Can the 126 GeV boson be a pseudoscalar?

TL;DR

This paper investigates whether the 126 GeV boson could be a pseudoscalar by analyzing decay correlations, finding the pseudoscalar hypothesis is disfavored but not ruled out, with specific experimental tests suggested.

Contribution

It provides a model-independent analysis of decay correlations to test the pseudoscalar nature of the boson, predicting specific experimental signatures to confirm or exclude this possibility.

Findings

Pseudoscalar hypothesis is disfavored but not excluded.

Certain decay rate measurements can conclusively test the pseudoscalar nature.

Existing data can potentially exclude the $Z extgamma$ decay hypothesis.

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 $\gamma\gamma$, $ZZ^*$, $Z\gamma$, and $WW^*$ final states in a model-independent way. These four decays are controlled by only two effective operators, so that the rates in $Z\gamma$ and $WW^*$ are predicted now that the rates in $\gamma\gamma$ and $ZZ^*,Z\gamma^* \to 4 \ell$ have been measured. We find that the pseudoscalar possibility is disfavored but not conclusively excluded. Experimental exclusion of the $Z\gamma$ decay to well below $\sigma/\sigma_{\rm SM} \sim 170$ or conclusive observation of the $WW^*$ decay near the Standard Model rate would eliminate the pseudoscalar possibility. The $Z\gamma$ 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.