Abstract
The PVLAS collaboration is presently assembling a new apparatus (at the INFN section of Ferrara, Italy) to detect vacuum magnetic birefringence (VMB). VMB is related to the structure of the quantum electrodynamics (QED) vacuum and is predicted by the Euler–Heisenberg–Weisskopf effective Lagrangian. It can be detected by measuring the ellipticity acquired by a linearly polarized light beam propagating through a strong magnetic field. Using the very same optical technique it is also possible to search for hypothetical low-mass particles interacting with two photons, such as axion-like (ALP) or millicharged particles. Here we report the results of a scaled-down test setup and describe the new PVLAS apparatus. This latter is in construction and is based on a high-sensitivity ellipsometer with a high-finesse Fabry–Perot cavity (>4 × 105) and two 0.8 m long 2.5 T rotating permanent dipole magnets. Measurements with the test setup have improved, by a factor 2, the previous upper bound on the parameter Ae, which determines the strength of the nonlinear terms in the QED Lagrangian: A(PVLAS)e < 3.3 × 10−21 T−2 at 95% c.l. Furthermore, new laboratory limits have been put on the inverse coupling constant of ALPs to two photons and confirmation of previous limits on the fractional charge of millicharged particles is given.
Highlights
The PVLAS collaboration is presently assembling a new apparatus to detect vacuum magnetic birefringence (VMB)
Quantum electrodynamics (QED) predicts nonlinear effects in vacuum leading to birefringence and light-by-light scattering through the four-photons box diagram [2,3,4,5,6,7,8,9]
Where the indices and ⊥ refer to light polarization parallel and perpendicular to Bext, respectively. From these sets of equations two important consequences are apparent: the velocity of light in the presence of an external magnetic field is no longer c and vacuum is birefringent with n(EHW)
Summary
To calculate the magnetic birefringence of vacuum, one can proceed by determining the electric displacement vector D and the magnetic intensity vector H from the Lagrangian density LEHW of equation (2) by using the constitutive relations [3]. Let us consider a linearly polarized beam of light propagating perpendicularly to an external magnetic field Bext. Where the indices and ⊥ refer to light polarization parallel and perpendicular to Bext, respectively. From these sets of equations two important consequences are apparent: the velocity of light in the presence of an external magnetic field is no longer c and vacuum is birefringent with n(EHW). This leads to the value given in equation (6). A magnetized vacuum behaves like a uniaxial crystal
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