Abstract
Off-shell Higgs at the high mass tail may shed light on the underlying mechanism of the electroweak symmetry breaking. Due to the large cancellation in the standard model (SM) between the box and Higgs-mediated triangle diagrams, the $gg\to WW(ZZ)$ process in the SM is dominated by the $V_T V_T$ transverse-mode at the high mass tail. The cancellation does not necessarily hold, when there is a sufficiently large new physics contribution resulting in $V_LV_L$ longitudinal mode, which is commonly the case when the Higgs sector is modified. Thus the $V_LV_L$ final states in the high mass tail can be utilized as a sensitive probe for such models. In the paper we focus on a study of the $gg \to ZZ$ process in the fully leptonic decay modes, proposing to utilize the polarization modes of the off-shell Higgs to probe new physics, whose contribution mainly shows in the longitudinal mode. As examples, we analyze three different Higgs sector new physics cases (Higgs portal with a light scalar, a broad-width scalar that mixes with the Higgs, and quantum critical Higgs models), and demonstrate that the angular information relating to the polarization serves as very sensitive probe for such new physics.
Highlights
With the discovery of the Higgs boson at the LHC [1,2], the Standard Model (SM) seems to be the true low energy effective theory (EFT), with the Higgs mechanism accounting for electroweak symmetry breaking (EWSB)
In the Standard Model, there is an exact cancellation of the logarithmic divergence between the box and Higgs-mediated triangle diagrams due to unitarity, such that the gg → ZZðWWÞ process is left with a negligible VLVL longitudinal mode contribution in the tail region of the offshell Higgs boson
We propose to test the unitarity of the Standard Model by probing this behavior, as new physics violating this cancellation at certain tail region can be probed by a precision study of the VLVL mode
Summary
With the discovery of the Higgs boson at the LHC [1,2], the Standard Model (SM) seems to be the true low energy effective theory (EFT), with the Higgs mechanism accounting for electroweak symmetry breaking (EWSB). Less known is to test unitarity by looking at the longitudinal modes towards the off-shell region of the Higgs mass distribution, as the logarithmic divergence from the box and Higgs-mediated triangle diagrams in this mode exactly cancel each other in the SM. For this reason, the off-shell Higgs boson deserves a careful study for possible unitarity violation behavior due to deviation from the theory, and an optimal place to start would be the hà → ZZ → 4l decay channel given its high precision [3,4,5]. We present a model-independent and effective method to cover generic NP scenarios modifying the Higgs sector
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