At LHC, the heavy Higgs searches are carried out in gg→H→WW→lνlν,lνqq, gg→H→ZZ→llll,llνν,llqq channels. The current searches for a heavy Higgs boson assume on-shell (stable) Higgs boson production. The Higgs boson is then decayed via an ad hoc Breit-Wigner (either fixed-width or running-width scheme) implemented in the MC simulations [1]. Therefore the question remains what is the limitation of narrow Higgs width approximation for decoupling Higgs production and decay, and what is the effect of interference due to large Higgs width with Standard Model (SM) backgrounds.

Recent studies show that the effects due to off-shell Higgs boson production and decay, and due to the interference with the SM backgrounds, may become sizable for Higgs boson masses m_{H} > 300 GeV.
The Higgs boson mass line shape is expected to be altered as well [2], [3],[4],.
In Ref. [5], [6], the default-scheme (purely Higgs signal cross-section) in iHixs program has been compared to zero-width approximation and Seymour-scheme (“improved s-channel approximation” which is a prescription based on the resummation of VV→VV scattering amplitudes). Seymour-scheme tries to simulate the effects of signal-background interference off the resonant peak.
In Figure 6 of Ref. [5], a deviation of default-scheme and Seymour-scheme with respect to the zero-width approximation is observed to be +30% ~ -20% difference in cross-section for M_{H} < 600 GeV.
Also in Figure 7, significant distortion of Higgs line shape of Seymour-scheme with respect to the default-scheme is demonstrated, and the effect becomes increasingly important for heavy Higgs.
The heavy Higgs line shape has being also studied in the CERN Report 2, Handbook of LHC Higgs cross sections: 2. Differential distributions [7]

Contrary to the narrow-width-approximation in Higgs boson production and decay, now the theory framework in complex-pole-scheme with proper Higgs propagator exists, based on studies reported in [3].

In the SM Higgs case, there are large interference effects with continuum backgrounds in gg->H->WW/ZZ channels, and it is know that the interference is very large for WW case in particular.
The interference effect has been studied in Ref.[8], where cross section of gg→W^{+}W^{-} has been studied for Higgs signal and background diagrams.
As shown in Figure 6 of Ref.[4], for M_{H} = 120 GeV, the prediction for the Higgs cross section including interference effects (σ_{H,i}, though non-physical quantity) is 10-15% lower than Higgs diagram only (σ_{H}).
For M_{H} > 400 GeV the interference becomes large and constructive, reaching above +30% for M_{H} = 600 GeV.
The effect of the interference has also been studied in [9], [10] and [11].

References

[1] LHC Higgs Cross Section Working Group, S. Dittmaier, C. Mariotti, G. Passarino, and R. Tanaka (Eds.). Handbook of LHC Higgs cross sections: 1. Inclusive observables. CERN-2011-002, CERN, Geneva, 2011, arXiv:1101.0593, Section 11 Higgs pseudo-observables.

[2] M.H. Seymour, The Higgs boson line shape and perturbative unitarity, Phys. Lett. B354 (1995) 409-414, hep-ph/9505211.

[3] S. Goria, G. Passarino, D. Rosco. The Higgs Boson Lineshape arXiv:1112.5517, 2011

[4] G. Passarino, C. Sturm, and S. Uccirati. Higgs pseudo-observables, second Riemann sheet and all that. Nucl.Phys., B834:77–115, 2010.

[5] C. Anastasiou, S. Buhler, F. Herzog, and A. Lazopoulos. Total cross-section for Higgs boson hadroproduction with anomalous Standard Model interactions. arXiv:1107.0683, 2011.

[6] C. Anastasiou, S. Buehler, F. Herzog and A. Lazopoulos, Inclusive Higgs boson cross-section for the LHC at 8 TeV, arXiv:1202.3638 [hep-ph].

[7] Handbook of LHC Higgs Cross Sections: 2. Differential Distributions / Dittmaier, S ; Mariotti, C (ed.) ; Passarino, G (ed.) ; Tanaka, R (ed.) arXiv:1201.3084 CERN-2012-002.

[8] J.M. Campbell, R.K. Ellis, and C. Williams. Gluon-gluon contributions to W^{+}W^{-} production and Higgs interference effects. JHEP, 1110:005, 2011.

[9] N. Kauer Signal-background interference in gg → H →VV. arXiv:1201.1667

[10] G. Passarino. Higgs Interference Effects in gg → ZZ and their Uncertainty 1206.3824v1

[11] N. Kauer, G. Passarino. Inadequacy of zero-width approximation for a light Higgs boson signal JHEP 1208 (2012) 116

-- ReiTanaka - 04-Dec-2011

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