PVLAS

PVLAS (Polarizzazione del Vuoto con LASer, "polarization of the vacuum with laser") is an experiment for the detection of dark matter at the Legnaro National Laboratory of the National Institute of Nuclear Physics near Padova, Italy. It searches for vacuum polarization causing nonlinear optical behavior in magnetic fields, indicating effects from unseen particles such as axions. Experiments began in 2001 and continue today with new equipment.

Background

As dark matter does not emit or reflect any electromagnetic radiation which may be directly detected, PVLAS searches for nonlinear effects on light in the vacuum, predicted by the Euler-Heisenberg Lagrangian in 1936. In the presence of a massive dark matter candidate that interacts with the photon, like the axion, virtual dark matter particles will be preferentially created by photons with polarization parallel to the external field. These virtual particles travel slower than light, so the effective speed of the photons with a parallel polarization is reduced. Thus the vacuum is birefringent, its index of refraction depends on polarization. PVLAS looks for the flattening of a linearly polarised laser beam after it passes through a vacuum with an intense magnetic field. This flattening is characteristic of light in a birefringent medium. The birefrigence of the vacuum in quantum electrodynamics by an external field is generally credited to Stephen L. Adler, who presented the first general derivation in Photon splitting and photon dispersion in a strong magnetic field in 1971.

Design

PVLAS uses a high-finesse Fabry-Perot optical cavity. The first setup, used until 2005, sent a linearly polarized laser beam though vacuum with 5T magnetic field from a superconducting magnet to an ellipsometer. After upgrades to avoid fringe fields, several runs were done at 2.3T and 5T, excluding a prior finding. It was determined an optimized optical setup was needed for discovery potential. A prototype with much improved sensitivity was tested in 2010. In 2013 the upgraded apparatus at INFN Ferrara with permanent magnets and horizontal ellipsometer was set up and began data taking in 2014

Results

PVLAS investigated vacuum polarization induced by external magnetic fields. An observation of the rotation of light polarization by the vacuum in a magnetic field was published in 2006. Data taken with an upgraded setup excluded the previous magnetic rotation in 2008 and set limits on photon-photon scattering. An improved limit on nonlinear vacuum effects was set in 2012: A_e < 2.9·10⁻²¹ T⁻² @ 95% C.L.

See also

• DAMA/NaI
• DAMA/LIBRA
• CAST

External links

• Exploring the Quantum Vacuum and its structure - Istituto Nazionale di Fisica Nucleare (INFN)
• Axion Interpretation of PVLAS Data? - 9th International Conference on Topics in Astroparticle and Underground Physics (TAUP 2005) September 10–14, 2005, University of Zaragoza, Zaragoza, Spain