Stellar cooling bounds on new light particles: plasma mixing effects
  Strong constraints on the coupling of new light particles to the Standard Model (SM) arise from their production in the hot cores of stars, and the effects of this on stellar cooling. For new light particles which have an effective in-medium mixing with the photon, plasma effects can result in parametrically different production rates to those obtained from a naive calculation. Taking these previously-neglected contributions into account, we make updated estimates for the stellar cooling bounds on light scalars and vectors with a variety of SM couplings. In particular, we improve the bounds on light (m <~ keV) scalars coupling to electrons or nucleons by up to 3 orders of magnitude in the coupling squared, significantly revise the supernova cooling bounds on dark photon couplings, and qualitatively change the mass dependence of stellar bounds on new vectors. Scalars with mass <~ 2 keV that couple through the Higgs portal are constrained to mixing angle <~ 3*10^-10, which gives the dominant bound for scalar masses above ~ 0.2 eV. 
Edward Hardy, Robert Lasenby
hep-ph