A strange gap between theoretical predictions and experimental results in a major neutrino research project could be a sign of the elusive 'sterile' neutrino – a particle so quiet, it can only be detected by the silence it leaves in its wake.
It's not the first time the anomaly has been seen,
adding to previous experimental data hinting at something odd in the world of
neutrino research. This time around, it's been detected at the Baksan
Experiment on Sterile Transitions (BEST).
Unambiguous evidence of the hypothetical sterile
neutrino could provide physicists with a solid candidate for the Universe's
mysterious supply of dark matter. On the other hand, it could simply all come
down to a problem in the models used to describe the quirky behaviors of old
school neutrinos.
Which would also make for a significant moment in the
history of physics.
"The results are very exciting," says Los
Alamos National Laboratory physicist Steve Elliott.
"This definitely reaffirms the anomaly we've seen
in previous experiments. But what this means is not obvious. There are now
conflicting results about sterile neutrinos. If the results indicate
fundamental nuclear or atomic physics are misunderstood, that would be very
interesting, too."
In spite of ranking among the most abundant particles
in the Universe, neutrinos are notoriously difficult to catch. When you've got
barely any mass, no electric charge, and only make your presence known through
the weak nuclear force, it's easy to slip through even the densest of materials
unimpeded.
The neutrino's ghost-like movement isn't its only
interesting quality. Each particle's quantum wave morphs as it zips along,
oscillating between characteristic 'flavors' that echo their negatively charged
particle cousins – the electron, muon, and tau.
Studies on the oscillations of neutrinos at the US Los
Alamos National Laboratory in the 1990s noticed gaps in the timing of this
flip-flopping that left room for a fourth flavor, one that wouldn't make so
much as a ripple in the weak nuclear field.
Cloaked in silence, the sterile flavor of neutrino
would only be conspicuous by a brief pause in its interactions.
BEST is shielded from cosmic neutrino sources beneath
a mile of rock in Russia's Caucasus Mountains. It features a double-chambered
tank of liquid gallium which patiently collects neutrinos erupting from a core
of irradiated chromium.
After measuring the amount of gallium that had
transformed into a germanium isotope in each tank, the researchers could work
backwards to determine the number of direct collisions with neutrinos while
they were oscillating through their electron flavor.
Similar to the Los Alamos experiment's own 'gallium
anomaly', researchers calculated a fifth to a quarter less germanium than
expected, hinting at a deficit in the expected number of electron neutrinos.
This isn't to say with certainty that the neutrinos
had briefly adopted a sterile flavor. Many other searches for the pale little
particle come up empty-handed, leaving open the possibility that the models
used to predict the transformations are on some level misleading.
That isn't itself a bad thing. Corrections in the
basic framework of nuclear physics could have significant ramifications,
potentially revealing gaps in the Standard Model which could lead to
explanations for some of science's big remaining mysteries.
If this is indeed the mark of the sterile neutrino, we
might at last have evidence of a material that exists in tremendous quantities,
yet makes only a gravitational dimple in the fabric of space.
Whether that is the sum of dark matter or a mere piece
of its puzzle would depend on further experimentation on the most ghostliest of
ghost particles.
Reference: Physical Review letter
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