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

Lc= 2.27 m

1’’

Ellipticity :

CMkλ

πΨ

π

F2BLB2

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))