The QED Vacuum Magnetic Birefringence (BMV) Experiment Laboratoire National Champs Magnétiques Intenses, Toulouse, France M. Fouché, Agathe Cadène, Paul Berceau, Rémy Battesti, Carlo Rizzo

Magnetic Birefringence x y E Epar B E Eperp Linear polarization Linear polarization Elliptic polarization Induced magnetic birefringence : Dn = ( n// – n┴ ) α B2 This effect exists in any medium

Vacuum Magnetic Birefringence QED 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 : At the lower orders in a Fondamental constants : Codata 2010 O(a3) Dn measurement to a few ppm  Pure QED test O(a4) ? O(a5) ? 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 with Extremely small Not yet experimentally observed  BMV project Development of a very sensitive ellipsometer in order to be able to observe this fundamental prediction for the first time

The ellipsometer P A laser M1 l=1064 nm B M2 LB Iext It 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 R. Battesti et al., EPJD 46, 323 (2008)

Pulsed transverse magnetic field X coil geometry  high transverse magnetic field Unconventional magnets developed at LNCMI L = 25 cm laser beam I B1 B2 B laser R. Battesti et al., EPJD 46, 323 (2008)

Pulsed transverse magnetic field Time evolution: Longitudinal profile: B (T) time (ms) I = 8300 A U = 4000 V tpulse = 4 ms B = 14.3 T B²Lmag = 28 T²m E = 100 kJ = 0.137 m Pulses in vacuum:

Coil in its cryostat External reinforcement to contain the magnetic pressure Hole to let the laser in 12mm Immersion in liquid nitrogen to avoid consequences of heating

The Fabry-Perot cavity Lc= 2.27 m Ellipticity : 1’’

The Fabry-Perot cavity Finesse : photon lifetime in the cavity (Lc = 2.27 m long) : t = 1.08 ms flight distance in the cavity = 325 km

Interferometers in the world Lc 3 km 6.4 m 4 km 2.27 m t 159 ms 442 ms 970 ms 1.08 ms F = 50 70 000 230 450 000 Dn = 1 kHz 360 Hz 164 Hz 147 Hz a p c τ Lc c 2LcF One of the sharpest cavities of the world

BMV experimental setup Pulsed transverse magnetic field B as high as possible Increase of the path of light in the medium : Fabry-Perot cavity high reflectivity mirrors work in a clean room

Data acquisition Laser locked on the cavity TEM00 mode Alignment of the cavity mirrors TEM00 mode Laser locked on the cavity Data analysis Shot

Data analysis Both signals It and Iext are used: for M1 B M2 P Iext A Lmag B M2 M1 Both signals It and Iext are used: Cavity static ellipticity ≈ 10-6 polarizers extinction ≈ 4.10-7 Ellipticity to be measured, proportional to B²(t) for

Data analysis Y(t) fitted by for with g = sign of G in T-2 takes into account the first order low pass filtering of the cavity cutoff frequency: nc=1/4pt = 75 Hz P. Berceau et al., Appl. Phys. B 100, 803 (2010)

Relative uncertainties Results in N2 Magnetic birefringence of nitrogen vs pressure 3.1×10-2 2.2×10-2 9×10-4 < 5×10-4 4×10-2 2×10-2 3.5×10-2 3×10-4 type A type B Relative uncertainties measurements linear fit 1s confidence level P. Berceau et al., Phys. Rev. A 85, 013837 (2012) Group PVLAS Q&A BMV

Results in vacuum at 3s confidence level Systematic effects Pulses realized in same experimental conditions, (rad) at 3s confidence level Systematic effects

Systematic effects Possible Cotton-Mouton effects: kCM = (2.1  0.1) 10-19 T-2 at 3s confidence level 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 mT No Cotton-Mouton systematic effect

New data acquisition procedure Using symmetry properties of  G>0, B parallel to Ox  G>0, B antiparallel to Ox 4 new data series YGB  G<0, B antiparallel to Ox  G<0, B parallel to Ox We then derive a more general expression for Y(t) = 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 A. Cadène et al., arXiv:1302.5389 (2013) at 3s confidence level

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 Future 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 Future Increase the transverse magnetic field : new XXL-coil B2Lmag > 300 T2m Improvement of the ellipticity sensitivity (Decrease of G2 and s2 (10-8))