Abstract
Abstract The Higgs boson is predicted to have spin zero. The ATLAS and CMS experiments have recently reported of an excess of events with mass ~ 125 GeV that has some of the characteristics expected for a Higgs boson. We address the questions whether there is already any evidence that this excess has spin zero, and how this possibility could be confirmed in the near future. The excess observed in the γγ final state could not have spin one, leaving zero and two as open possibilities. We review the angular distribution of γγ pairs from the decays of a graviton-like spin-two boson produced in gluon-gluon collisions, which is well-defined and distinct from the spin-zero case. We also calculate the distributions for lepton pairs that would be produced in the W W * decays of a spin-two boson, which are very different from those in Higgs decays, and note that the kinematics of the event selection currently used in the analysis of the W W * final state have reduced efficiency for spin two.
Highlights
The ATLAS [15,16,17] and CMS [18,19,20,21,22,23,24,25] collaborations have recently reported evidence for excesses in γγ and ZZ∗ that are consistent with expectations for a Standard Model Higgs boson, an interpretation supported by broader enhancements of less significance in W W ∗, τ τ and bb final states
We have presented in this paper analyses of the angular distributions that could be expected in the γγ and W −W + decays of a hypothetical spin-two state X2 produced at the LHC via gluon-gluon collisions, assuming that its couplings coincide with those expected for a massive Kaluza-Klein graviton
Under this hypothesis, such a spin-two particle would be produced in a definite combination of polarization states, and the polar angle distribution of the γγ final state would be predictable and non-isotropic in the X2 rest frame, and distinguishable in principle from the isotropic γγ decays of a hypothetical spin-zero boson X0
Summary
One would prefer to perform such a ‘Higgs’ spin analysis in the most modelindependent way possible. Neglecting orbital angular momentum, qq collisions can produce only spin-zero or -one states, so we are left with gg and. Neglecting initial-state transverse momentum and radiation, we may regard the gluons as massless spin-one particles whose momenta are aligned with the collision axis. If one quantizes angular momentum along this axis, they are likely to be in the helicity states |1, ±1. The gg initial states are a combination of the |2, +2 , |2, −2 and |2, 0 polarization states, described by a spin-two density matrix ρ2 that has only diagonal entries with relative weights ρi. The |20 20| component in the density matrix does not contribute if graviton-like couplings are assumed [26], as done here
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