Hiding a Heavy Higgs Boson at the 7 TeV LHC

TL;DR

The paper examines whether a heavy Standard Model Higgs can be reconciled with electroweak precision tests and 7 TeV LHC null results by adding a single new weak multiplet. It identifies two concrete hiding strategies: (i) introduce a color-singlet scalar that opens new Higgs decay channels, suppressing SM Higgs modes; or (ii) introduce a colored scalar that destructively interferes with the SM gluon-Higgs coupling and substantially reduces gluon-fusion production. EWPT constraints on S and T guide the allowed mass splittings, while collider phenomenology and current searches constrain the viable parameter space; in many cases sizable regions remain viable. The work also discusses alternative routes (vector-like fermions, $Z'$ bosons, and non-linear EWSB) and highlights future collider signatures, particularly multi-lepton and exotic final states, that could indirectly reveal or exclude a heavy Higgs scenario.

Abstract

A heavy Standard Model Higgs boson is not only disfavored by electroweak precision observables but is also excluded by direct searches at the 7 TeV LHC for a wide range of masses. Here, we examine scenarios where a heavy Higgs boson can be made consistent with both the indirect constraints and the direct null searches by adding only one new particle beyond the Standard Model. This new particle should be a weak multiplet in order to have additional contributions to the oblique parameters. If it is a color singlet, we find that a heavy Higgs with an intermediate mass of 200 - 300 GeV can decay into the new states, suppressing the branching ratios for the standard model modes, and thus hiding a heavy Higgs at the LHC. If the new particle is also charged under QCD, the Higgs production cross section from gluon fusion can be reduced significantly due to the new colored particle one-loop contribution. Current collider constraints on the new particles allow for viable parameter space to exist in order to hide a heavy Higgs boson. We categorize the general signatures of these new particles, identify favored regions of their parameter space and point out that discovering or excluding them at the LHC can provide important indirect information for a heavy Higgs. Finally, for a very heavy Higgs boson, beyond the search limit at the 7 TeV LHC, we discuss three additional scenarios where models would be consistent with electroweak precision tests: including an additional vector-like fermion mixing with the top quark, adding another U(1) gauge boson and modifying triple-gauge boson couplings.

Figures (12)

• Figure 1: The $S-T$ contour plot with the reference SM Higgs mass at 500 GeV (blue and upper) and 250 GeV (red and lower). For each mass, the two contours correspond to 68% and 95% C.L. constraints.
• Figure 2: The allowed regions in the $(m_1, m_2-m_1)$ plane in the scalar doublet model with $Y=\frac{1}{2}$ from a fit to the $S$ and $T$ parameters. The two contours correspond to 68% and 95% C.L. respectively.
• Figure 3: The allowed regions in the $(m_1, m_2-m_1)$ plane for a weak-triplet scalar model with $Y=1$ from a fit to the $S$ and $T$ parameters. The two contours correspond to 68% and 95% C.L. respectively.
• Figure 4: Decay branching ratio of the Higgs to the lightest component of an additional doublet (left) and triplet (right) as a function of the coefficient $\lambda_3$ which doesn't contribute to the mass splitting of different components of $\Phi$. For each curve, we fix the Higgs mass $m_h$ and the lowest component mass $m_{\phi_0}$. The mass splitting is $\delta$ =100 GeV (left); $\delta$ =50 GeV (right).
• Figure 5: Production cross sections of an additional doublet/triplet $\Phi$ at the LHC with $\sqrt{s}=7$ TeV where $m_0$ is the mass of the lightest component of $\Phi$. For a doublet, $\delta m =$100 GeV; for a triplet, $\delta m=$50 GeV. Left: $\phi_0\phi_0$ (purple and lower), $\phi_\pm \phi_0$ (black and middle) and $\phi_+\phi_-$ (blue and upper). Right: $\phi_{\pm\pm}\phi_{\mp\mp}$ (black and lower), $\phi_{\pm\pm}\phi_{\mp}$ (purple and middle) and $\phi_\pm \phi_0$ (blue and upper).