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 $(3.9\pm 0.5)\times {10}^{-12}\mathrm{rad}/\mathrm{\text{pass}}$, at 5 T with 44 000 passes through a 1 m long magnet, with $\lambda =1064\mathrm{nm}$. The relevance of this result in terms of the existence of a light, neutral, spin-zero particle is discussed.

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 $1.2\times {10}^{-8}\mathrm{rad}$ at 5 T and $1.0\times {10}^{-8}\mathrm{rad}$ 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 $1.4\times {10}^{-8}$ 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 ${\sigma }_{\gamma \gamma }<4.5\times {10}^{-34}\mathrm{barn}$ at 95% C.L.

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 $\lambda =1064$ and 532 nm lead to bounds on the parameter $A_e$, describing nonlinear effects in QED, of $A_e^{(1064)}<6.6\times {10}^{-21}{\mathrm{T}}^{-2}@1064\mathrm{nm}$ and $A_e^{(532)}<6.3\times {10}^{-21}{\mathrm{T}}^{-2}@532\mathrm{nm}$, respectively, at 95% confidence level, compared to the predicted value of $A_e=1.32\times {10}^{-24}{\mathrm{T}}^{-2}$. The total photon-photon scattering cross section may also be expressed in terms of $A_e$, setting bounds for unpolarized light of ${\sigma }_{\gamma \gamma }^{(1064)}<4.6\times {10}^{-62}{\mathrm{m}}^2$ and ${\sigma }_{\gamma \gamma }^{(532)}<2.7\times {10}^{-60}{\mathrm{m}}^2$. Compared to the expected QED scattering cross section these results are a factor of $\simeq 2\times {10}^7$ 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.

DOI: https://doi.org/10.1103/PhysRevLett.99.129901

At the Department of Physics of the University of Pisa, Italy, a new and fully engineered version of the YAP-(S)PET small animal scanner has been recently installed. The new YAP-(S)PET is able to perform both PET and SPECT studies on small animals. The scanner is made up of four heads: each one is composed of a 4/spl times/4 cm/sup 2/ YAlO/sub 3/:Ce (or YAP:Ce) matrix of 20/spl times/20 elements, 2/spl times/2/spl times/25 mm/sup 3/ each, coupled to a Position Sensitive Photomultiplier (PS-PMT) (Hamamatsu R2486). The four modules are positioned on a rotating gantry. The switching to the SPECT modality is easily made by mounting a high resolution parallel hole (0.6 mm O, 0.15 mm septum) lead collimator in front of each crystal. This paper reports the performance of the system in terms of absolute sensitivity and spatial resolution for both PET and SPECT modalities. The scatter fraction and noise-equivalent count rate (NEC) for a mouse phantom for different energy windows have been measured in PET modality. Images of phantoms and animals are also presented.