procedure. In general, it will not preserve resonance masses, so that in the $R / B$ ratio, unless the condition (\[eq:collimgamma\]) is met, the numerator and the denominator will not be on the resonance peak at the same time. In case when $R$ is on peak and $B$ is not, this will yield large ratios that badly violate the collinear approximation.

A further problem arises when interfacing the NLO+PS calculation to a Shower generator, in order to generate the next-to-hardest radiation. Shower Monte Carlo’s should be instructed to preserve the mass of the resonances. Thus, radiation should have a resonance assignment. This is generally not available in processes that include interference among radiation generated by different resonances, or by a resonance and the production process itself.

The method
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In order to solve the problems mentioned above, we should separate all contributions to the cross section into terms with definite resonance structure. Each term individually should have resonance peaks only in a single, well defined, resonance cascade chain. The mapping into an underlying Born configuration should be defined for these terms in such a way that the resonance masses are preserved. Thus, when looking for parton pairs that can give 