Observation of the diphoton decay of the Higgs boson and measurement of its properties

TL;DR

This CMS study reports the observation of the Higgs boson in the diphoton decay channel using the full 2011–2012 data set, achieving a local significance of 5.7σ near mH ≈ 125 GeV. It employs a sophisticated event-classification framework with a multivariate diphoton classifier and exclusive production-tagging (VBF, VH, ttH) to optimize sensitivity across production modes, while relying on data-driven background modeling. The measured mass is 124.70 GeV with uncertainties, and the signal strength is consistent with the SM prediction (μ ≈ 1.14), with detailed fits to production-mode modifiers and coupling benchmarks. A direct width limit (Γ < 2.4 GeV at 95% CL) and searches for additional Higgs-like states are reported, and spin tests strongly favor a spin-0 SM-like Higgs over a spin-2 alternative, confirming the SM Higgs interpretation within experimental precision.

Abstract

Observation of the diphoton decay mode of the recently discovered Higgs boson and measurement of some of its properties are reported. The analysis uses the entire dataset collected by the CMS experiment in proton-proton collisions during the 2011 and 2012 LHC running periods. The data samples correspond to integrated luminosities of 5.1 inverse femtobarns at sqrt(s) = 7 TeV and 19.7 inverse femtobarns at 8 TeV. A clear signal is observed in the diphoton channel at a mass close to 125 GeV with a local significance of 5.7 sigma, where a significance of 5.2 sigma is expected for the standard model Higgs boson. The mass is measured to be 124.70 +/- 0.34 GeV = 124.70 +/- 0.31 (stat) +/- 0.15 (syst) GeV, and the best-fit signal strength relative to the standard model prediction is 1.14 +0.26/-0.23 = 1.14 +/- 0.21 (stat) +0.09/-0.05 (syst) +0.13/-0.09 (theo). Additional measurements include the signal strength modifiers associated with different production mechanisms, and hypothesis tests between spin-0 and spin-2 models.

Figures (32)

• Figure 1: Invariant mass of $\mathrm{e}^+\mathrm{e}^-$ pairs in ${Z}\to\mathrm{e}^+\mathrm{e}^-\xspace$ events in the 8$\,\text{Te\spaceV}$ data (points), and in simulated events (histogram), in which the electron showers are reconstructed as photons, and the full set of photon corrections and smearings are applied. The comparison is shown for (left) events with both showers in the barrel, and (right) the remaining events. For each bin, the ratio of the number of events in data to the number of simulated events is shown in the lower main plot.
• Figure 2: Photon identification BDT score of the lower-scoring photon of diphoton pairs with an invariant mass in the range $100<m_{\gamma\gamma}\xspace<180\,\text{Ge\spaceV}\xspace$, for events passing the preselection in the 8$\,\text{Te\spaceV}$ dataset (points), and for simulated background events (histogram with shaded error bands showing the statistical uncertainty). Histograms are also shown for different components of the simulated background, in which there are either two, one, or zero prompt signal-like photons. The tall histogram on the right (righthand vertical axis) corresponds to simulated Higgs boson signal events.
• Figure 3: Comparison of the photon identification BDT score for electron showers in the barrel in ${Z}\to\mathrm{e}^+\mathrm{e}^-\xspace$ events in the 8$\,\text{Te\spaceV}$ dataset and MC simulated events, for events passing the preselection, but with the electron veto condition inverted. The systematic uncertainty assigned to the photon identification BDT score is shown as a band. The comparison is shown for two sets of events with different numbers of primary vertices, $N_\mathrm{vtx}\xspace$. For each bin, the ratio of the number of events in data to the number of simulated events is shown in the lower plot.
• Figure 4: Fraction of diphoton vertices (solid points) assigned, by the vertex assignment BDT, to a reconstructed vertex within 10$\,\text{mm}$ of their true location in simulated Higgs boson events, $m_{H}\xspace\xspace = 125\,\text{Ge\spaceV}\xspace$, $\sqrt{s}=8\,\text{Te\spaceV}\xspace$, as a function of $p_{\mathrm{T}}^{\gamma\gamma}\xspace$. Also shown is a band, the centre of which is the mean prediction, from the vertex probability BDT (described in Section \ref{['sec:vtx-prob']}), of the probability of correctly locating the vertex. The mean is calculated in $p_{\mathrm{T}}^{\gamma\gamma}$ bins, and the width of the band represents the event-to-event uncertainty in the estimates.
• Figure 5: Distribution of the vertex probability estimate in ${Z}\to{\mu^+}{\mu^-}\xspace$ events. The vertex probability estimates in 8$\,\text{Te\spaceV}$ data (points), are compared to the estimates in MC simulation (histograms). The comparison is made separately for events in which the vertex is assigned to the same (open circles and filled histogram), or to a different vertex (filled circles and outlined histogram), as that identified by the muons.