Precision studies of the Higgs boson decay channel H -> ZZ -> 4l with MEKD

TL;DR

This work evaluates the precision of Higgs property measurements in the H → ZZ → 4l channel using the matrix element method (MEM) and introduces MEKD, a public tool for LO matrix-element-based discriminants. By incorporating all relevant permutations and interference in SF final states, the study demonstrates enhanced separation between signal and ZZ background and among spin-parity hypotheses, with notable gains for non-SM spin-2 scenarios. The analysis shows that initial-state information provides limited extra discrimination after realistic lepton cuts, while full matrix-element-based discriminants outperform single-variable approaches. Overall, MEMKD enables more accurate mass, spin, and CP determinations in the golden channel and serves as a practical resource for experimental and phenomenological Higgs studies.

Abstract

The importance of the H -> ZZ -> 4l "golden" channel was shown by its major role in the discovery, by the ATLAS and CMS collaborations, of a Higgs-like boson with mass near 125 GeV. We analyze the discrimination power of the matrix element method both for separating the signal from the irreducible ZZ background and for distinguishing various spin and parity hypotheses describing a signal in this channel. We show that the proper treatment of interference effects associated with permutations of identical leptons in the four electron and four muon final states plays an important role in achieving the best sensitivity in measuring the properties of the newly discovered boson. We provide a code, MEKD, that calculates kinematic discriminants based on the full leading order matrix elements and which will aid experimentalists and phenomenologists in their continuing studies of the H -> ZZ -> 4l channel.

Figures (15)

• Figure 1: Comparison of different ROC curves based on: the value of $KD$ obtained from matrix elements (red curve), $m_{Z2}$ (blue curve), $\Phi$ (magenta curve), and $\theta^\ast$ (black curve). The black dashed diagonal line is the ROC curve obtained from cutting events indiscriminately (e.g., by flipping a fair coin or by only considering some fraction of the data set).
• Figure 2: Comparison of ROC curves obtained from the different implementations of the kinematic discriminant: from MadGraph/CalcHEP (red solid lines) or from NLOME (blue dotted lines), for DF (left) and SF (right) $4\ell$ events.
• Figure 3: Comparison of the signal (left column) and background (middle column) matrix elements, as well as the kinematic discriminant $KD_{\textsc{MAD}}$ (right column), for signal events (top row) and background events (bottom row) as calculated with the full matrix element for SF events $|{\cal M}|^2_{SF}$ (including the interference) versus the approximation $|{\cal M}|^2_{DF(sym)}$ obtained by the patch given in Eq. (\ref{['patch1']}). All quantities are calculated with MadGraph.
• Figure 4: Unit-normalized rapidity distributions of $ZZ^\ast$ events for signal (bluish colors) and background (reddish colors). Results are shown for two different LHC energies (7 and 8 TeV), before cuts (dashed lines) and after cuts (solid lines).
• Figure 5: Impact of parton distribution functions on ROC curves. The red curves are based on $KD(H;ZZ)$ from Eq. (\ref{['D simple']}) and do not account for the longitudinal boost of the event, while the blue curves are based on $KD(H;ZZ)$ from Eq. (\ref{['D final']}) and include the effect from the parton distribution functions. Solid (dashed) lines are obtained from event samples with (without) the lepton acceptance cuts.