[Experiments cannot confirm what theory predicts about neutrinos.
And particle physicists have no idea why. ]
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NEUTRINOS: THE “GHOSTLY CHAMELEONS” OF PARTICLE PHYSICS BECOME
EVEN MORE MYSTERIOUS  
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Don Lincoln
June 26, 2022
Big Think [[link removed]]

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_ Experiments cannot confirm what theory predicts about neutrinos.
And particle physicists have no idea why. _

Beautiful Neutrino drawn on paper, Wikememeia

 

KEY TAKEAWAYS

* Neutrinos are the ghosts of the physics world because they interact
so little with matter, easily passing through entire planets.  
* They also can oscillate between three different forms, known as
the electron neutrino, muon neutrino, and tau neutrino. 
* Current research in this area is extremely confusing, as is often
the case at the scientific frontier.

Neutrinos have long been one of the most perplexing and mysterious of
all the known subatomic particles — but they just got more
confusing, after two groups of scientists announced contradictory
results. 

Neutrinos are like ghostly chameleons

Neutrinos are often said to be the ghosts of the physics world
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they interact so little with matter, easily passing through entire
planets. But they are also quantum chameleons, changing their
identity, essentially as if a Chevrolet could change into a Ford, then
change into a Chrysler, before changing back into the original car.
The three distinct types of neutrinos are called the electron
neutrino, muon neutrino, and tau neutrino.

This subatomic switcheroo, called neutrino oscillation, is unique in
nature. And it has been observed in many different settings, including
in neutrinos generated from nuclear reactors, from high-energy
particle beams, and when cosmic rays from space slam into the
Earth’s atmosphere.

A neutrino in the ointment

The leading theory of subatomic physics, known as the Standard Model,
both accommodates neutrino oscillation and sets out a series of
universal parameters that govern the rate at which the various
neutrino types can transform into one another. Neutrino experiments
can then determine the actual value of the parameters. Assuming the
theory is correct, and the experiments are accurate, all experiments
should determine the same values for those parameters. Indeed, that is
how it worked out for many experiments.

However, in the 1990s, the Liquid Scintillator Neutrino Experiment
(LSND), located at Los Alamos National Laboratory in New Mexico, found
a fly in the ointment. LSND used a beam of muon neutrinos to look for
instances in which they had transformed into electron neutrinos. As
expected, the scientists observed
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neutrinos, but at a rate that was hard to reconcile with established
theory.

Interestingly, they _could_ reconcile their measurement with all
other measurements if there existed a fourth (yet undiscovered) type
of neutrino. This type of neutrino would differ from all the other
neutrinos. The known neutrinos interact via the weak nuclear force,
while the fourth type would not. Because of this non-interaction,
scientists have taken to calling this hypothetical fourth type of
neutrino “sterile neutrinos.”

The existence or non-existence of sterile neutrinos has been the topic
of fierce debate among particle physicists, with most experiments
seeming to rule out the existence of a fourth neutrino, while a few
supported the conjecture. But there was always the LSND elephant in
the room.

Confusion at the frontier of neutrino research

So, what is the answer? There are two possibilities. The first, and
least exciting, is that the LSND result was just wrong. This does not
imply any ineptitude by the experimenters. Sometimes rare things
happen, and sometimes things are overlooked or modeled incorrectly. In
what might well be an epic understatement, science at the frontier is
hard. On the other hand, if the disagreement between the predictions
and the various experiments arises because the current theory is
incomplete, then scientists will have to develop a new theory.
Obviously, this is more exciting, and sterile neutrinos are merely the
most popular possibility.

To resolve whether the LSND result is the first harbinger of new
physics, or just an unfortunate fluke, it is necessary to replicate
the experiment to see if the second experiment confirms LSND or not.
To that end, the MiniBooNE
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at Fermilab in Illinois was designed and performed. Like LSND,
MiniBooNE also used a muon neutrino beam, looking for an unexplained
excess of electron neutrinos. Furthermore, MiniBooNE was a newer and
more modern experiment, with improved capabilities. If the LSND
observation was correct, MiniBooNE would have confirmed it. However,
that was not the case. MiniBooNE didn’t see the same excess that
LSND did.

Ordinarily, this would be the end of the story, with the LSND
measurement being chalked up as a preliminary one that just turned out
to be wrong. This comforting conclusion would mean that the current
theoretical framework was enough, and scientists could be content that
they understood neutrino interactions. But, the history of neutrino
research is full of surprises and is rarely without a mystery or two.

While MiniBooNE ruled out the LSND observation, the new experiment
discovered a different anomaly. Because of the MiniBooNE
experiment’s enhanced capabilities, they could look for neutrinos in
more ways than LSND could. When MiniBooNE looked for electron
neutrinos in an energy range not explored by LSND, they found more
electron neutrinos than the accepted theory could explain. And, like
the LSND anomaly before it, the new MiniBooNE anomaly could be
explained by the existence of a sterile neutrino (or possibly more
than one). Though it would be different than the sterile neutrino
proposed to explain the LSND anomaly, it was still sterile.

Given that anomalies are often the first sign of discoveries,
researchers couldn’t just give up. It was imperative that
a _third_ experiment be done, hopefully one that could definitively
determine if the current theory, with its three known types of
neutrinos, is sufficient or not.

Thus, the MicroBooNE [[link removed]] experiment was
created. This experiment uses far more sophisticated detector
technology and should provide the definitive word on the MiniBooNE
anomaly. In addition, there was one remaining loophole in the
LSND/MiniBooNE comparison: They used different beams of neutrinos.
Maybe there were subtle differences in the beams that hadn’t been
properly considered. But in comparisons between MicroBooNE and
MiniBooNE measurements, there would be none of that. The two
experiments used the exact same beam line, simplifying comparisons
between the two experiments. MicroBooNE should be a definitive check
on MiniBooNE.

An unsolved mystery

How does it all end? MicroBooNE doesn’t detect the same excess
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MicroBooNE paper doesn’t conclude that MiniBooNE was wrong; it
merely says that the MiniBooNE anomaly remains unexplained
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that the two experiments are performed by many of the same scientists.

To this day, the situation remains unresolved — an unsatisfactory
state of affairs, to be sure, but this is often the case in frontier
scientific research, where the more confusing the mystery, the more
interesting it is to researchers. And neutrinos are always confusing.

DR. DON LINCOLN is a Senior Scientist at Fermilab, America’s leading
particle physics laboratory, who has coauthored over 1,500 scientific
papers
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He was a member of the teams that discovered the top quark in 1995 and
the Higgs boson in 2012.

Dr. Lincoln is also an avid popularizer of science. He has
written several books
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most recently _The Large Hadron Collider
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He also writes for many online venues, such as CNN and _Scientific
American_. He appears frequently on the Fermilab YouTube channel
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available through The Great Courses company
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Dr. Lincoln is a recipient of the 2013 Outreach Prize from the
European Physical Society and the 2017 Gemant Award from the American
Institute of Physics. He is a Fellow of the American Physical Society
and the American Association for the Advancement of Science.

You can learn more about Dr. Lincoln on his home page
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