2:1 for Naturalness at the LHC?

TL;DR

This paper analyzes whether a potential Higgs diphoton rate enhancement can be explained by loops of new charged fermions and derives strong vacuum-stability constraints on such scenarios. It shows that large Yukawa couplings required for $\mu_{\gamma\gamma}\sim1.5$–$2$ generically drive the Higgs quartic to negative values at low scales, forcing a low UV cutoff $\Lambda_{UV}$ and implying new bosons below $\sim10$ TeV unless the new fermions are extremely light. Two explicit vector-like fermion setups are studied in detail, revealing that achieving the required enhancement with a high $\Lambda_{UV}$ typically requires light charged states ($m$ around $100$ GeV) and that LHC searches at 8–14 TeV should probe or constrain these scenarios. If no such light states are found and the diphoton enhancement persists, the data would point to new bosons at low scale and argue against large classes of fine-tuned theories, whereas no enhancement would favor natural TeV-scale electroweak symmetry breaking.

Abstract

A large enhancement of a factor of 1.5 - 2 in Higgs production and decay in the diphoton channel, with little deviation in the ZZ channel, can only plausibly arise from a loop of new charged particles with large couplings to the Higgs. We show that, allowing only new fermions with marginal interactions at the weak scale, the required Yukawa couplings for a factor of 2 enhancement are so large that the Higgs quartic coupling is pushed to large negative values in the UV, triggering an unacceptable vacuum instability far beneath the 10 TeV scale. An enhancement by a factor of 1.5 can be accommodated if the charged fermions are lighter than 150 GeV, within reach of discovery in almost all cases in the 8 TeV run at the LHC, and in even the most difficult cases at 14 TeV. Thus if the diphoton enhancement survives further scrutiny, and no charged fermions beneath 150 GeV are found, there must be new bosons far beneath the 10 TeV scale. This would unambiguously rule out a large class of fine-tuned theories for physics beyond the Standard Model, including split SUSY and many of its variants, and provide strong circumstantial evidence for a natural theory of electroweak symmetry breaking at the TeV scale. Alternately, theories with only a single fine-tuned Higgs and new fermions at the weak scale, with no additional scalars or gauge bosons up to a cutoff much larger than the 10 TeV scale, unambiguously predict that the hints for a large diphoton enhancement in the current data will disappear.

Figures (6)

• Figure 1: Uncolored (smooth lines) and colored (dashed lines) particles, for generating a diphoton partial width enhancement $\Gamma(h\to\gamma\gamma)=\Gamma(h\to\gamma\gamma)_{SM}|1+\delta|^2$.
• Figure 2: Left: "vector-like lepton" model. Right: "wino-higgsino" model. The horizontal and vertical axes correspond to the light and heavy mass eigenvalues, respectively. Pink bands denote the diphoton enhancement $\mu_{\gamma\gamma}$. Gray bands denote the vacuum instability cut-off $\Lambda_{UV}$. Dark is for $y=y^c$; pale is for $y=2y^c$. The width of the bands (for both $\mu_{\gamma\gamma}$ and $\Lambda_{UV}$) correspond to varying the electroweak-conserving mass splitting term $\Delta_m$ (see Eq. (\ref{['eq:mspl']})) from zero to one. Green dashed band, on the right, denotes the SUSY wino-higgsino scenario.
• Figure 3: Same as Fig. \ref{['fig:stab']}, but for $\mathcal{N}=2$ copies of vector like fermions.
• Figure 4: Left: $\sigma(pp\to L_1^+L_1^-)$ as a function of the lightest charged state mass in the "vector-like lepton" model at the LHC7 (green, bottom), LHC8 (black, middle) and LHC14 (purple, top). Right: same, for the "wino-higgsino" model.
• Figure 5: Feynman diagrams for new fermion production and decay.