Light from the Hidden Sector

TL;DR

The paper investigates extensions of the standard model with a hidden U(1) sector featuring paraphotons that kinetically mix with the photon, naturally yielding minicharged particles (MCPs). It develops a comprehensive framework for optical probes, deriving photon–paraphoton oscillation and regeneration probabilities in light-shining-through-walls (LSW) setups, both with and without external magnetic fields, and analyzes polarization-based dichroism and birefringence signals. The authors introduce finite-field and multi-paraphoton generalizations (MR model) and provide explicit expressions for regeneration, rotation, and ellipticity, showing that oscillation lengths scale as $1/(\omega e_h\Delta n)$ and can be tuned by the magnetic field. They demonstrate that LSW experiments can be sensitive to paraphotons and can distinguish them from axion-like particles, while polarization tests offer complementary constraints, with BFRT data illustrating concrete bounds; finite-field effects and the two-paraphoton scenario enrich the phenomenology and motivate future high-intensity laser experiments to explore sub-eV hidden sectors.

Abstract

Optical precision experiments are a powerful tool to explore hidden sectors of a variety of standard-model extensions with potentially tiny couplings to photons. An important example is given by extensions involving an extra light U(1) gauge degree of freedom, so-called paraphotons, with gauge-kinetic mixing with the normal photon. These models naturally give rise to minicharged particles which can be searched for with optical experiments. In this paper, we study the effects of paraphotons in such experiments. We describe in detail the role of a magnetic field for photon-paraphoton oscillations in models with low-mass minicharged particles. In particular, we find that the upcoming light-shining-through-walls experiments are sensitive to paraphotons and can distinguish them from axion-like particles.

Figures (12)

• Figure 1: Two diagrams contributing to the coupling of the photon to the hidden-sector fermion $h$ in a situation where the paraphoton is massive. The first is the direct contribution via the charge $\epsilon e$ that arises from the shift \ref{['shift']} of the paraphoton field. The second is due to the non-diagonal mass term \ref{['masssimple']} and cancels the first diagram if the external photon is on shell and massless ($q^2=0$). Note that the second diagram is only present if the paraphoton has non-vanishing mass $\mu^2\neq 0$.
• Figure 2: Schematic picture of a "Light-shining-through-walls" experiment in absence of a magnetic field. The crosses denote the non-diagonal mass terms that convert photons into paraphotons. The photon $\gamma$ oscillates into the paraphoton $\gamma'$ and, after the wall, back into the photon $\gamma$ which can then be detected.
• Figure 3: Projected sensitivity (one expected event per indicated time; no background; $\eta =1$) of future LSW experiments to photon-paraphoton oscillations in the absence of a magnetic field. The shaded region shows the $95\%$ exclusion region of BFRT.
• Figure 4: The contribution of minicharged particles to the polarization tensor \ref{['convertAA']}. The real part leads to birefringence, whereas the imaginary part reflects the absorption of photons caused by the production of particle-antiparticle pairs. The analogous diagram \ref{['convertBB']} shows how minicharged particles mediate transitions between photons and paraphotons. Note that the latter diagram is enhanced with respect to the first one by a factor $\sim e_\mathrm{h}/(\epsilon e){=1/\chi}$. The double line represents the complete propagator of the minicharged particle in an external magnetic field $\mathbf{{\sf B}}$ as displayed in \ref{['convertCC']}Schwinger:1951nm.
• Figure 5: Dependence of the regeneration probability $P_\text{trans}$ (upper panels), rotation $\Delta \theta$ (center panels), and ellipticity $\psi$ (lower panels) on the magnetic filed ${\sf B}$ (left panels) and the length $\ell$ of the magnetic region inside the cavity (right panels). As a benchmark point we assume one massless paraphoton with kinetic mixing parameter $\chi=2\times10^{-6}$ and para-coupling $e_h=e$ with a hidden Dirac spinor with mass $m_\epsilon=0.1$ eV. The remaining experimental parameters are kept at $B=5$ T, $\omega=1$ eV, $N_\text{pass}=1$, and $\ell=5$ m in each plot. The photon regeneration probability is shown for the case of parallel $\theta=0$ (solid line) and orthogonal $\theta=\pi/2$ (dot-dashed line) laser polarization. The dotted line indicates the asymptotic behavior $P_\text{trans} = \chi^4$. The rotation and ellipticity signals assume a polarization of $\theta=\pi/4$. For comparison, the dashed line shows the result for rotation and ellipticity without massless paraphotons (see Ref. Gies:2006caAhlers:2006iz). The gray shaded band in each plot indicates the oscillation regime, corresponding to $\left|K^{\parallel,\perp}\right| < \ell^{-1}$ and $\left|\Delta k^{\parallel,\perp}\right| > \ell^{-1}$ (compare Eqs. (\ref{['defKD']})--(\ref{['ellKD']})).