PVLAS Results Research Articles

Results from the OSQAR photon-regeneration experiment: No light shining through a wall

A new method to amplify the photon-axion conversions in a magnetic field is proposed using a buffer gas at a specific pressure in a photon-regeneration experiment. As a first result, new bounds for mass and coupling constant for laboratory experiments aiming to detect any hypothetical scalars and pseudoscalars, which can couple to photons were obtained, excluding with 95% confidence level, the recently withdrawn PVLAS result.

New PVLAS results and limits on magnetically induced optical rotation and ellipticity in vacuum

In 2006 the PVLAS collaboration reported the observation of an optical rotation generated in vacuum by a magnetic field. To further check against possible instrumental artifacts, several upgrades to the PVLAS apparatus have been made during the past year. Two data taking runs, at the wavelength of 1064 nm, have been performed in the new configuration with magnetic field strengths of 2.3 and 5 T. The 2.3 T field value was chosen in order to avoid stray fields. The new observations do not show the presence of a rotation signal down to the levels of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{rad}$ at 5 T and $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{rad}$ at 2.3 T (at 95% C.L.) with 45 000 passes in the magnetic field zone. In the same conditions no ellipticity signal was detected down to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ at 2.3 T (at 95% C.L.), whereas at 5 T a signal is still present. The physical nature of this ellipticity as due to an effect depending on ${B}^{2}$ can be excluded by the measurement at 2.3 T. These new results completely exclude the previously published magnetically induced vacuum dichroism results, indicating that they were instrumental artifacts. These new results therefore also exclude the particle interpretation of the previous PVLAS results as due to a spin-zero boson. The background ellipticity at 2.3 T can be used to determine a new limit on the total photon-photon scattering cross section of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}l4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}34}\text{ }\text{ }\mathrm{barn}$ at 95% C.L.

Effective field theory approach to light propagation in an external magnetic field

The recent PVLAS experiment observed the rotation of polarization and the ellipticity when a linearly polarized laser beam passes through a transverse magnetic field. The phenomenon cannot be explained in the conventional QED. We attempt to accommodate the result by employing an effective theory for the electromagnetic field alone. No new particles with a mass of order the laser frequency or below are assumed. To quartic terms in the field strength, a parity-violating term appears besides the two ordinary terms. The rotation of polarization and ellipticity are computed for parity asymmetric and symmetric experimental set-ups. While rotation occurs in an ideal asymmetric case and has the same magnitude as the ellipticity, it disappears in a symmetric set-up like PVLAS. This would mean that we have to appeal to some low-mass new particles with nontrivial interactions with photons to understand the PVLAS result.

Compatibility of the Chameleon-Field Model with Fifth-Force Experiments, Cosmology, and PVLAS and CAST Results

We analyze the PVLAS results using a chameleon field whose properties depend on the environment. We find that, assuming a runaway bare potential V(phi) and a universal coupling to matter, the chameleon potential is such that the scalar field can act as dark energy. Moreover, the chameleon-field model is compatible with the CERN Axion Solar Telescope results, fifth-force experiments, and cosmology.

Windows over a new low energy axion

We outline some general features of possible extensions of the Standard Model that include anomalous U(1) gauge symmetries, a certain number of axions and their mixings with the CP-odd Higgs sector. As previously shown, after the mixing one of the axions becomes a physical pseudoscalar (the axi-Higgs) that can take the role of a modified QCD axion. It can be driven to be very light by the same non-perturbative effects that are held responsible for the solution of the strong CP-problem. At the same time the axi-Higgs has a sizeable gauge interaction, which is not allowed to the Peccei–Quinn axion, possibly explaining the PVLAS results. We point out that the Wess–Zumino term, typical of these models, can be both interpreted as an anomaly inflow from higher dimensional theories (second window) but also as a result of partial decoupling of an extra Higgs sector (and of a fermion) that leaves behind an effective anomalous Abelian theory (first window) in a broken Stückelberg phase. The possibility that the axi-Higgs can be heavy, of the order of the Higgs mass or larger, however, cannot be excluded. The potentialities for the discovery of this particle and of anomaly effects in the neutral current sector at the LHC are briefly discussed in the context of a superstring inspired model (second window), but with results that remain valid also if any of the two possibilities is realized in Nature.

Transparency of the Sun to Gamma Rays due to Axionlike Particles

It is shown that the Sun can become partially transparent to high energy photons in the presence of a pseudoscalar. In particular, if the axion interpretation of the PVLAS result were true, then up to 2% of GeV energy gamma rays might pass through the Sun, while an even stronger effect is expected for some axion parameters. We discuss the possibilities of observing this effect. Present data are limited to the observation of the solar occultation of 3C 279 by the Energetic Gamma-Ray Experiment Telescope in 1991; 98% C.L. detection of a nonzero flux of gamma rays passing through the Sun is not yet conclusive. Since the same occultation happens every October, future experiments, e.g., the Gamma-Ray Large Area Space Telescope, are expected to have better sensitivity.

Light Scalars Coupled to Photons and Non-Newtonian Forces

A particle phi coupling to two photons couples also radiatively to charged particles, like protons. If the particle is a light scalar, this induced coupling leads to spin-independent non-Newtonian forces. We show that the experimental constraints on exotic, fifth-type forces lead to stringent constraints on the phigammagamma coupling. We discuss the impact on the recent PVLAS results and the role of paraphoton models introduced to solve the PVLAS-CAST puzzle.

Compatibility of Cast Search with Axionlike Interpretation of PVLAS Results

The PVLAS Collaboration has results that may be interpreted in terms of a light axionlike particle, while the CAST Collaboration has not found any signal of such particles. We propose a particle physics model with paraphotons and with a low energy scale in which this apparent inconsistency is circumvented.

Evading astrophysical constraints on axion-like particles

Stellar energy loss arguments lead to strong constraints on the couplingϕγγ of a light axion-like particle to two photons. Helioscopes, like CAST, are able to imposecompetitive bounds. The PVLAS experiment has recently observed a rotation of the polarizationof a laser propagating in a magnetic field that can be interpreted as the effect of a quite strongϕγγ coupling. We present scenarios where the astrophysical and CAST bounds can be evaded,and we show that the PVLAS result can be accommodated in one of the models, providedthat the new physics scale is at very low energies.