At the 2018 International Conference on High Energy Physics (ICHEP) in Seoul, the ATLAS Collaboration announced the observation of the Higgs boson decaying into a pair of bottom quarks (Hàbb) with a significance of 5.4 standard deviations, which means the possibility for the background to fake such a signal is less than 6.7×10−8. This is the first time in the world that the decay mode of Hàbb is observed, as one of the most important physics achievements of the LHC experiment since the discovery of the Higgs boson in 2012. The result has been officially published in Phys. Lett. B 786 (2018) 59-86 on November 10, 2018. The research team led by Prof. Ma Lianliang in Shandong University made important contributions to this great observation. Ph.D. student Ma Yanhui reported this important result on behalf of the ATLAS Collaboration in Higgs Couplings 2018 at Tokyo in November 2018 and in the 4th China LHC Physics Conference in Wuhan in December 2018, respectively.
The Higgs mechanism was proposed to explain the origin of the mass of elementary particles, such as W/Z bosons and charged fermions. These fermions acquire masses by the interaction with the Higgs field through the "Yukawa coupling", with a strength proportional to the fermion mass. The Standard Model of particle physics predicts the branching fraction of the Higgs boson decay into a pair of bottom quarks to be 58% with the mass at 125 GeV, as its dominant decay final state. Detecting the Hàbb decay process is thus important for understanding the properties of Higgs boson and the Yukawa coupling between Higgs boson and fermions. To observe this Higgs decay channel is quite challenging and has taken six years since the discovery of the Higgs boson in 2012. The difficulty comes from the overwhelming background with respect to the signal in proton–proton collisions, which makes it hard to isolate the Higgs boson decay signal from the background.
Prof. Ma participated in the first VH, Hbb analysis in the ATLAS experiment, published in Phys. Lett. B 718 (2012) 369-390, since the beginning of ATLAS data-taking in 2010. In addition to the studies of the analysis event selection, he was in charge of the statistical analysis for the first published VH, Hà>bb result, and provided the VH, Hàbb statistical result for the Higgs boson discovery paper. After he joined Shandong University in 2013, Prof. Ma built a strong research team with two postdoctors Remi Zaidan and Mario Sousa, and three graduate students Ma Yanhui, Xu Zhongyukun and Li Tong. Important contributions have been made by the research team for the observation, including the group ntuple production, QCD background estimation, event selection optimization, WH process physics inputs production, and the final combined statistical analysis. Ma Yanhui was one of the internal supporting note editors, and gave approval talks twice in ATLAS Higgs working group on behalf of the Hbb group. Prof. Ma, Mario and Ma Yanhui presented the results in different international conferences on behalf of the ATLAS Collaboration respectively.
The above research work carried out at Shandong University is supported by the projects from the National Natural Science Foundation of China and Ministry of Science and Technology of the People's Republic of China.
Figure 1. Invariant mass distribution of the pair of bottom quarks (mbb) in the selected events of: the data (black points with error bars), the signal (red) and all the other different backgrounds. The bottom pad shows the mbb distribution of signal candidate events after subtraction of all backgrounds from data.
Figure 2: An event display of a selected candidate event of WH, Hbb. The electron is shown as a red track with a large energy deposit in the electromagnetic calorimeter, corresponding to light green bars, and has a transverse momentum of 151 GeV. The missing transverse momentum, shown as a white dashed line, has a magnitude of 320 GeV. These correspond to the W transverse momentum of 450 GeV. The two b-tagged jets are represented by the green and yellow bars corresponding to the energy depositions in the electromagnetic and hadronic calorimeters, respectively, and they have an invariant mass of 124 GeV.
ATLAS Paper: Phys.Lett.B.786.(2018)59-86 https://www.sciencedirect.com/science/article/pii/S0370269318307056?via=ihub
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Written by: Yin Na
Edited by: Xie Tingting