Old CERN Data Yields New Hunt for Ghostly Dark Matter Particles

Deep in the archives of CERN, a treasure trove of fifteen-year-old data has just been given a second life in the hunt for one of physics’ most elusive quarries. In a study published in The European Physical Journal C, physicist Mehran Dehpour of Charles University in Prague has reinterpreted data collected by the COMPASS experiment in 2009 to search for axion-like particles, or ALPs — hypothetical lightweight particles that rank among the most compelling candidates for dark matter and possible messengers from a hidden dark sector. The clever twist is that no new experiment was needed at all. Instead, the analysis exploits a subtle loophole in how the detector sees the world: under the right conditions, a decaying exotic particle can disguise itself as an ordinary photon, hiding in plain sight within data originally gathered for an entirely different purpose.

The theoretical backdrop to this search is one of the deepest puzzles in modern physics. The Standard Model of particle physics describes the known fundamental particles and forces with astonishing precision, yet it harbors an embarrassing flaw in its description of the strong nuclear force. The theory naturally permits a term that violates charge-parity symmetry, which should endow the neutron with a measurable electric dipole moment. Experiments have searched for such a moment and found nothing, constraining the offending term to a value unnaturally close to zero — a fine-tuning spanning nearly ten orders of magnitude. The celebrated solution, proposed by Roberto Peccei and Helen Quinn in 1977 and elaborated by Frank Wilczek and Steven Weinberg, postulates a new symmetry whose spontaneous breaking gives rise to a ghostly particle called the axion. But the story does not end there: extensions of the Standard Model, including string theory and supersymmetry, predict a whole zoo of axion-like particles with masses and couplings far freer than the original QCD axion allows, making them prime targets for experimental searches across an enormous range of energies.

COMPASS, mounted on the M2 beam line of CERN’s Super Proton Synchrotron, was not built to hunt for dark matter. Its 2009 campaign fired beams of negatively charged pions and muons, each carrying a hefty 190 GeV of energy, at a thin fixed target made of nickel. The original goal was a precision measurement of the charged pion polarizability — a fundamental property describing how the pion’s internal charge distribution deforms in an electric field — through a process called Primakoff Compton scattering, in which the beam particle radiates a single photon in the intense electromagnetic field of the nucleus. A parallel run with muons, which are point-like particles with no internal structure, served as a control measurement to validate the method against the exactly calculable predictions of quantum electrodynamics. The choice of a heavy nickel target was deliberate: the electromagnetic interaction scales with the square of the nuclear charge, coherently amplifying the signal while suppressing messy hadronic background processes.

The new analysis asks a provocative question: what if, among those seemingly ordinary single photons, some were actually impostors? If ALPs exist and couple to photons, they can be produced through the same Primakoff mechanism, with the beam particle emitting a virtual photon that converts into an ALP in the nuclear field. The ALP would then decay into a pair of real photons. But here the extreme beam energy becomes decisive. An ALP in the MeV mass range produced by a 190 GeV beam carries an enormous Lorentz boost, meaning its two decay photons are hurled forward in a tightly collimated jet. The electromagnetic calorimeter ECAL2, situated roughly 34 meters downstream of the target and built from lead-glass cells 3.8 centimeters across, simply cannot resolve two photon showers that arrive almost on top of each other. The pair merges into a single reconstructed cluster — indistinguishable, at first glance, from the lone photon of ordinary Compton scattering.

Quantifying exactly when this masquerade succeeds required a careful analytical model of the detector’s response. The study identifies two distinct scenarios in which an ALP decay slips through the single-photon selection. In the first, the decay is highly asymmetric: boosted kinematics can shift one photon’s energy below the calorimeter’s 2 GeV reconstruction threshold, leaving only its energetic partner to be registered. Because the scalar ALP decays isotropically in its rest frame, the probability of such asymmetric configurations can be computed exactly. In the second scenario, both photons are detectable but geometrically merge, which happens when their transverse separation at the calorimeter face falls within a single cell. A dedicated Monte Carlo simulation of the ECAL2 grid, projecting photon pairs at characteristic separations, yielded a smooth sigmoid-shaped merging probability as a function of the dimensionless separation. On top of these factors, the analysis corrects for a further subtlety: a single photon and a photon pair suffer different amounts of material attenuation from pair conversion as they traverse the nickel target and spectrometer, a correction that mildly penalizes promptly decaying ALPs and slightly favors long-lived ones.

To turn this machinery into actual limits, the study compares the observed event yields — published by COMPASS as ratios of data to simulation as a function of the photon’s energy fraction — with theoretical predictions that now include a possible ALP contribution. A profile likelihood approach, complete with a nuisance parameter that accounts for fully correlated systematic uncertainties, then scans the plane spanned by ALP mass and its coupling to photons. For each mass, the coupling is varied until the deviation from the Standard Model hypothesis crosses the statistical threshold corresponding to 95 percent confidence. The result: couplings larger than roughly 0.1 inverse GeV are excluded for ALP masses between about 0.2 and 600 MeV, carving out a swath of parameter space that sits squarely between the territory of low-mass beam dump experiments and that of high-energy colliders.

The shape of the excluded region tells a physically rich story. At low masses, ALPs become long-lived: their decay length grows so large that most of them sail clean through the detector without decaying at all, and the signal collapses exponentially. The boundary where the decay probability drops below one percent follows a simple contour in the coupling-mass plane. At the other extreme, for masses above roughly 30 MeV, the opening angle between the decay photons grows until geometrical merging becomes hopeless — yet the asymmetric-decay channel survives, sustaining a small but nonzero acceptance. This residual effect produces a distinctive high-mass tail in the pion-beam limits, stretching sensitivity all the way up to about 600 MeV, albeit at stronger couplings. Intriguingly, the pion beam outperforms the muon beam despite comparable statistics, for a beautifully simple reason: the Standard Model bremsstrahlung background scales inversely with the square of the beam particle’s mass, so the heavier pion suppresses the background and lets any ALP deviation shine through more clearly.

The work also highlights a genuine experimental advantage over competing approaches. Photoproduction experiments such as PrimEx and GlueX rely on real photon beams, which makes spotting merged diphoton signatures brutally difficult — the non-interacting beam itself floods the detector with background. COMPASS, by contrast, used charged beams, and the detection of the scattered charged particle provides a clean coincidence tag for the interaction vertex, naturally suppressing beam-related backgrounds and making the single-cluster search feasible. When the original COMPASS data were published, the mass region probed here was largely unexplored; subsequent reinterpretations of LEP data have since constrained it, but the new limits stand as an independent check built on a completely different production mechanism.

Looking ahead, the framework is anything but a one-off. The same single-cluster analysis can be applied directly to the much larger COMPASS dataset collected in 2012 with an identical setup, and the method carries forward to COMPASS’s successor, the AMBER experiment, whose planned measurement of the kaon polarizability with a 100 GeV kaon beam would modify the Standard Model background in ways that open fresh territory in the ALP parameter space. Meanwhile, an ongoing COMPASS analysis of Primakoff neutral pion production, which explicitly reconstructs resolved photon pairs, could extend sensitivity to higher masses where merging is no longer a constraint. What began as a calibration dataset for pion polarizability may yet have more secrets to surrender — proof that in particle physics, yesterday’s data can illuminate tomorrow’s dark matter.

Subject of Research: Search for axion-like particles via Primakoff production using reinterpreted COMPASS beam data

Article Title: Search for axion like particles produced via the Primakoff process at COMPASS

Article References: Dehpour, M. (2026). Search for axion like particles produced via the Primakoff process at COMPASS. The European Physical Journal C, 86(9), Article 1033. https://doi.org/10.1140/epjc/s10052-026-16296-6

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16296-6

Keywords: axion-like particles, dark matter, COMPASS experiment, CERN, Primakoff process, particle physics, photon coupling, electromagnetic calorimeter, pion polarizability, beam dump experiments, AMBER experiment, Standard Model