They do model backgrounds; the 5-sigma significance is a claim about the odds that background phenomena could have randomly caused this observation. However, t

They do model backgrounds; the 5-sigma significance is a claim about the odds that background phenomena could have randomly caused this observation. However, their main backgrounds are not from the sources you mention. OPERA knows exactly what time the beam was on, and only considers data during that time <insert joke about that one time they thought neutrinos were traveling faster than light>. If galactic/solar tau neutrinos were present, they would see them all the time (not just when the beam was on), but they don't. That's not to say that they aren't there, but at the energies measured by the OPERA experiment, they would be exceedingly rare, especially compared to the high intensity of the neutrino beam they are receiving from CERN.