Measurements of vacuum magnetic birefringence using permanent dipole magnets: the PVLAS experiment

TL;DR

The PVLAS experiment aims to detect vacuum magnetic birefringence predicted by QED using high-sensitivity optical techniques and permanent magnets, also setting limits on hypothetical particles like axion-like and millicharged particles.

Contribution

It reports the development of a new apparatus with advanced optical sensitivity and improved bounds on nonlinear QED effects and hypothetical particles.

Findings

Improved upper bound on the nonlinear QED parameter $A_e$ by a factor of 2.

Set new laboratory limits on axion-like particles' coupling to photons.

Confirmed previous limits on the fractional charge of millicharged particles.

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 QED vacuum and is predicted by the Euler-Heisenberg-Weisskopf effective Lagrangian. It can be detected by measuring the ellipticity acquired by a linearly polarised 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 (MCP). Here we report results of a scaled-down test setup and describe the new PVLAS apparatus. This latter one is in construction and is based on a high-sensitivity ellipsometer with a high-finesse Fabry-Perot cavity ($>4\times 10^5$) 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 $A_e$, which determines the strength of the nonlinear terms in the QED Lagrangian: $A_e^{\rm (PVLAS)} < 3.3 \times 10^{-21}$ T$^{-2}$ 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.