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WG2: Pseudo-observables and VBF+VH Higgs production
Introduction talk by Gino
Gino clarified the role of POs as the connection (bridge) between experimental measurements and theory calculations. The POs should allow to reduce the information of the experiment measurements to a limited number of general pseduo-observables. These pseudo-observables can then be calculated in any theory, including EFT. So POs are not intended to replace EFT, but are rather an input to EFT calculations
Gino also expanded on on-shell couplings, the problems with off-shell form-factors and limitations in momentum expansions when discussing VH and VBF. For the latter processes, the set of PO proposed for H->4f is still valid, but only in a limited kinematical regime (i.e. close to threshold for VH). His proposal is to keep measurements differential in shat (for VH) for regions of the phase space where a momentum expansion is not necessarily valid.
VH & VBF in the EFT at NLO in QCD
Fabio gave a summary of VH and VBF simulation at NLO QCD with MG5_aMC@NLO, showing the importance of NLO QCD corrections for several process and observables.
He stressed that the EFT provides a consistent approach to test the interaction of heavy new physics with the SM on a general basis. EFT is also the only way to provide MC simulations, including higher order corrections, to the experiments.
Vices and Virtues of high-energy VH processes
Francesco pointed out potential problems in the applicability of EFT in processes with large momentum transfer, such as VH and VBF. He presented a proposal to derive conservative bounds on the scale of higher-dim EFT operators. The proposal is based on 1) assuming a given EFT cut off (M*) and effective coupling (g*); 2) analyse data imposing kinematical cuts to remain in the EFT validity regime; 3) reports bounds in terms of (M* and g*). A similar approach is already employed in DM searches at LHC via monojets. For measurements that same cut-off needs to be applied to data and simulation, which can be tricky in case the needed kinematic properties cannot be fully reconstructed in data. For conservative bounds it is sufficient to apply the cut-offs only on the simulation.
VBF
Dieter gave an oral overview of BSM VBF production and the anomalous couplings contributing to it. The slides on the agenda page are from a meeting last year and were used only as reference material. Dieter pointed out that some issues in VBF production, as shown for example on slide 5 of the slides, would be mostly relevant if the Higgs was produced exclusively through a non-SM coupling. Given the current LHC results, the by far more likely scenario is SM production with some (small) admixture of anomalous couplings, which would lead to far less drastic modifications of the jet kinematics. Hence a description of VBF through an EFT or equivalent approach should be valid for the dominant part of the events. Regions where the EFT is not valid, should be treated as detailed in the “Discussion” section below.
EFT implementations in MC
Ken presented the new tool “Rosetta” that allows to translate between different dim-6 basis sets and provides an output format directly suitable for simulation with MG5_aMC@NLO. The tool is flexible and new contributions are very welcome.
Discussion
After some discussion, a general consensus emerged on the following two points:
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It would be useful to report differential bounds/measurements on the EW cross-sections (pp->VH) and (pp->Hjj) in terms of the “large-momentum variables”. These are shat =M(VH) for VH and the PT of the highest-PT jet in VBF. Similar, also for gg->H+jets the high pT regime should be explored through differential measurements in the pT of the highest pT jet.
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Measurements on the anomalous momentum structures, in both processes, can be performed in the kinematical region where the SM signal is visible (i.e. when the momentum transfer associated to the fermionic currents is not very large). For VH tihs correspond to the threshold region M(VH) ~ MV+MH. For VBF this correspond to maximal PT of the VBF jets below ~ 150 GeV (see plot on slide 5 by Dieter).
However, in both cases a more detailed discussion/work is needed in order to identify the impact of the cuts necessary to isolate the EW signal from bkg and/or different type of amplitudes (e.g. gg->H).
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