• Getting ready: Tools & MCs topics: Here, first circular about Tools & MCs topics. Discussion sessions on these topics will be organized as needed. Some suggested discussion topics:

1. Uncertainties in HEP event generators: Monte Carlo Event Generators come with a host of perturbative (PU) and non-perturbative (NPU) uncertainties. If PU and NPU are correlated, can we find recipes that do not underestimate the uncertainties, while at the same time not being overly conservative? Many of the PU scale uncertainties are again intertwined. Can we find a common recipe to assess perturbative uncertainties while retaining consistency for each variation?
2. Applicability of matched/merged fixed-order+parton shower calculations: If matched/merged calculations are presented as accurate & precise fixed-order + resummed calculations, how do they compare against dedicated calculations employing higher-order QCD resummation and/or electroweak corrections? What is the impact of calculations including beyond-narrow-width effects on precision extractions of SM parameters, and what are the uncertainties we expect in “resonance-aware” calculations? Can the details of matching/merging schemes interfere with/be exposed by jet substructure measurements?
3. Treatment of charm/bottom quarks in parton showers, and in fixed order+parton shower calculations: Do we need to worry about the treatment of the g → QQ~ in parton showers when describing/interpreting heavy flavor measurements? Should the description of g → QQ~ in timelike and spacelike evolution be improved, or its uncertainty assessed? Does the parton-shower modeling feed down into commonly used fixed-order+parton shower calculations?
4. Assessing higher-order parton shower effects: If a major uncertainty of matched/merged calculations is inherited from their parton shower parts, can we find observables that expose higher-order effects? If so, can we find observables (e.g. through jet substructure studies) that inform parton shower developments and improvements?
5. Addendums to common interfaces: Do we need to reconsider some features for the LHEF format? Or does a new more detailed - description of the current format suffice? Do we need to/want to think how to simulate long-lived particles which traverse active detector material? Are there particles for which a more dynamic interface of detector simulations and event generators is desirable (e.g. long-lived B-mesons)?

- New topics proposed at LH: ttbb shows large shower uncertainties, possibly from shower starting scales, and should be looked at (Christian Reuschle, Fabrizio Caola; a dedicated study in the style of shower uncertainty studies should be aimed for). Charmonium production within jets might offer a different way to assess the modelling of g→cc (Jesse Thaler).