Search for the standard model Higgs boson produced in association with a W or a Z boson and decaying to bottom quarks

Abstract: A search for the standard model Higgs boson (H) decaying to b b-bar when produced in association with a weak vector boson (V) is reported for the following channels: W(mu nu)H, W(e nu)H, W(tau nu)H, Z(mu mu)H, Z(e e)H, and Z(nu nu)H. The search is performed in data samples corresponding to integrated luminosities of up to 5.1 inverse femtobarns at sqrt(s) = 7 TeV and up to 18.9 inverse femtobarns at sqrt(s) = 8 TeV, recorded by the CMS experiment at the LHC. An excess of events is observed above the expected background with a local significance of 2.1 standard deviations for a Higgs boson mass of 125 GeV, consistent with the expectation from the production of the standard model Higgs boson. The signal strength corresponding to this excess, relative to that of the standard model Higgs boson, is 1.0 +/- 0.5.

• The paper demonstrates a detailed search for the SM Higgs boson produced with W/Z bosons, identifying its decay to bottom quarks with a 2.1σ excess.
• It employs advanced simulation, multivariate regression, and boosted decision trees to optimize event selection amid significant backgrounds.
• The analysis reports a signal strength of μ = 1.0 ± 0.5 for mH=125 GeV, reinforcing SM expectations and guiding future Yukawa coupling studies.

Overview of the Search for the Standard Model Higgs Boson Decaying to Bottom Quarks

This paper presents a comprehensive analysis of the search for the Standard Model (SM) Higgs boson (\PH) produced in association with a weak vector boson (\PW\ or \cPZ) and decaying into a pair of bottom quarks (\bbbar). The research utilizes data collected by the CMS experiment at the LHC, corresponding to integrated luminosities of up to 5.1\fbinv at a center-of-mass energy of 7\TeV and up to 18.9\fbinv at 8\TeV. The analysis is conducted across several channels depending on the decay modes of the vector bosons: $\PW(\Pgm\Pgn)\PH$, $\PW(\Pe\Pgn)\PH$, $\PW(\Pgt\Pgn)\PH$, $\cPZ(\Pgm\Pgm)\PH$, $\cPZ(\Pe\Pe)\PH$, and $\cPZ(\Pgn\Pgn)\PH$.

The analysis strategy involves a detailed reconstruction of vector bosons through their leptonic decay modes, and the Higgs boson via its main decay channel into bottom quarks. This process is further complicated by substantial backgrounds, most notably from \PW\ and \cPZ\ bosons produced in association with jets, top-quark pair production (\ttbar), singly produced top quarks, diboson production, and QCD multijet processes. The presence of an excess of events with a local significance of 2.1 standard deviations over the expected background for a Higgs boson mass of 125\GeV aligns with the SM Higgs boson production hypothesis.

Key Elements of the Analysis

1. Signal and Background Modeling: The modeling uses extensive simulation samples from various MC event generators, incorporating the CMS detector response through GEANT4. Simulated samples guide the optimization of the event selection criteria, with focus on ${\PW+}$ jets and $\cPZ+{}$ jets backgrounds, where the data-driven methods are applied to extract scale factors.
2. Selection Criteria: The selection criteria align closely with event characteristics expected for associated \PH\ production and decay into \bbbar. Jet energies are corrected using multivariate regression techniques, enhancing the dijet mass resolution crucial for identifying the Higgs signal.
3. Event Categorization: Events are categorized into low, intermediate, and high boost regions according to the transverse momenta (\ptV) of vector bosons, optimizing signal sensitivity across various kinematic regimes.
4. Boosted Decision Trees (BDTs): BDTs are essential to separate signal from background, trained with discriminant variables, capturing the nuances in signal-like and background-like event structures.
5. Systematic Uncertainties: Extensive treatment of systematic uncertainties is critical for robust results, including on b-tagging efficiencies, energy scales, luminosity, and background estimations. These are integrated into the statistical analysis framework via nuisance parameters.

Results and Implications

The combined analysis across all channels yields a signal strength of $\mu = 1.0 \pm 0.5$ for $m_\PH=125\GeV$, consistent with the SM expectation. The excess of events indicating \PH\ production with \bbbar\ decays marks a significant observation, enhancing our understanding of Yukawa couplings in the quark sector.

Future Directions

Continued improvements in detector technology, analysis techniques, and theoretical models can refine the search for the Higgs boson, particularly focusing on rare decay modes and precise coupling measurements. Upcoming LHC data at increased energies and luminosities will provide further validation and exploration of the diverse phenomenology associated with the Higgs sector in the SM and potential new physics scenarios.