Measurements of Higgs boson production and couplings in the four-lepton channel in $pp$ collisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector

TL;DR

This paper reports the final ATLAS Run 1 measurements of Higgs production and couplings in the H→ZZ^{*}→4ℓ channel, using 7 and 8 TeV data to achieve an 8.1σ observation and a precise mass determination near 125.36 GeV. The analysis employs inclusive and category-based strategies to disentangle ggF, VBF, and VH production, aided by three boosted decision trees and advanced signal/background modeling. Key results include a combined signal strength μ ≈1.44 relative to the SM, and coupling fits within a κ-framework showing consistency with the Standard Model, with a detailed breakdown across production modes. The work demonstrates robust data-driven background estimations, sophisticated object-reconstruction infrastructure, and multivariate techniques that enhance Higgs-property measurements in Run 1 data.

Abstract

The final ATLAS Run 1 measurements of Higgs boson production and couplings in the decay channel $H\to ZZ^{*}\to\ell^+\ell^-\ell^{'+}\ell^{'-}$, where $\ell,\ell^{'}=e$ or $μ$, are presented. These measurements were performed using $pp$ collision data corresponding to integrated luminosities of 4.5 fb$^{-1}$ and 20.3 fb$^{-1}$ at center-of-mass energies of 7 TeV and 8 TeV, respectively, recorded with the ATLAS detector at the LHC. The $H\to ZZ^{*}\to 4\ell$ signal is observed with a significance of 8.1 standard deviations, with an expectation of 6.2 standard deviations, at $m_{H}$ = 125.36 GeV, the combined ATLAS measurement of the Higgs boson mass from the $H\to γγ$ and $H\to ZZ^{*}\to 4\ell$ channels. The production rate relative to the Standard Model expectation, the signal strength, is measured in four different production categories in the $H\to ZZ^{*}\to 4\ell$ channel. The measured signal strength, at this mass, and with all categories combined, is 1.44 $^{+0.40}_{-0.33}$. The signal strength for Higgs boson production in gluon fusion or in association with $t\bar{t}$ or $b\bar{b}$ pairs is found to be 1.7 $^{+0.5}_{-0.4}$, while the signal strength for vector-boson fusion combined with $WH/ZH$ associated production is found to be 0.3 $^{+1.6}_{-0.9}$.

Figures (21)

• Figure 1: \ref{['fig:CollinearFSR']} The invariant mass distributions of $Z\rightarrow \mu^+\mu^-(\gamma)$ events in data before collinear FSR correction (filled triangles) and after collinear FSR correction (filled circles), for events with a collinear FSR photon satisfying the selection criteria as described in Sec. \ref{['sec:fsr']}. The prediction of the simulation is shown before correction (red histogram) and after correction (blue histogram). \ref{['fig:NonCollinearFSR']} The invariant mass distributions of $Z\rightarrow \mu^+\mu^-(\gamma)$ events with a noncollinear FSR photon satisfying the selection criteria as described in Sec. \ref{['sec:fsr']}. The prediction of the simulation is shown before correction (red histogram) and after correction (blue histogram).
• Figure 2: Schematic view of the event categorization. Events are required to pass the four-lepton selection, and then they are assigned to one of four categories which are tested sequentially: VBF enriched, VH-hadronic enriched, VH-leptonic enriched, or ggF enriched.
• Figure 3: Distributions of the dijet invariant mass for the events with at least two jets for the data (filled circles), the expected signal (solid and dot-dot-dashed histograms) and the backgrounds (filled histograms). The $WH$ and $ZH$ hadronic signals are scaled by a factor 50 and the $ZH$ distribution is added on top of the $WH$ distribution.
• Figure 4: The observed $m_{12}$ distributions (filled circles) and the results of the maximum likelihood fit are presented for the four control regions: \ref{['fig:OSad0smuSimFit']} inverted requirement on impact parameter significance, \ref{['fig:OSd0aisoSimFit']} inverted requirement on isolation, \ref{['fig:emuSimFit']}$e\mu$ leading dilepton, where the backgrounds besides $t\bar{t}$ are small and not visible, and \ref{['fig:SSSimFit']} same-sign subleading dilepton. The fit results are shown for the total background (black line) as well as the individual components: $Z+{\rm jets}$ decomposed into $Z+b\bar{b}$ (blue line) and $Z+$light-flavor jets (green line), $t\bar{t}$ (dashed red line), and the combined $WZ$ and $ZZ$ (dashed gray line), where the $WZ$ and $ZZ$ contributions are estimated from simulation.
• Figure 5: The distribution of the difference between the transverse momentum measured in the ID and in the MS normalized to the ID measurement, $(p_{\rm T_{ID}}-p_{\rm T_{MS}})/p_{\rm T_{ID}}$, for combined muons accompanying a $Z\to\ell\ell$ candidate. The data (filled circles) are compared to the background simulation (filled histograms) which has the $Z+$ light-flavor background shown separately to distinguish the contribution from $\pi/K$ in-flight decays. The additional muon is selected to be a combined muon with $p_{\mathrm{T}}$$> 6$Ge V, which fulfills the $\Delta R$ requirement for the lepton separation of the analysis and in the case of $Z (\to \mu^+ \mu^-) + \mu$ final state, the opposite sign pairs are required to have $m_{\mu^+\mu^-} > 5$Ge V to remove $J/\psi$ decays.