Helioscope Bounds on Hidden Sector Photons

TL;DR

This work analyzes a hidden photon $B_{\mu}$ with mass $m_{\gamma'}$ that kinetically mixes with the SM photon via a coupling $\chi$, deriving solar production rates and helioscope detection prospects. By modeling the Sun as a plasma with transverse and longitudinal plasmon modes and applying a weak-mixing approximation, the author computes the hidden-photon flux $d\Phi_T/d\omega$ and $d\Phi_L/d\omega$, including resonant production when $\omega_P=m_{\gamma'}$. Combining solar energy-loss bounds with CAST's non-observation of X-rays, the paper derives stringent bounds $\chi \lesssim 10^{-14}$ over a broad $m_{\gamma'}$ range, with CAST providing especially strong constraints at low masses and resonant production producing the dominant limits in the $1$–$295$ eV window. The results reveal that longitudinal hidden photons at low energies contribute notably to the solar flux but are largely invisible to current helioscopes, and they discuss prospects for improving bounds via gas-filled helioscopes or lowered energy thresholds. The study thus establishes leading constraints on hidden photons in the sub-eV to tens of keV mass range, highlighting the Sun and CAST as complementary probes of hidden-sector physics.

Abstract

The flux of hypothetical "hidden photons" from the Sun is computed under the assumption that they interact with normal matter only through kinetic mixing with the ordinary standard model photon. Requiring that the exotic luminosity is smaller than the standard photon luminosity provides limits for the mixing parameter down to 10^-14, depending on the hidden photon mass. Furthermore, it is pointed out that helioscopes looking for solar axions are also very sensitive to hidden photons. The recent results of the CAST collaboration are used to further constrain the mixing parameter at low masses m<1 eV where the luminosity bound is weaker. In this regime the solar hidden photon flux has a sizable contribution of longitudinally polarized hidden photons of low energy which are invisible for current helioscopes.

Figures (6)

• Figure 1: Values of the solar parameters relevant for this work plotted as a function of the normalized solar radial coordinate $r$ (left) and the plasma frequency $\omega_\mathrm{P}$ (right) in decimal logarithmic scale. From small to large dashing these are the electron density $n_e$ (red), temperature $T$ (blue), plasma frequency (black), $d\omega_\mathrm{P}^2/dr$ (brown), Hydrogen mass fraction $X$ (green) and radial coordinate $r$ (pink). Except for $X$, they are normalized to their largest values $6.07\times 10^{25}$ cm$^{-3}$, $1350$ eV, $295.5$ eV, $5.8\times 10^{-4}$ eV$^2$ m$^{-1}$ and $R_\odot=6.96\times 10^8$ m, respectively (taken from Bahcall:2004pz).
• Figure 2: The $F_1$ and $F_2$ functions give the flux of solar transverse and longitudinal $B$'s at the Earth for $m_{\gamma^\prime} \ll1$ eV. Notice the different energy scales, only $eV$ L-hidden photons are emitted while the spectrum of T-modes extends to X-ray energies, although considerably suppressed. See the text for details.
• Figure 3: The value of $\omega\Gamma/\omega_\mathrm{P}^2$ controls the enhancement of the probability of emission of hidden photons in resonant conditions. The shadowed region contains the values for the transverse modes in the whole solar model, with the boundary curves for the Solar center (up) and surface (down). The short line is a lower bound ($m_{\gamma^\prime} =\omega_\mathrm{P}$) for the longitudinal modes for which the relation between the energy and the position at the Sun is fixed. See the text for details.
• Figure 4: The function $G$ gives the flux of $m_{\gamma^\prime} \gg295$ eV hidden photons from the Sun. See the text for details.
• Figure 5: Schematics of an helioscope experiment looking for hidden photons. Hidden bosons are produced in the Sun's interior from X-ray plasmon conversion and get out almost freely. Only the sterile component ($S$) of $B$ will traverse the helioscope external shielding, leading to the possibility of reconversion into a detectable photon by $S-A$ oscillations.