  We investigate the question of whether the recent modulation signal claimed
by CoGeNT is best explained by the dark matter (DM) hypothesis from a Bayesian
model comparison perspective. We consider five phenomenological explanations
for the data: no modulation signal, modulation due to DM, modulation due to DM
compatible with the total CoGeNT rate, and a signal coming from other physics
with a free phase but annual period, or with a free phase and a free period. In
each scenario, we assign to the free parameters physically motivated priors. We
find that while the DM models are weakly preferred to the no modulation model,
but when compared to models where the modulation is due to other physics, the
DM hypothesis is favoured with odds ranging from 185:1 to 560:1. This result is
robust even when astrophysical uncertainties are taken into account and the
impact of priors assessed. Interestingly, the odds for the DM model in which
the modulation signal is compatible with the total rate against a DM model in
which this prior is not implemented is only 5:8, in spite of the former's
prediction of a modulation amplitude in the energy range 0.9 to 3.0 keVee that
is significantly smaller than the value observed by CoGeNT. Classical
hypothesis testing also rules out the null hypothesis of no modulation at the
1.6 sigma to 2.3 sigma level, depending on the details of the alternative.
Lastly, we investigate whether anisotropic velocity distributions can help to
mitigate the tension between the CoGeNT total and modulated rates, and find
encouraging results.
