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 $1.2\times {10}^{-8}\mathrm{rad}$ 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.

Figure 1

Schematic layout of the PVLAS apparatus. See text for description.

Figure 2

Schematic section of the PVLAS apparatus. The distance between the hall floor and the concrete platform is 2.9 m and the height of the granite tower is 7.7 m. See text for description.

Figure 3

Polar plot for the ellipticity signal generated with a 2.3 T magnetic field intensity when helium gas is present in the vacuum chamber. The figure shows the signal for four different gas pressures: 5, 10, 15, and 20 mbar. Each data point represents amplitude and phase of the signal peak observed in a 100 s long time record. For these data, an ellipticity amplitude of ${10}^{-6}$ corresponds to a birefringence $\Delta n\approx {10}^{-17}$.

Figure 4

Summary table of typical spectra observed in vacuum in the measurements reported here. Each spectrum corresponds to about 600 s of data acquisition time (see text).

Figure 5

QWP0 and QWP90 noise distributions in the magnet rotation frequency band $1.92{\Omega }_{\mathrm{Mag}}–2.08{\Omega }_{\mathrm{Mag}}$ for the 2.3 T rotation measurements. The vertical line indicates the resulting amplitude at $2{\Omega }_{\mathrm{Mag}}$ determined from a weighted average of 100 s long data subsets. The value of $\sigma$ for the two configurations is also shown (see text).

Figure 6

QWP0 and QWP90 noise distributions in the magnet rotation frequency band $1.92{\Omega }_{\mathrm{Mag}}–2.08{\Omega }_{\mathrm{Mag}}$ for the 5 T rotation measurements. The vertical line indicates the resulting amplitude at $2{\Omega }_{\mathrm{Mag}}$ determined from a weighted average of 100 s long data subsets. The value of $\sigma$ for the two configurations is also shown (see text).

Figure 7

Noise distributions in the magnet rotation frequency band $1.92{\Omega }_{\mathrm{Mag}}–2.08{\Omega }_{\mathrm{Mag}}$ for the 2.3 T (left) and 5 T (right) ellipticity measurements. The vertical line indicates the resulting amplitude at $2{\Omega }_{\mathrm{Mag}}$ determined from a weighted average of 100 s long data subsets. The value of $\sigma$ for the two field intensities is also shown (see text).

Figure 8

Upper bounds on mass and inverse coupling constant for scalar/pseudoscalar bosons coupled to two photons. These bounds are derived from the background values reported in Table 5 taking into account 45 000 passes in the FP cavity. Also shown are the regions calculated from the data published in [9] and compatible with the bounds reported in [11]. The new data completely exclude the previous 2006 results [9].