Search

Spin-dependent fifth-forces are associated with particles beyond the standard model. In particular, light pseudo-scalar bosons mediate long-range forces, allowing mass to interact with spins. The search of these interactions can be performed by periodically varying the distance between a source mass and a spin ensemble, in order to modulate the force intensity and detect it with precision magnetometry techniques. In our setup the force arises from room temperature lead masses and is detected in a paramagnetic crystal at 4.2\,K, whose magnetisation is monitored by a SQUID-based magnetometer with the sensitivity of $53\,\mathrm{aT/\sqrt{Hz}}$. Our measurement places the most stringent constraints on a spin-mass interaction in the ranges 1\,cm to 10\,m and 10\,km to 300\,km, improving existing limits up to more than two orders of magnitude. We show that this experimental technique may be further leveraged to explore a vast region of the fifth force's parameter space, with an interaction range longer than a few centimetres., Comment: 5 figures, 10 pages

The original version of this article unfortunately contained a mistake.

In this Letter we report a novel measurement of the magnetically induced birefringence (Cotton–Mouton effect) in krypton and xenon. The measurement is performed, with the apparatus of the PVLAS experiment, at the previously uninvestigated wavelength of 1064 nm.

Even at absolute zero, the vacuum is seething with activity and behaves as a fluid in which moving objects dissipate their kinetic energy. The dynamical Casimir effect foresees that a metallic mirror put in motion in quantum vacuum gives rise to 'dissipated' energy in the form of real photons, called the Motion Induced Radiation. The final aim of this study is to present a solution to the main experimental difficulty, that is to realize the oscillating motion of the metallic wall at high frequency (10^9 ÷ 10^10 Hz). To obtain a fast moving wall we propose to switch an effective microwave mirror on and off in very short intervals of time changing the reflectivity of the semiconductor by shining pulsed laser beam on its surface. In this way a microwave mirror can be created and disappears at very high frequency, giving rise to a system equivalent to that of a fast moving metallic slab.

The PVLAS experiment operates an ellipsometer which embraces a superconducting dipole magnet and can measure ellipticity and rotation induced by the magnetic field onto linearly polarized laser light. The sensitivity of the instrument is about 10 -7 rad Hz -1/2 . With a residual pressure less than 10 -7 mbar the apparatus gives both ellipticity and rotation signals at the 10-7rad level with more than 8 sigma sob ratio in runs that last about 1000 sec. These signals can be interpreted as being generated largely by vacuum ellipticity and dichroism induced by the transverse magnetic field. If this interpretation is correct, a tool has become available to characterize physical properties of vacuum as if it were an ordinary transparent medium. A microscopic effect responsible for this induced dichroism could be the existence of ultralight spin zero bosons with mass of the order of 10 −3 eV , that would couple to two photons and would be created in the experiment by interactions of photons of the laser beam with virtual photons of the magnetic field. The inverse of the coupling constant to two photons would correspond to a mass M of the order of 10 6 GeV .

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 $\mathrm{\ensuremath{\Delta}}{n}_{u}^{(\mathrm{vac})}=(4\ifmmode\pm\else\textpm\fi{}20)\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}23}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}$ (the error represents a $1\ensuremath{\sigma}$ deviation). This measurement is compatible with zero and hence represents a new limit on vacuum magnetic birefringence deriving from nonlinear electrodynamics. This result reduces to a factor of 50 the gap to be overcome to measure for the first time the value of $\mathrm{\ensuremath{\Delta}}{n}_{u}^{(\mathrm{vac},\mathrm{QED})}$ predicted by QED: $\mathrm{\ensuremath{\Delta}}{n}_{u}^{(\mathrm{vac},\mathrm{QED})}=4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}$. 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-2 improvement of the fractional charge limit on millicharged particles (fermions and scalars), including neutrinos.

We report on the resonant Fabry Perot cavity of the PVLAS (Polarization of the Vacuum with LASer) experiment operating at $\lambda = 1064$ nm with a record decay time of 2.7 ms, a factor more than two larger than any previously reported optical resonator. This corresponds to a coherence length of $8.1\cdot10^5$ m. The cavity length is 3.303 m, and the resulting finesse is 770 000.

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 collaboration is presently assembling a new apparatus to detect vacuum magnetic birefringence. This property 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. Here we report results of a scaled-down test setup and briefly 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 $(> 4\times10^5)$ 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: $A^{\rm (PVLAS)}_e < 3.3 \times10^{21}~{\rm T}^{-2}$ 95\% c.l.

The experiment PVLAS studies the optical properties of the vacuum, that behaves much like a material medium when it is permeated by an external (electric or magnetic) field. Using a strong superconducting magnet and a very sensitive ellipsometer we have searched for modifications of the index of refraction of the vacuum due to the presence of a magnetic field. A birefringence is predicted to arise because of the vacuum fluctuations of the electromagnetic field, and a similar effect (dichroism) could be due to the presence of yet undiscovered low mass particles interacting with two photons. At present PVLAS has set the best existing limits on such processes, its sensitivity being limited by external noise sources which have now been accounted for. A completely redesigned prototype apparatus is now under construction: it is based on rotating permanent magnets and an ellipsometer employing an ultra stable Fabry-Perot resonator. A 50-fold improvement in the sensitivity of the ellipsometer has now been achieved, and we hope to improve our best limits when the magnets system will be installed.

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 Polarizzazione del Vuoto con Laser apparatus [E. Zavattini et al., Phys. Rev. D 77, 032006 (2008)] at both $\ensuremath{\lambda}=1064$ and 532 nm lead to bounds on the parameter ${A}_{e}$, describing nonlinear effects in QED, of ${A}_{e}^{(1064)}l6.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}@1064\text{ }\text{ }\mathrm{nm}$ and ${A}_{e}^{(532)}l6.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}@532\text{ }\text{ }\mathrm{nm}$, respectively, at 95% confidence level, compared to the predicted value of ${A}_{e}=1.32\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}$. The total photon-photon scattering cross section may also be expressed in terms of ${A}_{e}$, setting bounds for unpolarized light of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}^{(1064)}l4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}62}\text{ }\text{ }{\mathrm{m}}^{2}$ and ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}^{(532)}l2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}60}\text{ }\text{ }{\mathrm{m}}^{2}$. Compared to the expected QED scattering cross section these results are a factor of $\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ 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.

We report new measurements of the linear polarization of thermal radiation emitted by incandescent thin tungsten wires, with thicknesses ranging from five to hundred microns. Our data show very good agreement with theoretical predictions, based on Drude-type fits to measured optical properties of tungsten., Comment: 12 pages, 4 encapsulated figures. This new version matches the one published in New. J. Phys.. Improved presentation, more references added, and one new figure included

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 D

The PVLAS experiment operates an ellipsometer based on a Fabry-Perot optical cavity that embraces a superconducting rotating dipole magnet and can measure the ellipticity induced by the magnetic field onto linearly polarized laser light. With a residual pressure less than 10 −7 mbar the apparatus gives ellipticity signals at the level of 10 −11 rad per passage through 1 m of 5 Tesla transverse magnetic field of 532 nm wavelength laser light. These signals can be interpreted as being generated largely by vacuum birefringence. If this interpretation is valid, a tool has become available to characterize physical properties of vacuum as if it were an ordinary transparent medium. The main source of the induced ellipticity could be the existence of ultralight bosons with mass of the order of 10 −3 eV that would couple to two photons and would be created in the experiment by interactions of photons of the laser beam with virtual photons of the magnetic field. The apparatus is calibrated in amplitude and in phase by measuring Cotton-Mouton ellipticity in gases. The ellipticity induced in vacuum has phase opposite to that of the CME ellipticity induced with noble gases in the interaction region. If the ellipticity signals observed in vacuum are due to authentic quantum vacuum birefringence and not to the apparatus, and a microscopic interpretation of the effect in terms of existence of spin zero ultralight bosons is valid, the observed phase of the ellipticity implies a positive parity of the bosons. The ultralight bosons would then be scalars.

When measuring very small ellipticities or rotations induced on a polarized beam of light, Fabry–Perot cavities are often used to increase the number of passes within the region of interest. In this paper we show that due to the intrinsic birefringence of the reflective surface of the cavity mirrors, a cross talk between dichroism and ellipticity is induced. We also show how such a cross talk may be measured and kept under control by means of an adequate phase locking scheme of the laser to the cavity.

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., 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 result

A novel experimental set-up for the measurement of the Casimir effect at separations larger than a few microns is presented. The apparatus is based on a mechanical resonator and will use a homodyne detection technique to sense the Casimir force in the plane–parallel configuration.

Nonlinear interactions of light with light are well known in quantum electronics, and it is quite common to generate harmonic or subharmonic beams from a primary laser with photonic crystals. One suprising result of quantum electrodynamics is that because of the quantum fluctuations of charged fields, the same can happen in vacuum. The virtual charged particle pairs can be polarized by an external field and vacuum can thus become birefringent: the PVLAS experiment was originally meant to explore this strange quantum regime with optical methods. Since its inception PVLAS has found a new, additional goal: in fact vacuum can become a dichroic medium if we assume that it is filled with light neutral particles that couple to two photons, and thus PVLAS can search for exotic particles as well. PVLAS implements a complex signal processing scheme: here we describe the double data acquisition chain and the data analysis methods used to process the experimental data., Comment: presented by E. Milotti to the WSEAS-ISCGAV '05 Conference, Malta, 15-17/9/2005

In this Letter, we report a novel measurement of the magnetically induced birefringence (Cotton–Mouton effect) in neon. Using a highly sensitive apparatus we were able to precisely measure the specific birefringence value of Δnu = (5.9 ± 0.2) × 10−16 at the wavelength of 1064 nm (for B = 1 T and atmospheric pressure) and T ≈ 290 K. The results reported here are in agreement with theory, while the only previous precise measurement differs significantly.

The PVLAS experiment aims at the observation and measurement of the effect of magnetic birefringence of vacuum (MBV) predicted by Quantum Electrodynamics. We describe here the new PVLAS apparatus which is currently being set up in INFN Ferrara. The apparatus features two rotating permanent dipole magnets and an ellipsometer operating under UHV with a high finesse Fabry–Perot cavity.

PVLAS is an experiment which aims at the direct detection of photon-field scattering: it employs optical methods and a large rotating superconducting magnet, and its large, compact structure is affected by both mechanical and electrical noises. This paper introduces briefly the data analysis methods used in the experiment and summarizes the mechanical and electrical noise situation., 8 pages, 14 figures, PDF only, talk given by E. Milotti at the QNDE 2002 conference, Bellingham (WA), July 14-19 2002

Nearly massless particles coupling weakly to two photons constitute suitable candidates as dark matter components: experimental searches investigating this effect by optical techniques have recently been conducted. PVLAS is an experiment designed to measure the vacuum magnetic birefringence and to improve the detection sensitivity for the above particles. It is based on a very sensitive ellipsometer and a 9 T superconducting dipole magnet especially developed for this purpose. With a sensitivity of about 10−8 rad/√Hz and an integration time of 106 s we plan to be able to detect a dichroism corresponding to the production of particles with a coupling constant to two photons of 10-8 GeV-1, provided that particles masses are less than 10−3 eV. With our experimental parameters, the vacuum birefringence effect would give to an ellipticity of 10−11 rad

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., presented at "Frontier Detectors for Frontier Physics - 8th Pisa Meeting on Advanced Detectors - May 21-27, 2000" to appear in: Nucl.Instr. and Meth. A

We have searched for light scalar and/or pseudoscalar particles that couple to two photons by studying the propagation of a laser beam ($\ensuremath{\lambda}=514$ nm) through a transverse magnetic field. A limit of 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ rad was set on a possible optical rotation of the beam polarization for an effective path length of 2.2 km in a 3.25 T magnetic field. We find that the coupling ${g}_{\ensuremath{\alpha}\ensuremath{\gamma}\ensuremath{\gamma}}l3.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ Ge${\mathrm{V}}^{\ensuremath{-}1}$ at the 95% confidence level, provided ${m}_{a}l{10}^{\ensuremath{-}3}$ eV. Similar limits can be set from the absence of ellipticity in the transmitted beam. We also searched for photon regeneration in a magnetic field and found the limit ${g}_{\ensuremath{\alpha}\ensuremath{\gamma}\ensuremath{\gamma}}l6.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ Ge${\mathrm{V}}^{\ensuremath{-}1}$ for the same range of particle mass.

We searched for light scalars/pseudoscalars (e.g., axions) that couple to two photons using three different techniques: 1) Optical rotation, where a polarized laser beam enters a dipole magnetic field region. The production of scalars/pseudoscalars has an effect the rotation of the polarization which is detected by an analyzer system giving a limit gαγγ

We have searched for the regeneration of photons propagating in a transverse magnetic field. Such an effect would reveal the existence of light scalar or pseudoscalar particles such as the axion that couple to two photons. We obtain for this coupling the limitgaγγ

An experimental set-up for the measurement of the Casimir effect at separations larger than a few microns is presented. The apparatus is based on a mechanical resonator and uses a homodyne detection technique to sense the Casimir force in the plane-parallel configuration. First measurements in the 3-10 micrometer range show an unexpected large force probably due to patch effects., Proceedings of the workshop On the 60 Years on Casimir Effect

A new signal extraction chain for gravitational wave bar detectors is proposed. Signal transduction is made by means of a Fabry–Perot cavity. Using a reference cavity for frequency comparison a sensitivity of hmin=3×10−20 with a bandwidth of about 50 Hz is estimated for an ultracryogenic bar detector. The first prototype implementation as well as room-temperature measurements on it are reported.