[Paper Review] Search for the Standard Model Higgs boson produced by vector boson fusion and decaying to bottom quarks

Giorgia Rauco|arXiv (Cornell University)|Dec 29, 2015

Particle physics theoretical and experimental studies15 references4 citations

TL;DR

This paper presents a search for the Standard Model Higgs boson produced via vector boson fusion and decaying into bottom quarks using 19.8 fb⁻¹ of proton-proton collisions at √s = 8 TeV with the CMS detector. The analysis observes a signal significance of 2.2 standard deviations at 125 GeV, with a measured signal strength of μ = 2.8⁺¹.⁶₋₁.⁴, which is compatible with the SM prediction at the 8% level after combination with other CMS H → bb̄ searches, yielding μ = 1.03⁺⁰.⁴⁴₋⁰.⁴² and a significance of 2.56σ.

ABSTRACT

The search for the Standard Model (SM) Higgs boson (H) produced through the Vector Boson Fusion (VBF) mechanism and decaying to a pair of bottom quarks is reported. The used data have been collected with the CMS detector and correspond to an integrated luminosity of 19.8 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=8$ TeV at the CERN LHC. Parked data have been exploited as well. This search resulted in an observed (expected) significance in these data samples for a H $ ightarrow$ b$\bar{b}$ signal at a mass of 125 GeV of 2.2 (0.8) standard deviations. The cited signal strength, $μ=σ/σ_{ extrm{SM}}$, was measured to be 2.8$^{+1.6}_{-1.4}$. This result has been combined with other CMS searches for the SM Higgs boson decaying in a pair of bottom quarks exploiting other Higgs production mechanisms. The obtained combined signal strength is 1.0 $\pm$ 0.4, corresponding to an observed signal significance of 2.6 standard deviations for a Higgs boson mass of 125 GeV.

Motivation & Objective

• To search for the Standard Model Higgs boson produced via vector boson fusion (VBF) and decaying into a bottom quark pair (bb̄), a dominant decay mode with high theoretical importance.
• To overcome the overwhelming QCD background in inclusive Higgs production by exploiting the distinctive VBF topology with two forward jets and a central bb̄ pair.
• To measure the signal strength μ = σ/σ_SM for VBF-produced H → bb̄ at 125 GeV, testing consistency with the Standard Model prediction.
• To combine results with other CMS H → bb̄ search channels (VH, ttH) to improve sensitivity and constrain the Higgs signal strength.

Proposed method

• Data from 19.8 fb⁻¹ of pp collisions at √s = 8 TeV collected by the CMS detector were analyzed, using dedicated and general-purpose triggers to select VBF-enriched events with four jets and b-tagged candidates.
• Event reconstruction used the Particle Flow algorithm and anti-kT jet clustering (R = 0.5), with b-jets identified via the CSV b-tagging algorithm.
• A multivariate discriminant was applied to distinguish signal from background, combining b-tag values, jet pseudorapidity, and ordering information to enhance signal sensitivity.
• The analysis employed a simultaneous fit to the invariant mass distribution of the two b-jet candidates in multiple signal-enriched categories, modeling backgrounds as QCD, top quark, and Z/W + jets contributions.
• Signal and background shapes were derived from Monte Carlo simulations, with jet energy scale (JES) and resolution (JER) uncertainties parameterized via calibration factors k_JES and k_JER.
• The signal strength μ = σ/σ_SM was extracted using the Asymptotic CLs method, with systematic uncertainties on shapes and yields treated as free parameters in the fit.

Experimental results

Research questions

• RQ1 What is the observed significance of a Higgs boson produced via VBF and decaying into bottom quarks at 125 GeV in 19.8 fb⁻¹ of 8 TeV data?
• RQ2 How does the measured signal strength μ = σ/σ_SM for VBF → H → bb̄ compare to the Standard Model prediction of μ = 1?
• RQ3 What is the combined signal strength and significance when integrating the VBF result with other H → bb̄ search channels (VH, ttH)?
• RQ4 How well do the observed data agree with the expected background-only hypothesis in the VBF → bb̄ channel?
• RQ5 What are the dominant systematic uncertainties affecting the signal strength measurement in the VBF H → bb̄ search?

Key findings

• The observed significance for a Higgs boson at 125 GeV in the VBF → bb̄ channel is 2.2 standard deviations, with an expected significance of 0.8 standard deviations under the background-only hypothesis.
• The measured signal strength for VBF-produced H → bb̄ is μ = 2.8⁺¹.⁶₋¹.⁴, indicating a signal excess relative to the SM prediction, though compatible at the 8% level.
• After combining with VH and ttH H → bb̄ search results, the combined signal strength is measured as μ = 1.03⁺⁰.⁴⁴₋⁰.⁴², consistent with the SM prediction of μ = 1.
• The combined observed significance for a 125 GeV Higgs boson across all H → bb̄ channels is 2.56 standard deviations, with an expected significance of 2.70 standard deviations.
• The background-only hypothesis is excluded at more than 2σ confidence level in the combined analysis, supporting the presence of a Higgs signal.
• The fit results show good agreement between data and the signal + background model, with uncertainties well-controlled by the simultaneous fit across multiple categories and systematic parameters.