Dark Matter Searches
Understanding the nature of dark matter is one of the most fundamental and burning questions in physics because it represents approximately 85% of the total matter in the universe, yet it remains entirely invisible to our current instruments as it does not emit, absorb, or reflect light. Its existence is for now only inferred through its gravitational effect which manifest most significantly in the structure of visible matter in the universe (CMB), and maybe more intuitively when we observe the rotation curves of galaxies and the way light bends around massive cosmic structures, suggesting that our current understanding of the standard model of particle physics is incomplete. Identifying the nature of dark matter–whether it consists of Weakly Interacting Massive Particles (WIMPs), axions, or something else entirely–would provide the missing link in our map of the universe’s evolution and help explain how galaxies formed and remain held together. We know that dark matter interacts very, very feebly but not much more, which makes opens up a huge phase space of options how to find it.
CMS role in this search initially was to search for signatures where dark matter is produced in the proton-proton collisions and escapes undetected, thus leaving a large missing transverse momentum signature. More recently with no signals identified in those signatures we have cast a wider net and searched for more complex models. These searches are often referred to as Dark Sector searches.
Mono-X and more
In a “Mono-X” type of analysis the primary objective is to search for new particles that do not interact with the detector (such as dark matter candidates) by looking for events where they are produced alongside a single, visible Standard Model particle (“X”). The “X” particle, which could be a jet of hadrons, a photon, a top quark , a vector boson (W or Z) or even a Higgs boson, serves as a “recoil” that allows the event to be triggered and recorded. Because the new particles escape undetected, the experimental signature is defined by the visible particle and a significant imbalance in the transverse momentum of the event, referred to as missing transverse momentum (pTmiss). The PPC has led the Mono-X search with all ‘X’ particles mentioned above.
The latest and most sensitive analysis in our Mono-X search portfolio looks for proton-proton collisions that produce energetic hadronic jets alongside large missing transverse momentum. We used the full CMS Run 2 dataset, totaling 137 fb−1 of luminosity, and employed advanced machine learning techniques to categorize events. These categories distinguished between narrow jets from initial-state radiation and larger-radius jets that could indicate the hadronic decay of a W or Z boson.
The results of the search showed no significant excess of events beyond the expectations of the standard model background. Consequently, the study established new or more stringent constraints on several theoretical models, including the invisible decay of the Higgs boson and various dark matter mediators. Specifically, the limits set for spin-1, pseudoscalar, and colored dark matter mediators, as well as for first-generation scalar leptoquarks, are among the most restrictive to date. The results also provided updated constraints on the fundamental Planck scale within the context of models involving large extra dimensions.
Dark Photons
Dark photons represent a compelling portal between the Standard Model and a hypothesized dark sector. While the Standard Model is highly successful, it fails to explain dark matter, which accounts for roughly 85% of the universe’s mass. The dark photon model suggests that dark matter may have its own fundamental forces, with the dark photon serving as the force carrier, much like the ordinary photon carries electromagnetism. This makes it an attractive target for CMS for several reasons. Starting with ‘Theoretical Versatility’. One of the main strengths of the dark photon model is the mechanism of kinetic mixing. In this framework, a dark photon and an ordinary photon can oscillate or mix into one another with a very small probability. This mixing allows the dark sector to communicate with our own through a faint electromagnetic signature. Furthermore, the dark photon is often predicted to have a small but non-zero mass, unlike the ordinary photon. This mass is not predicted by the theory, meaning it could exist anywhere from the sub-eV range to hundreds of GeV, providing a vast parameter space for experiments like CMS to explore. Another reason is the ‘Unique Experimental Signatures’. The interaction between dark photons and the Standard Model is so weak that these particles can be long-lived by particle physics standards. While a Higgs boson decays almost instantly, a dark photon might travel a measurable distance before decaying. This creates a striking and distinguishable signature in the CMS detector. And finally there is ‘The Higgs Connection’ which makes dark photons interesting. CMS is particularly well-suited to look for dark photons through the Higgs portal. According to some models, the Higgs boson could decay into a photon and a dark photon or even a pair of dark photons. The PPC has investigated both options mentioned above: kinetic mixing and Higgs portal production mechanisms for the dark photon.
Several BSM predicted particles could give rise to resonant particle pair production, with dimuon being one of the most promising final states. In those predictions, a new particle emerges in the pp collision and subsequently decays into dimuon. One of the most exciting candidates is the dark photon, a new gauge field predicted by the dark matter theory. Another intriguing candidate is a pseudo-scalar particle predicted in the extended two-Higgs-doublet model. Recent searches have so far yielded null results for the new particles in the large mass value, interest has therefore grown in extending resonance searches to explore the phase space of lower masses. Our analysis thus broadens the exploration of dimuon resonances, now reaching scales as low as GeV.
Soft Unclustered Energy Patterns (SUEP)
As many simpler models and signatures of possible beyond Standard Model physics have been ruled out in recent searches at the LHC and beyond, models like Hidden Sectors have gained increasingly more attention. These models assume a dark sector that is only weakly coupled to our Standard Model sector via some ‘portal’ – a particle – perhaps a new one or perhaps something like our Higgs. This particular dark matter model assumes a QCD-like group in the dark sector that would generate ‘dark showers’, which promptly decay back to Standard Model particles, in a diffuse and isotropic ‘soft bomb’ that would be very hard to distinguish from typical QCD events. Our analysis thus seeks to identify very high multiplicity and the particular shape of the jets to gain sensitivity to possible SUEPs.
