[Paper Review] A 750 GeV graviton and the Higgs as a pNGB

Ezequiel Álvarez, Leandro Da Rold+2 more|arXiv (Cornell University)|Jun 16, 2016

Particle physics theoretical and experimental studies26 references3 citations

TL;DR

This paper proposes that the 750 GeV diphoton excess observed by ATLAS and CMS could be a massive spin-2 graviton in a two-site model, with the Higgs as a pseudo-Nambu-Goldstone boson (pNGB). The pNGB nature suppresses couplings to longitudinal electroweak vectors, evading stringent $VV$ decay bounds, while partial compositeness suppresses leptonic channels. Loop corrections from third-generation quarks and composite fermions are negligible, allowing a natural, large region of parameter space to reproduce the diphoton signal and satisfy all experimental constraints.

ABSTRACT

We study the diphoton excess at 750 GeV reported by ATLAS and CMS, by assuming that it corresponds to a new spin-two resonant state. We model this state as a massive graviton in a two-site model. We show that the very stringent bounds from $VV$ final states can be evaded naturally by considering that the Higgs is a pseudo Nambu-Goldstone boson. In this case the couplings of the graviton to the longitudinal electroweak gauge bosons can be parametrically suppressed. On the other hand, partial compositeness allows to suppress the leptonic channels. We compute loop-induced contributions to the graviton couplings by the presence of the SM third generation of quarks and composite partners of the SM fermions and obtain that they are not important. We find that the diphoton signal and the experimental constraints from other decay channels can be reproduced in a large and natural region of the parameter space of the theory.

Motivation & Objective

• To explain the 750 GeV diphoton resonance observed by ATLAS and CMS as a massive spin-2 state, avoiding stringent experimental bounds from $VV$ final states.
• To show that the Higgs as a pseudo-Nambu-Goldstone boson (pNGB) naturally suppresses couplings to longitudinal electroweak gauge bosons, evading $VV$ constraints without fine-tuning.
• To demonstrate that partial compositeness suppresses leptonic decay channels, further improving consistency with experimental data.
• To compute loop-induced corrections to graviton couplings from SM third-generation quarks and composite fermions, showing they are negligible in the relevant parameter space.

Proposed method

• Model the resonance as a massive graviton in a two-site holographic-like model, with a strongly interacting composite sector breaking global symmetries.
• Assume the Higgs arises as a pNGB from spontaneous global symmetry breaking, enabling natural suppression of $WW$ and $ZZ$ couplings.
• Use partial compositeness to suppress couplings to leptons, reducing branching ratios in leptonic final states.
• Compute one-loop corrections to the graviton-photon and graviton-gluon couplings using dimensional regularization with $D=4-\epsilon$, including contributions from top quarks and heavy fermionic partners.
• Apply renormalization at $\mu = m_X \approx 750$ GeV for SM fermions and at $\mu = m_{\psi_1} \sim \text{TeV}$ for composite fermions to avoid large logarithms.
• Use effective couplings $C_{\gamma}^{\text{eff}}$ and $C_g^{\text{eff}}$ with loop functions $A_G(\tau, \mu)$ for $\tau < 1$ (SM) and $\tau > 1$ (composite fermions), ensuring finite results at the physical scale.

Experimental results

Research questions

• RQ1 Can a massive spin-2 resonance at 750 GeV explain the diphoton excess while evading strong constraints from $VV$ final states?
• RQ2 Does the pNGB nature of the Higgs naturally suppress couplings to longitudinal $W$ and $Z$ bosons, thereby avoiding $VV$ decay bounds?
• RQ3 To what extent do loop corrections from SM third-generation quarks and composite fermions affect the graviton couplings to photons and gluons?
• RQ4 Is there a large, natural region of parameter space where the diphoton signal and all other experimental constraints are simultaneously satisfied?
• RQ5 What are the distinguishable predictions of the model, such as correlations between $\sigma_{ZZ}$, $\sigma_{HH}$, and $\sigma_{Z\gamma}$?

Key findings

• The pNGB Higgs mechanism provides a parametrically suppressed coupling to longitudinal $W$ and $Z$ bosons, naturally evading $VV$ decay bounds without fine-tuning.
• Partial compositeness suppresses leptonic decay channels, further reducing branching ratios and improving agreement with data.
• One-loop corrections to the graviton-photon coupling from SM top quarks and composite fermions are small and do not alter the tree-level phenomenology in the favored parameter region.
• For $C_H \sim 0.5$, the composite scale $M_1$ can be as low as 1–2 TeV, while for $C_H = 1$, $M_1$ must be 4–5 TeV, indicating a natural parameter range.
• The model predicts a correlation between $\sigma_{ZZ}$, $\sigma_{HH}$, and $\sigma_{Z\gamma}$, allowing future measurements to test the model’s consistency.
• The effective coupling $C_{\gamma}^{\text{eff}}$ is stable under loop corrections, with the loop function $A_G(\tau, \mu)$ properly handled for both $\tau < 1$ and $\tau > 1$ cases.