Editor’s Note: This article was published as part of the inaugural edition of The Commonwealth Times and reflects events as reported at the time of the referenced news coverage.
Of all the particles that thread the fabric of the observable universe, none is so prolific and yet so profoundly elusive as the neutrino — a particle so vanishingly reluctant to interact with matter that tens of trillions of them pass through every human body each second without disturbing a single atom. To detect such a particle at all is an achievement of considerable ingenuity. To detect it at the energies produced by the most powerful accelerator ever constructed by human hands is something more: it is the opening of a door that physics has long known existed but could never before unlock. That door has now been opened. Scientists from the FASER collaboration at CERN have published new results confirming the detection and measurement of high-energy neutrinos generated in proton-proton collisions at the Large Hadron Collider, results that carry implications not only for neutrino physics but for the broader enterprise of understanding the fundamental forces governing nature.
FASER — the Forward Search Experiment — occupies a singular position in the geography of the LHC complex. Situated some 480 meters downstream from the ATLAS interaction point, in a disused service tunnel beneath the Franco-Swiss border, the detector was designed to intercept particles that travel along the beam collision axis, a forward region that the large general-purpose detectors like ATLAS and CMS were never built to observe. The collaboration, led by University of California, Irvine physicist Jamie Boyd and conceived in large part by theorist Jonathan Feng, also of UC Irvine, recognized that this neglected corridor of particle trajectories represented an untapped scientific resource. Among the particles streaming through this forward region are neutrinos of all three known flavors — electron, muon, and tau — produced at energies ranging from hundreds of giga-electron-volts to several tera-electron-volts, a regime that has remained almost entirely unexplored.
The collaboration first announced the detection of collider neutrinos in 2023, a result that confirmed the basic viability of the experimental approach. The new results substantially deepen that initial finding. Using the FASERν sub-detector — a compact emulsion detector composed of interleaved tungsten plates and nuclear emulsion films, a technology descended from the photographic techniques that helped discover the pion in the 1940s — the team has now measured neutrino interaction cross-sections at energies that bridge a critical gap between accelerator-based experiments and observations of cosmic-ray neutrinos. These measurements populate a region of the energy-cross-section parameter space that had, until now, contained no data points whatsoever.
The significance of this achievement extends well beyond the filling of a gap on a graph. Neutrino cross-sections at these energies bear directly on our understanding of quantum chromodynamics, the theory governing the strong nuclear force that binds quarks within protons and neutrons. The neutrinos detected by FASER are produced primarily through the decay of charm mesons and other heavy hadrons generated in the forward scattering of LHC collisions — a process that is exquisitely sensitive to the distribution of gluons within the proton at very small values of the momentum fraction known as Bjorken-x. Precise measurements of these neutrino fluxes and their interaction rates thus serve as an indirect but powerful probe of proton structure in a kinematic regime where existing knowledge is poor and where discrepancies between theoretical models are largest.
The implications ripple outward into astrophysics with equal force. The IceCube Neutrino Observatory at the South Pole routinely detects neutrinos from distant cosmic sources — active galactic nuclei, gamma-ray bursts, and other cataclysms of the high-energy universe — at energies overlapping with those now measured by FASER. Yet the interpretation of every IceCube observation depends on knowledge of how neutrinos interact with matter, knowledge that until FASER’s results rested on theoretical extrapolations rather than direct measurement. The FASER data thus furnish an empirical foundation beneath an entire field of multi-messenger astronomy.
It is worth pausing to consider the material modesty of the instrument that has accomplished this. The FASER detector, in its entirety, cost approximately two million dollars — a figure that would scarcely cover the catering budget of a major LHC experiment. Its emulsion films, once exposed, are developed and scanned at partner institutions including laboratories in Bern, Naples, and Japan, using automated microscopes that trace the sub-micrometer tracks left by charged particles traversing the tungsten absorbers. This technique, at once ancient in its photographic lineage and modern in its computational analysis, has proven capable of distinguishing between the three neutrino flavors, including the notoriously difficult tau neutrino, with a clarity that electronic detectors alone could not match at this scale.
The detection of tau neutrinos in the forward region of the LHC is itself a matter of particular interest. Prior to FASER, the total number of tau neutrino interactions ever directly observed by any experiment in the history of physics could be counted in the single digits — the DONuT experiment at Fermilab reported nine candidate events in the year 2000. FASER’s growing sample of tau neutrino candidates promises to multiply the world’s dataset of these interactions many times over by the conclusion of the current LHC run, transforming tau neutrino physics from a statistical curiosity into a field amenable to serious quantitative study.
The collaboration’s published results also carry implications for the search for physics beyond the Standard Model. FASER was designed from its inception with a dual mandate: to detect neutrinos, and to search for new, long-lived, weakly interacting particles — so-called dark photons, axion-like particles, and other hypothetical objects — that might be produced in LHC collisions and travel the full 480 meters to the detector before decaying into visible particles. While no such exotic particles have been observed to date, the neutrino measurements serve to validate the detector’s performance and constrain backgrounds in ways that sharpen the sensitivity of ongoing searches.
Looking forward, CERN has approved an upgraded successor experiment, FASER2, as part of the proposed Forward Physics Facility that would accompany the High-Luminosity LHC era beginning in the latter half of this decade. FASER2, with a substantially larger detector volume and improved instrumentation, is projected to collect tens of thousands of neutrino interactions at these extreme energies, enabling precision measurements of a quality that would have seemed fantastical even five years ago.
What FASER has demonstrated, beneath the Alpine meadows through which the Rhône begins its journey to the sea, is that great discoveries need not require great expenditures — that a small, sharply conceived experiment, positioned with care at the right place in the architecture of an existing machine, can illuminate corners of nature that the largest instruments were never designed to reach. In the long tradition of particle physics, where every generation’s certainty becomes the next generation’s approximation, the ghost particle has once more reminded us that the universe discloses its structure only to those patient and clever enough to look where no one thought to look before.