[Paper Review] Linking radiative seesaw-type mechanism of fermion masses and non-trivial quark mixing with the 750 GeV diphoton excess

Carolina Arbeláez, A. E. Cárcamo Hernández+2 more|arXiv (Cornell University)|Feb 11, 2016

Particle physics theoretical and experimental studies20 citations

TL;DR

This paper proposes a predictive inert 2HDM extended by $S_4 \otimes Z_2 \otimes Z_{12}$ symmetry, introducing exotic charged fermions and a right-handed Majorana neutrino. The model explains the 750 GeV diphoton excess via a singlet scalar decaying to photons through a triangular loop with exotic fermions, while simultaneously generating hierarchical fermion masses and mixing via one-loop mechanisms and radiative seesaw neutrino masses at tree and one-loop levels.

ABSTRACT

We propose a predictive inert 2 Higgs doublet model, where the Standard Model (SM) symmetry is extended by $S_{4}\otimes Z_{2}\otimes Z_{12}$ and the field content is enlarged by extra scalar fields, charged exotic fermions and one heavy righthanded Majorana neutrino. Our model successfully explains the $750$ GeV diphoton excess recently registered at the LHC as well as the observed SM fermion mass and mixing pattern. This excess with respect to the SM Higgs diphoton decay arises from the decay of a singlet scalar field neutral under the symmetries of the model. It decays into gluon and photon pairs via the triangular loop with the internal charged exotic fermions. The latter also generate a nontrivial quark mixing and provide one-loop-level masses for the first- and second-generation charged fermions. The masses of the light active neutrinos are generated from a combination of tree level type-I and one loop-level radiative seesaw mechanisms.

Motivation & Objective

• To explain the 750 GeV diphoton excess observed at the LHC within a minimal yet predictive extension of the Standard Model.
• To simultaneously account for the hierarchical masses and non-trivial mixing pattern of SM fermions using one-loop mechanisms.
• To generate light active neutrino masses through a combination of tree-level type-I and one-loop radiative seesaw mechanisms.
• To unify the origin of fermion masses, mixing, and the diphoton resonance within a single symmetric framework.

Proposed method

• Introduce an inert 2Higgs doublet model extended by $S_4 \otimes Z_2 \otimes Z_{12}$ global symmetry to protect the scalar potential and fermion interactions.
• Postulate the existence of charged exotic fermions that mediate the diphoton decay of a singlet scalar via a triangular loop diagram.
• Use the same exotic fermions to generate one-loop masses for first- and second-generation charged fermions, reproducing the observed quark and lepton mass hierarchy.
• Introduce a heavy right-handed Majorana neutrino to enable tree-level type-I seesaw and one-loop radiative seesaw contributions to light neutrino masses.
• Ensure the singlet scalar is neutral under all gauge and global symmetries, allowing it to decay into $\gamma\gamma$ and $gg$ via loop-induced couplings.
• Construct a scalar potential and Yukawa interactions that preserve the symmetry and allow for a stable vacuum with a light Higgs and a 750 GeV resonance.

Experimental results

Research questions

• RQ1 Can the 750 GeV diphoton excess be explained by a singlet scalar decaying into photons via a loop involving exotic charged fermions within a symmetric extension of the SM?
• RQ2 How can the same exotic fermions simultaneously generate hierarchical masses for first- and second-generation charged fermions at one-loop level?
• RQ3 What is the role of the right-handed Majorana neutrino in generating light active neutrino masses through a mixed tree-level and one-loop seesaw mechanism?
• RQ4 Is it possible to unify the explanation of the diphoton excess, fermion masses, and neutrino masses within a single, predictive model with a consistent global symmetry structure?

Key findings

• The 750 GeV diphoton excess is explained by a singlet scalar decaying into $\gamma\gamma$ via a triangular loop involving charged exotic fermions, with the decay rate consistent with LHC data.
• The same exotic fermions generate one-loop masses for the first- and second-generation charged fermions, reproducing the observed hierarchical mass pattern.
• Light active neutrino masses arise from a combination of tree-level type-I seesaw and one-loop radiative seesaw mechanisms, providing a natural explanation for small neutrino masses.
• The model realizes a predictive framework with $S_4 \otimes Z_2 \otimes Z_{12}$ symmetry, ensuring the stability of the scalar potential and the absence of flavor-changing neutral currents.
• The scalar sector includes a 750 GeV resonance that is neutral under all symmetries and couples to photons and gluons through loop diagrams involving exotic fermions.
• The model successfully unifies the origin of the diphoton excess, fermion mass generation, and neutrino mass generation within a single symmetric framework.