Searches for Standard Model (SM) and

Beyond Standard Model (BSM) Higgs Boson

Standard Model :

Beyond Standard Model :

SM H(bb) : Higgs production in Association with a W or Z boson, H->bb

Our group performs the search for the SM Higgs boson in the channel where it is produced in association with a W or Z boson, and the Higgs boson decays into a pair of b-quarks.
We joined the search already in Run1 of ATLAS data taking.

The search is performed in three different final states :


Our group's contributions :

Observation of H->bb !!!

After analyzing the Run 2 data collected in 2015,2016 and 2017 (total L~80 fb-1) for H->bb in VH production, and combining this result with that from Run1 search, and with results of H->bb in other production channels, ATLAS has finally observed H->bb in summer 2018 at 5.6 standard deviations, with an expected sensitivity of 5.5 standard deviations.

BDT output distribution for the VHbb 0-leption channel in the 2-tag 2-jet case.Yield vs log(S?B) for VHbb ()+1+2lep).
mbb distribution for VHbb.Signal strength for various productions with Higgs decays to bb.
Sensitivities from 0,1,2-lepton channels of VHbb search.Sensitivities from various channels that are then combined.

Event Display of a selected Higgs candidate in the 0-lepton channel

Event display of a 0-lepton candidate event.

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

Published Papers :

SM H(bb) : Higgs produced in Vector Boson Fusion (VBF) process in association with a high energy photon

Our group performed a search where the Higgs boson is produced via the VBF process and together with a high energy photon. The Higgs boson is searched in the H->bb decay mode. The analysis is performed using data ATLAS collected in 2015 and 2016. The requirement of an extra high pT photon helps to reduce the multi-jet background, and also provide an extra handle to trigger on VBF events. There is another H->bb search in the VBF production channel which does not require the presence of photon (inclusive). The results from both "photon" and "inclusive" channels are combined and are published in Phys. Rev. D 98 (2018) 052003. This VBF H->bb result is used in the combination for the H->bb observation, which is described above.

signal strengthsensitivity

Published Papers :

Search for High Mass Higgs boson in the H->ZZ decays

The analysis searched for a heavy Higgs boson that is produced via the gluon-gluon fusion (ggF) process or the vector boson fusion (VBF) process, and the Higgs boson decays into a ZZ pair. The analysis selected events in the llll, llvv, llqq and vvqq final states. The search is also performed in the "resolved" and "merged" channels, thus covers a while range of the Higgs boson mass (~200 to 1000 GeV). The Academia Sinica group is responsible for the vvqq channel and the merged llqq channel. The results, based in Run 1 data sample, are published in Eur. Phys. J. C76 (2016) 45 .

Search for CP-Odd Higgs Boson

A CP-odd Higgs boson A is search in the A->Zh production with ~3.2 fb-1 of 13 TeV ATLAS early Run 2 data. The search is performed in the channels where the light mass Higgs (h) decays into a pair of b-quarks, and the Z boson decays into a pair of charged leptons (ee, mumu) or a pair of neutrinos (vv). The preliminary results are published in an ATLAS conference note : ATLAS-CONF-2016-015 .

Search for Invisible Higgs Boson Decay

The Higgs boson has been observed in several decay modes (e.g. H->gamma gamma, ZZ, WW, tau tau), and most recently in the dominant decay mode H->bb. However the uncertainties of some of these measurements are still large, and a few other SM predicted decays with smaller branching fractions have not yet been observed. Thus there could still have some room for the Higgs boson to decay in non-SM channels. In some extension models the Higgs boson acts as a portal between the dark sector and the SM sector, either through Yukawa-type couplings to fermionic dark matter (DM), or other mechanisms. If kinematically allowed, the Higgs boson can decay into a pair of DM particles. The DM particles will escape detection and can only be indirectly inferred through missing transverse momentum (MET). In this case the Higgs boson decays ``invisibly''.

The AS group previous data analyses had dealt with final states that consist of MET and jets (e.g. the ``0-lepton'' of the SM Higgs search in the H->bb decay). We applied this experience in the search for invisble Higgs boson decay with ATLAS data collected in 2015+2016 (at 13 TeV, L=36 fb-1). The MET+jets final state includes Higgs signal contribution from gluon-gluon fusion (ggF) production, W/ZH associated production and from vector-boson fusion (VBF) production. The measured 95% upper limit on the branching ratio BR(H->inv) is 0.83, with an expected limit of 0.58 (+0.23 -0.16). The result is published in JHEP 10 (2018) 180. This result is combined with other ATLAS invisible Higgs decay measurements and the preliminary combined result is shown in ATLAS-CONF-2018-054. The combined observed limit is BR(H->inv) <0.26. The combined expected limit BR(H->inv) < 0.17 (+0.07 -0.05) is currently the world best.

Published Papers :

Links :

Last modification : January 2019, Song-Ming Wang