August 14, 2020


Aim for Excellence

Hidden Neutrino Particles May Be a Link to the Dark Sector

The physicists who arrived to our presentation at the 2010 Neutrino convention in Athens, Greece,...

The physicists who arrived to our presentation at the 2010 Neutrino convention in Athens, Greece, probably predicted us to place to rest a controversial acquiring from a 10 years prior. As an alternative we left them in a shocked silence.

The tale commences in 1996, when we unveiled info, obtained at the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos Countrywide Laboratory, suggesting a challenge with the greatly approved notion that neutrinos—tiny, ubiquitous particles that pass suitable through most matter—come in three styles, or flavors. Our success showed that there might be a fourth neutrino flavor that had gone undetected. The scientific community was skeptical, and in reality, early info from a adhere to-up experiment hinted that our 1996 success were off—there was no fourth neutrino flavor following all. That working day in Athens it was very clear that the viewers predicted our latest results to shut the doorway on the LSND success once and for all. We unveiled, nevertheless, that the evidence for a fourth variety of neutrino had turn out to be even additional powerful.

We had not learned the particle, but our do the job, done as section of the Mini Booster Neutrino Experiment (MiniBooNE) at Fermi Countrywide Accelerator Laboratory in Batavia, Unwell., showed that there was practically unquestionably a challenge with the modern day understanding of particle physics. The most probable answer was a new neutrino—a “sterile” neutrino, so termed because it would not interact with other matter in any way besides through gravity. In the 10 years given that our presentation in Greece, info from MiniBooNE have bolstered the situation for a fourth flavor of neutrino even further.

We now imagine there is additional than a 99.999999 % opportunity that some thing is heading on beyond the scope of regarded physics, and sterile neutrinos are a potent contender. The notion that our experiments might be detecting a fourth neutrino remains controversial, nevertheless, because the Common Product of particle physics is a single of the most tested and extensively confirmed theoretical frameworks in history—and it enables for only three neutrinos. Nevertheless, we know the Common Product is not finish, because it can not describe dim matter or dim electricity, the invisible stuff that would seem to dominate the cosmos. And a new neutrino flavor might just be a connection to that hidden realm. Last but not least, following many years of uncertainty, a number of assignments are starting about the world—including our have Coherent CAPTAIN-Mills (CCM) experiment—that could place this mystery to bed.

What Is a Sterile Neutrino?

All neutrinos are ghostly. Trillions of them fly through you every second at just about the pace of light-weight. Nevertheless a sterile neutrino would be the ghostliest of them all. Since it does not expertise the potent, weak and electromagnetic forces through which other particles interact, it would be in essence undetectable. This top quality would render it section of the invisible realm physicists call the dim sector, which involves the dim electricity and dim matter that make up 95 % of the electricity density of the universe. Sterile neutrinos may perhaps be ready to interact with dim matter through new forces of character. They might even be dim matter: some hypotheses recommend that sterile neutrinos make up some or even most of the invisible matter in the cosmos.

If sterile neutrinos exist, it will be the latest in a series of surprises this puzzling spouse and children of particles has thrown at physicists. The initially arrived in the sixties, when experiments developed to catch the neutrinos traveling towards us from the sun measured significantly much less than experts predicted. All stars are fueled by nuclear fusion reactions in which protons mix to type helium nuclei, which in convert fuse to produce heavier aspects. Among the merchandise of these reactions are electron neutrinos—one of the three regarded flavors, along with muon and tau neutrinos. Concept predicted that a regular stream of the particles from the sun would make their way to observatories on Earth, however experiments measured only a tiny fraction of the anticipated range. The resulting deficit was regarded as the photo voltaic neutrino challenge.

Credit score: Lucy Examining-Ikkanda

Numerous physicists initially assumed that we just did not definitely comprehend how the sun operates. The actual issue turned out to be the two simpler and a lot additional problematic. It was not that the sun emitted much less neutrinos than predicted. It was that the neutrinos were not earning it to our terrestrial detectors—or relatively they were changing en route.

What experts ultimately figured out is that a neutrino is not a pure object. Rather every neutrino is made up of a blend of all neutrino styles and can oscillate through the numerous flavors as it travels. This discovery was surprising for a selection of motives. For starters, the reality that neutrinos can alter their flavor usually means they can not be massless particles traveling at light-weight pace, as the Common Product predicted. The cause is a consequence of Einstein’s specific idea of relativity, which tells us that time moves additional slowly and gradually for an object in movement than for a stationary a single. As an object’s pace improves, time continues to sluggish right until it actually stops. That is the point when the object reaches the pace of light—meaning that if you could journey at light-weight pace, time would appear to be to stand however, with the full universe frozen in location. If neutrinos change their flavor, while, they should bear alter and for that reason expertise time. Hence, they should be traveling additional slowly and gradually than light-weight, which usually means they can not be massless. Particles transferring at the pace of light-weight would have no mass, according to relativity, so if they are slower than that, they should have some mass—and the Common Product has a challenge. This revelation, and the discovery that neutrinos oscillate, won Takaaki Kajita and Arthur B. McDonald the 2015 Nobel Prize in Physics.

Stunning Alerts

The sudden means of neutrinos to change flavors is what we and other people were studying back in the nineteen nineties and 2000s at LSND and MiniBooNE, when we saved acquiring hints of further neutrinos. The two experiments were attached to particle accelerators that produced regular streams of muon neutrinos, and the two used detectors placed some length absent that were tuned to observe electron neutrinos.

If you could sail along beside a single neutrino as it traveled through room, you would see it oscillate from a single variety to yet another, cycling through all the flavors. The electron, muon and tau flavors would all be observable, at least in principle. If sterile neutrinos exist, nevertheless, neutrinos could also change into this fourth flavor. To a traveling-along observer, the particle would simply just appear to be to vanish for this section of its flight. In the most straightforward situation, the neutrino would reappear some time later on as a single of the typical flavors (while there is a theoretical likelihood that a sterile neutrino could decay, ending the oscillation cycle completely).

COHERENT CAPTAIN-MILLS researcher T. J. Schaub
COHERENT CAPTAIN-MILLS researcher T. J. Schaub lifts out a photomultiplier tube to be changed as section of a neutrino detector update (left). Staff elevate the CCM neutrino detector out of its container, which is stuffed with liquid argon when the experiment is working (suitable). Credit score: Los Alamos Countrywide Laboratory

Oscillations amongst the three regular neutrino flavors most generally consider location more than prolonged distances. Since sterile neutrinos are probable to be additional large than the typical flavors, nevertheless, particles could make the change to this variety additional speedily and could also alter back from sterile to a single of the three typical flavors more than shorter distances. Hence, if sterile neutrinos exist, they really should pace the oscillation system and significantly cut down the length that a muon neutrino, for illustration, would journey just before reworking into an electron neutrino.

That is particularly what we discovered in our before experiments: it seemed that muon neutrinos were disappearing a lot more rapidly than we predicted as they traveled from their sources, and electron neutrinos were turning up in figures bigger than anticipated. We noticed oscillations more than lengths of merely tens to hundreds of meters relatively than the tens to hundreds of kilometers that we predicted. Such large figures of muon neutrinos really should not have been ready to change into electron neutrinos more than the length of the experiments except, perhaps, they were reworking into sterile neutrinos on the way.

Our experiments are not the only types with anomalous success. Some neutrino detectors set up close to nuclear reactors have also discovered hints of some thing amiss. Like the sun, nuclear reactors produce neutrinos as a by-merchandise, and a number of experiments have been carried out close to them to analyze the particles. They have unveiled much less neutrinos than predicted, which implies that some of the particles may perhaps be oscillating into sterile neutrinos on their way from the reactors. These success are more durable to interpret, nevertheless, because physicists do not know particularly how numerous neutrinos really should be generated in fission reactors. For that reason, the reduce-than-predicted neutrino depend could be the end result of sterile neutrinos, or experts might have simply just overestimated how numerous particles they really should discover.

Physicists in Russia are preventing these uncertainties with an experiment termed DANSS (detector of the reactor antineutrino dependent on solid-condition plastic scintillator). This undertaking takes location beneath a nuclear reactor, but the experts range the length involving the reactor and the neutrino detector every several days to see no matter whether the electron neutrinos are actually morphing into other flavors as they shift absent from their supply. This approach may perhaps notify the scientists no matter whether short-vary oscillations are transpiring even if they do not have exact estimates of the range of neutrinos generated in the reactor.

The great IceCube Neutrino Observatory in Antarctica is also looking for sterile neutrinos. This cubic-kilometer array of photodetectors buried in the polar ice documents a light-weight sign termed Cherenkov radiation, produced when a large-electricity neutrino from the higher ambiance interacts with ice and sets off a shower of particles. Researching the Cherenkov light-weight gathered in the photodetectors tells experts about the variety, electricity and flight way of the neutrino that sparked the particle shower.

The IceCube crew will soon report an eight-calendar year assessment of neutrinos passing through Earth to the IceCube array. The analyze will look for for signs of muon neutrinos disappearing, which, if discovered, could imply the existence of sterile neutrinos reliable with the success from LSND and MiniBooNE.

All of the evidence for sterile neutrinos is intriguing and suggestive, but it is not however conclusive. The neutrinos researched in IceCube come in a broad vary of energies, which helps make examining their oscillations complex. It is also tough in reactor experiments to distinguish the neutrinos scientists search for from history neutrinos generated in the sun and from radioactive decays in common detector elements that can masquerade as alerts.

In experiments this kind of as those at LSND and MiniBooNE, the accelerators that generate the neutrinos can be turned on and off to determine the amount of history sounds. Even in those styles of experiments, we have been constrained in section by our lack of ability to glimpse for neutrinos at additional than a tiny vary of distances. Approaches for catching neutrinos in the previous commonly have relied on large, motionless detectors that limit our flexibility. The assignments are comparable to using a frame from a single point in a movie, and what we require is a sequence of frames to get the full tale.

A Novel Strategy

A new breed of experiment now coming on the net really should be ready to seize the several frames we require. Ideally, as stated before, we would fly along with a neutrino and check out it oscillate. We can not do that, but these experiments give us a way of using snapshots all through the oscillation system that could expose traces of sterile neutrinos if they exist. Such assignments involve the Brief-Baseline Neutrino application at Fermilab and the CCM experiment that we are just starting at Los Alamos.

CCM is housed in a hall in the Los Alamos Neutron Science Centre (LANSCE) at the end of an 800-meter-prolonged particle accelerator. The accelerator fires a beam of protons at a tungsten focus on. When the protons hit the tungsten, they kick showers of neutrons out of the focus on atoms through a system termed neutron spallation. Some of the electricity introduced all through this system leads to the creation of short-lived pion particles. The pions speedily come to rest and decay into muons and, additional critical for our uses, muon neutrinos with a really specific electricity.

Coherent CAPTAIN-Mills Experiment at Los Alamos National Lab searches for sterile neutrinos
Credit score: Lucy Examining-Ikkanda

CCM detects neutrinos through coherent neutrino scattering, an outcome that depends on the reality that all particles (together with neutrinos) act not just like minimal marbles but also like waves. This wave-particle duality is a cornerstone of quantum mechanics. The wavelength affiliated with a particle is dependent on the particle’s electricity. Significant-electricity, rapidly-transferring particles have short wavelengths, and sluggish-transferring, minimal-electricity particles have prolonged wavelengths. When short-wavelength neutrinos strike an atomic nucleus, they interact with a single neutron or proton within it. But some thing specific takes place when a neutrino’s electricity is minimal plenty of that its wavelength is comparable to the diameter of an atomic nucleus. As an alternative of striking a single proton or neutron in an atom, a minimal-electricity neutrino interacts with the whole nucleus. It is somewhat analogous to waves passing a boat. A series of short ripples on h2o has barely any outcome on the movement of a large boat, but really prolonged waves on the open ocean will lift that exact same boat significantly. Since a prolonged-wavelength neutrino interacts with an atomic nucleus as a full in its place of as a assortment of tiny bits, the likelihood of the neutrino hitting the whole nucleus are a lot bigger at minimal energies than at large energies.

Unlike large-electricity neutrinos, a minimal-electricity neutrino bounces off an atom’s whole nucleus. This variety of scattering is termed “coherent” because the wavelength and the nucleus are very similar in dimensions. The atom recoils when it is hit. If the interaction takes location in a appropriate content, the recoiling atom generates moment flashes of light-weight, and the neutrino continues on its way, albeit in a distinctive way than it was heading in initially. By capturing the light-weight flash with a sensitive photomultiplier-tube detector, we can determine when and where the neutrino scattered from the atom, as nicely as the atom’s kinetic electricity. Despite the fact that coherent scattering can not expose an personal neutrino’s flavor, it can evaluate the sum of all three regarded neutrino flavors throughout interactions. This reality is important: if the sum is not the predicted range of neutrinos, unmeasured sterile neutrinos—which would not scatter off argon and for that reason would not produce any flash of light—may be associated.

Offered that our likelihood of a neutrino hitting a full atomic nucleus are bigger than the likelihood of its hitting just a single of its nucleons, we can use smaller sized detectors in this kind of experiments than the gargantuan types that have been necessary for numerous neutrino detectors in many years previous. In contrast to MiniBooNE’s 800-metric-ton detector vat of mineral oil, the CCM detector contains 10 metric tons of liquid argon. And because the detector is compact, we can shift it from location to location to observe neutrinos at a vary of distances from their supply. If neutrinos oscillate frequently plenty of more than the tens of meters readily available in the LANSCE experimental hall, we will observe the overall sum of neutrino interactions to range by length. Such an observation would be smoking cigarettes gun evidence that oscillations into sterile neutrinos are occurring—because with only three flavors, we really should see no oscillations at all more than this kind of tiny distances.

The likelihood exists, of training course, that CCM will discover no evidence of sterile neutrinos. In that situation, the observations of the regarded neutrinos would surface to reduce steadily as we moved the CCM detector farther from the tungsten focus on, a lot as the evident brightness of a lightbulb decreases as you walk absent from it.

Even though disappointing, nondetection would not rule out the existence of a single or additional sterile neutrinos. It would, nevertheless, enable us to location limitations on their likely qualities. Since neutrino oscillations depend on the relative masses of neutrinos and a parameter regarded as the mixing angle, a failure to discover signs of a sterile neutrino can give us a tackle on what the relative masses and mixing angles are not probable to be, successfully narrowing the vary of parameters that future queries for sterile neutrinos should think about.

Further than the Common Product

Experimentalists who offer you success at odds with the Common Product are appropriately deemed responsible right until confirmed harmless because traditionally experts who have challenged the Common Product have been completely wrong. Nevertheless, it is particular that the Common Product is not the whole tale. Neutrino oscillations alone are evidence of that reality.

Neutrino exploration has been a harbinger of enjoyable new developments in physics given that the initially neutrinos were learned in 1956 by our Los Alamos predecessors Frederick Reines and Clyde Cowan. Despite the fact that sterile neutrinos stay a controversial subject matter twenty five many years following we initially discovered signs of them, experiments are now on the verge of resolving the controversy a single way or the other.