Laser experiments explore the hidden sector

TL;DR

Laser polarization and light-shining-through-walls experiments probe hidden-sector physics beyond axion-like particles by constraining minicharged particles and hidden-sector photons through vacuum birefringence, dichroism, and photon regeneration. The authors derive expressions for the relevant observables, map experimental limits to the MCP and hidden-photon parameter spaces, and demonstrate that recent PVLAS results tighten MCP charges to $m_\epsilon \lesssim 0.05$ eV with $\epsilon \lesssim (3-4)\times 10^{-7}$, while BMV and GammeV improve kinetic-mixing bounds by about a factor of two. They also discuss MCPs arising from kinetic mixing with a massless hidden photon (Masso–Redondo type) and the implications for future LSW experiments, including potential order-of-magnitude gains with high-power lasers and long baselines. Overall, the work shows that optical experiments provide competitive laboratory constraints on sub-eV hidden-sector physics and complement astrophysical and cosmological bounds.

Abstract

Recently, the laser experiments BMV and GammeV, searching for light shining through walls, have published data and calculated new limits on the allowed masses and couplings for axion-like particles. In this note we point out that these experiments can serve to constrain a much wider variety of hidden-sector particles such as, e.g., minicharged particles and hidden-sector photons. The new experiments improve the existing bounds from the older BFRT experiment by a factor of two. Moreover, we use the new PVLAS constraints on a possible rotation and ellipticity of light after it has passed through a strong magnetic field to constrain pure minicharged particle models. For masses <~0.05 eV, the charge is now restricted to be less than (3-4)x10^(-7) times the electron electric charge. This is the best laboratory bound and comparable to bounds inferred from the energy spectrum of the cosmic microwave background.

Figures (4)

• Figure 1: 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 hidden-sector photons $\gamma^{\prime}$. 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 $B$ as displayed in \ref{['convertCC']}Schwinger:1951nm.
• Figure 2: New constraints for the MCP mass $m_\epsilon$ and charge fraction $\epsilon$ for Dirac spinor MCPs (left panel) and complex scalar MCPs (right panel) as deduced from the new PVLAS data Zavattini:2007ee. Also shown are the constraints obtained from the BFRT Cameron:1993mr and Q&A Chen:2006cd data, as derived in Ref. Ahlers:2006iz. The constraints result from the absence of ellipticity $\psi$ and rotation $\Delta\theta$ signals, with the shaded parameter space being excluded at 95% C.L.. The 2$\sigma$ region for the ellipticity signal $\psi$ at $B=5$ T is marked with solid lines; apart from a marginally allowed region at larger masses, this signal is excluded in the MCP scenario.
• Figure 3: Limits on hidden-sector photons $\gamma^\prime$ mixing with the photon from searches for photon regeneration in LSW experiments. Left panel: New limits from the non-observation in the LSW experiments BMV and GammeV compared to the old BFRT results. The bounds relax by a factor $1\sim 3$ in the MR model Masso:2006gc depending on $m_{\epsilon}$. See Sect. \ref{['hiddenphotons+MCP']} for details. Right panel: Forecast of future experiments searching for $\gamma^\prime$s. The current limit from LSW experiments (grey shaded) can be extended by dedicated experiments exploiting high power, $\sim 200$ W, lasers with long beamlines, e.g.$L_1=L_2=40$ m for ALPS Ehret:2007cm Phase PETRA (black solid) or $L_1=L_2=170$ m for ALPS Phase HERA (blue dotted). Inserting "phase-shift plates" into the beamline as suggested in Ref. Jaeckel:2007gk could improve the LSW results at larger masses. A substantial improvement in the sensitivity to $\chi$ by several orders of magnitude can be achieved, in the mass range from $m_{\gamma^{\prime}}\sim 10^{-7}\,\rm{eV}$ to $m_{\gamma^{\prime}}\sim 10^{-4}\,\rm{eV}$, through experiments exploiting high-quality microwave cavities Jaeckel:2007ch. These experiments are complementary to searches for deviations of the Coulomb law Williams:1971msBartlett:1988yy and for photon regeneration of hidden-sector photons produced in the Sun within the CAST magnet Redondo (the limit arising from the lifetime of the Sun is slightly worse (see also Ref. Popov:1991)).
• Figure 4: Left panel: Limits on the kinetic mixing of hidden-sector photons with the ordinary electromagnetic photon in a model with a minicharged particle with charge $\epsilon=\chi$, i.e. $e_h=-e$.Right panel: Limits on the kinetic mixing parameter $\chi$ as a function of the hidden sector gauge coupling $e_{h}$ in a model with a minicharged particle with $m_{\epsilon}\rightarrow 0$.