Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV

TL;DR

This CMS analysis searches for the Standard Model Higgs boson decaying to two photons in proton–proton collisions at $\sqrt{s}=7$ TeV using 4.8 fb$^{-1}$ of data. By classifying events into five categories to optimize mass resolution and signal-to-background, the study constructs a data-driven diphoton mass spectrum with simulated $H\to\gamma\gamma$ shapes corrected to data. The results exclude the SM Higgs decay to two photons in the mass window 128–132 GeV at 95% CL and reveal a local excess at 124 GeV with a significance of 3.1$\sigma$ (global 1.8$\sigma$ after look-elsewhere). The combined best-fit signal strength near 124 GeV is $\mu=2.1\pm0.6$, indicating a fluctuation that requires more data for clarification, while showcasing the analysis approach's sensitivity to diphoton final states.

Abstract

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded by the CMS experiment at the LHC from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross section of the standard model Higgs boson decaying to two photons. The expected exclusion limit at 95% confidence level is between 1.4 and 2.4 times the standard model cross section in the mass range between 110 and 150 GeV. The analysis of the data excludes, at 95% confidence level, the standard model Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The largest excess of events above the expected standard model background is observed for a Higgs boson mass hypothesis of 124 GeV with a local significance of 3.1 sigma. The global significance of observing an excess with a local significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is estimated to be 1.8 sigma. More data are required to ascertain the origin of this excess.

Figures (4)

• Figure 1: Background model fit to the $m_{\gamma\gamma}$ distribution for the five event classes, together with a simulated signal ($m_{{H}\xspace}\xspace$=120$\,\text{Ge\spaceV}$). The magnitude of the simulated signal is what would be expected if its cross section were twice the SM expectation. The sum of the event classes together with the sum of the five fits is also shown. a) The sum of the five event classes. b) the dijet-tagged class, c) both photons in the barrel, $R_\mathrm{9}^\text{min} >$ 0.94, d) both photons in the barrel, $R_\mathrm{9}^\text{min} <$ 0.94, e) at least one photon in the endcaps, $R_\mathrm{9}^\text{min} >$ 0.94, f) at least one photon in the endcaps, $R_\mathrm{9}^\text{min} <$ 0.94.
• Figure 2: Exclusion limit on the cross section of a SM Higgs boson decaying into two photons as a function of the boson mass (upper plot). Below is the same exclusion limit relative to the SM Higgs boson cross section, where the theoretical uncertainties on the cross section have been included in the limit setting.
• Figure 3: The local $p$-value as a function of Higgs boson mass, calculated in the asymptotic approximation. The point at 124$\,\text{Ge\spaceV}$ shows the value obtained with a pseudo-data ensemble.
• Figure 4: The best fit signal strength, in terms of the standard model Higgs boson cross section, for the combined fit to the five classes (vertical line) and for the individual contributing classes (points) for the hypothesis of a SM Higgs boson mass of 124$\,\text{Ge\spaceV}$. The band corresponds to $\pm 1\,\sigma$ uncertainties on the overall value. The horizontal bars indicate $\pm 1\,\sigma$ uncertainties on the values for individual classes.