Projet ANR-06-BLAN-0048 – BMV : Biréfringence Magnétique du Vide
Programme : Programme blanc (BLANC) 2006
Année de financement : 2006
Coordinateur du projet : UNIVERSITE TOULOUSE III [PAUL SABATIER] (UNIVERSITE TOULOUSE III [PAUL SABATIER])
Partenaires : Centre national de la recherche scientifique; Université de Toulouse III - Paul Sabatier
Montant : 320000.0 euros
Date de début : 2006
Durée : 36 mois
Résumé : The understanding of the nature of dark matter and of the origin of vacuum energy is one of the major objectives of modern fundamental physics. During the last years, some research activities in this field have been developed in Toulouse, mainly in two institutes (LCAR-IRSAMC, LNCMP), in collaboration with institutes outside Toulouse (LMA of Lyon), in France and abroad. One finds indeed in Toulouse a wide spectrum of knowledge, ranging from pulsed magnetic fields to quantum optics. The scientific program we present in this application benefits from synergies between these different research areas. It is based on an ambitious experimental project aiming at a laboratory test of the Quantum Electro-Dynamics predictions concerning vacuum energy: the BMV (Biréfringence Magnétique du Vide) project. This project combines very intense pulsed magnetic fields, developed at LNCMP, and a very sensitive optical device to detect the effects induced on a laser beam by such fields. This device is developed at LCAR-IRSAMC in collaboration with LMA-VIRGO from IN2P3 in Lyon. Our goal is to assemble a first version of our experience in 2006, and to obtain first results in 2007. In particular, we plan to test the results from the PVLAS collaboration which has observed a dichroism and an ellipticity induced by a magnetic field on a linearly polarized laser beam propagating in a vacuum. The existence of scalar or pseudoscalar particles coupling with two photons has been suggested to explain such a phenomenon. Indeed, some components of non baryonic dark matter, such as the axion, can also induce detectable optical effects in such a device.  The major physical interest of a possible confirmation of the observation of such a particle has given rise to stronger and stronger competition. Numerous experimental projects begin in large international institutes, such as CERN in Geneva and at DESY in Hamburg. However, our experiment remains the most advanced one in the world race to prove or disprove these results.  Classical electrodynamics, modified at the beginning of XXth century to take into account the quantum principles, gave birth to quantum electrodynamics (QED). Among the new phenomena predicted by QED but never observed, one finds the vacuum magnetic birefringence, also known as the Cotton-Mouton effect of vacuum. The velocity of light propagating in the presence of a transverse magnetic field B depends on light polarization, i.e. the index of refraction npar for light polarized parallel to the magnetic field is different from the index of refraction nperp for light polarized perpendicular to the magnetic field. The difference Dn = (npar - nperp) is proportional to B2. Thus, an incident linearly polarized light exits elliptically polarized from the magnetic field region.  Starting from the Dirac model of the quantum vacuum, Euler and Kochel in 1935, and Heisenberg and Euler in 1936 [2] established the effective lagrangian of the interaction between electromagnetic fields. Thanks to their work, one can calculate that Dn=4x10-24 T-2. This is the challenge to which experimentalists are confronted. The observation of such an effect would constitute the first direct evidence of the non linear propagation of light in a quantum vacuum. Moreover, it would be a novel test of QED. In particular, this would prove that QED predicts a correct value for the vacuum energy density, which could be the cause of the expansion of the universe.   On the other hand, in 1986 Maiani, Petronzio and Zavattini showed that the existence of a light, spinless, neutral boson (pseudoscalar or scalar) can induce an ellipticity and an apparent rotation due to a dichroism on a linearly polarized light propagating in the presence of a transverse magnetic field [3]. This is due to the fact that a photon, depending on its polarization, can oscillate into one of these particles either real (dichroism) or virtual (ellipticity). In particular, this is the case for the axion. This boson,