In the 1930s, Swiss astronomer Fritz Zwicky observed that the velocities of galaxies in the Coma Cluster were too high to be maintained solely by the gravitational pull of luminous matter. He proposed the existence of some non-luminous matter within the galaxy cluster, which he called dark matter. This discovery marked the beginning of humanity's understanding and study of dark matter.
Today, the most precise measurements of dark matter in the universe come from observations of the cosmic microwave background. The latest results from the Planck satellite indicate that about 5% of the mass in our universe comes from visible matter (mainly baryonic matter), approximately 27% comes from dark matter, and the rest from dark energy.
Despite extensive astronomical observations confirming the existence of dark matter, we have limited knowledge about the properties of dark matter particles. From a microscopic perspective, the Standard Model of particle physics, established in the mid-20th century, has been hugely successful and confirmed by numerous experiments. However, the Standard Model cannot explain the existence of dark matter in the universe, indicating the need for new physics beyond the Standard Model to account for dark matter candidate particles, and the urgent need to find experimental evidence of these candidates.
Consequently, dark matter research is not only a hot topic in astronomy but also at the forefront of particle physics research. Searching for dark matter particles in colliders is one of the three major experimental approaches to detect the interaction between dark matter and regular matter, complementing other types of dark matter detection experiments such as underground direct detection experiments and space-based indirect detection experiments.
Recently, the ATLAS collaboration searched for dark matter using the 139 fb-1 of proton-proton collision data accumulated during LHC's Run 2, within the 2HDM+a dark matter theoretical framework. The search utilized a variety of dark matter production processes and experimental signatures, including some not considered in traditional dark matter models. To further enhance the sensitivity of the dark matter search, this work statistically combined the three most sensitive experimental signatures: the process involving a Z boson decaying into leptons with large missing transverse momentum, the process involving a Higgs boson decaying into bottom quarks with large missing transverse momentum, and the process involving a charged Higgs boson with top and bottom quark final states.
This is the first time ATLAS has conducted a combined statistical analysis of final states including dark matter particles and intermediate states decaying directly into Standard Model particles. This innovation has significantly enhanced the constraint on the model parameter space and the sensitivity to new physics.
"This work is one of the largest projects in the search for new physics at the LHC, involving nearly 20 different analysis channels. The complementary nature of different analysis channels to constrain the parameter space of new physics highlights the unique advantages of collider experiments," said Zirui Wang, a postdoctoral researcher at the University of Michigan.
This work has provided strong experimental constraints on multiple new benchmark parameter models within the 2HDM+a theoretical framework, including some parameter spaces never explored by previous experiments. This represents the most comprehensive experimental result from the ATLAS collaboration for the 2HDM+a dark matter model.
Lailin Xu, a professor at the University of Science and Technology of China stated, "2HDM+a is one of the mainstream new physics theoretical frameworks for dark matter in the world today. It has significant advantages in predicting dark matter phenomena and compatibility with current experimental constraints, predicting a rich variety of dark matter production processes in LHC experiments. This work systematically carried out multi-process searches and combined statistical analysis based on the 2HDM+a model framework, providing results that exclude a large portion of the possible parameter space for dark matter, offering important guidance for future dark matter searches."
Vu Ngoc Khanh, a postdoctoral researcher at Tsung-Dao Lee institute, stated: "Although we have not yet found dark matter particles at the LHC, compared to before the LHC's operation, we have put stringent constraints on the parameter space where dark matter might exist, including the mass of the dark matter particles and their interaction strengths with other particles, further narrowing the search scope." Tsung Dao Lee Fellow Li Shu, added: "So far, the data collected by the LHC only accounts for about 7% of the total data the experiment will record. The data that the LHC will generate over the next 20 years presents a tremendous opportunity to discover dark matter. Our past experiences have shown us that dark matter might be different from what we initially thought, which motivates us to use more innovative experimental methods and techniques in our search."
ATLAS is one of the four large experiments at CERN's Large Hadron Collider (LHC). The ATLAS experiment is a multipurpose particle detector with a forward-backward symmetric cylindrical geometry and nearly 4π coverage in solid angle. It consists of an inner tracking detector surrounded by a thin superconducting solenoid, high-granularity sampling electromagnetic and hadronic calorimeters, and a muon spectrometer with three superconducting air-core toroidal magnets. The ATLAS Collaboration consists of more than 5900 members from 253 institutes in 42 countries on 6 continents, including physicists, engineers, students, and technical staff.
