New results from the LUX-ZEPLIN (LZ) experiment, accepted by Physical Review Letters, set the world’s most sensitive limits on light dark matter and deliver the first statistically significant detection of coherent neutrino-nucleus scattering from an astrophysical source: the Sun. UCL physicists played a leading role.
A mile underground in a former South Dakota gold mine, the LUX-ZEPLIN (LZ) experiment holds seven tonnes of ultra-pure liquid xenon, watched by hundreds of light sensors and shielded by 1,500 metres of rock and 230 tonnes of water. At its centre sits the quietest place on Earth: nowhere in the world offers a lower-background environment for catching such rare events. LZ needs exactly that, hunting signals so feeble that a whisper of natural radioactivity would drown them out.
Its quarry is dark matter, the invisible substance that outweighs ordinary matter five to one and holds galaxies together. The prime suspects, WIMPs, should very occasionally clip a xenon nucleus and produce a tiny flash of light. LZ already holds the world-leading limits for heavier WIMPs, from a search also led by UCL’s Dr. Amy Cottle. The new analysis pushes into far harder territory: particles of three to nine proton masses, whose collisions are gentler still. None appeared in data taken from 2023 to 2025, but the search sets the strongest constraints in the world on these light candidates, ruling out swathes of territory where they might have been hiding.
Something else did appear. Of the 19 events surviving the data selection, most carry the signature of neutrinos from boron-8 decays in the Sun’s core bouncing off entire xenon nuclei at once. Daniel Freedman predicted this process, coherent elastic neutrino-nucleus scattering, in 1974, doubting it would ever be seen: “Our suggestion may be an act of hubris, because the inevitable constraints of interaction rate, resolution, and background pose grave experimental difficulties for elastic neutrino-nucleus scattering.” Only in 2017 did the COHERENT experiment observe it at all, using an intense accelerator source. LZ has now detected it from an astrophysical source at 4.5 sigma significance; earlier hints elsewhere sat below 3 sigma.
These are the lowest-energy nuclear recoils ever measured, each depositing less energy than a single X-ray photon. The feat is enabled by LZ’s liquid xenon time projection chamber, a technology pioneered in the UK, and because the process registers all three neutrino flavours equally it opens a new window on the Sun and on neutrino physics: the team even measured the weak mixing angle, a fundamental parameter of the Standard Model, at among the lowest energy scales yet probed. The detection also marks the arrival of the long-predicted ‘neutrino fog’, in which experiments become so sensitive that solar, atmospheric and supernova neutrinos form part of the backgrounds.
UCL was central throughout. Dr. Amy Cottle is a lead author and enabled much of the science programme; Prof. Chamkaur Ghag was international spokesperson of the 250-strong collaboration during the data taking. Postdocs Aiham Al Musalhi and Josh Green optimised the background modelling and statistical inference, while PhD students Jacopo Siniscalco, Simran Dave and Issy Darlington delivered innovations in characterising the detector’s low-energy response and removing rare-topology backgrounds, at a level never before achieved.
LZ continues towards a 1,000-day exposure, chasing dark matter deeper into the fog. Half a century on, Freedman’s act of hubris looks more like an act of foresight.
“Searches for Light Dark Matter and Evidence of Coherent Elastic Neutrino-Nucleus Scattering of Solar Neutrinos with the LUX-ZEPLIN (LZ) Experiment”, the LZ Collaboration, accepted by Physical Review Letters. Preprint: arxiv.org/abs/2512.08065
More about dark matter research at UCL: UCL Dark Matter Searches