Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC (1207.7214v2)

Abstract: A search for the Standard Model Higgs boson in proton-proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb^-1 collected at sqrt(s) = 7 TeV in 2011 and 5.8 fb^-1 at sqrt(s) = 8 TeV in 2012. Individual searches in the channels H->ZZ^*->llll, H->gamma gamma and H->WW->e nu mu nu in the 8 TeV data are combined with previously published results of searches for H->ZZ^*, WW^*, bbbar and tau^+tau^- in the 7 TeV data and results from improved analyses of the H->ZZ^*->llll and H->gamma gamma channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0 +/- 0.4(stat) +/- 0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7x10^-9, is compatible with the production and decay of the Standard Model Higgs boson.

• The paper presents a significant excess of events near 126.5 GeV with a 5.9 sigma significance, indicating a discovery consistent with the SM Higgs boson.
• It combines innovative event selection and large-scale Monte Carlo simulations across multiple decay channels, including H→ZZ(*) and WW(*), to maximize signal sensitivity.
• The findings validate the electroweak symmetry breaking mechanism in the Standard Model and set the stage for future precision measurements and beyond-SM physics exploration.

Overview of the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC

Introduction

The paper presents a detailed analysis of the search for the Standard Model (SM) Higgs boson using the ATLAS detector at the Large Hadron Collider (LHC). The data were obtained from proton-proton collisions at center-of-mass energies of 7 TeV in 2011 and 8 TeV in 2012, with integrated luminosities of approximately 4.8 fb$^{-1}$ and 5.8 fb$^{-1}$, respectively. The focus of this analysis is on several decay channels, including $H\rightarrow ZZ^{(*)}$, $WW^{(*)}$, $b\bar{b}$, and $\tau^+\tau^-$.

Methodology

The search combines results from individual channels for both 7 TeV and 8 TeV datasets, utilizing updated analyses for some channels and improved reconstruction techniques to mitigate the effects of increased pile-up in high-luminosity data.

• Data Analysis and Simulation: The analyses relied on large-scale Monte Carlo simulations to predict both signal and background processes. Different event generators and cross-section estimations were used to model the expected distributions for the Higgs production processes through gluon-gluon fusion (ggF), vector boson fusion (VBF), and associated production (VH), among others.
• Event Selection and Categorization: Distinct event selection criteria and categorization strategies were employed for different channels to maximize the sensitivity to potential Higgs signals. For instance, tighter identification and isolation cuts were applied in the $H\rightarrow ZZ^{(*)}\rightarrow 4\ell$ channel to reduce background contamination effectively.

Results

A significant excess of events compatible with the SM Higgs boson hypothesis was observed at a mass near 126.5 GeV. The combined significance for the discovery in this mass region reached 5.9 standard deviations, with individual contributions from the $H\rightarrow \gamma\gamma$, $H\rightarrow ZZ^{(*)}$, and $H\rightarrow WW^{(*)}$ channels. This significance corresponds to a background fluctuation probability of $1.7\times 10^{-9}$, strongly indicating the presence of a new particle consistent with the predicted Higgs boson.

Implications and Future Developments

The discovery of a new neutral boson marks a pivotal breakthrough in particle physics, confirming the existence of a Higgs-like particle responsible for electroweak symmetry breaking in the SM. The measured properties of this particle, such as its mass and production cross-sections, align with theoretical predictions for the SM Higgs boson, reinforcing the validity of the SM framework.

However, the paper also raises important questions for future work, including precision measurements of the boson's properties and the investigation of its couplings and interactions. Further studies are essential to verify if the observed particle is indeed the Higgs boson as envisioned in the SM or if it could be a part of a more complicated theory, such as those involving more Higgs bosons or alternative mechanisms of mass generation.

The ongoing and future runs of the LHC will provide more data to refine these measurements and could offer new insights into physics beyond the Standard Model, thus opening up potential new avenues of research into the fundamental laws of the universe.