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
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.
© Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica, 2013