The CMS experiment at CERN is pursuing a novel approach to dark matter detection, targeting an unusual signature: two low-momentum electrons appearing some distance from proton-proton collisions. This search focuses on a theory proposing a partner state that allows for delayed decay and potential visibility, addressing the reason “no existing experiment has been able to observe dark matter directly.” “It’s entirely possible that the universe is full of this kind of dark matter, but it just doesn’t have enough energy to interact with our experiments,” explains Bryan Cardwell, a postdoc at the University of Virginia, as the experiment also looks for accompanying missing transverse momentum, a key indicator of invisible particles carrying energy. Researchers are employing a specialized reconstruction method, originally designed for B hadron decays, to identify these faint electron signals and expand the search for particles beyond the Standard Model.
CMS Targets Inelastic Dark Matter via Displaced Electron Pairs
This specialized algorithm, capable of detecting electrons with transverse momenta as low as 1 GeV, significantly lower than the 5 GeV threshold of standard methods, recovers events otherwise missed by conventional dark matter searches. Kyungmin Park, a doctoral student at Carnegie Mellon University, explains, “One of the interesting aspects of this search is that a reconstruction technique originally designed for a different physics context turns out to be very powerful for this dark-matter signature.” The ability to resolve these displaced electrons is key to distinguishing the signal from background noise.
A distinctive signature of this search is the presence of two low-momentum electrons accompanied by missing transverse momentum, indicating that invisible particles are carrying away energy. This combination is predicted by theories of inelastic dark matter, where a dark-matter particle interacts with its partner state before decaying.
Bryan Cardwell, a postdoc at the University of Virginia, highlights the significance of this approach, stating, “It makes the search for inelastic dark matter at the LHC particularly exciting – producing it directly in LHC collisions may give us a chance to see something that would otherwise be invisible.” The team’s analysis focuses on scenarios where the dark matter particle travels a measurable distance before decaying, allowing for the identification of this unique electron signature. Machine-learning techniques further refine the search by distinguishing potential signal events from Standard Model backgrounds, identifying the specific pattern of two closely spaced electrons emerging away from the collision point. Very few background events are expected to exhibit this topology, enhancing the sensitivity of the experiment.
While the initial data analysis has not revealed any significant excess of events beyond those predicted by known physics, the CMS collaboration has established the first collider limits on inelastic dark matter in the electron channel. These limits constrain theoretical models with small mass differences and measurable displacements, narrowing the range of possible dark matter candidates. This search builds upon a previous CMS investigation utilizing muons, expanding the exploration of inelastic dark matter into regions uniquely accessible at colliders and potentially undetectable by other experiments.
The analysis, detailed in the CMS Physics Analysis Summary “Search for inelastic dark matter with low-momentum displaced electrons in proton-proton collisions at 13 TeV”, demonstrates the power of repurposing established techniques for new physics searches. “It allows us to identify low-momentum electrons that originate far from the collision point – a particularly challenging combination to reconstruct in the detector – and search for this signature that would otherwise be inaccessible,” Park adds.
With the larger dataset anticipated from Run 3 of the LHC, future analyses promise to probe even lower dark matter masses and greater displacement scenarios in both the electron and muon channels, extending the boundaries of dark matter exploration. The team anticipates that these expanded searches will further refine the constraints on inelastic dark matter models, potentially revealing the elusive nature of this mysterious substance.
The ongoing work represents a novel approach to a long-standing problem in particle physics, and it uses the unique capabilities of the CMS detector and advanced reconstruction techniques to unveil the secrets of the dark universe.
It’s entirely possible that the universe is full of this kind of dark matter, but it just doesn’t have enough energy to interact with our experiments.
Bryan Cardwell, postdoc at the University of Virginia
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