Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

TL;DR

This ATLAS study analyzes the full Run 1 proton-proton dataset to test the Higgs Yukawa coupling to tau leptons via H→ττ decays across ggF, VBF, and VH production. A two‑pronged approach—multivariate boosted decision trees (BDTs) and a cross-check cut-based analysis—employs category-specific strategies, Z→ττ embedding for the dominant background, and data-driven methods for misidentified leptons. The MMC-based mass reconstruction and a global likelihood fit across all categories extract the signal strength μ, yielding μ = 1.43^{+0.27}_{-0.26} (stat.)^{+0.32}_{-0.25} (syst.) ±0.09 (theory) with an observed significance of 4.5σ, consistent with the Standard Model expectation. The results provide evidence for Higgs decays to τ leptons and support the fermionic nature of the Higgs coupling, with cross-checks confirming robustness against analysis method.

Abstract

Results of a search for $H \to ττ$ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 $\rm{fb}^{-1}$ and 20.3 $\rm{fb}^{-1}$ at centre-of-mass energies of $\sqrt{s}$ = 7 TeV and $\sqrt{s}$ = 8 TeV respectively. All combinations of leptonic ($τ\to \ell ν\bar ν$ with $\ell = e, μ$) and hadronic ($τ\to \rm{hadrons} ν$) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of $μ= 1.43 ^{+0.43}_{-0.37}$ is consistent with the predicted Yukawa coupling strength in the Standard Model.

Figures (16)

• Figure 1: The reconstructed invariant $\tau\tau$ mass, $m_{\tau\tau}^{\mathrm{MMC}}$ for $H \to \tau \tau$ ($m_H$ = 125 Ge V) and $Z \to \tau \tau$ events in MC simulation and embedding respectively, for events passing (a) the VBF category selection and (b) the boosted category selection in the $\tau_{\mathrm{ lep}}\tau_{\mathrm{ had}}$ channel.
• Figure 2: Distributions of important BDT input variables for the three channels and the two categories (VBF, left) and (boosted, right) for data collected at $\sqrt{s}$ = 8 Te V. The distributions are shown for (a) the separation in pseudorapidity of the jets, $\Delta\eta(j_1,j_2)$, and (b) the transverse momentum of the leading jet $p_{\mathrm{T}}^{j_1}$ in the $\tau_{\mathrm{ lep}}\tau_{\mathrm{ lep}}$ channel, for (c) $\Delta\eta(j_1,j_2)$ and (d) $\Delta{}R(\tau_1,\tau_2)$, the distance $\Delta{}R$ between the lepton and $\tau_{\mathrm{ had}}$, in the $\tau_{\mathrm{ lep}}\tau_{\mathrm{ had}}$ channel and for (e) $\Delta\eta(j_1,j_2)$ and (f) $\Delta{}R(\tau_1,\tau_2)$, the distance $\Delta{}R$ between the two $\tau_{\mathrm{ had}}$ candidates, in the $\tau_{\mathrm{ had}}\tau_{\mathrm{ had}}$ channel. The contributions from a Standard Model Higgs boson with $m_H$ = 125 Ge V are superimposed, multiplied by a factor of 50. These figures use background predictions made without the global fit defined in section \ref{['sec:fit']}. The error band includes statistical and pre-fit systematic uncertainties.
• Figure 3: Distributions of important BDT input variables for the three channels and the two categories (VBF, left) and (boosted, right) for data collected at $\sqrt{s}$ = 8 Te V. The distributions are shown for (a) the product of the lepton centralities, $C_{\eta_1, \eta_2}(\eta_{\ell_1})\cdot C_{\eta_1, \eta_2}(\eta_{\ell_2})$, and (b) the sphericity in the $\tau_{\mathrm{ lep}}\tau_{\mathrm{ lep}}$ channel, for (c) the centrality of the lepton, $C_{\eta_1, \eta_2}(\eta_{\ell})$, and (d) the $E_\mathrm{T}^\mathrm{miss}\phi$ centrality in the $\tau_{\mathrm{ lep}}\tau_{\mathrm{ had}}$ channel, and for (e) the centrality of the subleading tau, $C_{\eta_1, \eta_2}(\eta_{\tau_2})$, and (f) the $E_\mathrm{T}^\mathrm{miss}\phi$ centrality in the $\tau_{\mathrm{ had}}\tau_{\mathrm{ had}}$ channel. The contributions from a Standard Model Higgs boson with $m_H$ = 125 Ge V are superimposed, multiplied by a factor of 50. These figures use background predictions made without the global fit defined in section \ref{['sec:fit']}. The error band includes statistical and pre-fit systematic uncertainties.
• Figure 4: (a) The distribution of the calorimeter isolation energy $I (E_{\mathrm{T}}, 0.3) \cdot p_{\mathrm{T}} ^{\mu}$ within a cone of radius $\Delta R$ = 0.3 around the muons in $Z \to \mu \mu$ events from data, before and after the embedding of simulated muons. (b) The distribution of the reconstructed invariant $\tau \tau$ mass, $m_{\tau\tau}^{\mathrm{MMC}}$, in the $\tau_{\mathrm{ lep}}\tau_{\mathrm{ had}}$ final state, for simulated $Z \to \tau \tau$ events, compared to the one obtained from simulated $Z \to \mu \mu$ events after tau embedding. The ratios of the values before and after the embedding and between the embedded $Z \to \mu \mu$ and $Z \to \tau \tau$ events are given in (a) and (b) respectively. The errors in (a) and (b) on the ratios (points) represent the statistical uncertainties, while the systematic uncertainties are indicated by the hatched bands in (b). The shaded bands represent the statistical uncertainties from the $Z \to \mu \mu$ data events in (a) and from the $Z \to \tau \tau$ simulation in (b).
• Figure 5: (a) The distribution of the reconstructed invariant $\tau \tau$ mass, $m_{\tau\tau}^{\mathrm{MMC}}$, for events in the $W{\rm+jets}$ control region, for the $\tau_{\mathrm{ lep}}\tau_{\mathrm{ had}}$ channel. (b) The separation in pseudorapidity of the $\tau_{\mathrm{ had}}$ candidates, $\Delta{}\eta(\tau_{\mathrm{ had}},\tau_{\mathrm{ had}})$, for the $\tau_{\mathrm{ had}}\tau_{\mathrm{ had}}$ channel in the rest control region. The expected SM Higgs boson signal contribution is superimposed, multiplied by a factor 50. These figures use background predictions made without the global fit defined in section \ref{['sec:fit']}. The error band includes statistical and pre-fit systematic uncertainties.