Eur. Phys. J. C (2013) 73: 2488
https://doi.org/10.1140/epjc/s10052-013-2488-5

Regular Article - Theoretical Physics

Distinguishing ‘Higgs’ spin hypotheses using γγ and WW ^∗ decays

¹ Theoretical Particle Physics and Cosmology Group, Physics Department, King’s College London, London, WC2R 2LS, UK
² TH Division, Physics Department, CERN, 1211, Geneva 23, Switzerland
³ Department of Physics and Astronomy, York University, Toronto, ON, Canada M3J 1P3,
⁴ Department of Physics, Sejong University, Seoul, 143–747, South Korea

^* e-mail: veronica.hirn@gmail.com

Received: 13 March 2013
Revised: 29 May 2013
Published online: 16 July 2013

Abstract

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 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.