CM Bkn 224
CM T 104 .kwith
The ellipsometer
A
P
It
Iext
BM2
M1
• P and A : polarisers crossed at maximum extinction
• Ellipticity :
• Key elements :
Magnetic field : special magnets developed at LNCMI to have a B2LB as high as possible
Fabry-Perot cavity : to increase the optical path in B
CMkλ
πΨ
π
F2BLB2
laser=1064 nm
LB
R. Battesti et al., EPJD 46, 323 (2008)
Pulsed transverse magnetic field
L = 25 cm
laser beam
Unconventional magnets developed at LNCMI
X coil geometry high transverse magnetic field
I
B1 B2
B
I
laser
R. Battesti et al., EPJD 46, 323 (2008)
Pulsed transverse magnetic fieldB
(T
)
time (ms)
I = 8300 AU = 4000 Vtpulse = 4 ms
B = 14.3 TB²Lmag = 28 T²m
E = 100 kJ
Pulses in vacuum:mT 75
T 5622
max
max
.
.
BLB
B
• Time evolution: • Longitudinal profile:
= 0.137 m
Hole to let the laser in
12mm
Coil in its cryostat
Immersion in liquid nitrogen to avoid consequences of heating
External reinforcement to contain the magnetic pressure
The Fabry-Perot cavity
Finesse :
• photon lifetime in the cavity (Lc = 2.27 m long) : = 1.08 ms
• flight distance in the cavity = 325 km
000 054cL
cF
One of the sharpest cavities of the world
Interferometers in the world
Lc 3 km 6.4 m 4 km 2.27 m
159 ms 442 ms 970 ms 1.08 ms
F = 50 70 000 230 450 000
= 1 kHz 360 Hz 164 Hz 147 Hz
aa
2LcF2LcF
LcLc
c τc τ
c c
high reflectivity mirrors work in a clean room
BMV experimental setup
• Increase of the path of light in the medium : Fabry-Perot cavity
• Pulsed transverse magnetic field B as high as possible
Data acquisition
Alignment of the cavity mirrors
Shot
Data analysis
TEM00 mode
Laser locked on the cavity
Data analysis
Both signals It and Iext are used:
A
P
It
Iext
B M2
M1
Ellipticity to be measured, proportional to B²(t)
polarizers extinction≈ 4.10-7
cavitywithout t
ext2
I
I
Cavity static ellipticity≈ 10-6
)( tI
Ψ2σI
22
t
ext for Ψ
22 ΓΨ(t)σI
t
ext I
Data analysis
)( tI
Ψ2σI
22
t
ext for Ψ
Ψ(t) Y(t) 22
t
e
I
I
with = sign of
CMCM2filteredCM 2
and BFL
ktBY
in T-2
takes into account the first order low pass filtering of the cavity
cutoff frequency: c=1/4 = 75 Hz
2filteredB
Y(t) fitted by tBCM2filtered
P. Berceau et al., Appl. Phys. B 100, 803 (2010)
Magnetic birefringence of nitrogen vs pressure
Results in N2
3.1×10-2
2.2×10-2
9×10-4
< 5×10-4
4×10-2
2×10-2
3.5×10-2
2.2×10-2
3×10-4
type A type B
Relative uncertainties
Pf
Ln
Bb
2sin
1
4 ISLCM
P. Berceau et al., Phys. Rev. A 85, 013837 (2012)
Group
PVLAS
Q&A
BMV
• measurements linear fit 1 confidence level
Results in vacuum
Ψ(t) Y(t)22
t
e
I
I
Pulses realized in same experimental conditions,
at 3 confidence level
Systematic effects
(rad
)
Ox
0
//B
-219CM T 101012 ..k
Systematic effects
Possible Cotton-Mouton effects:
• Residual gaz :
P<10-7 mbar
Gaz analyser: most important contributions come from N2 and O2
• CM effect of cavity mirrors
Bmirror = 150 T
kCM = (2.1 0.1) 10-19 T-2 at 3 confidence level
No Cotton-Mouton systematic effect
-223CM T 1051 .k
-224CM T 101 k
>0, B antiparallel to Ox
New data acquisition procedure
Using symmetry properties of Ψ(t) Y(t)22
t
e
I
I
4 new data series YB
Y(t) >0, B parallel to Ox
Y(t)Y(t)Y(t) <0, B parallel to Ox
<0, B antiparallel to Ox
We then derive a more general expression for Y(t)
SdScSbSa
SdScSbSa
SdScSbSa
SdScSbSa
Y
Y
Y
Y BS = function with a given symmetry
+ even parity- odd parity
Cotton-mouton effect S-+
New data acquisition procedure
• Symmetry functions can be measured with a linear combination of functions Y
• Allow to measure and to overcome systematic effects that might mimic the CM effect
at 3 confidence level
A. Cadène et al., arXiv:1302.5389 (2013)
-221CM T 107847 ..k
Comparison
BFRT Collaboration: R. Cameron et al., Phys. Rev. D 47, 3707 (1993)
PVLAS, 2008: E. Zavattini et al., Phys. Rev. D 77, 032006 (2008)
PVLAS, 2012: G. Zavattini et al., Int. J. of Mod. Phys. A 27, 1260017 (2012)
A. Cadène et al., arXiv:1302.5389 (2013)
Conclusion
Status
• Coupled high magnetic field and one the best Fabry-Perot cavities• Measurements performed on gases and in vacuum
Needed sensitivity improvement: almost 4 orders of magnitude
B2Lmag > 300 T2m
Future• Increase the transverse magnetic field : new XXL-coil
• Improvement of the ellipticity sensitivity (Decrease of 2 and 2 (10-8))