The NA64 experiment at CERN has searched for dark matter since 2016 by looking for energy that goes missing when particles strike its detector
Written by:
Davide De Biasio
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In particle physics, energy is never lost. When particles collide or decay, all the initial energy they carry must be conserved in the resulting products, and a shortfall signals that something has escaped unseen. It was the energy missing from radioactive decays that led Wolfgang Pauli to propose the neutrino in 1930, a particle so elusive that it evaded detection for a quarter of a century.
Similar approaches are now employed in searches for physics beyond the Standard Model. Evidence ranging from the afterglow of the Big Bang to the way galaxies formed and clustered, for instance, indicates that around 85% of the matter in the Universe emits no light, revealing itself only through gravity. For decades, the leading candidate for this dark matter was a new heavy particle, roughly a hundred times more massive than the proton. Increasingly sensitive experiments have searched for it. So far, without success.
However, the absence of a signal may simply suggest that dark matter is lighter and interacts more weakly than first assumed. A particle of this kind could naturally fit into a broader dark sector – a hidden family of new particles and forces, in contact with known matter only through a handful of feeble interactions. These interactions would, on rare occasions, allow collisions of ordinary particles to produce dark matter particles.
Dark matter particles of this kind are what NA64 has spent the past ten years looking for. The experiment sits in CERN’s North Area, where 400 GeV protons from the Super Proton Synchrotron hit a beryllium target, producing a spray of secondary particles that are sorted into different beams. The required type of beam is then selected and guided to NA64. On the way in, each particle’s energy is precisely measured before it strikes the detector. If the energy recorded after the interaction falls short of that of the incoming particle, the difference must have been carried off by something the detector cannot see. Perhaps by a dark matter particle.
For most of the experiment’s operation, the probes have been electrons, and their collisions have revealed no missing energy beyond what known processes can account for. However, the measurements have placed an upper limit on the strength of the coupling between the dark sector and ordinary matter, which sets how often dark matter particles should have been produced. For some of the lightest candidates, no other experiment has pushed this boundary further. The programme has since grown beyond electrons, taking in positrons (the electrons’ antimatter partners), muons (their heavier cousins) and hadrons, to test different dark matter models.
Since dark matter particles would be produced only rarely, the next step in this endeavour is to increase the number of collisions. During the third long shutdown of CERN’s accelerator complex, the NA64 Collaboration will upgrade its apparatus to run with more intense beams, while making sure that no ordinary energy slips through unmeasured and mimics a signal. The dataset is then expected to grow by up to two orders of magnitude, enough to test a wide range of scenarios for light dark matter.
Find out more in the article “Ten years at the missing-energy frontier” in the July/August issue of the CERN Courier.
Facts Only
* The NA64 experiment searches for dark matter since 2016.
* The search looks for energy that is missing when particles strike the detector.
* Energy conservation dictates that a shortfall signals something escaped detection.
* The neutrino was proposed due to missing energy from radioactive decays.
* Evidence from cosmology suggests about 85% of the Universe's matter emits no light, revealed by gravity.
* Dark matter is currently considered a heavy particle roughly a hundred times more massive than the proton.
* Searches for dark matter have been conducted without success so far.
* The experiment uses 400 GeV protons hitting a beryllium target at CERN's North Area.
* Energy is precisely measured for each particle before striking the detector.
* Measurements set an upper limit on the coupling strength between the dark sector and ordinary matter.
* The program has expanded to test different models by using electrons, positrons, muons, and hadrons.
* The plan involves upgrading the apparatus during a shutdown to run with more intense beams.
Executive Summary
The NA64 experiment at CERN searches for dark matter by looking for missing energy when particles interact with a detector. The principle relies on the conservation of energy: if the resulting energy is less than expected from known processes, the deficit suggests an unseen particle has carried energy away. This search mirrors historical efforts, such as the discovery of the neutrino, where missing energy pointed to new physics.
The experiment involves directing 400 GeV protons into a beryllium target at CERN's North Area. The energy of incoming particles is measured before they strike the detector. A discrepancy between the expected and recorded energy points toward potential dark matter interaction. Initially, electron probes yielded no missing energy beyond known processes. The measurements have established an upper limit on the coupling strength between the dark sector and ordinary matter, constraining how often dark matter particles should be produced.
The experimental program has since expanded to include positrons, muons, and hadrons to test various dark matter models. Future plans involve increasing beam intensity during a shutdown to gather more data, aiming to increase the dataset by up to two orders of magnitude to test scenarios for light dark matter.
Full Take
The narrative frames the search for dark matter as an exercise in fundamental conservation laws extended beyond the Standard Model, effectively positioning the missing energy as evidence for physics beyond current understanding. The progression from the neutrino to dark matter mirrors a persistent pattern: when a theoretical particle is elusive (e.g., neutrino), it requires experimental searches based on conservation principles.
A critical tension exists between the observed absence of signals and the theoretical possibility that dark matter interacts only weakly, inhabiting a hidden sector. This introduces an epistemic gap: the lack of signal might reflect unknown properties rather than non-existence. The focus shifts from confirming a specific particle to constraining interaction strengths with known matter. The continuous expansion across particle types (electrons to hadrons) demonstrates a necessary, iterative approach driven by limitations in existing constraints.
The call to increase beam intensity during shutdown is a pragmatic response to the current limits; it suggests that the next phase of knowledge depends on pushing the boundary defined by current coupling limits rather than simply waiting for a new discovery. The underlying assumption, which requires scrutiny, is that if dark matter exists, its interaction strength must be severely limited for it to remain undetected across so many experimental avenues, demanding a re-evaluation of what constitutes "allowed" physics within this broader dark sector.
Bridge Questions: What are the specific theoretical boundaries that define "weak interaction" in the context of dark sector coupling? How does the known limits on electron/positron interactions constrain the search space for hadronic dark matter candidates? If no signal is found, what constitutes a meaningful constraint on the existence or properties of light dark matter particles?
Sentinel — Human
The text reads like high-quality journalistic science reporting, demonstrating a sophisticated synthesis of physics history and current experimental methodology rather than simple data regurgitation.