Georg G. Raffelt

Astrophysical methods to constrain axions and other novel particle phenomena

Various extensions of the standard model of elementary particle interactions predict the existence of new particles such as axions, or “exotic” properties of known particles such as neutrino magnetic moments. If these particles are sufficiently light, they emerge in large numbers from the hot and dense interior of stellar bodies. For appropriate ranges of particle parameters, this “invisible” energy loss would lead to observable changes in the evolution of stars. We review the theoretical methods as well as the observational data that have been employed in order to use stars as “particle physics laboratories” in the spirit of this argument. The resulting constraints on the properties of axions are systematically explored, and the application of the general methods to other cases are mentioned and referenced. Cosmological axion bounds and experiments involving galactic or solar axions are briefly reviewed.

General kinetic description of relativistic mixed neutrinos

Abstract We derive a general Boltzmann-type collision integral for mixed neutrinos interacting with each other and with a medium. Our treatment is fully relativistic in that antineutrino degrees of freedom are included. This collision integral allows one to account for the simultaneous effects of neutrino oscillations in a medium and for the effects of collisions. Our results generalizes previous attempts of unify the first- and second-order interaction effects in a single self-consistent equation. Most importantly, our equation includes effects non-linear in the neutrino density matrices (or occupation numbers) such as Pauli blocking of neutrino final states or neutrino refraction in a medium of neutrinos. We apply the definition of the entropy of a non-equilibrium Fermi gas to the case of mixed neutrinos, and we prove that our collision integrals obey the relevant thermodynamic inequalities.

Astrophysical Axion Bounds

Axion emission by hot and dense plasmas is a new energy-loss channel for stars. Observable consequences include a modification of the solar sound-speed profile, an increase of the solar neutrino flux, a reduction of the helium-burning lifetime of globular-cluster stars, accelerated white-dwarf cooling, and a reduction of the supernova SN 1987A neutrino burst duration. I review and update these arguments and summarize the resulting axion constraints.

An improved limit on the axion–photon coupling from the CAST experiment

We have searched for solar axions or similar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup with improved conditions in all detectors. From the absence of excess X-rays when the magnet was pointing to the Sun, we set an upper limit on the axion-photon coupling of 8.8 x 10^{-11} GeV^{-1} at 95% CL for m_a<~ 0.02 eV. This result is the best experimental limit over a broad range of axion masses and for m_a<~ 0.02 eV also supersedes the previous limit derived from energy-loss arguments on globular-cluster stars.

Neutrino Dispersion at Finite Temperature and Density

We investigate the interaction of neutrinos with a thermal background in the framework of the standard model of weak interactions. We calculate the lowest order contribution to the refractive index, including the imaginary part and corrections from the finite gauge boson masses. These corrections are the dominant refractive effect in a CP-symmetric plasma. In the early universe, the occurence of neutrino oscillations, and particularly of magnetically induced left-right transitions, is determined by these effects. In an expanding universe, “level crossing” between left-and right-handed neutrinos occurs near the weak-decoupling temperature (Tweak ≈ 2 MeV), with no such effect for anti-neutrinos.

Cosmological bounds on neutrino degeneracy improved by flavor oscillations

Abstract We study three-flavor neutrino oscillations in the early universe in the presence of neutrino chemical potentials. We take into account all effects from the background medium, i.e., collisional damping, the refractive effects from charged leptons, and in particular neutrino self-interactions that synchronize the neutrino oscillations. We find that effective flavor equilibrium between all active neutrino species is established well before the big-bang nucleosynthesis (BBN) epoch if the neutrino oscillation parameters are in the range indicated by the atmospheric neutrino data and by the large mixing angle (LMA) MSW solution of the solar neutrino problem. For the other solutions of the solar neutrino problem, partial flavor equilibrium may be achieved if the angle θ13 is close to the experimental limit tan2θ13≲0.065. In the LMA case, the BBN limit on the νe degeneracy parameter, |ξν|≲0.07, now applies to all flavors. Therefore, a putative extra cosmic radiation contribution from degenerate neutrinos is limited to such low values that it is neither observable in the large-scale structure of the universe nor in the anisotropies of the cosmic microwave background radiation. Existing limits and possible future measurements, for example in KATRIN, of the absolute neutrino mass scale will provide unambiguous information on the cosmic neutrino mass density, essentially free of the uncertainty of the neutrino chemical potentials.

Cosmology Favoring Extra Radiation and Sub-eV Mass Sterile Neutrinos as an Option

Jan Hamann, Steen Hannestad, Georg G. Raffelt, Irene Tamborra, 3, 4 and Yvonne Y. Y. Wong Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Föhringer Ring 6, 80805 München, Germany Dipartimento Interateneo di Fisica “Michelangelo Merlin”, Via Amendola 173, 70126 Bari, Italy INFN, Sezione di Bari, Via Orabona 4, 70126 Bari, Italy Institut für Theoretische Teilchenphysik und Kosmologie, RWTH Aachen, 52056 Aachen, Germany (Dated: 25 June 2010, revised 23 September 2010)

Updated bounds on milli-charged particles

We update the bounds on fermions with electric charge e and mass m .F or m . m ewe nd 10 15 . <1 is excluded by laboratory experiments, astrophysics and cosmology. For larger masses, the limits are less restrictive and depend onm. For milli-charged neutrinos, the limits are stronger, especially if the dierentflavors mix as suggested by current experimental evidence.

PARTICLE PHYSICS FROM STARS

Low-mass particles such as neutrinos, axions, other Nambu-Goldstone bosons and gravitons are produced in the hot and dense interior of stars. Therefore, astrophysical arguments constrain the properties of these particles in ways which are often complementary to cosmological arguments and to laboratory experiments. This review provides an update on the most important stellar-evolution limits and discusses them in the context of other information from cosmology and laboratory experiments.

Standard and nonstandard plasma neutrino emission revisited

On the basis of Braaten and Segels representation of the electromagnetic dispersion relations in a QED plasma we check the numerical accuracy of several published analytic approximations to the plasma neutrino emission rates. As we find none of them satisfactory we derive a new analytic approximation which is accurate to within 4\%\ where the plasma process dominates. The correct emission rates in the parameter regime relevant for the red giant branch in globular clusters are larger by about