IRC-safe jet flavour at leading power

We derive the leading power quark mass effects in cross sections involving flavour modulo-2 jets at next-to-next-to-leading order (NNLO) in QCD. Including these leading power terms recovers, up to power corrections, the infrared-collinear-safe massive-quark cross section from the infrared-collinear-unsafe massless-quark one. The method is applicable to all common jet algorithms and significantly more practical than computing fully massive cross sections. We explicitly demonstrate the approach for flavoured jets produced in lepton collisions, inclusive $b$-jet production at the LHC and the associated production of a $b$-jet and a $Z$-boson at the LHC. Through NNLO, we do not observe any breakdown of perturbative convergence resulting from the presence of logarithms of the quark mass, though such effects might become significant at higher orders. The most important feature of our approach is that it does not require any changes to the definition of the jet or its flavour, nor does it modify the definition of the cross section. Consequently, the predictions can be compared to measurements performed using standard jet clustering algorithms, provided that jet flavour is assigned according to the flavour modulo-2 scheme or an unfolding to this scheme is performed, without the need for experimental collaborations to adapt their analyses to some new, infrared-collinear-safe definition of jet flavour, as would be the case for most - if not all - solutions presented in the literature thus far. We further demonstrate that power corrections in the quark mass, which are typically neglected in the literature, can be significant.