Combination of searches for invisible Higgs boson decays with the ATLAS experiment (1904.05105v2)

Abstract: Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for $H\to\textrm{invisible}$ decays where $H$ is produced according to the Standard Model via vector boson fusion, $Z(\ell\ell)H$, and $W!/!Z(\textrm{had})H$, all performed with the ATLAS detector using 36.1 fb$^{-1}$ of $pp$ collisions at a center-of-mass energy of $\sqrt s = 13$ TeV at the LHC. In combination with the results at $\sqrt s = 7$ and 8 TeV, an exclusion limit on the $H\to\textrm{invisible}$ branching ratio of $0.26 (0.17^{+0.07}_{-0.05})$ at 95% confidence level is observed (expected).

• The paper combines multiple ATLAS search analyses to set upper limits on the invisible decay rate of the Higgs boson.
• It integrates data from 13 TeV collisions (36.1 fb⁻¹) with earlier runs using robust statistical combination methods.
• The results constrain Higgs portal dark matter models by lowering the invisible branching ratio limit to 0.26 at 95% CL.

Overview of the Statistical Combination of Searches for Invisible Higgs Boson Decays

The paper "Combination of searches for invisible Higgs boson decays with the ATLAS experiment" addresses the potential decay of the Higgs boson into dark matter (DM) particles, which is a significant topic in understanding the nature of dark matter in the universe. The ATLAS collaboration has conducted a combination of various search analyses to investigate the invisible decays of the Higgs boson. This paper considers multiple production modes including vector boson fusion (VBF), associated production with a Z boson leptonically decaying ($Z(\ell\ell)H$), and associated production with hadronically decaying vector bosons ($W/Z(\text{had})H$).

Data and Methodology

The analysis employs data accumulated from proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV, recorded by the ATLAS detector at the Large Hadron Collider (LHC), amounting to an integrated luminosity of 36.1 fb$^{-1}$. The combined results also incorporate previous measurements from $\sqrt{s} = 7$ and 8 TeV. Searches from each topology are statistically combined, assuming the production rates as predicted by the Standard Model (SM).

The analysis identifies signal regions (SR) in events characterized by missing transverse energy ($\met$), indicative of undetectable particles consistent with DM candidates. Three event topologies are explored:

1. Vector Boson Fusion (VBF): Events are selected with high $\met$, accompanied by two jets with large pseudorapidity separation, targeting events with typical VBF characteristics.
2. Higgsstrahlung with Z decay to leptons ($\boldsymbol{Z(\ell\ell)H}$): Characterized by two high-$p_T$ leptons forming a Z boson and large $\met$, reducing background from dilepton events.
3. Higgsstrahlung with W/Z decay to Hadrons ($\boldsymbol{W/Z(\text{had})H}$): Focuses on events with large $\met$ and jet structure from hadronically decaying vector bosons, employing jet mass criteria to discern signal from background processes.

Results and Statistical Combination

The combined analysis places an upper limit on the invisible branching ratio of the Higgs boson to $\brinv < 0.38$ at 95% confidence level (CL) for data taken at $\sqrt{s} = 13$ TeV. When combined with analyses at $\sqrt{s} = 7$ and 8 TeV, this exclusion limit is refined to 0.26. These results do not suggest significant excesses over the SM predictions, corresponding to a $p_{\text{SM}}$-value of 3% under the null hypothesis.

A robust combination methodology is employed, considering systematic uncertainties correlated across the diverse analyses within the ATLAS framework. This involves nuisance parameters and profiling techniques to integrate the likelihood across individual analyses, yielding a comprehensive constraint on potential invisible Higgs decays.

Implications and Future Prospects

This analysis capitalizes on the improved sensitivity of the ATLAS detector at higher collision energies, extending the constraints on DM models that involve Higgs-mediated interactions. The derived exclusion on the invisible Higgs decay width significantly impacts models where the Higgs boson acts as a portal to a dark sector. Specifically, these measurements contribute meaningful constraints when juxtaposed against direct detection experiments for DM, underscoring the complementary role collider searches play in probing the nature of dark matter.

Future developments, such as increased luminosity and advancements in data analysis techniques (e.g., machine learning approaches), promise even more stringent limits on invisible decays and enriched understanding of the Higgs sector. As the LHC continues operations, ongoing analyses will further refine the parameters of existing models and potentially guide the exploration of new physics beyond the SM.