Off-shell effects in Higgs processes at a linear collider and implications for the LHC

Abstract

The importance of off-shell contributions is discussed for H → V V ( * ) with V ∈ {Z, W } for large invariant masses m V V involving a standard model (SM)-like Higgs boson with m H = 125 GeV at a linear collider (LC). Both dominant production processes e + e − → ZH → ZV V ( * ) and e + e − → ννH → ννV V ( * ) are taken into account, and the signal processes are compared with background yielding the same final state. The relative size of the off-shell contributions is strongly dependent on the centre-of-mass energy. These contributions can have an important impact on the determination of cross sections and branching ratios. However, the combination of on-and off-shell contributions can also be utilised to lift degeneracies allowing to test higher-dimensional operators, unitarity and light and heavy Higgs interferences in extended Higgs sectors. The latter is demonstrated in the context of a 2-Higgs-Doublet model. We also discuss the impact of these aspects for the Large Hadron Collider (LHC) where they are relevant. The importance of a precise measurement of the Higgs mass for on-shell contributions in H → V V ( * ) is emphasized. A particular focus is put on methods for extracting the Higgs width at a LC. Off-shell contributions are shown to have a negligible impact on the width determination at low √ s when applying the Z recoil method to extract branching ratios in combination with an appropriate determination of a partial width. On the other hand, off-shell contributions can be exploited to constrain the Higgs width in a similar fashion as in recent analyses at the LHC. It is demonstrated that this approach, besides relying heavily on theoretical assumptions, is affected by the negative interference of Higgs and background contributions that may limit the sensitivity that is achievable with the highest foreseeable statistics at the LHC and a LC.

Model-independent measurement of the e $$^{{+}}$$ + e $$^{-}$$ - $$\rightarrow $$ → HZ cross section at a future e $$^{{+}}$$ + e $$^{-}$$ - linear collider using hadronic Z decays

“…It has been pointed out [8] that off-shell effects in H → VV * with V = W, Z can break the model independence of the Higgs cross section and branching ratio measurements. However, for the centre-of-mass energies considered in this paper, off-shell effects, which depend strongly on √ s, are found to be small compared to the expected statistical uncertainties from the recoil mass measurements.…”

Heavy Higgs signal–background interference in $$gg\rightarrow VV$$ g g → V V in the Standard Model plus real singlet

“…4 For Higgs production in vector boson fusion, heavy-light interference in a two-Higgs model was studied in Ref. [35] for an e + e − collider and in more detail including heavy-continuum interference in Ref. [44] for the LHC.…”

Interference contributions to gluon initiated heavy Higgs production in the two-Higgs-doublet model

“…Since we obtained the results of In Ref. [16] the relevance of interferences for a heavy Higgs boson in e + e − → Z V V/ννV V in the context of a 2HDM at a linear collider was discussed. Since in these processes the Higgs is produced and decays through the couplings to heavy gauge bosons, the coupling g H V occurs twice in each Feynman diagram, whereas gg → Z Z only comes with one appearance.…”

“…Of particular importance for the discovery of the new particle and the subsequent investigations of its mass and couplings were the decays into heavy gauge bosons h → V V with V ∈ {W, Z }, which appear to be of relevance also for off-shell Higgs bosons [15] (see also Ref. [16] for an investigation of off-shell contributions at a linear collider and the LHC). Given the small theory uncertainties and because of their importance for the unitarization of the process, interference effects between the off-shell Higgs boson and the continuum background in gg → h * → V V were studied for leptonic decays in Refs.…”