Light shining through walls

TL;DR

Redondo and Ringwald review light shining through walls (LSW) as a probe of hidden-sector physics, focusing on axion-like particles, hidden photons, and minicharged particles. They articulate the photon-WISP oscillation framework and summarize a century of laboratory and astrophysical constraints, including HB-star bounds, SN1987a limits, BBN, and CMB constraints, while highlighting hints of cosmic photon regeneration that could be explained by WISPs. The article surveys historical experiments (BFRT, PVLAS) and modern efforts (ALPS, BMV, GammeV, LIPSS, OSQAR), emphasizing the pivotal role of optical cavities, strong magnets, and high-sensitivity detectors in achieving extreme suppression of the LSW probability. It further discusses the prospects for the next generation of LSW experiments, which aim to surpass astrophysical bounds for several WISPs by several orders of magnitude and to provide a critical, laboratory-based test of beyond-SM physics motivated by string theory and hidden sectors.

Abstract

Shining light through walls? At first glance this sounds crazy. However, very feeble gravitational and electroweak effects allow for this exotic possibility. Unfortunately, with present and near future technologies the opportunity to observe light shining through walls via these effects is completely out of question. Nevertheless there are quite a number of experimental collaborations around the globe involved in this quest. Why are they doing it? Are there additional ways of sending photons through opaque matter? Indeed, various extensions of the standard model of particle physics predict the existence of new particles called WISPs - extremely weakly interacting slim particles. Photons can convert into these hypothetical particles, which have no problems to penetrate very dense materials, and these can reconvert into photons after their passage - as if light was effectively traversing walls. We review this exciting field of research, describing the most important WISPs, the present and future experiments, the indirect hints from astrophysics and cosmology pointing to the existence of WISPs, and finally outlining the consequences that the discovery of WISPs would have.

Figures (11)

• Figure 1: In the standard model, light shining through walls can happen via conversion of photons ($\gamma$) into gravitons ($g$) or neutrino-antineutrino pairs ($\nu,\bar{\nu}$) in the background of a magnetic field (marked by a cross).
• Figure 2: The interaction of photons with a neutrino-antineutrino pair in a magnetic field proceeds through two intermediate states: an electron (which has electric charge and weak charge) and and a $Z$ boson (which has no electric charge). If the photon energy is smaller than the electron and $Z$ mass, the amplitude of this process is suppressed by the electron and the $Z$ masses. Other possibilities for this transition involve $W^\pm$ bosons.
• Figure 3: Explicit processes contributing to LSW for various WISPs. From left to right we have photon -- ALP, photon -- hidden photon and photon -- hidden photon oscillations facilitated by MCPs.
• Figure 4: The conversion of a photon into a WISP through mass mixing can happen at any position along the distance of the photon emitter and WISP receiver. The amplitude of all these processes differs only by a phase which depends on the relative "speed" of propagation of the photon and WISP waves. If this difference in phase velocities is small, all the amplitudes interfere constructively and enhance the photon-WISP probability. This is what we call a coherent production mechanism.
• Figure 5: Schematic of a light-shining-through a wall experiment.