Observation of Higgs boson decays to bb at the LHC

Post Date:2018-08-29

After six years since the discovery of the Higgs boson through the decays into gauge bosons (photon, W, Z), the ATLAS and CMS experiments at the LHC have finally observed the Higgs boson decaying into a pair of b-quarks (H→bb), which accounts for the largest fraction (about 58%) of all possible decays. Both experiments jointly announced this observation at a CERN seminar on August 28th [1], which was followed by a press release [2].  This observation is very important as it further strengthen the understanding of the mass generation mechanism predicted in the Standard Model (SM), and provides an additional confirmation of the “Yukawa-couplings” in which the charged fermion particles (leptons, quarks) acquire masses through coupling with the Higgs boson. The Taiwan Academia Sinica (AS) group is part of the team that contributes to the ATLAS’ observation result.

At ATLAS the H→bb signal is observed with a significance of 5.4 standard deviations. This result is obtained through analyzing the ATLAS Run 1 and Run 2 data samples (total integrated luminosity of about 100 fb-1) with the application of advanced techniques to extract the tiny signal in the data. To detect the signal several of its properties are exploited to separate the signal from the background. For example the long lifetime of the b-hadrons in the jets of the b-quarks from the Higgs decay, and the invariant mass resonance form by the two-b-jet system (see Figure 1). It takes a considerably longer time and larger data sample to observe the Higgs boson in this decay channel due to the overwhelming b-quark pair background that is produced via the strong interaction at the LHC. Thus making this search one of the most challenging analyses at the LHC. The ATLAS preliminary observation result [3] was first presented at the 2018 International Conference on High Energy Physics (ICHEP) at Seoul in July [4]. A paper was submitted on the same day as the joint CERN seminar to the Physics Letter B journal for publication [5]. This significant result is the outcome of the dedicated work of many physicists of the ATLAS collaboration and the fantastic performance of the LHC machine. A more general public explanation of this significant result can be found at [6].

The AS group has been active in the Higgs boson search via the H→bb decay channel for many years starting in the CDF experiment at the Tevatron and later continued the search in the ATLAS experiment since the beginning of the data taking by ATLAS at the LHC. The data analysis effort by the AS group is led by associate research fellow Song-Ming Wang. The AS team, including also the post-doctorial research Adrian Buzatu and PhD student Wei Wang, contributed significantly to the “0-lepton” sub-channel of this search. In this sub-channel the Higgs boson is produced in association with a Z boson and that the Z boson decays into a pair of neutrinos. The contributions from the AS team include:

  • Improving the resolution of the measured two-b-jet system mass,

  • Measuring the performance of the trigger used to collect the data for the “0-lepton” channel,

  • Training the multivariate analysis algorithm to separate the H→bb signal from background,

  • Development of the analysis software and performing the analysis.

Comparing among the sub-channels for this H→bb search, the 0-lepton sub-channel has one of the best sensitivity. Song-Ming Wang had also served as a co-convener of the ATLAS Higgs H→bb working group in 2015-2016.

Figure 1 : Mass distribution of the two-b-jet system from the analysis using ATLAS Run 2 data sample. The predicted background contributions, except the di-boson WZ and ZZ, have been subtracted from the data. The grey histogram is the predicted WZ and ZZ background. The red histogram is the measured H→bb signal.

Figure 2 : An ATLAS candidate event for Higgs boson (H) decaying to two b-quarks, in association with a Z boson decaying to a pair of neutrinos, which escape undetected.

Links to news and results of this analysis :
[1] https://indico.cern.ch/event/750541/
[2] http://press.cern/press-releases/2018/08/long-sought-decay-higgs-boson-observed/
[3] https://cds.cern.ch/record/2630338/
[4] https://indico.cern.ch/event/686555/contributions/3028060/ , https://indico.cern.ch/event/686555/contributions/3028073/
[5] https://arxiv.org/abs/1808.08238
[6] http://atlas.cern/updates/physics-briefing/higgs-observed-decaying-b-quarks

Important links :
CERN seminar : https://indico.cern.ch/event/750541/
CERN press release : http://press.cern/press-releases/2018/08/long-sought-decay-higgs-boson-observed
Submitted ATLAS paper : https://arxiv.org/abs/1808.08238


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