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Posted by eno •
d_sigma, yes, that can happen, even though we would usually regard this value as a probability for the event to happen. But it's a bit more complicated than this in practice. The negative values here come from the so-called parton density function, which can become negative (at higher orders in perturbation theory). Note that this is only a statement about individual points in the sample. Overall, with enough statistics, you will always get a positive sigma for any meaningful physics observable.
Posted by eno •