How I read this measurement is as follows:
– \beta = 1 would the case for an unoriented ensemble
– \beta < 1 is for less betarays (less oriented)
– \beta > 1 is for more betarays (more oriented)
But that is a bit besides the case. We can deduce the sign from the fact that the bigger the field is, the lower the asymmetry. This means that for higher fields, the spins will orient more randomly. I think the sign is thus opposed to the hyperfinefield, since otherwise they would just orient themselves parallel to the field. This would result in a more oriented state for higher magnetic field, which is not the case here.