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CMS-HIG-17-013 ; CERN-EP-2018-207
Search for a standard model-like Higgs boson in the mass range between 70 and 110 GeV in the diphoton final state in proton-proton collisions at $\sqrt{s}=$ 8 and 13 TeV
Phys. Lett. B 793 (2019) 320
Abstract: The results of a search for a standard model-like Higgs boson in the mass range between 70 and 110 GeV decaying into two photons are presented. The analysis uses the data set collected with the CMS experiment in proton-proton collisions during the 2012 and 2016 LHC running periods. The data sample corresponds to an integrated luminosity of 19.7 (35.9) fb$^{-1}$ at $\sqrt{s}=$ 8 (13) TeV. The expected and observed 95% confidence level upper limits on the product of the cross section and branching fraction into two photons are presented. The observed upper limit for the 2012 (2016) data set ranges from 129 (161) fb to 31 (26) fb. The statistical combination of the results from the analyses of the two data sets in the common mass range between 80 and 110 GeV yields an upper limit on the product of the cross section and branching fraction, normalized to that for a standard model-like Higgs boson, ranging from 0.7 to 0.2, with two notable exceptions: one in the region around the Z boson peak, where the limit rises to 1.1, caused by the presence of Drell-Yan dielectron production where both electrons are misidentified as isolated photons, and a second due to an observed excess with respect to the standard model prediction, which is maximal for a mass hypothesis of 95.3 GeV with a local (global) significance of 2.8 (1.3) standard deviations.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Full parameterized signal shape, integrated over all event classes, in simulated signal events with $ {m_{\mathrm {H}}} = $ 90 GeV at $\sqrt {s} = $ 8 TeV (left) and 13 TeV (right). The open points are the weighted MC events and the blue lines the corresponding parametric models. Also shown are the $\sigma _{\text {eff}}$ values and the shaded region limited by $\pm \sigma _{\text {eff}}$, along with the FWHM values, indicated by the position of the arrows on each distribution.

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Figure 1-a:
Full parameterized signal shape, integrated over all event classes, in simulated signal events with $ {m_{\mathrm {H}}} = $ 90 GeV at $\sqrt {s} = $ 8 TeV. The open points are the weighted MC events and the blue lines the corresponding parametric models. Also shown are the $\sigma _{\text {eff}}$ values and the shaded region limited by $\pm \sigma _{\text {eff}}$, along with the FWHM values, indicated by the position of the arrows on each distribution.

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Figure 1-b:
Full parameterized signal shape, integrated over all event classes, in simulated signal events with $ {m_{\mathrm {H}}} = $ 90 GeV at $\sqrt {s} = $ 13 TeV. The open points are the weighted MC events and the blue lines the corresponding parametric models. Also shown are the $\sigma _{\text {eff}}$ values and the shaded region limited by $\pm \sigma _{\text {eff}}$, along with the FWHM values, indicated by the position of the arrows on each distribution.

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Figure 2:
Background model fits using the chosen "best-fit" parametrization to data in the four event classes at $\sqrt {s} = $ 8 TeV. The corresponding signal model for each class for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

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Figure 2-a:
Background model fits using the chosen "best-fit" parametrization to data in class 0 at $\sqrt {s} = $ 8 TeV. The corresponding signal model for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

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Figure 2-b:
Background model fits using the chosen "best-fit" parametrization to data in class 1 at $\sqrt {s} = $ 8 TeV. The corresponding signal model for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

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Figure 2-c:
Background model fits using the chosen "best-fit" parametrization to data in class 2 at $\sqrt {s} = $ 8 TeV. The corresponding signal model for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

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Figure 2-d:
Background model fits using the chosen "best-fit" parametrization to data in class 3 at $\sqrt {s} = $ 8 TeV. The corresponding signal model for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

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Figure 3:
Background model fits using the chosen "best-fit" parametrization to data in the three event classes at $\sqrt {s} = $ 13 TeV. The corresponding signal model for each class for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

png pdf
Figure 3-a:
Background model fits using the chosen "best-fit" parametrization to data in class 0 at $\sqrt {s} = $ 13 TeV. The corresponding signal model for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

png pdf
Figure 3-b:
Background model fits using the chosen "best-fit" parametrization to data in class 1 at $\sqrt {s} = $ 13 TeV. The corresponding signal model for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

png pdf
Figure 3-c:
Background model fits using the chosen "best-fit" parametrization to data in class 2 at $\sqrt {s} = $ 13 TeV. The corresponding signal model for $ {m_{\mathrm {H}}} = $ 90 GeV, multiplied by 10, is also shown. The one- and two-$\sigma $ bands reflect the uncertainty in the background model normalization associated with the statistical uncertainties of the fits, and are shown for illustration purposes only. The difference between the data and the best-fit model is shown in the lower panels.

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Figure 4:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, from the analysis of the 8 (left) and 13 (right) TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis. The corresponding theoretical prediction for the product of the cross section and branching fraction into two photons for an additional SM-like Higgs boson is shown as a solid line with a hatched band, indicating its uncertainty [60].

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Figure 4-a:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, from the analysis of the 8 TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis. The corresponding theoretical prediction for the product of the cross section and branching fraction into two photons for an additional SM-like Higgs boson is shown as a solid line with a hatched band, indicating its uncertainty [60].

png pdf
Figure 4-b:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, from the analysis of the 13 TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis. The corresponding theoretical prediction for the product of the cross section and branching fraction into two photons for an additional SM-like Higgs boson is shown as a solid line with a hatched band, indicating its uncertainty [60].

png pdf
Figure 5:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, for the $ {\mathrm {g}} {\mathrm {g}} {\mathrm {H}} $ plus $ {{\mathrm {t}\overline {\mathrm {t}}}} {\mathrm {H}} $ (left) and VBF plus $\mathrm {V} {\mathrm {H}} $ (right) processes, from the analysis of the 8 (top) and 13 (bottom) TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis.

png pdf
Figure 5-a:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, for the $ {\mathrm {g}} {\mathrm {g}} {\mathrm {H}} $ plus $ {{\mathrm {t}\overline {\mathrm {t}}}} {\mathrm {H}} $ processes, from the analysis of the 8 TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis.

png pdf
Figure 5-b:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, for the VBF plus $\mathrm {V} {\mathrm {H}} $ processes, from the analysis of the 13 TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis.

png pdf
Figure 5-c:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, for the $ {\mathrm {g}} {\mathrm {g}} {\mathrm {H}} $ plus $ {{\mathrm {t}\overline {\mathrm {t}}}} {\mathrm {H}} $ processes, from the analysis of the 8 TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis.

png pdf
Figure 5-d:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson, for the VBF plus $\mathrm {V} {\mathrm {H}} $ processes, from the analysis of the 13 TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis.

png pdf
Figure 6:
Expected and observed exclusion limits (95% CL, in the asymptotic approximation) on the product of the production cross section and branching fraction into two photons for an additional Higgs boson, relative to the expected SM-like value, from the analysis of the 8 and 13 TeV data. The inner and outer bands indicate the regions containing the distribution of limits located within $\pm $1 and 2$\sigma $, respectively, of the expectation under the background-only hypothesis.

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Figure 7:
Expected and observed local $p$-values as a function of $ {m_{\mathrm {H}}} $ for the 8 and 13 TeV data and their combination (solid curves) plotted together with the relevant expectations for an additional SM-like Higgs boson (dotted curves).
Tables

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Table 1:
The expected number of SM-like Higgs boson signal events ($ {m_{\mathrm {H}}} = $ 90 GeV) per event class and the corresponding percentage breakdown per production process, for the 8 and 13 TeV data. The values of $\sigma _{\text {eff}}$ and $\sigma _{\text {HM}}$ are also shown, along with the number of background events ("Bkg.'') per GeV estimated from the background-only fit to the data, including the number from the Drell-Yan process ("DY Bkg.''), in a $\sigma _{\text {eff}}$ window centered on $ {m_{\mathrm {H}}} = $ 90 GeV.
Summary
A search for an additional, SM-like, low-mass Higgs boson decaying into two photons has been presented. It is based upon data samples corresponding to integrated luminosities of 19.7 and 35.9 fb$^{-1}$ collected at center-of-mass energies of 8 TeV in 2012 and 13 TeV in 2016, respectively. The search is performed in a mass range between 70 and 110 GeV. The expected and observed 95% CL upper limits on the product of the production cross section and branching fraction into two photons for an additional SM-like Higgs boson as well as the expected and observed local $p$-values are presented. No significant (${>}$3$\sigma$) excess with respect to the expected number of background events is observed. The observed upper limit on the product of the production cross section and branching fraction for the 2012 (2016) data set ranges from 129 (161) fb to 31 (26) fb. The statistical combination of the results from the analyses of the two data sets in the common mass range between 80 and 110 GeV yields an upper limit on the product of the cross section and branching fraction, normalized to that for a standard model-like Higgs boson, ranging from 0.7 to 0.2, with two notable exceptions: one in the region around the Z boson peak, where the limit rises to 1.1, caused by the presence of Drell-Yan dielectron production where both electrons are misidentified as isolated photons, and a second due to an observed excess with respect to the standard model prediction, which is maximal for a mass hypothesis of 95.3 GeV with a local (global) significance of 2.8 (1.3) standard deviations. More data are required to ascertain the origin of this excess. This is the first search for new resonances in the diphoton final state in this mass range based on LHC data at a center-of-mass energy of 13 TeV.
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