Abstract
We interpret the recently observed excess in diphoton invariant mass as a new spin-0 resonant particle. On the theoretical ground, an interesting question is whether this new scalar resonance belongs to a strongly coupled sector or a well-defined weakly coupled theory. A possible UV-completion that has been widely considered in literature is based on the existence of new vector-like fermions whose loop contributions---Yukawa-coupled to the new resonance---explain the observed signal rate. The large total width preliminarily suggested by data seems to favor a large Yukawa coupling, at the border of a healthy perturbative definition. This potential problem can be fixed by introducing multiple vector-like fermions or large electric charges, bringing back the theory to a weakly coupled regime. However, this solution risks to be only a low-energy mirage: Large multiplicity or electric charge can dangerously reintroduce the strong regime by modifying the renormalization group running of the dimensionless couplings. This issue is also tightly related to the (in)stability of the scalar potential. First, we study---in the theoretical setup described above---the parametric behavior of the diphoton signal rate, total width, and one-loop $\beta$ functions. Then, we numerically solve the renormalization group equations, taking into account the observed diphoton signal rate and total width, to investigate the fate of the weakly coupled theory. We find that---with the only exception of few fine-tuned directions---weakly coupled interpretations of the excess are brought back to a strongly coupled regime if the running is taken into account.
Highlights
The postulated new scalar resonance is very likely part of some unknown dynamics, related or not to the electroweak symmetry breaking
We show the allowed parameter space in the plane (NX, QX ) once the constraints coming from our RG analysis are imposed
In the left panel of figure 14, we focus on the impact of the Renormalization Group Equations (RGEs) related to the gauge-Yukawa sector of the theory, and to this end we put λS = 0
Summary
The cross section of diphoton production via s-channel exchange of a spin-0 resonance with mass M and total width Γ, assuming narrow width, is σ(pp → S → γγ) = M Γs. If the main production process is due to gluon fusion (see [7, 10] for related discussion), the cross section in eq (2.1) reduces to σ(pp. This assumption is favored by data, but it remains interesting to consider other production processes as well.. Production of the spin-0 resonance and its decay to diphoton can be studied in a model-independent way via the following effective Lagrangian, e2 16π2 csγγ M. where loop suppression factors account for possible loop-induced origins of the effective operators. We will rephrase these constraints in the context of a simple UV-complete model
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