Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Abstract: Results of a search for $H \to \tau \tau$ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 $\rm{fb}^{-1}$ and 20.3 $\rm{fb}^{-1}$ at centre-of-mass energies of $\sqrt{s}$ = 7 TeV and $\sqrt{s}$ = 8 TeV respectively. All combinations of leptonic ($\tau \to \ell \nu \bar \nu$ with $\ell = e, \mu$) and hadronic ($\tau \to \rm{hadrons} \nu$) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of $\mu = 1.43 ^{+0.43}_{-0.37}$ is consistent with the predicted Yukawa coupling strength in the Standard Model.

• The paper provides evidence for the Higgs boson decaying into tau leptons with a 4.5-sigma excess over the expected Standard Model background.
• It employs boosted decision trees and robust statistical methods to isolate the Higgs signal from complex collision data.
• The measured signal strength of 1.43 (+0.43/-0.37) supports the Standard Model’s expectations for the Yukawa coupling in the lepton sector.

Evidence for the Higgs-Boson Yukawa Coupling to Tau Leptons with the ATLAS Detector

This paper presents an empirical analysis of the Higgs boson decay into tau leptons, observed via the ATLAS detector, leveraging data from proton-proton collisions at the Large Hadron Collider (LHC). The study utilizes collision data collected in 2011 and 2012 under center-of-mass energies of 7 TeV and 8 TeV, with integrated luminosities of 4.5 fb$^-{1}$ and 20.3 fb$^-{1}$ respectively. The primary aim is to substantiate the presence of the Higgs-boson Yukawa coupling to fermions, specifically through evidence of tau lepton decay channels.

Methodology and Results

The research encompasses various tau lepton decay paths—leptonic (tau decay into leptons and neutrinos) and hadronic (tau decay into hadrons and a neutrino). These decay modes help address different background processes expected from Standard Model (SM) predictions. The analysis detects an excess of events over the anticipated SM background, showing observed significance at 4.5 standard deviations, compared to the expected significance of 3.4 standard deviations, providing robust evidence for the fermionic decay of the Higgs boson.

The measured signal strength, denoted $\mu$, which contrasts the observed signal yield to the SM predictions, is analyzed, yielding $\mu = 1.43^{+0.43}_{-0.37}$. This value is in alignment with the Standard Model’s expectation for the Yukawa coupling, suggesting congruence with established theoretical models for the Higgs boson.

Analytical Techniques

The investigation employs boosted decision trees (BDTs) for the discrimination between signal and background data. These BDTs are trained on a diverse set of kinematic and angular variables that optimize the separation of Higgs signal events from the backdrop of non-Higgs events. Systematic uncertainties, such as those from jet energy scales and tau energy decays, are incorporated into the theoretical model, improving the confidence in the statistical interpretations deduced from the data.

Furthermore, a comprehensive assessment of potential sources of systematic errors was conducted, including background modeling, theoretical uncertainties, and experimental constraints such as luminosity measurement disparities.

Implications and Future Prospects

The confirmation of Higgs-tau coupling is a significant milestone in confirming the Higgs mechanism's role in mass generation within the SM framework, particularly in the lepton sector. This research not only reaffirms the consistency of current physics models but also poses as a pivotal basis for future experimental and theoretical investigations aiming to explore and quantify Yukawa interactions further.

Future developments might involve enhancing detector sensitivity and data analysis algorithms, intending to reduce uncertainties and expand the collision dataset's scope. Continued scrutiny of these interactions could unveil physics beyond the Standard Model, potentially uncovering anomalies or new particles associated with Higgs-like interactions. Thus, the study fortifies the foundational understanding of Higgs boson interactions in high-energy physics, prompting deeper explorations into uncharted territories within particle physics.