Linking infrared and ultraviolet parameters of pion-like states in strongly coupled gauge theories

It has been shown previously that in a relativistic constituent-quark model, predictions for the electromagnetic form factor of the pi meson match not only experimental data but also, in the limit of large momentum transfers, the asymptotics derived from Quantum Chromodynamics (QCD). This is remarkable since no parameters are introduced to provide for this infrared-ultraviolet link. Here, we follow this approach, going beyond QCD. We obtain numerical relations between the gauge coupling constant, the decay constant and the charge radius of the pion-like meson in general strongly-coupled theories. These relations are compared to published lattice results for SU(2) gauge theory with two fermion flavours, and a good agreement is demonstrated. Further applications of the approach, to be explored elsewhere, include composite Higgs and dark-matter models.Comment: V2: 19 pages, 6 figures, version accepted by EPJ

Solar paraphotons

I revisit the question of production of paraphotons, or hidden photons, in the Sun and suggest that a simultaneous observations of solar flares by conventional instruments and by axion helioscopes may provide a discovery channel for paraphotons.Comment: 4 pages, 3 plots. Talk at the 7th Patras Workshop on Axions, WIMPs and WISPs, Mykonos, 26 June - 1 July 201

Physical conditions in potential sources of ultra-high-energy cosmic rays: Updated Hillas plot and radiation-loss constraints

We review basic constraints on the acceleration of ultra-high-energy (UHE) cosmic rays (CRs) in astrophysical sources, namely the geometrical (Hillas) criterion and restrictions from radiation losses in different acceleration regimes. Using the latest available astrophysical data, we redraw the Hillas plot and figure out potential UHECR accelerators. For the acceleration in central engines of active galactic nuclei, we constrain the maximal UHECR energy for a given black-hole mass. Among active galaxies, only the most powerful ones, radio galaxies and blazars, are able to accelerate protons to UHE, though acceleration of heavier nuclei is possible in much more abundant lower-power Seyfert galaxies.Comment: 27 pages, 12 figures. V.2: numerous minor changes, results untouched. Accepted in Physics Uspekhi (UFN)