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 :
Key analyzer for the 0-lepton channel
Understanding the missing transverse energy (MET) trigger that
is used to collect the data sample for 0-lepton channel
Study the jet energy resolution
Study the b-tagging efficiency for b-jets and rejection rates
for c- and light-flavor jets
Perform search in both cut-based analysis and multi-variant
Development of the analysis framework software use for the analysis by the Hbb group
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.
Event Display of a selected Higgs candidate in the 0-lepton
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
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.
Search for High Mass Higgs boson in the H->ZZ
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
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
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.