Updated: Oct 30, 2021

Since the early 2000's physicists have been continuously perplexed by the idea of neutrinos. A neutrino is a subatomic particle that has no charge and a very small mass. They are one of the most abundant particles in the universe, however, because they do not interact with matter, they are very difficult to detect. Neutrinos, like electrons, are not affected by the strong nuclear force and are therefore termed leptons.

A component of MicroBooNE’s time projection chamber being installed. Credit: Reidar Hahn, Fermilab

Neutrinos are known to be able to switch 'flavours'(electron-neutrino, muon-neutrino and tau-neutrino), and physicists have theorised that there may be a fourth flavour(the sterile neutrino) that would allow the Standard Model verified. Currently 'flavour-flipping' cannot be fully explained in the Standard Model without the idea of a sterile neutrino. Sterile neutrinos differ from other neutrinos as, unlike the other three, they would only interact via gravity and not the other fundamental forces. This theory was promising was it could have potentially explained several anomalies in the current theory and also give hints to the mystery of dark matter. Because they only interact with gravity, an experiment was built called the Micro Booster Neutrino Experiment (Microboone), which involved almost 200 scientists from 5 different countries. It weighs 150 tonnes and is based in Illinois. It began taking data at the end of 2015. The data that was collected disproved the theory of a fourth flavour, with the team saying that there "is no hint of the sterile neutrino".

Part of the MicroBooNE neutrino experiment. Credit: Cindy Arnold

Justin Evans, a spokesperson for Microboone from the University of Manchester said, “I would have really expected to see an excess in either the photons or the electrons and we haven’t seen one in either, which opens up more questions.” Currently, there is no way to explain the results which opens up the prospect of a new chapter of physics theories that could tie into account neutrino flavour-switching without a fourth flavour. Dr Sam Zeller from Fermilab says that the non-detection doesn't contradict previous findings stating, "The earlier data doesn't lie, There's something really interesting happening that we still need to explain. Data is steering us away from the likely explanations and pointing toward something more complex and interesting, which is really exciting."

The Fermi National Accelerator Laboratory, or Fermilab. Credit:

The big question is now, is the hunt for the sterile neutrino over? “We definitely are not seeing any evidence for a sterile neutrino,” says Evans. “As to whether it is completely dead, I think that depends who you ask, but you’ve certainly got to get a lot more inventive to get a sterile neutrino into your particle physics models now.” The exciting prospect of a new chapter of physics is interesting to anyone, physicist and not alike. Let me know if you have any ideas about possible theories in the comments below and if you liked this blog. :)

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