Distinguishing 'Higgs' spin hypotheses using γγ and WW* decays

The new particle X recently discovered by the ATLAS and CMS Collaborations in searches for the Higgs boson has been observed to decay into γγ, ZZ ∗ and WW ∗, but its spin and parity, J P , remain a mystery, with J P =0+ and 2+ being open possibilities. We use PYTHIA and Delphes to simulate an analysis of the angular distribution of gg→X→γγ decays in a full 2012 data set, including realistic background levels. We show that this angular distribution should provide strong discrimination between the possibilities of spin zero and spin two with graviton-like couplings: ∼3σ if a conservative symmetric interpretation of the log-likelihood ratio (LLR) test statistic is used, and ∼6σ if a less conservative asymmetric interpretation is used. The WW and ZZ couplings of the Standard Model Higgs boson and of a 2+ particle with graviton-like couplings are both expected to exhibit custodial symmetry. We simulate the present ATLAS and CMS search strategies for X→WW ∗ using PYTHIA and Delphes, and show that their efficiencies in the case of a spin-2 particle with graviton-like couplings are a factor ≃1.9 smaller than in the spin-0 case. On the other hand, the ratio of X2+→WW∗ and ZZ ∗ branching ratios is larger than that in the 0+ case by a factor ≃1.3. We find that the current ATLAS and CMS results for X→WW ∗ and X→ZZ ∗ decays are compatible with custodial symmetry under both the spin-0 and -2 hypotheses, and that the data expected to become available during 2012 are unlikely to discriminate significantly between these possibilities.