• in the value of alphaS
• in the b-jet production rate: at O(alphaS) showering S-events with 5F PDFs generates tt+3b configurations for which there is no counterpart in the subtraction term

Legend

• 4F NLO = 4F NNPDF30_nlo_af_0118 4F (used for NLO calculation)
• 5F LO PY8 = 5F NNPDF2.3 QCD+QED LO (Monash tune PDFs)
• 5F LO HW = 5F MMHT14 LO (HW tune)

Corresponding values of strong coupling

Given as input scale muQ the various tools set scalup as follows

• In MG5, for the showering of S-events, scalup_S is distributed between [0.1,1]*muQ, while H-events are showered with scalup_H=muQ
• In Sherpa H-events are showered by setting scalup_H=kT of 1st emission (independent of muQ)
• Note that also in Powheg the finite remnant (analogous to H-events) is showered with scalup_H=kT of 1st emission
• In Herwig scalup_H and scalup_S are distributed according to a "resummation profile" [3] that ranges between [1-2*rho,1]*muQ, with default width rho=0.3, i.e. scalup_mean=0.7*muQ;
• For H-events in Herwig, muQ' is computed including also the ET of the 1st reconstructed jet in the definition of HT/2 Relevant Herwig references (suggested by C.Reuschle and S. Plaetzer): [1] https://arxiv.org/abs/1705.06919, [2] https://arxiv.org/abs/1512.01178, [3] https://arxiv.org/abs/1605.01338,

The above choices may be responsible for the observed differences. Their motivation and impact should be discussed in detail

The following technical studies for (mainly for MC@NLO tools) are recommended

• separate contributions form S/H events and quantify their relative weight to check if MC differences arise from regions dominated by S or H events
• Study variations of scalup_H and scalup_S separately to find which one dominates (reducing scalup_H in MG5 was found to augment the enhancement. This suggests that S-events dominate and H-events tend to be negative)
• synchronise scalup choices as much as possible to understand if differences are mainly due to matching choices or to the shower itself.

Enhancements in the jet-pT spectrum can be explained with three possible mechanisms:

• By large NLO K-factor entering S-event weights (in regions where S-events tend to dominate)
• As side effect of the shower recoil, which can kick b-jets above the threshold resulting in migrations from bins with low to higher b-jet multiplicity. This hypothesis can be checked by studying the jet-pT distribution in the absence of b-jet acceptance cuts (to be added to the Rivet analysis)
• The different impact of g-> bb splittings in different showers. This can be checked by switching off g->bb splittings in the parton shower (to be studied for stable ttbb only).

Choice of scalup labelling scheme in MG5 and MatchBox

• An extra version of the lepton-pT distributions without lepton isolation was added
• It was proposed scalup should be added to the rivet analyses
• A new version of the light-jet pT distribution in log scale up to 1 TeV was added

Fixed-order predictions ( new )

• Optionally, as a sanity check, we recommend to provide also fixed-order NLO results for the analysis with stable top quarks
• Inclusive NLO XS

• errors in this table are copy-pasted from the Yoda files and are not realistic
• the Powhel result is still about 4% higher due to a radiation-dependent scale choice in the subtraction terms. New NLO and NLOPS predictions with a consistent Born-like scale choice in the counterterm will be generated.

Issues with HWG7

• In the case of decaying tops it was found that the lepton pT spectra predicted by MG5+HWG7 deviate quite strongly from MG5+HW++. This is probably due with an issue with top decays in HW7. It was suggested to try the spin-correlated version of top decays in HW7.
• At the moment MG5+HW predictions are based on HW++ for the case of decaying tops and HW7 for stable tops. In the latter case the matching is done with HW++ subtraction terms. Due to the different recoil schemes in HW++ and HW7 this is not entirely consistent and should be fixed.

Next steps before plenary meeting

• Everybody but only for stable ttbb analysis
  • update analysis from svn (now including also jet-pT and ttbb-pT spectrum without b-jet cuts) and regenerate default predictions
  • repeat analysis switching off g->bb splittings in the shower
• Everybody (optional)
  • separation of S/H events in MC@NLO matching
  • unless already done: factor-2 variations of resummation scale (in MC@NLO matching) and hdamp (in Powheg matching)

• Powhel
  • fix issue in scale choice for subtraction terms
  • deliver missing predictions
• MatchBox
  • deliver missing predictions
  • provide details on PDF+alphaS used for showering
• Sherpa:
  • clarify definition of starting scale in H-events

Plots for HXSWG plenary meeting (March 26, 2018) ( remember to refresh your browser)

Stable ttbb production:

• nominal NLOPS predictions and ratios wrt Sherpa all plots selected plots
• nominal NLO+PY8 and ratios wrt Sherpa all plots selected plots
• nominal NLO+HW7 and ratios wrt Sherpa all plots selected plots
• NLO+PY8/NLO+HW7 ratios (tool by tool) all plots selected plots
• relative effect of double g->bb splittings (tool by tool): sigma/sigma(g->bb off in PS) all plots selected plots

Stable ttbb production: muQ and hdamp variations

• all tools all plots selected plots
• Sherpa all plots selected plots
• MG5+PY8 all plots selected plots
• MG5+HWG all plots selected plots
• PowhegBox +PY8 all plots selected plots

Decayed ttbb:

• nominal parton-level NLOPS predictions and ratios wrt Sherpa all plots selected plots
• relative effects of hadronisation+MPI+QED shower tool by tool all plots selected plots

Open questions and next steps discussed at April 16 meeting

(page numbers refer to the slides presented at the meeting)

General observations that may explain MC differences

• The local K-factor that enters the NLOPS matching procedure is quite large: K=1.9 (see p.8)
• Hypothesis: MC differences may arise from the interplay between the large local K-factor (applied to S-events), the differences in the S/H separation, and b-jet bin migrations induced by jet-recoil effects (see p.8, 11 and 24)
• The only irreducible difference between Powheg and MC@NLO lies in the approximation used for soft/collinear radiation: parton shower (in MC@NLO) or matrix element (in Powheg), while the profile function used to separate S/H terms can (at least in principle) "synchronised" (see p.11)
• For a while we propose to restrict further studies to parton-level ttbb production w.o. hadronication/UE (p21)

Sherpa recoil scheme (p7)

• the change in the default Sherpa recoil scheme from v2.1 to v2.2 has a significant impact. The Sherpa authors should provide some details on nature an motivations behind these recoil schemes.
• Shall we regard the difference as uncertainty?

Scalup-S profile (p17a)

• In order to test if the MC differences arise from the S/H separation, a narrower profile for the S/H separation should be used throughout (both in MC@NLO and Powheg matching)
• Contributions from S and H events should be separated in order to check if S/H shape differences are consistent with the observed MC differences
• Jonas will try to implement an on-the-fly separation of S/H events through the Rivet analysis

Scalup-H profile (p17b)

• The Sherpa authors should clarify the choice of starting scale for H-events

PDF choice for showering in the MC@NLO approach (p19)

• MC@NLO matching to PY8 with Monash tune implies a very large value of alphaS (0.1365) for the 1st emission in S-events
• As a purely technical check, in order to assess the effect of the large alphaS value, one should compare the default MC@NLO+PY8 predictions against alternative ones where the Monash-tune PDFs (5F NNPDF2.3 QCD+QED LO) are replaced by PDFs of the same NNPDF release but with alphaS(MZ)=0.118 and, alternatively, alphaS(MZ)=0.130. The shower should employ the alphaS value taken from the actual PDFs (it is not entirely clear whether this is the case by default). Top quarks should be kept stable. (The results should be interpreted keeping in mind that changing alphaS+PDFs can mess up certain aspects of the tune).
• Later we should try dedicated PY8 tunes based on ttbar data, which feature more reasonable alphaS values
• What about Herwig?
• The relation between alphaS and the tuning of other parameters should be clarified with the PY8 authors. A tune with alphaS(MZ)=.118 would be desirable.

hdamp in Powhel (p22)

• The Powhel authors will consider the option of extending the hdamp separation to FSR

New Rivet analyses targeted at B-jets from g->bb splittings (p24)

• we have introduced new variants of the Rivet analyses targeted at jets with > 1 b-quark constituent (denoted here as bb-jets):
  • Hxswg_ttbjets_stable_v2 = standard analysis with cuts+observables in terms of usual b-jets (with one or more b-quarks inside)
  • Hxswg_ttbjets_stable_v2_0bb = same with extra veto against events with bb-jets
  • Hxswg_ttbjets_stable_v2_1bb = standard analysis with usual cuts+observables applied to bb-jets plus extra requirement of exactly one bb-jet
  • Hxswg_ttbjets_stable_v2_2bb = same as "1bb" but requiring exactly two bb-jets
• In practice you should just update the usual hxswg_ttbjets_stable_v2 analysis from svn and compile it. This will generate hooks for the four parellel analyses that all have to be requested in Rivet during event generation

From now on, please always run these 4 analyses in parallel.

muQ dependence (p28)

• Sherpa features a counter-intuitive muQ dependence that should be understood: the "excess" in the light-jet pT grows/decreases when muQ is reduced/increased.

-- StefanoPozzorini - 2018-02-09