Abstract
We propose a new approach for combining next-to-leading order (NLO) and parton shower (PS) calculations so as to obtain three core features: (a) applicability to general showers, as with the MC@NLO and POWHEG methods; (b) positive-weight events, as with the KrkNLO and POWHEG methods; and (c) all showering attributed to the parton shower code, as with the MC@NLO and KrkNLO methods. This is achieved by using multiplicative matching in phase space regions where the shower overestimates the matrix element and accumulative (additive) matching in regions where the shower underestimates the matrix element, an approach that can be viewed as a combination of the MC@NLO and KrkNLO methods.
Highlights
JHEP01(2022)[067] has the undesirable feature of negative weights.[1]
We propose a new approach for combining next-to-leading order (NLO) and parton shower (PS) calculations so as to obtain three core features: (a) applicability to general showers, as with the MC@NLO and POWHEG methods; (b) positive-weight events, as with the KrkNLO and POWHEG methods; and (c) all showering attributed to the parton shower code, as with the MC@NLO and KrkNLO methods
By comparing the MC@NLO and KrkNLO method using a common language, it becomes clear that the two methods have much in common, and can be merged in such a way that the KrkNLO positivity is maintained, unweighted events can be generated on the fly, and no issues arise from the limited coverage of the phase space by the parton shower code
Summary
By comparing the MC@NLO and KrkNLO method using a common language, it becomes clear that the two methods have much in common, and can be merged in such a way that the KrkNLO positivity is maintained, unweighted events can be generated on the fly, and no issues arise from the limited coverage of the phase space by the parton shower code. The NLO program is responsible for generating a second (positive definite) sample of events, corresponding to the θ(R − Rs) term of eq (3.1), covering the part of the real phase space where the shower Rs underestimates the true real matrix element. These events are passed to the parton shower code for normal showering. The scheme outlined above requires additional action from the NLO code after the first parton shower emission This is a technical consideration, which in our view is a small price to pay for an approach that, like the MC@NLO scheme, leaves responsibility for the first emission with the parton shower, while eliminating negative weights. One might envisage an approach in which one runs the complete shower, uses a jet algorithm to map the full event to the real phase space and applies the acceptance probability
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