Particle interpretation of the PVLAS data: Neutral versus charged particles

Recently the PVLAS Collaboration reported the observation of a rotation of linearly polarized laser light induced by a transverse magnetic field—a signal being unexpected within standard QED. Two mechanisms have been proposed to explain this result: production of a single (pseudo-)scalar particle coupled to two photons or pair production of light millicharged particles. In this work, we study how the different scenarios can be distinguished. We summarize the expected signals for vacuum magnetic dichroism (rotation) and birefringence (ellipticity) for the different types of particles—including new results for the case of millicharged scalars. The sign of the rotation and ellipticity signals as well as their dependencies on experimental parameters, such as the strength of the magnetic field and the wavelength of the laser, can be used to obtain information about the quantum numbers of the particle candidates and to discriminate between the different scenarios. We perform a statistical analysis of all available data, resulting in strongly restricted regions in the parameter space of all scenarios. These domains suggest clear target regions for upcoming experimental tests. As an illustration, we use preliminary PVLAS data to demonstrate that near-future data may already rule out some of these scenarios.

Figure 1

Schematic view of a “light-shining-through-a-wall” experiment. (Pseudo-)scalar production through photon conversion in a magnetic field (left), subsequent travel through a wall, and final detection through photon regeneration (right).

Figure 2

Dependence of the rotation and ellipticity signals on the strength of the magnetic field $B$, the wavelength $\lambda$ of the laser and the length $L$ of the magnetic region inside the cavity for ALPs (dark green) and MCPs (light red). The crossing of the blue dotted lines corresponds to the rotation published by the PVLAS group and their preliminary ellipticity signal for $B=5\mathrm{T}$, $\lambda =1064\mathrm{nm}$ and $L=1\mathrm{m}$.

Figure 3

ALP: The $5\sigma$ confidence level of the model parameters (red). The blue-shaded regions arise from the BFRT upper limits for regeneration (dark blue), rotation (blue) and ellipticity (light blue). The gray-shaded region is the Q&A upper limit for rotation. The bands show the PVLAS $5\sigma$ C.L.’s for rotation (coarse hatched) and ellipticity (fine hatched) with $\lambda =532\mathrm{nm}$ (left hatched) and $\lambda =1064\mathrm{nm}$ (right hatched), respectively. The dark-green band shows the published result for rotation with $\lambda =1064\mathrm{nm}$. The light-green bands result from an inclusion of preliminary data from PVLAS. The upper panels show the fit to the published data; the center panels include also the preliminary data from PVLAS and the lower panels depict the fit using only PVLAS data. The preliminary data is only used to demonstrate the potential to distinguish between the different scenarios.

Figure 4

MCP: The $5\sigma$ confidence level of the model parameters (red). The blue-shaded regions arise from the BFRT upper limits for rotation (blue) and ellipticity (light blue). The gray shaded region is the Q&A upper limit for rotation. The bands show the PVLAS $5\sigma$ C.L.’s for rotation (coarse hatched) and ellipticity (fine hatched) with $\lambda =532\mathrm{nm}$ (left hatched) and $\lambda =1064\mathrm{nm}$ (right hatched), respectively. The dark-green band shows the published result for rotation with $\lambda =1064\mathrm{nm}$. The light-green bands result from an inclusion of preliminary data from PVLAS. The upper panels show the fit to the published data; the center panels also include the preliminary data from PVLAS, and the lower panels depict the fit using only PVLAS data. The preliminary data is only used to demonstrate the potential to distinguish between the different scenarios. The preliminary PVLAS value for the sign of the ellipticity singles out the large-$\chi$ (small-mass) branch of the fermionic MCP $\frac{1}{2}$ and the small-$\chi$ (large-mass) branch of the scalar MCP 0, cf. Table , as is visible in the center and lower panels.