Scrutinizing the Higgs Signal and Background in the $2e2μ$ Golden Channel

TL;DR

The paper delivers analytic, fully differential LO predictions for the golden 2e2μ channel, incorporating all interference among ZZ, Zγ, and γγ with on- and off-shell gauge bosons for both signal (general spin-0 CP mixtures) and background (q q̄ → 4ℓ). It derives a compact, Lorentz-structure–based formalism that separates gauge, propagator, and tensor-coupling information, enabling precise studies of invariant-mass and angular distributions. By examining singly and doubly differential spectra and highlighting interference effects, the work provides a robust framework for discriminating between SM-like and exotic scalar couplings, potentially via matrix element methods. The results underscore the importance of exploring off-shell regions (low M2) and relaxing Z-window constraints to maximize sensitivity, and they offer analytic tools that can be integrated with NLO/PDF inputs for LHC analyses.

Abstract

Kinematic distributions in the decays of the newly discovered resonance to four leptons are a powerful test of the tensor structure of its couplings to electroweak gauge bosons. We present an analytic calculation for both signal and background of the fully differential cross section for the `Golden Channel' $e^+e^-μ^+μ^-$ final state. We include all interference effects between intermediate gauge bosons and allow them to be on- or off-shell. For the signal we compute the fully differential cross section for general scalar couplings to $ZZ$, $γγ$, and $Zγ$. For the background we compute the leading order fully differential cross section for the dominant contribution coming from $q\bar{q}$ annihilation into $Z$ and $γ$ gauge bosons, including the contribution from the resonant $Z\rightarrow 2e2μ$ process. We also present singly and doubly differential projections and study the interference effects on the differential spectra. These expressions can be used in a variety of ways to uncover the nature of the newly discovered resonance or any new scalars decaying to neutral gauge bosons which might be discovered in the future.

Figures (21)

• Figure 1: Definition of angles in the four lepton CM frame $X$.
• Figure 2: Feynman diagrams contributing to $\varphi \rightarrow 2\ell_12\ell_2$. The arrows are to indicate the direction of momentum flow.
• Figure 3: In the top plot we take 4 GeV $< M_{1,2} < 120$ GeV while in the bottom plot we take the range 40 GeV $< M_1 < 120$ GeV, 4 GeV $< M_2 < 120$ GeV at $\sqrt{s}=m_h=125$ GeV when integrating over phase space. The SM is shown in blue, but is essentially indistinguishable from hypothesis 3 (see text).
• Figure 4: Feynman diagrams contributing to $q\bar{q} \rightarrow ZZ \rightarrow 2e2\mu$ and $q\bar{q} \rightarrow Z \rightarrow 2e2\mu$. The arrows are to indicate the direction of momentum flow.
• Figure 5: Feynman diagrams contributing to $q\bar{q} \rightarrow V_i V_j\rightarrow 2\ell_1 2\ell_2$ and $q\bar{q} \rightarrow V_i\rightarrow 2\ell_1 2\ell_2$. Note that diagrams $( c)-(f)$ are in fact s-channel diagrams so the fermions labeled by $1$ and $2$ are not to be confused as being in the initial state. This is taken into account in how the various momenta are assigned as indicated by the arrows.