The Low-Energy Frontier of Particle Physics

Abstract

Most embeddings of the Standard Model into a more unified theory, in particular the ones based on supergravity or superstrings, predict the existence of a hidden sector of particles which have only very weak interactions with the visible sector Standard Model particles. Some of these exotic particle candidates (such as e.g. "axions", "axion-like particles" and "hidden U(1) gauge bosons") may be very light, with masses in the sub-eV range, and have very weak interactions with photons. Correspondingly, these very weakly interacting sub-eV particles (WISPs) may lead to observable effects in experiments (as well as in astrophysical and cosmological observations) searching for light shining through a wall, for changes in laser polarisation, for non-linear processes in large electromagnetic fields and for deviations from Coulomb's law. We present the physics case and a status report of this emerging low-energy frontier of fundamental physics.

Figures (12)

• Figure 1: In compactifications of type II string theories the Standard Model is locally realized by a stack of space-time filling D-branes wrapping topologically non-trivial submanifolds in the compact dimensions. In general, there can also be hidden sectors localized at different places. They can arise from branes of different dimension (D3 or D7 branes) which can be either of large extent or localized at singularities. Light visible and hidden matter particles arise from strings located at intersection loci and stretching between brane stacks.
• Figure 2: Summary of cosmological and astrophysical constraints for axions (top) (for the mass $m_a$ or decay constant $f_a$) Raffelt:2006cw and axion-like-particles (bottom) (two photon coupling $g$ vs. mass $m_{a}$ of the ALP) Andriamonje:2007ewSchlattl:1998fzInoue:2008zp. See the text for details. Note that the mass region, where the axion can be the cold dark matter (the orange regions labeled "CDM" in the plots), can be extended towards smaller masses (larger $f_a\lesssim 10^{16}$ GeV) by anthropic reasoning. Moreover, in the first plot the areas marked "ADMX" and "CAST" show the near future search ranges. In the second plot the axion band is shown hatched. We have also marked other areas with interesting astrophysical hints in orange. For comparision, we also show laboratory limits from photon regeneration experiments (ADMX and LSW) as discussed in Section \ref{['searches']}. (Both compilations extended from Ref. Redondo:2008en.) Note that the limit from ADMX is valid only under the assumption that the local density of ALPs at earth is given by the dark matter density.
• Figure 3: Summary of cosmological and astrophysical constraints for minicharged particles (fractional charge $\epsilon =Q_\epsilon/e$ vs. mass $m_{\epsilon}$) (compilation from Ref. Goodsell:2009xc). See the text for details. In addition we also show the laboratory limits discussed in Sect. \ref{['searches']}. Moreover, at relatively large masses and couplings we also have the bounds from accelerator and fixed target experiments (SLAC).
• Figure 4: Summary of cosmological and astrophysical constraints for hidden photons (kinetic mixing $\chi$ vs. mass $m_{\gamma^\prime}$) (compilation from Ref. Redondo:priv). See the text for details. In addition we also show laboratory limits (see Sect. \ref{['searches']} for details on the constraints in the sub-eV regions; at higher mass we have electroweak precision measurements (EW), bounds from upsilon decays ($\Upsilon_{3S}$) and fixed target experiments (EXXX)). Areas that are especially interesting are marked in light orange.
• Figure 5: Schematic of a "light-shining-through a wall" experiment. An incoming photon $\gamma$ is converted into a new particle $X$ which interacts only very weakly with the opaque wall. It passes through the wall and is subsequently reconverted into an ordinary photon which can be detected.