Steril neutrinos really only interact via gravity. You can detect steril neutrinos by observing an excess or a deficit of active neutrinos, i.e. you could detec

Steril neutrinos really only interact via gravity. You can detect steril neutrinos by observing an excess or a deficit of active neutrinos, i.e. you could detect more than the expected number of active neutrinos due to steril neutrinos oscillating into active neutrinos but you could also detect fewer than the expected number of active neutrinos due to active neutrinos oscillating into steril neutrinos. I did not read the paper and don't know what they are measuring but it certainly could be either or even both, i.e. they could detect an excess and a deficit under differing circumstances and both could point towards the existence of steril neutrinos.