[Paper Review] A spartan model for the LHC diphoton excess

Thomas Appelquist, James Ingoldby+2 more|arXiv (Cornell University)|Jun 2, 2016

Particle physics theoretical and experimental studies20 references3 citations

TL;DR

This paper proposes a minimal, renormalizable extension of the Standard Model with a new $SU(2)_V$ gauge sector and a complex scalar doublet $\Phi_V$ that acquires a vacuum expectation value to generate a heavy scalar resonance at 750 GeV, explaining the LHC diphoton excess. The model features an accidental $U(1)$ symmetry that stabilizes new vector bosons and suppresses electroweak precision corrections, allowing sub-TeV vector masses and strong couplings while remaining consistent with existing constraints.

ABSTRACT

We propose a simple model accommodating the reported 750 GeV diphoton excess seen in the first 13-TeV run of the LHC. It leads to testable predictions, in particular for di-lepton production, at higher integrated luminosity. We append to the minimal standard model a new gauge sector with its own SU(2) symmetry group. A new complex doublet scalar field provides mass for the new vectors and describes the 750-GeV resonance. An adequate rate for the diphoton signals, with resonant production via photon fusion, requires the VEV of the new scalar field to be somewhat less than the electroweak scale. This in turn requires the new heavy vectors to have sub-TeV masses and be relatively strongly coupled. A new global U(1) symmetry plays a key role. Current precision-electroweak constraints are respected.

Motivation & Objective

• To explain the 750 GeV diphoton resonance observed in the LHC's 13 TeV run using a minimal, renormalizable extension of the Standard Model.
• To maintain consistency with precision electroweak measurements despite the presence of new sub-TeV vector and scalar states.
• To identify a mechanism that suppresses new contributions to electroweak precision observables without requiring fine-tuning.
• To predict testable signals, particularly in di-lepton production, at higher luminosity runs of the LHC.
• To explore the role of an accidental global $U(1)$ symmetry in stabilizing the new vector bosons and suppressing their decay widths.

Proposed method

• Introduce a new $SU(2)_V$ gauge sector coupled to a complex scalar doublet $\Phi_V$ that breaks the symmetry via a vacuum expectation value $f_V$.
• Construct the model such that the physical Higgs-like scalar $H$ from $\Phi_V$ is the 750 GeV resonance, with its mass and couplings determined by $f_V$ and the gauge coupling $g_V$.
• Implement a global $U(1)$ symmetry that emerges as an accidental symmetry after $SU(2)_R \times SU(2)_V$ breaking, which suppresses tree-level contributions to electroweak precision observables.
• Use effective field theory (EFT) techniques to compute the diphoton production cross section via photon fusion, dominated by loops of the new vector bosons.
• Compute parton luminosities $C_{\gamma\gamma}$ using NNPDF23_nlo_as_0119_qed PDF sets to estimate the diphoton signal rate at $\sqrt{s} = 8$ and $13$ TeV.
• Assess constraints from electroweak precision data and Higgs couplings, showing that the model remains viable even with sub-TeV vector masses.

Experimental results

Research questions

• RQ1 Can a simple, renormalizable model explain the 750 GeV diphoton excess without requiring fine-tuning or very heavy new states?
• RQ2 How can a new $SU(2)_V$ gauge sector with sub-TeV vector bosons be consistent with electroweak precision measurements?
• RQ3 What role does the accidental $U(1)$ symmetry play in suppressing new contributions to $S$ and $T$ parameters?
• RQ4 What are the predictions for di-lepton and photon fusion production rates at the LHC, and are they testable in future runs?
• RQ5 What constraints does the requirement of a 750 GeV resonance place on the vacuum expectation value $f_V$ and the gauge coupling $g_V$?

Key findings

• The model explains the 750 GeV diphoton excess via photon fusion, with the resonance arising from a massive scalar $H$ produced via loops of new $SU(2)_V$ vector bosons.
• The requirement of a sufficiently large diphoton rate implies $f_V \lesssim f$, where $f \approx 244$ GeV is the electroweak scale, forcing the new vectors to be relatively light and strongly coupled.
• The new $SU(2)_V$ gauge coupling must be relatively strong to generate the required vector masses, with $g_V \gtrsim 2$ to achieve sub-TeV masses without fine-tuning.
• The accidental $U(1)$ symmetry suppresses tree-level contributions to $S$ and $T$ parameters, allowing the model to satisfy precision electroweak constraints despite the new states.
• Production cross sections for the $H$ resonance via photon fusion are estimated to be in the range of $\sim 11$ fb at $\sqrt{s} = 8$ TeV and $\sim 54$ fb at $\sqrt{s} = 13$ TeV, depending on $M_H$, with $C_{\gamma\gamma}$ values of order 10–100.
• The new charged vector bosons are stabilized by the accidental $U(1)$ symmetry, leading to long lifetimes and potential signatures in di-lepton final states at future LHC runs.