Evidence for the 125 GeV Higgs boson decaying to a pair of tau leptons

TL;DR

This CMS study analyzes proton-proton collisions at 7 and 8 TeV to search for the SM Higgs boson decaying to a τ-lepton pair across six final states. Using a comprehensive categorization by jet activity and a likelihood-based SVFit mass reconstruction, the analysis observes an excess over background with local significance >3σ in the 115–130 GeV mass range and finds a best-fit signal strength of $\hat{\mu}=0.78\pm0.27$ at $m_H=125$ GeV, with a mass estimate of $m_H=122\pm7$ GeV. The results provide evidence for $H\to\tau\tau$ and corroborate the fermionic coupling of the 125 GeV Higgs boson observed in other decay channels. Data-driven background estimation and a detailed treatment of systematic uncertainties underpin the robustness of the result.

Abstract

A search for a standard model Higgs boson decaying into a pair of tau leptons is performed using events recorded by the CMS experiment at the LHC in 2011 and 2012. The dataset corresponds to an integrated luminosity of 4.9 inverse femtobarns at a centre-of-mass energy of 7 TeV and 19.7 inverse femtobarns at 8 TeV. Each tau lepton decays hadronically or leptonically to an electron or a muon, leading to six different final states for the tau-lepton pair, all considered in this analysis. An excess of events is observed over the expected background contributions, with a local significance larger than 3 standard deviations for m[H] values between 115 and 130 GeV. The best fit of the observed H to tau tau signal cross section for m[H] = 125 GeV is 0.78 +- 0.27 times the standard model expectation. These observations constitute evidence for the 125 GeV Higgs boson decaying to a pair of tau leptons.

Figures (32)

• Figure 1: Leading-order Feynman diagrams for Higgs boson production through gluon-gluon fusion (left), vector boson fusion (middle), and the associated production with a $\mathrm{W}$ or a ${Z}$ boson (right).
• Figure 2: Observed and predicted distributions for the visible $\tau_{\rm h}\xspace$ mass, $m_\text{vis}\xspace^{\tau_{\rm h}\xspace}$, in the ${\mu}\tau_{\rm h}\xspace$ channel after the baseline selection described in section \ref{['sec:event_selection']}. The yields predicted for the ${Z} \to \tau \tau$, ${Z} \to \mu \mu$, electroweak, ${t}\overline{{t}}\xspace$, and QCD multijet background contributions correspond to the result of the final fit presented in Section \ref{['sec:results']}. The ${Z} \to \tau \tau$ contribution is then split according to the decay mode reconstructed by the hadron-plus-strips algorithm as shown in the legend. The mass distribution of the $\tau_{\rm h}$ built from one charged hadron and photons peaks near the mass of the intermediate $\rho\mathrm{(770)}$ resonance; the mass distribution of the $\tau_{\rm h}$ built from three charged hadrons peaks around the mass of the intermediate $\mathrm{a_1(1260)}$ resonance. The $\tau_{\rm h}$ built from one charged hadron and no photons are reconstructed with the $\pi^\pm$ mass, assigned to all charged hadrons by the PF algorithm, and constitute the main contribution to the third bin of this histogram. The first two bins correspond to $\tau^\pm$ leptons decaying into $\mathrm{e}^\pm\nu\nu$ and ${\mu}^\pm\nu\nu$, respectively, and for which the electron or muon is misidentified as a $\tau_{\rm h}\xspace$. The electroweak background contribution is dominated by $\mathrm{W}+\text{jets}$ production. In most selected $\mathrm{W}+ \text{jets}$, ${t}\overline{{t}}\xspace$, and QCD multijet events, a jet is misidentified as a $\tau_{\rm h}\xspace$. The "bkg. uncertainty" band represents the combined statistical and systematic uncertainty in the background yield in each bin. The expected contribution from the SM Higgs signal is negligible.
• Figure 3: Normalized distributions obtained in the ${\mu}\tau_{\rm h}\xspace$ channel after the baseline selection for (left) the invariant mass, $m_\text{vis}\xspace$, of the visible decay products of the two $\tau$ leptons, and (right) the svfit mass, $m_{\tau\tau}\xspace$. The distribution obtained for a simulated sample of ${Z}\to \tau \tau$ events (shaded histogram) is compared to the one obtained for a signal sample with a SM Higgs boson of mass $m_{{H}\xspace}\xspace=125\,\text{Ge\spaceV}\xspace$ (open histogram).
• Figure 4: Event categories for the $LL'$ channels. The $p_{\mathrm{T}}\xspace^{\tau\tau}\xspace$ variable is the transverse momentum of the Higgs boson candidate. In the definition of the VBF-tagged categories, $\lvert \Delta \eta_\mathrm{jj}\xspace \rvert$ is the difference in pseudorapidity between the two highest-$p_{\mathrm{T}}$ jets, and $m_\mathrm{jj}\xspace$ their invariant mass. In the ${\mu}{\mu}$ and $\mathrm{e}\mathrm{e}$ channels, events with two or more jets are not required to fulfil any additional VBF tagging criteria. For the analysis of the 7$\,\text{Te\spaceV}$$\mathrm{e}\tau_{\rm h}\xspace$ and ${\mu}\tau_{\rm h}\xspace$ data, the loose and tight VBF-tagged categories are merged into a single VBF-tagged category. In the $\mathrm{e}\tau_{\rm h}\xspace$ channel, the $E_{\mathrm{T}}^{\text{miss}}$ is required to be larger than $30\,\text{Ge\spaceV}\xspace$ in the 1-jet category. Therefore, the high-$p_{\mathrm{T}}\xspace^{\tau_{\rm h}\xspace}$ category is not used and is accordingly crossed out. The term "baseline" refers to the baseline selection described in section \ref{['sec:event_selection']}.
• Figure 5: Observed and predicted distributions in the ${\mu}\tau_{\rm h}\xspace$ channel after the baseline selection, for (left) the transverse momentum of the Higgs boson candidates and (right) the transverse momentum of the $\tau_{\rm h}\xspace$. The yields predicted for the various background contributions correspond to the result of the final fit presented in Section \ref{['sec:results']}. The electroweak background contribution includes events from $\mathrm{W}+ \text{jets}$, diboson, and single-top-quark production. The "bkg. uncertainty" band represents the combined statistical and systematic uncertainty in the background yield in each bin. In each plot, the bottom inset shows the ratio of the observed and predicted numbers of events. The expected contribution from the SM Higgs signal is negligible.