Experimental observation of optical rotation generated in vacuum by a magnetic field

We report the experimental observation of a light polarization rotation in vacuum in the presence of a transverse magnetic field. Assuming that data distribution is Gaussian, the average measured rotation is (3.9+/-0.5)e-12 rad/pass, at 5 T with 44000 passes through a 1m long magnet, with lambda = 1064 nm. The relevance of this result in terms of the existence of a light, neutral, spin-zero particle is discussed.Comment: 11 pages, 4 figures, submitted to Physical Review Letters Comment to version 2: minor changes to abstract and final discussion. Added 2 references Comment to version 3: corrected minor typographical errors, eliminated the distinction between scalar and pseudoscalar in the particle interpretation of the resul

Towards a direct measurement of vacuum magnetic birefringence: PVLAS achievements

Nonlinear effects in vacuum have been predicted but never observed yet directly. The PVLAS collaboration has long been working on an apparatus aimed at detecting such effects by measuring vacuum magnetic birefringence. Unfortunately the sensitivity has been affected by unaccounted noise and systematics since the beginning. A new small prototype ellipsometer has been designed and characterized at the Department of Physics of the University of Ferrara, Italy entirely mounted on a single seismically isolated optical bench. With a finesse F = 414000 and a cavity length L = 0.5 m we have reached the predicted sensitivity of psi = 2x10^-8 1/sqrt(Hz) given the laser power at the output of the ellipsomenter of P = 24 mW. This record result demonstrates the feasibility of reaching such sensitivities and opens the way to designing a dedicated apparatus for a first detection of vacuum magnetic birefringence

Optical production and detection of dark matter candidates

The PVLAS collaboration is at present running, at the Laboratori Nazionali di Legnaro of I.N.F.N., Padova, Italy, a very sensitive optical ellipsometer capable of measuring the small rotations or ellipticities which can be acquired by a linearly polarized laser beam propagating in vacuum through a transverse magnetic feld (vacuum magnetic birefringence). The apparatus will also be able to set new limits on mass and coupling constant of light scalar/pseudoscalar particles coupling to two photons by both producing and detecting the hypothetical particles. The axion, introduced to explain parity conservation in strong interactions, is an example of this class of particles, all of which are considered possible dark matter candidates. The PVLAS apparatus consists of a very high finesse (> 140000), 6.4 m long, Fabry-Perot cavity immersed in an intense dipolar magnetic field (~6.5 T). A linearly polarized laser beam is frequency locked to the cavity and analysed, using a heterodyne technique, for rotation and/or ellipticity acquired within the magnetic field.Comment: presented at "Frontier Detectors for Frontier Physics - 8th Pisa Meeting on Advanced Detectors - May 21-27, 2000" to appear in: Nucl.Instr. and Meth.

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

IIn 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 last 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 T 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 $1.2\cdot 10^{-8}$ rad at 5 T and $1.0\cdot 10^{-8}$ rad at 2.3 T (at 95% c.l.) with 45000 passes in the magnetic field zone. In the same conditions no ellipticity signal was detected down to $1.4\cdot 10^{-8}$ 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 $\sigma_{\gamma\gamma} < 4.5 \cdot10^{-34}$ barn at 95% c.l..Comment: 25 pages, 7 figures Main changes rel. to v.2: minor changes to abstract, replaced Figures 4,5,6, corrected typographical errors. Paper submitted to Physical Review

Axion interpretation of the PVLAS data?

The PVLAS collaboration has recently reported the observation of a rotation of the polarization plane of light propagating through a transverse static magnetic field. Such an effect can arise from the production of a light, m_A ~ meV, pseudoscalar coupled to two photons with coupling strength g_{A\gamma} ~ 5x10^{-6} GeV^{-1}. Here, we review these experimental findings, discuss how astrophysical and helioscope bounds on this coupling can be evaded, and emphasize some experimental proposals to test the scenario.Comment: 4 pages, 1 figure, jpconf.cls, talk presented at the ninth International Conference on Topics in Astroparticle and Underground Physics, TAUP 2005, Zaragoza, Spain, September 10-14, 200

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

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

The PVLAS experiment: measuring vacuum magnetic birefringence and dichroism with a birefringent Fabry-Perot cavity

Vacuum magnetic birefringence was predicted long time ago and is still lacking a direct experimental confirmation. Several experimental efforts are striving to reach this goal, and the sequence of results promises a success in the next few years. This measurement generally is accompanied by the search for hypothetical light particles that couple to two photons. The PVLAS experiment employs a sensitive polarimeter based on a high finesse Fabry-Perot cavity. In this paper we report on the latest experimental results of this experiment. The data are analysed taking into account the intrinsic birefringence of the dielectric mirrors of the cavity. Besides the limit on the vacuum magnetic birefringence, the measurements also allow the model-independent exclusion of new regions in the parameter space of axion-like and milli-charged particles. In particular, these last limits hold also for all types of neutrinos, resulting in a laboratory limit on their charge

Limits on Low Energy Photon-Photon Scattering from an Experiment on Magnetic Vacuum Birefringence

Experimental bounds on induced vacuum magnetic birefringence can be used to improve present photon-photon scattering limits in the electronvolt energy range. Measurements with the PVLAS apparatus (E. Zavattini {\it et al.}, Phys. Rev. D {\bf77} (2008) 032006) at both $\lambda = 1064$ nm and 532 nm lead to bounds on the parameter {\it A$_{e}$}, describing non linear effects in QED, of $A_{e}^{(1064)} < 6.6\cdot10^{-21}$ T$^{-2}$ @ 1064 nm and $A_{e}^{(532)} < 6.3\cdot10^{-21}$ T$^{-2}$ @ 532 nm, respectively, at 95% confidence level, compared to the predicted value of $A_{e}=1.32\cdot10^{-24}$ T$^{-2}$. The total photon-photon scattering cross section may also be expressed in terms of $A_e$, setting bounds for unpolarized light of $\sigma_{\gamma\gamma}^{(1064)} < 4.6\cdot10^{-62}$ m$^{2}$ and $\sigma_{\gamma\gamma}^{(532)} < 2.7\cdot10^{-60}$ m$^{2}$. Compared to the expected QED scattering cross section these results are a factor of $\simeq2\cdot10^{7}$ higher and represent an improvement of a factor about 500 on previous bounds based on ellipticity measurements and of a factor of about $10^{10}$ on bounds based on direct stimulated scattering measurements

New PVLAS model independent limit for the axion coupling to γγ\gamma\gamma for axion masses above 1meV

During 2014 the PVLAS experiment has started data taking with a new apparatus installed at the INFN Section of Ferrara, Italy. The main target of the experiment is the observation of magnetic birefringence of vacuum. According to QED, the ellipticity generated by the magnetic birefringence of vacuum in the experimental apparatus is expected to be $\psi^{\rm(QED)} \approx 5\times10^{-11}$. No ellipticity signal is present so far with a noise floor $\psi^{\rm(noise)} \approx 2.5\times10^{-9}$ after 210 hours of data taking. The resulting ellipticity limit provides the best model independent upper limit on the coupling of axions to $\gamma\gamma$ for axion masses above $10^{-3}$eV