Accurate Predictions for Heavy Quark Jets

Abstract

When looking at the current comparison between the inclusive b-jet spectra measured by CDF and the corresponding next-to-leading order (NLO) predictions, Fig. 1 [2], one notices two striking features. Firstly, one sees a tension between data and theory: the ratio of data over NLO is around 1.2-1.5 over the whole range of accessible transverse momenta pt of the jets. Secondly, one notices that the uncertainties associated with the theoretical predictions are embarrassingly large (∼ 40 − 50%) for a NLO calculation and in particular they are larger than the corresponding experimental uncertainties. To understand why this happens it is useful to examine Fig. 2. The top plots show that the large uncertainty is associated with very largeK-factors. The middle plots confirm that the uncertainty is the same both with MCFM [3] and MCNLO [4]. Finally, the bottom plots illustrate the origin of the poor convergence of the perturbative expansion: when breaking down the Herwig [5] b-jet spectrum into the hard underlying channels it turns out that two NLO channels, flavour excitation, where a b-quark is kicked out of the sea-quarks, and gluon splitting, where a gluon in the final state splits into a bbpair, are larger than the leading order heavy quark production mechanism, flavour creation, when two incoming light partons produce a heavy quark pair. The reason why supposedly higher order contributions are actually larger than the leading order channel can be clarified by counting soft and collinear logarithms associated with the splitting of gluons into bb-pairs. It turns out that flavour excitation contributes with (αs ln pt/mb) n and gluon splitting contributes with (αs ln pt/mb) 2n−1 relative to the leading order, O(αs) process. Since mb pt these contributions are enhanced. Moreover, the