Bounds on Very Weakly Interacting Sub-eV Particles (WISPs) from Cosmology and Astrophysics

Abstract

Many weakly interacting sub-electronVolt particles (WISPs) are easily accommodated in extensions of the standard model. Generally the strongest bounds on their existence come from stellar evolution and cosmology, where to the best of our knowledge observations seem to agree with the standard budget of particles. In this talk I review the most demanding constraints for axions and axion-like-particles, hidden photons and mini-charged particles. There is little doubt in the particle physics community about the need of complementing the already very successful standard model (SM) to pursue a completely satisfactory final theory of elementary particles. On the other hand, and with the exception of the dark matter, our increasingly precise knowledge of the universe shows no trace of physics beyond the SM. If new light particles exist they should be very weakly interacting, probably only accessible to extremely precise experiments. Experiments such as the ones presented in this conference. Astrophysics and cosmology are often strong probes of weakly interacting particles. The reason is clear: the huge magnitudes of the typical sizes, time scales, densities or temperatures in the early universe or in stars can convert a tiny “microscopic” eect in a big qualitative change in the evolution of the whole system. This conclusion is specially emphasized when we note that the only weakly interacting sub-eV particles (WISPs) in the standard model are neutrinos, whose production cross sections are strongly energy-dependent and therefore their role is increasingly inhibited as temperatures drop below the electroweak scale. Thus, in an non-extreme range of temperatures the early universe and stellar plasmas are very opaque to standard particles and WISPs can be the most ecient way of en ergy transfer. Whenever such an anomalous energy transfer has an observable implication we can derive strong constraints on the WISP interactions with the standard particles constituting the relevant plasma. The oldest picture of the universe we have is a dense and hot plasma of elementary particles that expanded against gravity. As this plasma cooled down, the three long range forces clustered the particles into the structures which nowadays are found: the color force first confined quarks into protons and neutrons and later merged them into light nuclei (at BBN), the Coulomb force combined them with electrons into atoms (releasing the CMB) that gravity finally clustered into galaxies, then into clusters, etc... After the first galaxies formed, the conditions for stars to be born were settled. During all these steps of structure formation (in a broad sense) the role of WISPs can be constrained. Let us start this review in chronological order. Big Bang Nucleosynthesis.- BBN left an invaluable probe of the early universe environment imprinted in today’s observable light nuclei abundances [1]. Below T ∼ 0.7 MeV the weak