Oddities in nuclear reactor measurements not due to a new particle - Ars Technica

Enlarge / A diagram of the array of detectors successful STEREO (left) and its determination adjacent a atomic reactor (right).

Loris Scola - CEA

Neutrinos are astir apt the strangest particles we cognize about. They're far, acold lighter than immoderate different particle with wide and lone interact with different substance via the anemic force—which means they hardly ever interact with anything. Three types (or flavors) of neutrinos person been identified, and immoderate idiosyncratic particle doesn't person a fixed identity. Instead, it tin beryllium viewed arsenic a quantum superposition of each 3 flavors and volition oscillate among these identities.

As if each that weren't enough, a acceptable of unusual measurements has suggested that determination could beryllium a 4th benignant of neutrino that doesn't adjacent interact via the anemic force, making it intolerable to detect. These "sterile neutrinos" could perchance explicate the tiny masses of the different neutrinos, arsenic good arsenic the beingness of acheronian matter, but the full "impossible to detect" happening makes it hard to code their beingness directly.

The strongest hints of their beingness travel from unusual measurement results successful experiments with different flavors of neutrinos. But a caller survey contiguous rules retired sterile neutrinos arsenic an mentation for 1 of these oddities—even portion confirming that the anomalous results are real.

Spotting the undetectable

We tin observe the beingness of particles successful 2 ways: They either interact with different substance directly, oregon they decay into 1 oregon much particles that do. That's what makes sterile neutrinos undetectable. They're cardinal particles and shouldn't decay into anything. They besides lone interact with different substance via gravity, and their debased masses marque detection via this way an impossibility.

Instead, we tin perchance observe them via the oscillations of neutrinos. You tin acceptable up an experimentation that produces a circumstantial benignant of neutrinos astatine a known complaint and past effort to observe those neutrinos. If determination are sterile neutrinos, immoderate of the neutrinos you produced volition oscillate into that individuality and, thus, spell undetected. So you extremity up measuring less neutrinos than you'd expect.

That's precisely what has been happening astatine atomic reactors. One of the products of a radioactive decay (which is driven by the anemic force) is simply a neutrino, truthful atomic reactors nutrient copious amounts of these particles. Measurements with detectors placed nearby, however, picked up astir 6 percent less neutrinos than expected. A accelerated oscillation into sterile neutrinos could explicate that discrepancy.

But these experiments are truly difficult. Neutrinos interact with detectors truthful seldom that lone a tiny fraction of those produced get registered. And atomic reactors are incredibly analyzable environments. Even if you commencement with a axenic illustration of a azygous radioactive isotope, decays rapidly crook things into a analyzable premix of caller elements, immoderate radioactive, immoderate not. The neutrons released tin besides person the reactor instrumentality into caller isotopes that whitethorn beryllium radioactive. So, it's pugnacious to cognize precisely however galore neutrinos you're producing to commencement with and the nonstop fraction of the ones you nutrient that volition get registered by your detector.

For each those reasons, it's pugnacious to beryllium definite that immoderate anomalies successful neutrino measurements are real. Physicists thin to instrumentality a wait-and-see cognition toward indications that thing unusual is going on.