Heavy Higgs Line Shape

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]. In Ref. [3], 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. [3], 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 interference effect has been studied in Ref.[4], 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.

For heavy Higgs line shape, theoretical uncertainty of 150 x M_H³[%] (M_H in TeV) has been proposed for conservative estimate. The cubic Higgs mass dependence is based upon the naive assumption of the effects proportional to the Higgs natural width growth. The corresponding numbers are listed below.

The heavy Higgs line shape is now being studied in the CERN Report 2, Handbook of LHC Higgs cross sections: 2. Differential distributions.

Figure 1: In the figure the blue curve gives the off-shell production cross section sampled over the(complex) Higgs propagator while the red curves is sampled over a Breit-Wigner distribution.

Figure 2: The normalised invariant-mass distribution in the OFFP-scheme (red9 with running QCD scales at 800 GeV in the window M_peak+-2Γ_os .

Scaling factor OFFP/OFFBW schemes for the invariant-mass windows.

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.

[1] LHC Higgs Cross Section Working Group, S. Dittmaier, C. Mariotti, G. Passarino, and R. Tanaka (Eds.). Handbook of LHC Higgs cross sections: 2. Differential Distributions. CERN-2012-002, CERN, Geneva, 2012, arXiv:1101.0593, Section 11 Higgs pseudo-observables. [2] G. Passarino, C. Sturm, and S. Uccirati. Higgs pseudo-observables, second Riemann sheet and all that. Nucl.Phys., B834:77–115, 2010.

[3] 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.

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

-- Main.Daniel Hedberg- 09-Feb-2012