This project proposes a search for a novel signature of dark matter (DM) production at the Large Hadron Collider (LHC)
resulting from the emission of an additional Higgs boson in the dark sector. The presence of such a dark Higgs boson is
motivated both by the need to generate the masses of the particles in the dark sector and the possibility to relax constraints
from the dark matter relic abundance. If the dark Higgs boson is the lightest state in the dark sector and has a small mixing
with the Standard Model (SM) Higgs boson, it predominantly decays into b-quarks for a dark Higgs boson mass below 160
GeV. This gives a distinct mono-dark-Higgs signature to search for consisting of a two collimated b-jets in association with
missing transverse momentum from a pair of undetected dark matter particles. The new search channel probes large parts
of phase space which are inaccessible to conventional mono-jet and di-jet searches and is complementary to the existing
dark matter search programme at the LHC. The potential discovery of a dark Higgs boson would provide a unique window to
the dark sector with excellent opportunities for further exploration, e.g. by probing the couplings of the dark Higgs boson to
other SM particles. The implications would even go beyond the question of DM as it would suggest that the Higgs
mechanisms is more fundamental that what is currently believed. The project described here proposes to use the data
collected with ATLAS at the LHC by the end of Run-2 to search for a dark Higgs boson decaying to b-quarks. In addition, the
project has a hardware part on the upgrade of the ATLAS inner tracker (ITk) for High-Luminosity LHC. This is an important
element for the preparations for future searches for the dark Higgs boson, either as a means to further explore a potential
positive outcome of the search with Run-2 data, or to improve the sensitivity of the search in case no evidence is found for a
dark Higgs boson with the Run-2 data.
The overall objectives for this project was:
- Study how the dark Higgs boson decaying to b-quarks can be identified at the LHC.
- Perform a mono-dark-Higgs search using Run-2 ATLAS data.
- Prepare for further exploration of the dark Higgs boson
