Constraints on new phenomena via Higgs boson couplings and invisible decays with the ATLAS detector

TL;DR

ATLAS performs a global fit of Higgs production and decay rates across multiple channels to test SM consistency and constrain BSM scenarios. By parameterizing couplings with mass-dependent scaling and mixing frameworks, the study derives limits on models including MCHM, EW singlets, and 2HDMs, as well as the hMSSM. The analysis also directly probes invisible Higgs decays, combining visible and invisible channels to set BR$_{\rm inv}$ < 0.23 at 95% CL, and translates this into Higgs-portal dark matter constraints. Overall, results align with SM expectations and place tight indirect bounds on new physics in the Higgs sector, with meaningful implications for dark matter in Higgs-portal scenarios.

Abstract

The ATLAS experiment at the LHC has measured the Higgs boson couplings and mass, and searched for invisible Higgs boson decays, using multiple production and decay channels with up to 4.7 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=7$ TeV and 20.3 fb$^{-1}$ at $\sqrt{s}=8$ TeV. In the current study, the measured production and decay rates of the observed Higgs boson in the $γγ$, $ZZ$, $WW$, $Zγ$, $bb$, $ττ$, and $μμ$ decay channels, along with results from the associated production of a Higgs boson with a top-quark pair, are used to probe the scaling of the couplings with mass. Limits are set on parameters in extensions of the Standard Model including a composite Higgs boson, an additional electroweak singlet, and two-Higgs-doublet models. Together with the measured mass of the scalar Higgs boson in the $γγ$ and $ZZ$ decay modes, a lower limit is set on the pseudoscalar Higgs boson mass of $m_{A}>370$ GeV in the "hMSSM" simplified Minimal Supersymmetric Standard Model. Results from direct searches for heavy Higgs bosons are also interpreted in the hMSSM. Direct searches for invisible Higgs boson decays in the vector-boson fusion and associated production of a Higgs boson with $W/Z$ ($Z\to ll$, $W/Z \to jj$) modes are statistically combined to set an upper limit on the Higgs boson invisible branching ratio of 0.25. The use of the measured visible decay rates in a more general coupling fit improves the upper limit to 0.23, constraining a Higgs portal model of dark matter.

Figures (9)

• Figure 1: Two-dimensional confidence regions as a function of the mass scaling factor $\epsilon$ and the vacuum expectation value parameter $M$. The likelihood contours where $-2\ln\Lambda=2.3$ and $-2\ln\Lambda=6.0$, corresponding approximately to the 68% CL (1 std. dev.) and the 95% CL (2 std. dev.) respectively, are shown for both the data and the prediction for a SM Higgs boson. The best fit to the data and the SM expectation are indicated as $\times$ and $+$ respectively.
• Figure 2: Observed (solid) and expected (dashed) likelihood scans of the Higgs compositeness scaling parameter, $\xi$, in the MCHM4 and MCHM5 models. The expected curves correspond to the SM Higgs boson. The line at $-2\ln\Lambda=0$ corresponds to the most likely value of $\xi$ within the physical region $\xi \ge 0$. The line at $-2\ln\Lambda=3.84$ corresponds to the one-sided upper limit at approximately the 95% CL (2 std. dev.), given $\xi \ge 0$.
• Figure 3: Two-dimensional likelihood contours in the $[\kappa_{V},\kappa_{F}]$ coupling scale factor plane, where $-2\ln\Lambda=2.3$ and $-2\ln\Lambda=6.0$ correspond approximately to the 68% CL (1 std. dev.) and the 95% CL (2 std. dev.), respectively. The coupling scale factors predicted in the MCHM4 and MCHM5 models are shown as parametric functions of the Higgs boson compositeness parameter $\xi = v^2 / f^2$. The two-dimensional likelihood contours are shown for reference and should not be used to estimate the exclusion for the single parameter $\xi$.
• Figure 4: Observed and expected upper limits at the 95% CL on the squared coupling scale factor, $\kappa'^{2}$, of a heavy Higgs boson arising through an additional EW singlet, shown in the $[\mu_{H}, {\rm BR}_{H, \rm new}]$ plane. The light shaded and hashed regions indicate the observed and expected exclusions, respectively. Contours of the scale factor for the total width, $\Gamma_{H}/\Gamma_{H, \rm SM}$, of the heavy Higgs boson are also illustrated based on Eqs. (\ref{['eqn:EWSinglet_H']}) and (\ref{['eqn:EWSinglet_mu']}).
• Figure 5: Regions of the $[\cos(\beta-\alpha),\ \tan\beta]$ plane of four types of 2HDMs excluded by fits to the measured rates of Higgs boson production and decays. The likelihood contours where $-2\ln\Lambda=6.0$, corresponding approximately to the 95% CL (2 std. dev.), are indicated for both the data and the expectation for the SM Higgs sector. The cross in each plot marks the observed best-fit value. The light shaded and hashed regions indicate the observed and expected exclusions, respectively. The $\alpha$ and $\beta$ parameters are taken to satisfy $0 \le \beta \le \pi/2$ and $0 \le \beta-\alpha \le \pi$ without loss of generality.