A new result from the LHCb experiment at CERN could help explain why our universe is made of matter and not antimatter.

Matter particles, such as protons and electrons, all have an antimatter twin. These antimatter twins appear identical in nearly every respect except that their electric and magnetic properties are opposite.

Cosmologists predict that the Big Bang produced an equal amount of matter and antimatter, which is a conundrum because matter and antimatter annihilate into pure energy when they come into contact. Particle physicists are looking for any minuscule differences between matter and antimatter, which might explain why our universe contains planets and stars and not a sizzling broth of light and energy instead.

The Large Hadron Collider doesn’t just generate Higgs bosons during its high-energy proton collisions—it also produces antimatter. By comparing the decay patterns of matter particles with their antimatter twins, the LHCb experiment is looking for miniscule differences in how these rival particles behave.

“Many antimatter experiments study particles in a very confined and controlled environment,” says Nicola Neri, a researcher at Italian research institute INFN and one of the leaders of the study. “In our experiment, the antiparticles flow and decay, so we can examine other properties, such as the momenta and trajectories of their decay products.”

The result, published today in Nature Physics, examined the decay products of matter and antimatter baryons (a particles containing three quarks) and looked at the spatial distribution of the resulting daughter particles within the detector. Specifically, Neri and his colleagues looked for a very rare decay of the lambda-b particle (which contains an up quark, down quark and bottom quark) into a proton and three pions (which contain an up quark and anti-down quark).

Based on data from 6000 decays, Neri and his team found a difference in the spatial orientation of the daughter particles of the matter and antimatter lambda-bs.

“This is the first time we’ve seen evidence of matter and antimatter baryons behaving differently,” Neri says. “But we need more data before we can make a definitive claim.”

Statistically, the result has a significant of 3.3 sigma, which means its chances of being a just a statistical fluctuation (and not a new property of nature) is one out of a thousand. The traditional threshold for discovery is 5 sigma, which equates to odds of one out of more than a million.

For Neri, this result is more than early evidence of a never before seen process—it is a key that opens new research opportunities for LHCb physicists.

“We proved that we are there,” Neri says, “Our experiment is so sensitive that we can start systematically looking for this matter-antimatter asymmetry in heavy baryons at LHCb. We have this capability, and we will be able to do even more after the detector is upgraded next year.”

The Large Hadron Collider is the world’s most powerful particle accelerator. Buried in the bedrock beneath the Franco-Swiss boarder, it whips protons through its nearly 2000 magnets 11,000 times every second.

As you might expect, the subterranean tunnel which houses the LHC is not always the friendliest place for human visitors.

“The LHC contains 120 tons of liquid helium kept at 1.9 Kelvin,” says Ron Suykerbuyk, an LHC operator. “This cooling system is used to keep the electromagnets in super conducting state capable of carrying up to 13,000 Amps of current through its wires. Even with all the safety systems we have in place, we prefer to limit our underground access when the cryogenic systems are on”.

But as with any machine, sometimes the LHC needs attention: inspections, repairs, tuning. The LHC is so secure that even with perfect conditions, it takes 30 minutes after the beam is shut off for the first humans to even arrive at the entrance to the tunnel.

But the robotics team at CERN asks: Why do we need humans for this job anyway?

Enter TIM—the Train Inspection Monorail. TIM is a chain of wagons, sensors and cameras that snake along a track bolted to the LHC tunnel’s ceiling. In the 1990s, the track held a cable car that transported machinery and people around the Large Electron-Position Collider, the first inhabitant of the tunnel. With the installation of the LHC, there was no longer room for both accelerator and the cable car, so the monorail was reconfigured for the sleeker TIM robots.

There are currently two TIM robots and plans to install two more in the next couple of years. These four TIM robots will patrol the different quadrants of the LHC, enabling operators to reach any part of the 17-mile tunnel within 20 minutes. As TIM slithers along the ceiling, an automated eye keeps watch for any changes in the tunnel and a robotic arm drops down to measure radiation. Other sensors measure the temperature, oxygen level and cell phone reception.

“In addition to performing environmental measurements, TIM is a safety system which can be the eyes and ears for members of the CERN Fire Brigade and operations team,” says Mario Di Castro, the leader of CERN’s robotics team. “Eventually we’d like to equip TIM with a fire extinguisher and other physical operations so that it can be the first responder in case of a crisis.”

TIM isn’t alone in its mission to provide a safer environment for its human coworkers. CERN also has three teleoperated robots that can assess troublesome areas, provide assessments of hazards and carry tools.

The main role of these three robots is to access radioactive areas.

Radiation is a type of energy carried by free-moving subatomic particles. As protons race around CERN’s accelerator complex, special equipment called collimators constrict their passage and absorb particles that have wondered away from the center of the beam pipe. This trimming process ensures that the proton stream is compact and tidy.

After a couple weeks of operation, the collimators have absorbed so many particles that they will reemit their energy—even after the beam is shut off. There is no radiation hazard to humans unless they are within a few meters of the collimators, and because the machine is fully automated, humans rarely need to perform check-ups. But occasionally, material in these restricted areas required attention.

By replacing humans with robots, engineers can quickly fix small problems without needing to wait long periods of time for the radiation to dissipate or sending personnel into potentially unsafe environments.

“CERN robots help perform repetitive and dangerous tasks that humans either prefer to avoid or are unable to do because of hazards, size constraints or the extreme environments in which they take place, such CERN experimental areas,” Di Castro says.

About half the time, these tasks are very simple, such as performing a visual assessment of the area or taking measurements. “Robots can replace humans for these simple tasks and improve the quality and timeliness of work,” he says.

Last year the SPS accelerator (which starts the acceleration process for particles that eventually move to the LHC) needed an oil refill to keep its parts running smoothly. But the accelerator itself was too radioactive for humans to visit, so one of the CERN robotics team’s robots rolled in gripping an oil can in its flexible arm.

In June 2016, scientists needed to dispose of radioactive Cobalt, Cesium and Americium they had used to calibrate radiation sensors. Two CERN robots cycled in with several tools, extracted the radioactive sources and packed them in thick protective containers for removal.

Over the last two years, these two robots have performed more than 30 interventions, saving humans both time and radiation doses.

As the LHC increases the power and particle collisions over the next decade, Di Castro and his team are preening these robot companions to increase their capabilities. “We are putting a strong commitment to adapt and develop existing robotic solutions to fit CERN’s evolving needs,” Di Castro says.

As a massive star dies, expelling most of its guts across the universe in a supernova explosion, its iron heart, the star’s core, collapses to create the densest form of observable matter in the universe: a neutron star. 

A neutron star is basically a giant nucleus, says Mark Alford, a professor at Washington University.

“Imagine a little lead pellet with cotton candy around it,” Alford says. “That’s an atom. All the of mass is in the little lead pellet in the middle, and there’s this big puffy cloud of electrons around it like cotton candy.” 

In neutron stars, the atoms have all collapsed. The electron clouds have all been sucked in, and the whole thing becomes a single entity with electrons running around side-by-side with protons and neutrons in a gas or fluid.

Neutron stars are pretty small, as far as stellar objects go. Although scientists are still working on pinning down their exact diameter, they estimate that they’re somewhere around 12 to 17 miles across, just about the length of Manhattan. Despite that, they have about 1.5 times the mass of our sun.

If a neutron star were any denser, it would collapse into a black hole and disappear, Alford says. “It’s the next to last stop on the line.” 

These extreme objects offer intriguing test cases that could help physicists understand the fundamental forces, general relativity and the early universe. Here are some fascinating facts to get you acquainted:

1. In just the first few seconds after a star begins its transformation into a neutron star, the energy leaving in neutrinos is equal to the total amount of light emitted by all of the stars in the observable universe.

Ordinary matter contains roughly equal numbers of protons and neutrons. But most of the protons in a neutron star convert into neutrons—neutron stars are made up of about 95 percent neutrons. When protons convert to neutrons, they release ubiquitous particles called neutrinos. 

Neutron stars are made in supernova explosions which are giant neutrino factories. A supernova radiates 10 times more neutrinos than there are particles, protons, neutrons and electrons in the sun.

2. It’s been speculated that if there were life on neutron stars, it would be two-dimensional.

Neutron stars have some of the strongest gravitational and magnetic fields in the universe.  The gravity is strong enough to flatten almost anything on the surface. The magnetic fields of neutron stars can be a billion times to a million billion times the magnetic field on the surface of Earth. 

“Everything about neutron stars is extreme,” says James Lattimer, a professor at Stony Brook University. “It goes to the point of almost being ridiculous.” 

Because they’re so dense, neutron stars provide the perfect testbed for the strong force, allowing scientists to probe the way quarks and gluons interact under these conditions. Many theories predict that the core of a neutron star compresses neutrons and protons, liberating the quarks of which they are constructed. Scientists have created a hotter version of this freed “quark matter” in the Relativistic Heavy Ion Collider and the Large Hadron Collider. 

The intense gravity of neutron stars requires scientists to use the general theory of relativity to describe the physical properties of neutron stars. In fact, measurements of neutron stars give us some of the most precise tests of general relativity that we currently have.

Despite their incredible densities and extreme gravity, neutron stars still manage to maintain a surprising amount of internal structure, housing crusts, oceans and atmospheres. “They’re a weird mixture of something the mass of a star with some of the other properties of a planet,” says Chuck Horowitz, a professor at Indiana University.

But while here on Earth we’re used to having an atmosphere that extends hundreds of miles into the sky, because a neutron star’s gravity is so extreme, its atmosphere may stretch up less than a foot.

3. The fastest known spinning neutron star rotates about 700 times each second.

Scientists believe that most neutron stars either currently are or at one point have been pulsars, stars that spit out beams of radio waves as they rapidly spin. If a pulsar is pointed toward our planet, we see these beams sweep across Earth like light from a lighthouse.

Scientists first observed neutron stars in 1967, when a graduate student named Jocelyn Bell noticed repeated radio pulses arriving from a pulsar outside our solar system. (The 1974 Nobel Prize in Physics went to her thesis advisor, Anthony Hewish, for the discovery.)

Pulsars can spin anywhere from tens to hundreds of times per second. If you were standing on the equator of the fastest known pulsar, the rotational velocity would be about 1/10 the speed of light.

The 1993 Nobel Prize in Physics went to scientists who measured the rate at which a pair of neutron stars orbiting each other were spiraling together due to the emission of gravitational radiation, a phenomenon predicted by Albert Einstein's general theory of relativity.

Scientists from the Laser Interferometer Gravitational-Wave Observatory, or LIGO, announced in 2016 that they had directly detected gravitational waves for the first time. In the future, it might be possible to use pulsars as giant, scaled-up versions of the LIGO experiment, trying to detect the small changes in the distance between the pulsars and Earth as a gravitational wave passes by.

4. The wrong kind of neutron star could wreak havoc on Earth.

Neutron stars can be dangerous because of their strong fields. If a neutron star entered our solar system, it could cause chaos, throwing off the orbits of the planets and, if it got close enough, even raising tides that would rip the planet apart.

But the closest known neutron star is about 500 light-years away. And considering Proxima Centauri, the closest star to Earth at a little over 4 light-years away, has no bearing on our planet, it’s unlikely we’ll feel these catastrophic effects anytime soon.

Probably even more dangerous would be radiation from a neutron star’s magnetic field. Magnetars are neutron stars with magnetic fields a thousand times stronger than the extremely strong fields of “normal” pulsars. Sudden rearrangements of these fields can produce flares somewhat like solar flares but much more powerful.

On December 27, 2004, scientists observed a giant gamma-ray flare from Magnetar SGR 1806-20, estimated to be about 50,000 light years away. In 0.2 seconds the flare radiated as much energy as the sun produces in 300,000 years. The flare saturated many spacecraft detectors and produced detectable disturbances in the Earth’s ionosphere.

Fortunately, we are not aware of any nearby magnetars powerful enough to cause any damage.

5. Despite the extremes of neutron stars, researchers still have ways to study them.

There are many things we don’t know about neutron stars—including just how many of them are out there, Horowitz says. “We know of about 2000 neutron stars in our own galaxy, but we expect there to be billions more. So most neutron stars, even in our own galaxy, are completely unknown.”

Many radio, X-ray and optical light telescopes are used to investigate the properties of neutron stars. NASA’s upcoming Neutron Star Interior Composition ExploreR Mission (NICER), which is scheduled to attach to the side of the International Space Station in 2017, is one mission devoted to learning more about these extreme objects. NICER will look at X-rays coming from rotating neutron stars to try to more accurately pin down their mass and radii.

We could also study neutron stars by detecting gravitational waves. LIGO scientists hope to detect gravitational waves produced by the merger of two neutron stars. Studying those gravitational waves might clue scientists in to the properties of the extremely dense matter that neutron stars are made of.

Studying neutron stars might help us figure out the origin of the heavy chemical elements, including gold and platinum, in our universe. There’s a possibility that when neutron stars collide, not everything gets swallowed up into a more massive neutron star or black hole, but instead some fraction gets flung out and forms these heavy nuclei.

“If you want to use the lab of 24th or 25th century,” says Roger Romani, a professor at Stanford University, “then studying neutron stars is a way of looking at conditions that we cannot produce in labs on Earth.”

There is little wiggle room for disparities between matter and antimatter protons, according to a new study published by the BASE experiment at CERN.

Charged matter particles, such as protons and electrons, all have an antimatter counterpart. These antiparticles appear identical in every respect to their matter siblings, but they have an opposite charge and an opposite magnetic property. This recalcitrant parity is a head-scratcher for cosmologists who want to know why matter triumphed over antimatter in the early universe.

“We’re looking for hints,” says Stefan Ulmer, spokesperson of the BASE collaboration. “If we find a slight difference between matter and antimatter particles, it won’t tell us why the universe is made of matter and not antimatter, but it would be an important clue.”

Ulmer and his colleagues working on the BASE experiment at CERN closely scrutinize the properties of antiprotons to look for any miniscule divergences from protons. In a paper published today in the journal Nature Communications, the BASE collaboration at CERN reports the most precise measurement ever made of the magnetic moment of the antiproton.

“Each spin-carrying charged particle is like a small magnet,” Ulmer says. “The magnetic moment is a fundamental property which tells us the strength of that magnet.”

The BASE measurement shows that the magnetic moments of the proton and antiproton are identical, apart from their opposite signs, within the experimental uncertainty of 0.8 parts per million. The result improves the precision of the previous best measurement by the ATRAP collaboration in 2013, also at CERN, by a factor of six. This new measurement shows an almost perfect symmetry between matter and antimatter particles, thus further constricting leeway for incongruencies which might have explained the cosmic asymmetry between matter and antimatter.

The measurement was made at the Antimatter Factory at CERN, which generates antiprotons by first crashing normal protons into a target and then focusing and slowing the resulting antimatter particles using the Antiproton Decelerator. Because matter and antimatter annihilate upon contact, the BASE experiment first traps antiprotons in a vacuum using sophisticated electromagnetics and then cools them to about 1 degree Celsius above absolute zero. These electromagnetic reservoirs can store antiparticles for long periods of time; in some cases, over a year. Once in the reservoir, the antiprotons are fed one-by-one into a trap with a superimposed magnetic bottle, in which the antiprotons oscillate along the magnetic field lines. Depending on their North-South alignment in the magnetic bottle, the antiprotons will vibrate at two slightly different rates. From these oscillations (combined with nuclear magnetic resonance methods), physicists can determine the magnetic moment.

The challenge with this new measurement was developing a technique sensitive to the miniscule differences between antiprotons aligned with the magnetic field versus those anti-aligned.

“It’s the equivalent of determining if a particle has vibrated 5 million times or 5 million-plus-one times over the course of a second,” Ulmer says. “Because this measurement is so sensitive, we  stored antiprotons in the reservoir and performed the measurement when the antiproton decelerator was off and the lab was quiet.”

BASE now plans to measure the antiproton magnetic moment using a new trapping technique that should enable a precision at the level of a few parts per billion—that is, a factor of 200 to 800 improvement.

Members of the BASE experiment hope that a higher level of precision might provide clues as to why matter flourishes while cosmic antimatter lingers on the brink of extinction.

“Every new precision measurement helps us complete the framework and further refine our understanding of antimatter’s relationship with matter,” Ulmer says.

Before building any large piece of infrastructure, potential investors or representatives from funding agencies or governments have to decide whether it’s worth it. Teams of economists perform a cost-benefit analysis to help them determine how a project will affect a region and whether it makes sense to back and build it. 

But when it comes to building infrastructure for basic science, the process gets a little more complicated. It’s not so easy to pin an exact value on the benefits of something like the Large Hadron Collider.

“The main goal is priceless and therefore has no price attached,” says Stefano Forte, a professor of theoretical physics at the University of Milan and part of a team that developed a new method of economic analysis for fundamental science. “We give no value to discovering the Higgs boson in the past or supersymmetry or extra dimensions in the future, because we wouldn’t be able to say what the value of the discovery of extra dimensions is.”

Forte’s team was co-led by two economists, academic Massimo Florio, also of the University of Milan, and private business consultant Silvia Vignetti. They answered a 2012 call by the European Investment Bank’s University Sponsorship Program, which provides grants to university research centers, for assistance with this issue. The bank funded their research into a new way to evaluate proposed investments in science.

Before anyone can start evaluating any sort of impact, they have to define what they’re measuring. Generally, economic impact analyses are highly local, measuring exclusively money flowing in and out of a particular area. 

Because of the complicated nature of financing any project, the biggest difficulty for economists performing an analysis is usually coming up with an appropriate counterfactual: If the project isn’t built, what will happen? As Forte asks, “If you hadn’t spent the money there, where else would you have spent it, and are you sure that by spending it there rather than somewhere else you actually gain something?” 

Based on detailed information about where a scientific collaboration intends to spend their money, economists can take the first step in painting a picture of how that funding will affect the region. The next step is accounting for the secondary spending that this brings.

Companies are paid to do construction work for a scientific project, “and then it sort of cascades throughout the region,” says Jason Horwitz of Anderson Economic Group, which regularly performs economic analyses for universities and physics collaborations. “As they hire more people, the employees themselves are probably going to local grocery stores, going to local restaurants, they might go to a movie now and then—there’s just more local spending.”  

These first parts of the analysis account only for the tangible, concrete-and-steel process of building and maintaining an experiment, though. 

“If you build a bridge, the main benefit is from people who use the build—transportation of goods over the bridge and whatnot,” Forte says. But the benefit of constructing a telescope array or a huge laser interferometer is knowledge-formation, “which is measured in papers and publications, references and so on,” he says. 

One way researchers like Horwitz and Forte have begun to assign value to such projects is by measuring the effect of the project on the people who run it. Like attending university, working on a scientific collaboration gives you an education—and an education changes your earning capabilities. 

“Fundamental research has a huge added value in terms of human capital formation, even if you work there for two years and then you go and work in a company on Wall Street,” Forte says. Using the same methods used by universities, they found doing research at the LHC would raise students’ earning potential by about 11 percent over a 40-year career.

This method of measuring the value of scientific projects still has limitations. In it, the immeasurable, grander purpose of a fundamental science experiment is still assigned no value at all. When it comes down to it, Forte says, if all we cared about were a big construction project, technology spinoffs and the earning potential of students, we wouldn’t have fundamental physics research. 

“The actual purpose of this is not a big construction project,” Horwitz says. “It’s to do this great research which obviously has other benefits of its own, and we really don’t capture any of that.” Instead, his group appends qualitative explanations of the knowledge to be gained to their economic reports. 

Forte explains, “The fact that this kind of enterprise exists is comparable and evaluated in the same way as, say, the value of the panda not being extinct. If the panda is extinct, there is no one who’s actually going to lose money or make money—but many taxpayers would be willing to pay money for the panda not to be extinct.” 

Forte and his colleagues found a 90 percent chance of the LHC’s benefits exceeding its costs (by 2.9 billion euros, they estimate). But even in the 10 percent chance that its economics aren’t quite so Earth-shaking, its discoveries could change the way we understand our universe.

CERN, home to the Large Hadron Collider, is known for high-speed, high-energy feats of coordination, so it’s only fitting that the touring percussion group STOMP would stop by for a visit.

After taking a tour of the research center, STOMP performers were game to share their talent by turning three pieces of retired scientific equipment into a gigantic drum set. Check out the video below to hear the beat of an LHC dipole magnet, the Gargamelle bubble chamber and a radiofrequency cavity from the former Large Electron-Positron Collider.

As CERN notes, these are trained professionals who were briefed on how to avoid damaging the equipment they used. Lab visitors are generally discouraged from hitting the experiments.

Almost every worthwhile performance is preceded by a rehearsal, and scientific performances are no exception. Engineers test a car’s airbag deployment using crash test dummies before incorporating them into the newest model. Space scientists fire a rocket booster in a test environment before attaching it to a spacecraft in flight.

One of the newest “training grounds” for astrophysicists is called Twinkles. The Twinkles dataset, which has not yet been released, consists of thousands of simulated, highly realistic images of the night sky, full of supernovae and quasars. The simulated-image database will help scientists rehearse a future giant cosmological survey called LSST.

LSST, short for the Large Synoptic Survey Telescope, is under construction in Chile and will conduct a 10-year survey of our universe, covering the entire southern sky once a year. Scientists will use LSST images to explore our galaxy to learn more about supernovae and to shine a light on the mysterious dark energy that is responsible for the expansion of our universe.

It’s a tall order, and it needs a well prepared team. Scientists designed LSST using simulations and predictions for its scientific capabilities. But Twinkles’ thousands of images will give them an even better chance to see how accurately their LSST analysis tools can measure the changing brightness of supernovae and quasars. That’s the advantage of using simulated data. Scientists don’t know about all the objects in the sky above our heads, but they do know their simulated sky— there, they already know the answers. If the analysis tools make a calculation error, they’ll see it.

The findings will be a critical addition to LSST’s measurements of certain cosmological parameters, where a small deviation can have a huge impact on the outcome.

“We want to understand the whole path of the light: From other galaxies through space to our solar system and our planet, then through our atmosphere to the telescope – and from there through our data-taking system and image processing,” says Phil Marshall, a scientist at the US Department of Energy's SLAC National Accelerator Laboratory who leads the Twinkles project. “Twinkles is our way to go all the way back and study the whole picture instead of one single aspect.”

Scientists simulate the images as realistically as possible to figure out if some systematic errors add up or intertwine with each other. If they do, it could create unforeseen problems, and scientists of course want to deal with them before LSST starts.

Twinkles also lets scientists practice sorting out a different kind of problem: A large collaboration spread across the whole globe that will perform numerous scientific searches simultaneously on the same massive amounts of data.

Richard Dubois, senior scientist at SLAC and co-leader of the software infrastructure team, works with his team of computing experts to create methods and plans to deal with the data coherently across the whole collaboration and advise the scientists to choose specific tools to make their life easier.

“Chaos is a real danger; so we need to keep it in check,” Dubois says. “So with Twinkles, we test software solutions and databases that help us to keep our heads above water.”

The first test analysis using Twinkles images will start toward the end of the year. During the first go, scientists extract type 1a supernovae and quasars and learn how to interpret the automated LSST measurements.

“We hid both types of objects in the Twinkles data,” Marshall says. “Now we can see whether they look the way they’re supposed to.”

LSST will start up in 2022, and the first LSST data will be released at the end of 2023.

“High accuracy cosmology will be hard,” Marshall says. “So we want to be ready to start learning more about our universe right away!”

Neutrinos are elementary particles first discovered six decades ago. 

Over the years, scientists have learned several surprising things about them. But they have yet to answer what might sound like a basic question: How much do neutrinos weigh? The answer could be key to understanding the nature of the strange particles and of our universe.

To understand why figuring out the mass of neutrinos is such a challenge, first you must understand that there’s more than one way to picture a neutrino.

Neutrinos come in three flavors: electron, muon and tau. When a neutrino hits a neutrino detector, a muon, electron or tau particle is produced. When you catch a neutrino accompanied by an electron, you call it an electron neutrino, and so on. 

Knowing this, you might be forgiven for thinking that there are three types of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. But that’s not quite right. 

That’s because every neutrino is actually a quantum superposition of all three flavors. Depending on the energy of a neutrino and where you catch it on its journey, it has a different likelihood of appearing as electron-flavored, muon-flavored or tau-flavored.

Armed with this additional insight, you might be forgiven for thinking that, when all is said and done, there is actually just one type of neutrino. But that’s even less right. 

Scientists count three types of neutrino after all. Each one has a different mass and is a different mixture of the three neutrino flavors. These neutrino types are called the three neutrino mass states.

A weighty problem

We know that the masses of these three types of neutrinos are small. We know that the flavor mixture of the first neutrino mass state is heavy on electron flavor. We know that the second is more of an even blend of electron, muon and tau. And we know that the third is mostly muon and tau.

We know that the masses of the first two neutrinos are close together and that the third is the odd one out. What we don’t know is whether the third one is lighter or heavier than the others. 

The question of whether this third mass state is the heaviest or the lightest mass state is called the neutrino mass hierarchy (or neutrino mass ordering) problem.

Easy as 1,2,3—or 3,1,2?

Some models that unify the different forces in the Standard Model of particle physics predict that the neutrino mass ordering will follow the pattern 1, 2, 3—what they call a normal hierarchy. Other models predict that the mass ordering will follow the pattern 3, 1, 2—an inverted hierarchy. Knowing whether the hierarchy is normal or inverted can help theorists answer other questions.

For example, four forces—the strong, weak, electromagnetic and gravitational forces—govern the interactions of the smallest building blocks of matter. Some theorists think that, in the early universe, these four forces were united into a single force. Most theories about the unification of forces predict a normal neutrino mass hierarchy. 

Scientists’ current best tools for figuring out the neutrino mass hierarchy are long-baseline neutrino experiments, most notably one called NOvA.

Electron drag

The NOvA detector, located in Minnesota near the border of Canada, studies a beam of neutrinos that originates at Fermi National Accelerator Laboratory in Illinois.

Neutrinos very rarely interact with other matter. That means they can travel 500 miles straight through the Earth from the source to the detector. In fact, it’s important that they do so, because as they travel, they pass through trillions of electrons.

This affects the electron-flavor neutrinos—and only the electron-flavor neutrinos—making them seem more massive. Since the first and second mass states contain more electron flavor than the third, those two experience the strongest electron interactions as they move through the Earth. 

This interaction has different effects on neutrinos and antineutrinos—and the effects depend on the mass hierarchy. If the hierarchy is normal, muon neutrinos will be more likely to turn into electron neutrinos, and muon antineutrinos will be less likely to turn into electron antineutrinos. If the hierarchy is inverted, the opposite will happen. 

So if NOvA scientists see that, after traveling through miles of rock and dirt, more muon neutrinos and fewer muon antineutrinos than expected have shifted flavors, it will be a sign the mass hierarchy is normal. If they see fewer muon neutrinos and more muon antineutrinos have shifted flavors, it will be a sign that the mass hierarchy is inverted. 

The change is subtle. It will take years of data collection to get the first hint of an answer. Another, shorter long-baseline neutrino experiment, T2K, is taking related measurements. The JUNO experiment under construction in China aims to measure the mass hierarchy in a different way. The definitive measurement likely won’t come until the next generation of long-baseline experiments, DUNE in the US and the proposed Hyper-Kamiokande experiment in Japan.

Neutrinos are some of the most abundant particles in the universe. As we slowly uncover their secrets, they give us more clues about how our universe works.