Likelihood for Nuclear Recoil Ionization Variance for Dark Matter Searches

The ionization variance for electron recoils is given by their Fano factor, and nuclear recoils can be similarly parameterized with an effective Fano factor. Such a parameter has not been precisely constrained, yet has a major impact on the expected signal shape for detectors with single electron-hole resolution. The expected signal distribution can be modeled and constrained by CDMS-II data down to recoil energies of around 5 keV to extract the effective Fano factor in both Silicon and Germanium. Likelihood and Markov chain Monte Carlo analysis techniques are being employed by scientists and students from University of Colorado Denver and University of Minnesota to accomplish this, and will result in a measurement of the effective nuclear recoil Fano factor at energies relevant to dark matter search.