Constraints on new phenomena via Higgs boson couplings and invisible decays with the ATLAS detector (1509.00672v2)

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 $\gamma\gamma$, $ZZ$, $WW$, $Z\gamma$, $bb$, $\tau\tau$, and $\mu\mu$ 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 $\gamma\gamma$ 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.

• The paper demonstrates that measured Higgs boson couplings consistently match Standard Model predictions using a multi-channel analysis.
• It sets a lower bound of mA > 370 GeV for a pseudoscalar Higgs in the hMSSM framework, limiting possible new physics models.
• The study constrains the Higgs invisible decay branching ratio to be between 0.23 and 0.25, effectively limiting potential dark matter signatures.

Overview of Higgs Boson Constraints via ATLAS Detector

The paper "Constraints on new phenomena via Higgs boson couplings and invisible decays with the ATLAS detector," authored by the ATLAS Collaboration, presents an extensive investigation of Higgs boson properties as observed in proton-proton collisions at the LHC. The paper utilizes data at energies of √s = 7 TeV and 8 TeV to perform detailed measurements and searches, aiming to constrain possible extensions to the Standard Model (SM).

Methodology and Results

The authors employ a comprehensive analysis, using various production and decay channels of the Higgs boson, specifically examining the γγ, ZZ, WW, Zγ, bb, ττ, and μμ decay channels. The investigation also integrates production associated with top-quark pairs to probe the scaling of couplings with mass. The findings indicate no significant deviations from SM expectations, thereby supporting the SM hypothesis, which attributes electroweak symmetry breaking to a Higgs doublet.

Higgs Boson Coupling and Mass

The research incorporates coupling measurements to test the mass scaling of the Higgs boson. The work concludes that the couplings are consistent with the SM's predictions, with limits being set on the parameters that would indicate the presence of a composite Higgs boson or an additional electroweak singlet.

Pseudoscalar Higgs and hMSSM Model

Moreover, the paper sets a lower limit on the mass of a hypothetical pseudoscalar Higgs within the "hMSSM" (simplified Minimal Supersymmetric Standard Model) framework, estimating m_A > 370 GeV. This model remains a benchmark for analyzing supersymmetry in Higgs sectors.

Invisible Decay Modes

In direct searches for invisible Higgs decays — potential indicators of dark matter via unseen decay channels — the paper combines results from vector-boson fusion and associated production modes. These efforts yield an upper limit on the Higgs boson's invisible branching ratio of 0.25. When visible decay rates are incorporated into a broader coupling fit, this constraint improves slightly to 0.23.

Implications

The research provides significant contributions to the understanding of the Higgs boson's role in the SM and potential new physics. By constraining beyond the SM theories, the findings have implications for the development of Higgs sector models that include particles such as dark matter candidates or heavier Higgs-like bosons.

Future Developments in AI and Machine Learning

The techniques used in the ATLAS analysis, which deal with massive volumes of data from high-energy particle collisions, suggest potential advancements in AI and machine learning for future analyses. In particular, improving algorithms to highlight subtle deviations from expected SM results could be instrumental in discovering new physics. As computational capacity enhances, one can anticipate increasingly sophisticated methods for data analysis and simulation within particle physics research and related computational fields.

In conclusion, this work represents a methodically comprehensive approach to understanding the constraints and characteristics of the Higgs boson as realized by the ATLAS detector, thereby reinforcing the foundational structure of the SM while paving pathways for new theoretical investigations.