A framework for Higgs characterisation

TL;DR

The paper introduces an effective-field-theory framework to characterise the LHC boson across all relevant channels, using a minimal, systematically improvable Lagrangian for spins 0, 1, and 2 and embedding it into FeynRules/MadGraph for LO and NLO QCD simulations. It demonstrates the framework's capabilities via ME+PS merging and aMC@NLO, validates against benchmark scenarios, and explores three applications: unitarity concerns for spin-2 with non-universal couplings, higher-order QCD effects on spin observables, and a matrix-element-based method to determine CP mixing in a spin-0 state. The results highlight the importance of higher-order corrections for reliable phenomenology and the utility of automated tools for comprehensive resonance characterisation. The work sets the stage for extended EFT operators, improved loop automation, and advanced merging strategies to enhance Higgs analyses at the LHC and future colliders.

Abstract

We introduce a framework, based on an effective field theory approach, that allows one to perform characterisation studies of the boson recently discovered at the LHC, for all the relevant channels and in a consistent, systematic and accurate way. The production and decay of such a boson with various spin and parity assignments can be simulated by means of multi-parton, tree-level matrix elements and of next-to-leading order QCD calculations, both matched with parton showers. Several sample applications are presented which show, in particular, that beyond-leading-order effects in QCD have non-trivial phenomenological implications.

Figures (13)

• Figure 1: Normalised distributions in $pp\to X_0\to\mu^+\mu^-e^+e^-$ for different choices of $X_0ZZ$ couplings: the invariant masses of the two lepton pairs $m_1$, $m_2$ (with $m_1>m_2$), $\cos\theta^*$, $\cos\theta_1$, and $\Delta\phi$, as defined in ref. Bolognesi:2012mm. Event simulation performed at the leading order, parton level only (no shower/hadronisation).
• Figure 2: Normalised distributions in $pp\to X_1\to\mu^+\mu^-e^+e^-$ for different choices of $X_1ZZ$ couplings: the invariant masses of the two lepton pairs $m_1$, $m_2$ (with $m_1>m_2$), $\cos\theta_1$, $\cos\theta_2$, and $\phi_1$, as defined in ref. Bolognesi:2012mm. Event simulation performed at the leading order, parton level only (no shower/hadronisation).
• Figure 3: Representative diagrams for the decay of $X_2\to 4\ell$.
• Figure 4: Normalised distributions in $pp\to X_2\to\mu^+\mu^-e^+e^-$ for different $\kappa_{\ell}$ values: the invariant masses of the two lepton pairs $m_1$, $m_2$ (with $m_1>m_2$), $\cos \theta^*$, $\cos\theta_2$, and $\phi_1$, as defined in ref. Bolognesi:2012mm. Event simulation performed at the leading order, parton level only (no shower/hadronisation).
• Figure 5: The transverse momentum $p_{T}^X$, pseudorapidity $\eta^X$, and jet rates of the new boson $X(J^P)=0^+,0^-,1^+,1^-,2^+$ as obtained from aMC@NLO. The lower inset shows the bin-by-bin ratio of the same distribution obtained via ME+PS merging and that of aMC@NLO.