First results from the new PVLAS apparatus: a new limit on vacuum magnetic birefringence

TL;DR

The PVLAS experiment has set a new upper limit on vacuum magnetic birefringence, approaching the predicted QED value, and provided improved constraints on axion-like particles and millicharged particles.

Contribution

First experimental measurement with the new PVLAS apparatus providing a new limit on vacuum magnetic birefringence and improved bounds on axion-like and millicharged particles.

Findings

Measured vacuum birefringence noise floor: (4±20)×10⁻²³ T⁻²

Reduced the gap to QED prediction by a factor of 50

Improved bounds on axion-like particles and millicharged particles

Abstract

Several groups are carrying out experiments to observe and measure vacuum magnetic birefringence, predicted by Quantum Electrodynamics (QED). We have started running the new PVLAS apparatus installed in Ferrara, Italy, and have measured a noise floor value for the unitary field magnetic birefringence of vacuum $\Delta n_u^{\rm (vac)}= (4\pm 20) \times 10^{-23}$ T$^{-2}$ (the error represents a 1$\sigma$ deviation). This measurement is compatible with zero and hence represents a new limit on vacuum magnetic birefringence deriving from non linear electrodynamics. This result reduces to a factor 50 the gap to be overcome to measure for the first time the value of $\Delta n_u^{\rm (vac,QED)}$ predicted by QED: $\Delta n_u^{\rm (vac,QED)}= 4\times 10^{-24}$ ~T$^{-2}$. These birefringence measurements also yield improved model-independent bounds on the coupling constant of axion-like particles to two photons, for masses greater than 1 meV, along with a factor two improvement of the fractional charge limit on millicharged particles (fermions and scalars), including neutrinos.