[Paper Review] Search for pair production of Higgs bosons in the $b\bar{b}b\bar{b}$ final state using proton--proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

ATLAS Collaboration|arXiv (Cornell University)|Jun 15, 2016

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a search for Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state using 3.2 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector. Using both resolved and boosted $b$-jet reconstruction techniques, the study sets upper limits on the $hh$ production cross section times branching ratio, constraining nonresonant production to less than 1.22 pb at 95% CL and resonant Kaluza-Klein graviton production to 24–91 fb for masses between 600–3000 GeV.

ABSTRACT

A search for Higgs-boson pair production in the $b\bar{b}b\bar{b}$ final state is carried out with 3.2 fb$^{-1}$ of proton--proton collision data collected at $\sqrt{s} = 13$ TeV with the ATLAS detector. The data are consistent with the estimated background and are used to set upper limits on the production cross section of Higgs-boson pairs times branching ratio to $b\bar{b}b\bar{b}$ for both nonresonant and resonant production. In the case of resonant production of Kaluza--Klein gravitons within the Randall--Sundrum model, upper limits in the 24 to 91 fb range are obtained for masses between 600 and 3000 GeV, at the 95% confidence level. The production cross section times branching ratio for nonresonant Higgs-boson pairs is also constrained to be less than 1.22 pb, at the 95% confidence level.

Motivation & Objective

• To search for nonresonant and resonant Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state using 13 TeV proton-proton collisions.
• To improve sensitivity to high-mass resonances by leveraging increased production cross sections at $\sqrt{s} = 13$ TeV compared to previous 8 TeV data.
• To enhance detection efficiency for highly boosted Higgs bosons by employing large-radius jets and small-radius track jets in the boosted analysis.
• To extend sensitivity to resonant states above 2000 GeV by including a three-$b$-tagged jet channel, addressing reduced $b$-jet identification efficiency at high $p_T$.
• To set 95% confidence level upper limits on the $hh$ production cross section times branching ratio for both nonresonant and resonant scenarios.

Proposed method

• Utilizes a two-pronged analysis strategy: the 'resolved' analysis for low-mass $hh$ systems where four distinct $b$-jets are resolved, and the 'boosted' analysis for high-mass systems where $b$-jets merge into large-radius jets.
• Employs large-radius jets (anti-$k_t$, $R=1.0$) to reconstruct highly boosted Higgs bosons, combined with small-radius track jets ($R=0.3$) for $b$-hadron tagging.
• Applies advanced $b$-jet tagging algorithms and kinematic reconstruction techniques to distinguish signal from dominant $t\bar{t}+\text{jets}$ and $gg\to b\bar{b}$ backgrounds.
• Uses a multivariate analysis (MVA) approach to combine kinematic and topological variables, improving signal-to-background separation.
• Performs a profile-likelihood fit to the observed data to set upper limits on the signal cross section at 95% confidence level.
• Incorporates a new three-$b$-tagged channel in the boosted analysis to improve sensitivity for high-mass resonances where $b$-tagging efficiency drops.

Experimental results

Research questions

• RQ1 What are the upper limits on the production cross section times branching ratio for nonresonant Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state at $\sqrt{s} = 13$ TeV?
• RQ2 How does the sensitivity to resonant Higgs pair production vary across different masses in the Randall–Sundrum model with Kaluza–Klein gravitons?
• RQ3 To what extent does the inclusion of a three-$b$-tagged channel in the boosted analysis improve sensitivity for high-mass resonances?
• RQ4 How do the resolved and boosted analysis strategies compare in sensitivity across the $hh$ mass spectrum?
• RQ5 What is the expected sensitivity gain from using smaller track-jet radii and updated $b$-tagging at 13 TeV compared to 8 TeV?

Key findings

• The observed data are consistent with the expected background, with no significant excess observed in the $b\bar{b}b\bar{b}$ final state.
• The upper limit on the nonresonant $hh$ production cross section times branching ratio is set at 1.22 pb at 95% confidence level.
• For resonant production of Kaluza–Klein gravitons in the Randall–Sundrum model, upper limits range from 24 fb at 600 GeV to 91 fb at 3000 GeV, at 95% CL.
• The boosted analysis achieves improved sensitivity for resonances above 2000 GeV due to the inclusion of the three-$b$-tagged channel, compensating for reduced $b$-tagging efficiency at high $p_T$.
• The resolved analysis provides higher sensitivity than the boosted analysis for $hh$ masses below 1100 GeV, justifying the use of different strategies in different mass regions.
• The overall sensitivity gain at 13 TeV over 8 TeV is significant, particularly in the high-mass region, due to the increased production cross section and improved jet reconstruction techniques.