Can the 126 GeV boson be a pseudoscalar?

TL;DR

The paper investigates whether the 126 GeV boson could be a pseudoscalar by parameterizing its loop-induced couplings to gauge bosons with a minimal two-operator effective Lagrangian. By fitting to the observed γγ and 4ℓ rates (including substantial Zγ*→4ℓ contributions), it fixes the coupling ratio and makes sharp predictions for WW* and Zγ decays. It finds a dramatic suppression of φ→WW* and a substantial enhancement of φ→Zγ relative to the SM Higgs, implying that Zγ searches should easily exclude the pseudoscalar, while WW* measurements would also challenge it if observed near SM levels. A potential loophole remains if the 4ℓ signal arises from φ→Z′Z′ decays near the Z pole, but even then the strong WW* suppression remains a characteristic feature; overall, the pseudoscalar interpretation is disfavored but not definitively ruled out.

Abstract

We test the possibility that the newly-discovered 126 GeV boson is a pseudoscalar by examining the correlations among the loop-induced pseudoscalar decay branching fractions to $γγ$, $ZZ^*$, $Zγ$, and $WW^*$ final states in a model-independent way. These four decays are controlled by only two effective operators, so that the rates in $Zγ$ and $WW^*$ are predicted now that the rates in $γγ$ and $ZZ^*,Zγ^* \to 4 \ell$ have been measured. We find that the pseudoscalar possibility is disfavored but not conclusively excluded. Experimental exclusion of the $Zγ$ decay to well below $σ/σ_{\rm SM} \sim 170$ or conclusive observation of the $WW^*$ decay near the Standard Model rate would eliminate the pseudoscalar possibility. The $Zγ$ exclusion should be possible using existing data. The only loophole in our argument is the possibility that the $4\ell$ signal comes from pseudoscalar decays to a pair of new neutral gauge bosons with mass near the $Z$ pole.

Figures (2)

• Figure 1: Theoretical prediction for the partial widths of $\phi$ to $\gamma\gamma$ (blue), $Z\gamma$ (red), $WW^{*}$ (green), and $ZZ^{*}$ (magenta) as a function of $b$, for $M_{\phi} = 126$ GeV and $a = 1$. The normalization on the $y$-axis can be changed by varying $a$; all the partial widths shown scale with $a^2$. The vertical dotted line shows the value of $b$ that we will use to match the SM rates in $\gamma\gamma$ and $4\ell$.
• Figure 2: Invariant mass distributions $M_{12}$ (left) and $M_{34}$ (right) for the $4e$ final state event selection (see text for definitions), after application of the $\Delta R$ cut. We show separately the contributions from $\phi \to Z\gamma^* \to 4e$ (red) and $\phi \to ZZ^* \to 4e$ (blue, multiplied by 1000). The vertical lines indicate the lower cuts on $M_{12}$ at 50 GeV and on $M_{34}$ at 20.5 GeV.