Search for Higgs boson pair production in the γ γ W W ∗ channel using pp collision data recorded at s = 13 TeV with the ATLAS detector

Abstract

Searches for non-resonant and resonant Higgs boson pair production are performed in the $\gamma \gamma W{W}^{\ast }$ channel with the final state of $\gamma \gamma \ell \nu jj$ using 36.1 ${\text{fb}}^{-1}$ of proton-proton collision data recorded at a centre-of-mass energy of $\sqrt{s}=$ 13 TeV by the ATLAS detector at the Large Hadron Collider. No significant deviation from the Standard Model prediction is observed. A 95% confidence-level observed upper limit of 7.7 pb is set on the cross section for non-resonant production, while the expected limit is 5.4 pb. A search for a narrow-width resonance X decaying to a pair of Standard Model Higgs bosons HH is performed with the same set of data, and the observed upper limits on $\sigma (pp\to X)\times B(X\to HH)$ range between 40.0 and 6.1 pb for masses of the resonance between 260 and 500 GeV, while the expected limits range between 17.6 and 4.4 pb. When deriving the limits above, the Standard Model branching ratios of the $H\to \gamma \gamma$ and $H\to W{W}^{\ast }$ are assumed.

Fig. 1

Feynman diagrams for leading-order Higgs boson pair production in the SM through a a heavy-quark loop, b the Higgs self-coupling, and c an intermediate heavy resonance in a BSM scenario. The total SM contribution is the sum of the two modes depicted in a and b, which have significant destructive interference. Physics beyond the SM can enhance Higgs boson pair production either by modifying the Higgs boson coupling constants from their SM values in a and/or b, or by an additional s-channel exchange of an intermediate scalar resonance in c

Fig. 2

Distributions of the reconstructed transverse momenta of the diphoton system with all event selections, except the $p_{\text{T}}^{\gamma \gamma }$selection, applied for various signal models, as well as sideband data, normalised to unit area

Fig. 3

Invariant mass spectrum of the diphoton system in the searches for both resonant and non-resonant Higgs boson pair production, with the corresponding backgrounds for a $m_X=260$ GeV without any $p_{\text{T}}^{\gamma \gamma }$selection and b the non-resonant case with a $p_{\text{T}}^{\gamma \gamma }$> 100 GeV selection. Fits to $m_{\gamma \gamma }$ are performed using the full signal-plus-background model. In each plot, only the background component is present. The shape parameters and normalisation of the continuum background model are determined in the fits. The “SM Higgs boson” in a contains the single-Higgs-boson background and SM di-Higgs-boson background. The band shows the uncertainty of the “Total background” in the upper panel and is calculated by a sampling method. The bottom panel shows the difference between the number of events in data and the estimated number of background events, as determined by the fits

Fig. 4

$95\%$ CL expected (dashed line) and observed (solid line) limits on the resonant Higgs boson pair production cross section times the branching fraction of $X\to HH$ as a function of $m_X$ (a) with and b without assuming the SM branching fractions of $H\to W{W}^{\ast }$ and $H\to \gamma \gamma$. The ${\text{CL}}_{\text{s}}$ method and the asymptotic approximation are used. The $\pm 1\sigma$ and $\pm 2\sigma$ bands on the expected limit are also presented. To the right of the vertical dashed line at $m_X=400$ GeV, the $p_{\text{T}}^{\gamma \gamma }>100$ GeV selection is applied in both plots, but not to the left