The QED Vacuum Magnetic Birefringence BMV Experiment Laboratoire

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 E Epar B E Eperp y Linear Ellipticpolarization Linear polarization Induced magnetic birefringence : Dn = ( n// – n┴ ) α B 2 q This effect exists in any medium

Vacuum Magnetic Birefringence q 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 70 s : At the lower orders in a Fondamental constants : Codata 2010 O(a 3) O(a 4) ? O(a 5) ? Z. Bialynicka-Birula and I. Bialynicki-Birula, Phys. Rev. D 2, 2. 34 (1970) V. I. Ritus, Sov. Phys. JETP 42, 774 (1975) Dn measurement to a few ppm Pure QED test

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 laser l=1064 nm M 1 B P M 2 LB Iext It A • P and A : polarisers crossed at maximum extinction • Ellipticity : • Key elements : Magnetic field : special magnets developed at LNCMI to have a B 2 LB 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 las er be am I laser I L B 1 =2 5 c m R. Battesti et al. , EPJD 46, 323 (2008) B 2 B

Pulsed transverse magnetic field B (T) • Time evolution: • Longitudinal profile: I = 8300 A U = 4000 V E = 100 k. J tpulse = 4 ms B = 14. 3 T B²Lmag = 28 T²m = 0. 137 m time (ms) Pulses in vacuum:

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

The Fabry-Perot cavity Lc= 2. 27 m q Ellipticity : 1’’

The Fabry-Perot cavity q 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 t 159 ms 442 ms 970 ms 1. 08 ms 50 70 000 230 450 000 1 k. Hz 360 Hz 164 Hz 147 Hz F= Dn = pcτ Lc a 4 km 2. 27 m c 2 Lc. F 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 Alignment of the cavity mirrors Shot Data analysis TEM 00 mode

Data analysis M 1 q Both signals It and Iext are used: P B M 2 Iext It polarizers extinction ≈ 4. 10 -7 Cavity static ellipticity ≈ 10 -6 Ellipticity to be measured, proportional to B²(t) for A

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

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

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

Systematic effects k. CM = (2. 1 0. 1) 10 -19 T-2 at 3 s confidence level q Possible Cotton-Mouton effects: • Residual gaz : P<10 -7 mbar Gaz analyser: most important contributions come from N 2 and O 2 • CM effect of cavity mirrors Bmirror = 150 m. T No Cotton-Mouton systematic effect

New data acquisition procedure q Using symmetry properties of 4 new data series YGB G>0, B parallel to Ox G>0, B antiparallel to Ox G<0, B parallel to Ox q 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. , ar. Xiv: 1302. 5389 (2013) at 3 s 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. , ar. Xiv: 1302. 5389 (2013)

Conclusion q 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 q Future • Increase the transverse magnetic field : new XXL-coil B 2 Lmag > 300 T 2 m • Improvement of the ellipticity sensitivity (Decrease of G 2 and s 2 (10 -8))