## October 26, 2011

In High Energy Physics on October 26, 2011 by physthjc

1) Fourth Generation of Neutrinos

• Baryon asymmetry of the universe and new neutrino states
Abstract: The presence of additional neutrino states with masses in the GeV range is allowed by electro-weak precision observables. However, these additional states can lead to lepton number violating interactions which potentially can wash out considerably any GUT scale generated or elsewise pre-existing baryon asymmetry. We discuss the resulting bounds on neutrino parameters and find that – unless the baryon asymmetry is created at or below the electroweak scale or in some flavor which is decoupled from interactions to the new neutrino states – these have to be pseudo-Dirac with mass splittings between the right-handed and left-handed states of less than about ten keV.
• Fourth Generation Majorana Neutrinos
Abstract: We investigate the possibility of a fourth sequential generation in the lepton sector. Assuming neutrinos to be Majorana particles and starting from a recent – albeit weak – evidence for a non-zero admixture of a fourth generation neutrino from fits to weak lepton and meson decays we discuss constraints from neutrinoless double beta decay, radiative lepton decay and like-sign dilepton production at hadron colliders. Also an idea for fourth generation neutrino mass model building is briefly outlined. Here we soften the large hierarchy of the neutrino masses within an extradimensional model that locates each generation on different lepton number violating branes without large hierarchies.

2)  Wimps

• Galactic synchroton emission from WIMPs at radio frequencies
Abstract: Dark matter annihilations in the Galactic halo inject relativistic electrons and positrons which in turn generate a synchrotron radiation when interacting with the galactic magnetic field. We calculate the synchrotron flux for various dark matter annihilation channels, masses, and astrophysical assumptions in the low-frequency range and compare our results with radio surveys from 22 MHz to 1420 MHz. We find that current observations are able to constrain particle dark matter with “thermal” annihilation cross-sections, i.e. (\sigma v) = 3 x 10^-26 cm^3/s, and masses M_DM < 10 GeV. We discuss the dependence of these bounds on the astrophysical assumptions, namely galactic dark matter distribution, cosmic rays propagation parameters, and structure of the galactic magnetic field. Prospects for detection in future radio surveys are outlined.