Second resonance of the Higgs field: motivations, experimental signals, unitarity constraints

TL;DR

This paper explores the theoretical possibility of a second Higgs resonance at around 690 GeV, discusses experimental signals at the LHC, and considers unitarity constraints to refine predictions, challenging the metastability scenario of the SM potential.

Contribution

It proposes a non-perturbative approach predicting a second Higgs resonance at 690 GeV and analyzes its potential experimental signatures and theoretical constraints.

Findings

Hints of a new resonance in LHC data near 690 GeV

Predicted resonance couples similarly to the 125 GeV Higgs

Resonance width estimated between 30 and 38 GeV

Abstract

Perturbative calculations predict that the Standard Model (SM) effective potential should have a new minimum, well beyond the Planck scale, much deeper than the electroweak vacuum. As it is not obvious that gravitational effects can get so strong to stabilize the potential, most authors have accepted the metastability scenario in a cosmological perspective. This perspective is needed to explain why the theory remains trapped into our electroweak vacuum, but requires to control the properties of matter in the extreme conditions of the early universe. Alternatively, one can consider the completely different idea of a non-perturbative effective potential which, as at the beginning of the SM, is restricted to the pure $\Phi^4$ sector yet consistent with the now existing analytical and numerical studies. In this approach, where the electroweak vacuum is the lowest-energy state, besides the resonance of mass $m_h=125$ GeV defined by the quadratic shape of the potential at its minimum, the Higgs field should exhibit a second resonance with mass $690\pm10({\rm stat})\pm20({\rm sys})$ GeV associated with the zero-point energy determining the potential depth. Despite its large mass, this would couple to longitudinal $W$s with the same typical strength as the low-mass state at 125 GeV and represent a relatively narrow resonance of width $\Gamma_H=30\div 38$ GeV, mainly produced at LHC by gluon-gluon fusion. So it is interesting that, in the LHC data, one can find various indications for a new resonance in the expected mass range with a non-negligible statistical significance. As this could become an important new discovery by just adding two missing samples of RUN2 data, we outline further refinements of the theoretical predictions that could be obtained by implementing unitarity constraints, in the presence of fermion and gauge fields, with coupled-channel calculations used for meson spectroscopy.