Teams prepare to move the MicroBooNE cryostat from DZero to the Liquid Argon Test Facility (LArTF).

Results from a global science experiment have cast doubt on the existence of a theoretical particle beyond the Standard Model.

The results were gathered by an international team at the MicroBooNE experiment in the United States, with leadership from a UK team including researchers from the University of Cambridge.

The two most likely explanations for anomalies that were seen in two previous physics experiments: one which suggests a sterile neutrino, and one which points at limitations in those experiments, have been ruled out by MicroBooNE. 

The fourth neutrino

For more than two decades, this proposed fourth neutrino has remained a promising explanation for anomalies seen in earlier physics experiments. In these previous experiments, neutrinos were observed acting in a way not explained by the Standard Model of Physics – the leading theory to explain the building blocks of the universe and everything in it.

Neutrinos are the most abundant particle with mass in our universe, but they rarely interact with other matter, making them hard to study. But these elusive particles seem to hold answers to some of the biggest questions in physics – such as why the universe is made up of more matter than antimatter.

A 170-ton neutrino detector the size of a bus was created to study these particles – and became known as MicroBooNE. The international experiment has close to 200 collaborators from 36 institutions in five countries, and is supported by the Science and Technology Facilities Council (STFC) in the UK.

Standard Model holds up

The team used cutting-edge technology to record precise 3D images of neutrino events and examine particle interactions in detail. Four complementary analyses released by the international MicroBooNE collaboration, at the Fermi National Accelerator Laboratory (Fermilab), deal a blow to the fourth neutrino hypothesis.

All four analyses show no sign of the sterile neutrino, and instead the results align with the Standard Model. The data is consistent with what the Standard Model predicts: three kinds of neutrinos only. But the anomalies are real and still need to be explained. Crucially, MicroBooNE has also ruled out the most likely explanation to explain these anomalies without requiring new physics. 

These results mark a turning point in neutrino research. With the evidence for sterile neutrinos becoming weaker, scientists are investigating other possibilities for anomalies in perceived neutrino behaviour.

“This result is incredibly exciting as it suggests something far more interesting than we expected is happening – it’s now our goal to find out what this could be,” said Dr Melissa Uchida, who leads the Neutrino Group at Cambridge’s Cavendish Laboratory.

“This heralds the start of a new era of precision for neutrino physics, in which we will deepen our understanding of how the neutrino interacts, how it impacted the evolution of the universe, and what it can reveal to us about physics beyond our current Standard Model of how the universe behaves at the most fundamental level,” said Professor Justin Evans from the University of Manchester, co-spokesperson of the experiment.

“Cambridge has played an integral part in this experiment both through the software — the reconstruction algorithms that allow us to distinguish particles and their interactions in MicroBooNE and through the analysis itself,” said Uchida. “With half the data still to analyse and more exotic avenues to pursue, there is an exciting journey ahead.”

The UK at MicroBooNE

The UK has taken a leading role in MicroBooNE, leading the development of state-of-the-art pattern recognition algorithms, making world-leading contributions to the understanding of neutrino interactions in the argon, and bringing a broad range of expertise to these searches for the elusive sterile neutrinos.

UK universities involved in MicroBooNE are Manchester, Edinburgh, Cambridge, Lancaster, Warwick and Oxford.

Mission to understand neutrinos

With our understanding of neutrinos still incomplete, the UK through STFC has invested in a science programme to address these key science questions, as well as invest in new technologies.

The UK government has already invested £79 million in the Deep Underground Neutrino Experiment, Long-Baseline Neutrino Facility (LBNF), and the new PIP-II accelerator, all hosted by Fermilab.

This investment has given UK scientists and engineers the chance to take leading roles in the management and development of the DUNE far detector, the LBNF neutrino beam targetry and PIP-II accelerator.

Professor Mark Thomson, Executive Chair of STFC and one of the first UK physicists to join MicroBooNE, said: “This much-awaited result is a significant step our understanding of neutrinos. This extremely challenging measurement is also important in that the MicroBooNE experiment used a new technology to record detailed images of individual neutrino interactions.

“The successful use the liquid argon imaging technology is a major stepping stone towards DUNE.

“Once complete by the end of this decade, DUNE will use several detectors each of the size of an extra-deep Olympic swimming pool, but with liquid argon replacing the water, to measure the movements and behaviours of neutrinos.”

Adapted from an STFC press release

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