The nature of dark matter is one of the biggest puzzles in contemporary particle physics. Despite being five times more abundant than regular matter, it has never been directly observed in any experiment. Many searches are ongoing – at LHC experiments and elsewhere – but so far traditional approaches have yet to turn up any sightings. Physicists at the ATLAS Experiment are taking a new approach to the hunt for dark matter particles, by looking for composite dark particles.

What does this entail? As a comparison, regular matter consists of just three fundamental particles: up and down quarks, and electrons. Similarly, dark-matter particles could combine into a dark sector of composite particles. Much like the proton, some of these particles might be stable and could make up the dark matter of the Universe.

ATLAS researchers have explored this possibility in the first direct search for new composite dark particles called dark mesons, presented this week at the Large Hadron Collider Physics (LHCP) Conference in Boston. Researchers searched for the dark rho and dark pion particles, which are analogous to the Standard-Model rho and pion mesons (themselves combinations of up and down quarks). Through their interaction with the Higgs field, these dark mesons could decay into heavy Standard-Model particles.

To narrow the possibilities, ATLAS researchers focused on dark-meson decays to top and bottom quarks, in final states including many “jets” of particles and either 0 or 1 lepton. Further, they focused on collision events where the top or bottom quarks are “boosted”. This means that they are produced quite close to each other and so can be reconstructed as large jets with a characteristic substructure. There are different ways to reconstruct these large jets; in this case, the ATLAS team re-clustered the regular jets, b-jets (originating from a bottom quark) and leptons all into bigger structures (see Figure 1).

The new search uses the full dataset recorded by ATLAS during LHC Run 2 (2015-2018) and covers many possible masses of dark pions and dark rhos. To narrow down the data to areas enriched with possible dark meson candidates, researchers selected events based on particular kinematic variables, such as the masses of the re-clustered jets and the angles between them. Nonetheless, the busy collision environment was very difficult to reconstruct and even harder to model, making it extremely challenging to identify a signal from those that may mimic it (“background”). In the final state without leptons, physicists focused on events with 8 to 10 jets, where at least 4 are b-jets. The main background in this channel comes from Standard-Model particles interacting via the strong force, and was estimated by comparing the recorded data in 99 regions with orthogonal selections. When considering the final state with one lepton, they focused on events with 6 to 8 jets where at least 4 are b-jets and are produced simultaneously. In this case, the main background comes from top-quark pairs produced with bottom-quark pairs – a notoriously challenging process to model. Researchers defined different control regions, similar enough to the signal region but with minimal signal contributions, to best understand this background and include it in the final statistical fit.

The search found no hints of dark mesons in data and so exclusion limits were set on the dark pion mass (𝑚𝜋𝐷) and the dark pion to dark rho ratio of masses (𝜂𝐷=𝑚𝜋𝐷/𝑚𝜌𝐷) (see Figure 2). Dark pions with masses under 940 GeV are excluded for signals where 𝜂𝐷 is 0.45. Dark pions with masses under 740 GeV are excluded where 𝜂𝐷 is 0.25. These are the first direct collider constraints on such dark-sector particles. They significantly extend the phase space previously excluded through indirect collider studies, and pave the way for further searches for a dark sector of matter.

Learn more

• Search for dark mesons decaying to top and bottom quarks in proton-proton collisions at 13 TeV with the ATLAS detector (submitted to JHEP, arXiv:2405.20061, see figures)