Fermilab’s MicroBooNE rules out existence of an elusive sterile neutrino

Imperial researchers, as part of the global MicroBooNE collaboration, challenge the existence of the once-believed sterile neutrino.

Neutrinos have mystified scientists for decades. Often referred to as ‘ghost particles’ due to their elusive nature, these elementary particles are abundant across the universe yet rarely interact with other matter, making them notoriously difficult to study.

Despite this, neutrinos are believed to hold the key to uncovering phenomena not fully explained by the Standard Model of particle physics, such as the matter-antimatter asymmetry of the Universe.

For years, anomalies in neutrino experiments have suggested the existence of a fourth type of “sterile” neutrino,  beyond the three known flavours: electron, muon and tau, which might be responsible for these discrepancies.  

However, results from the MicroBooNE experiment, an international endeavour involving over 150 scientists from 40 institutions including Imperial, strongly challenge this hypothesis. The findings show no evidence for the particle in the experimental data and have ruled  it out with 95% certainty.

This paper was published today in Nature.

Closing a decades long mystery

The Standard Model, while the most successful theory of particle physics to date, is incomplete. It explains three neutrino flavours and their ability to oscillate, changing from one to another as they travel.

However, past experiments such as LSND (Los Alamos, 1995) and MiniBooNE (Fermilab, 2002) observed anomalies that didn’t fit this picture.

These experiments hinted, for example, that muon neutrinos might transform into electron neutrinos over distances that couldn’t be explained by three flavours alone.

To account for these anomalies, physicists proposed the sterile neutrino, a particle even more elusive than its counterparts, interacting only with gravity. MicroBooNE was designed to put this hypothesis to the test.

The MicroBooNE experiment

Managed by Fermi National Accelerator Laboratory, MicroBooNE is part of Fermilab’s short-baseline neutrino programme, which uses cutting-edge liquid-argon time projection chambers (LArTPCs) to study neutrino interactions with world-leading precision.

Its 85-tonne detector began collecting data in 2015 and is notable for being the first to use a single detector to receive neutrinos from two beams simultaneously - the Booster Neutrino Beam (BNB) and the Neutrinos at the Main Injector (NuMI).

This enabled the capture of detailed 3D images of neutrino events within its liquid argon chamber as ionisation charges drift to wire planes under an electric field of 273V/cm.

This dual-beam approach significantly reduced uncertainties in MicroBooNE’s results. By observing millions of interactions, researchers were able to exclude almost the entire favoured region where sterile neutrinos were expected.

Nitish Nayak, a postdoctoral research associate at Brookhaven National Laboratory and collaborator on the MicroBooNE experiment, said “MicroBooNE is finally closing the chapter on one of the strongest explanations over the past few decades for those anomalies.”

These results allowing physicists to redirect their focus towards new physics that may uncover some of the universe’s deepest secrets.

Imperial’s involvement

Imperial researchers have played a key role in the operation and data analysis for MicroBooNE and have been integral members of the collaboration for over a decade.

“The MicroBooNE experiment has produced a treasure trove of data that allows us to probe models that go beyond testing the sterile neutrino hypothesis. Anyssa Navrer-Agasson Research Associate at Imperial College

MicroBooNE’s new findings reshape neutrino physics and redirect efforts towards other explanations for the anomalies. At the forefront of Imperial’s contributions is the MicroBooNE Astroparticles and Exotics group, led by Anyssa Navrer-Agasson. The Imperial team specialises in searching for new phenomena related to the dark sector by training AI to analyse complex data recorded by the MicroBooNE detector.  

These tools help distinguish potential dark matter signals from other known processes, accelerating the analysis and enabling researchers to test previously unexplored dark-sector models.

Anyssa Navrer-Agasson, Research Associate at Imperial College, said “the MicroBooNE experiment has produced a treasure trove of data that allows us to probe models that go beyond testing the sterile neutrino hypothesis.”

The Imperial group also focuses on further developing the techniques proven in MicroBooNE for the Deep Underground Neutrino Experiment (DUNE), a next-generation project aiming to probe neutrino behaviour on a larger scale. DUNE will use a much larger liquid-argon detector, located one-mile underground in South Dakota, to explore fundamental questions about matter-antimatter asymmetry and the origins of the universe. Pip Hamilton, Imperial College Research Fellow, coordinates the prototyping of the UK-built anode readout planes at the ProtoDUNE detector at CERN.

Professor Stefan Söldner-Rembold from Imperial College, who previously served as Spokesperson of the international DUNE Collaboration, emphasised that  ‘MicroBooNE has demonstrated the excellent performance of the liquid-argon technology, which will be employed by DUNE at an unprecedented scale . At Imperial College, we play a leading role in the design and construction of DUNE.’

Why this matters

Ruling out the sterile neutrino closes one of the most compelling explanations for anomalies observed over the past 30 years. While this result strengthens the Standard Model, the mystery of the anomalies still remains.

This result used only 60% of MicroBooNE’s total data set, and scientists are now beginning to explore the remaining data.

“The MicroBooNE data might still contain many surprises – as the sterile neutrino explanation of the observed anomalies is now very unlikely, we are now turning towards other new physics scenarios.” said Professor Soldner-Rembold.

The international MicroBooNE collaboration is hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. The collaboration consists of 193 scientists from 40 institutions including national labs and universities from six countries.