Physicists trying to find proof of a “gentle sterile neutrino”, a hypothetical particle that would give clues to cosmic puzzles equivalent to the character of darkish matter and why the Universe is made from matter in any respect, have introduced their hunt has come again empty-handed.
The MicroBooNE experiment at Fermilab was designed to comply with up on earlier hints of neutrinos behaving oddly, however the detrimental consequence offers a blow to the thought of such a brand new elementary particle.
Explainer: the elusive neutrino
Neutrinos are light-weight, elusive subatomic particles, and present theories recognise three differing kinds. In 1995, nonetheless, the Liquid Scintillator Neutrino Detector (LSND) experiment in Los Alamos detected extra of 1 kind than anybody anticipated.
Most makes an attempt to elucidate the anomaly proposed the existence of a fourth type of neutrino that hardly interacts with regular matter in any respect: a so-called “sterile” neutrino.
Newer experiments have additionally reported outcomes broadly in keeping with the sterile neutrino speculation, however the MicroBooNE consequence casts the entire thought into doubt.
What’s a sterile neutrino?
Neutrinos are subatomic particles postulated by Austrian physicist Wolfgang Pauli in 1930 to elucidate how some radioactive atoms hearth out electrons.
Their existence wasn’t confirmed till 1956, when Individuals Clyde Cowan and Frederick Reines noticed tiny flashes of sunshine made by neutrinos crashing into the atoms in a tank of water.
At the moment, neutrinos are an integral a part of the “Normal Mannequin of particle physics”. That is our greatest idea of the Universe’s particles, describing the 17 identified elementary particles and the way they work together by way of three elementary forces (electromagnetism and the robust and weak forces).
Explainer: Normal Mannequin of Particle Physics
The Normal Mannequin divides the 17 particles into two fundamental teams: 12 fermions, which make up matter, and 5 bosons, which carry the forces.
Not all fermions work together with all of the forces. For instance, neutrinos are solely affected by the weak pressure (and gravity, which doesn’t match into the Normal Mannequin).
The fermions are cut up into three households, every of which has a neutrino: the electron, muon, and tau neutrinos.
MissMJ / Wikimedia, CC BY
All of those neutrinos are “left-handed” with respect to the weak pressure. It’s onerous to elucidate merely what meaning, however suffice to say left- and right-handed particles are mirror pictures of each other, and they’re affected in a different way by the weak pressure.
All different identified fermions are available in each left- and right-handed variations. This encourages us to suppose that right-handed neutrinos must also exist in nature.
Being right-handed, these hypothetical neutrinos are blind even to the weak pressure and are on this sense “sterile”.
However like all identified particles, they need to nonetheless really feel gravity. Sterile neutrinos are additionally predicted by “grand unified theories” that attempt to mix the three forces into one.
Trying to find sterile neutrinos
If sterile neutrinos exist, how would we discover them? A method is to make use of a course of referred to as “neutrino oscillation”, by which the three identified sorts of neutrinos can rework into each other.
Experiments measuring these oscillations often take a look at both what number of of a given type of neutrino seem in some state of affairs, or what number of disappear.
The LSND experiment which initially impressed the sterile neutrino speculation was an “look” experiment, as are MicroBooNE (which produced the brand new detrimental consequence) and its predecessor MiniBooNE.
Fred Ullrich / Fermilab, CC BY
They hearth a beam of muon-neutrinos over a comparatively quick distance (between 30 and 500 metres) and measure what number of electron-neutrinos are detected on the different finish.
At LSND and MiniBooNE, they noticed extra electron-neutrinos than anticipated. We all know from different experiments that muon-neutrinos can’t oscillate straight into electron-neutrinos over these distances.
But when a few of muon-neutrinos flip into very gentle sterile neutrinos after which into electron-neutrinos, it may clarify how these further electron-neutrinos appeared.
That is the sterile neutrino speculation.
What if there are sterile neutrinos?
If experiments did affirm the existence of a lightweight sterile neutrino, there could be a superb probability that heavier sterile neutrinos exist as nicely.
These heavier cousins may reply a number of main puzzles in particle physics, equivalent to the character of the “darkish matter” that appears to make up many of the Universe, why neutrinos have any mass in any respect, and why the Universe accommodates a lot extra matter than antimatter.
There’s however one drawback. The sunshine sterile neutrino we began with is a headache for cosmologists.
If it exists, we ought to be capable of observe traces of sterile neutrinos fashioned simply after the Massive Bang.
ESA / Planck Collaboration
Nevertheless, no latest surveys of the cosmic microwave background radiation or the distribution of galaxies and light-weight components in between them present any signal these sterile neutrinos existed.
This might imply the sterile neutrino speculation is wrong. However additionally it is attainable that one thing else in our understanding of the Universe is amiss.
MicroBooNE and the worldwide image
MicroBooNE analysed its ends in 4 alternative ways, and none of them turned up indicators of additional electron-neutrinos. That is disappointing for the researchers behind the LSND and MiniBooNE collaborations, and for proponents of the sterile neutrino speculation.
It additionally raises the query of precisely what triggered the outcomes noticed by the sooner experiments. Additional evaluation of the MicroBooNE information could assist unravel this thriller.
Emanuel Jacobi / IceCube / NSF
Globally, nonetheless, MicroBooNE’s newest result’s in step with the findings of two massive “disappearance” experiments, MINOS+ and IceCube. Neither of those noticed proof of disappearing muon-neutrinos because the sterile neutrino speculation predicts.
Recognizing astrophysical neutrinos is simply the tip of the IceCube
Elsewhere, there have been claims of disappearing neutrinos in nuclear reactor experiments. Nevertheless, calculating what number of neutrinos a nuclear reactor will emit is notoriously tough, so these claims are greatest taken with a grain of salt.
The MicroBooNE collaboration has up to now analysed solely half of its collected information.
Some have additionally questioned whether or not no extra of electron-neutrinos essentially means no neutrino oscillations, given the measurement has been made at just one distance. Technically, we want measurements at two distances or extra to definitively set up oscillations or in any other case.
These measurements are prone to come within the subsequent few years, when Fermilab switches on two extra detectors as a part of the Fermilab Quick Baseline Neutrino program. The trio of detectors will check for disappearance of muon-neutrinos and look of electron-neutrinos utilizing a single beam of supply neutrinos.
The prospects for a ultimate verdict on gentle sterile neutrinos within the subsequent decade are subsequently good.