Not sure I entirely understand your question but I'll try to explain a bit anyway: they're using a dataset of 9fb^-1. That means it includes 9 events for a cros

Not sure I entirely understand your question but I'll try to explain a bit anyway: they're using a dataset of 9fb^-1. That means it includes 9 events for a crosssection of 1fb (femtobarn). Now the paper doesn't seem to include what crosssection their events have but if we assume something of the order of magnitude of 1pb then the dataset would contain around 9000 events of interest. Now all that doesn't really matter as this is already factored into the standard deviation of their result. They have enough data to say it's a 3 sigma (being the standard deviation) deviation from the standard model. Now this "1 in 1000" really just means that the null hypothesis (the Standard Model) says there's a 0.1% chance for that result to happen. Of course it could also be that they forgot to factor in some uncertainty of their measurements and their sigma is actually much bigger. But it surely won't be the size of their dataset as this is quite obvious and easy to take into account.