On the spin and parity of a single-produced resonance at the LHC

TL;DR

The paper addresses how to extract the spin, parity, and tensor structure of a 125 GeV LHC resonance by exploiting complete angular and mass distributions in X→ZZ, WW, and γγ decays, including off-shell vector bosons. It develops a comprehensive framework of helicity amplitudes for spin-0, -1, and -2, and validates this with analytic calculations and a full Monte Carlo generator that preserves spin correlations. By coupling these analyses with matrix-element techniques, it demonstrates potential 8 TeV LHC sensitivity to discriminate extreme spin-parity hypotheses and explores non-parity-eigenstate scenarios. The work provides a practical, generalizable toolset for robust property measurements of the Higgs-like boson and other resonances, with implications for future discoveries.

Abstract

The experimental determination of the properties of the newly discovered boson at the Large Hadron Collider is currently the most crucial task in high energy physics. We show how information about the spin, parity, and, more generally, the tensor structure of the boson couplings can be obtained by studying angular and mass distributions of events in which the resonance decays to pairs of gauge bosons, $ZZ, WW$, and $γγ$. A complete Monte Carlo simulation of the process $pp \to X \to VV \to 4f$ is performed and verified by comparing it to an analytic calculation of the decay amplitudes $X \to VV \to 4f$. Our studies account for all spin correlations and include general couplings of a spin $J=0,1,2$ resonance to Standard Model particles. We also discuss how to use angular and mass distributions of the resonance decay products for optimal background rejection. It is shown that by the end of the 8 TeV run of the LHC, it might be possible to separate extreme hypotheses of the spin and parity of the new boson with a confidence level of 99% or better for a wide range of models. We briefly discuss the feasibility of testing scenarios where the resonances is not a parity eigenstate.

Figures (2)

• Figure 1: Illustration of a $X$ particle production and decay in $pp$ collision $gg$ or $q\bar{q}\to X\to V_1(q_1) V_2(q_2)$, $V_1 \to f(q_{11}) \bar{f}(q_{12})$, $V_2 \to f(q_{21}) \bar{f}(q_{22})$. The three-momenta of the fermions ($f$) and antifermions ($\bar{f}$), ${\hbox{\boldmath$q$}}_{11}$, ${\hbox{\boldmath$q$}}_{12}$, ${\hbox{\boldmath$q$}}_{21}$, and ${\hbox{\boldmath$q$}}_{22}$, are shown in their parent $V_i$ rest-frames, and the three-momenta of the $V_i$ bosons, ${\hbox{\boldmath$q$}}_{i}$, are shown in the $X$ rest-frame. For sign convention of the angles between planes see text.
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