Showdown: Two huge neutrino detectors will vie to probe matter’s origins

Showdown: Two huge neutrino detectors will vie to probe matter’s origins

Science

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A model of this story appeared in Science, Vol 377, Issue 6614.Download PDF

Among physicists, these learning elusive particles known as neutrinos might set the usual for dogged willpower—or stubborn stubbornness. For 12 years, scientists in Japan have fired trillions of neutrinos a whole lot of kilometers by Earth to a big subterranean detector known as Super-Kamiokande (Super-Okay) to examine their shifting properties. Yet the almost massless particles work together with different matter so feebly that the experiment, often called T2K, has captured fewer than 600 of them.

Nevertheless, so alluring are neutrinos that physicists usually are not simply persisting, they’re planning to vastly scale up efforts to make and lure them. At stake could also be perception into one of the crucial profound questions in physics: how the new child universe generated extra matter than antimatter, in order that it’s full of one thing as a substitute of nothing.

That prospect, amongst others, has sparked a race to construct two large subterranean detectors, at prices starting from a whole lot of hundreds of thousands to billions of {dollars}. In an outdated zinc mine close to the previous city of Kamioka in Japan, physicists are gearing up to construct Hyper-Kamiokande (Hyper-Okay), a gargantuan successor to Super-Okay, which will scrutinize neutrinos fired from a particle accelerator on the Japan Proton Accelerator Research Complex (J-PARC) in Tokai 295 kilometers away. In the United States, scientists are creating the Deep Underground Neutrino Experiment (DUNE) in a former gold mine in Lead, South Dakota, which will snare neutrinos from Fermi National Accelerator Laboratory (Fermilab) 1300 kilometers away in Batavia, Illinois.

Researchers with each experiments acknowledge they’re in competitors—and that Hyper-Okay might have a bonus as a result of it will seemingly begin to take information a 12 months or two earlier than DUNE. Yet except for their objectives, “Hyper-Okay and DUNE are vastly totally different,” says Chang Kee Jung, a neutrino physicist at Stony Brook University and a T2K member who now additionally works on DUNE.

science Inside Hyper-K, two researchers in a small raft appear tiny compared to the rows and rows of photosensors, each of which is a golden dome about half a meter in diameter.

Hyper-Kamiokande will be a good greater model of the famed Super-Kamiokande neutrino detector, an enormous, water-filled tank lined with phototubes.Kamioka Observatory/Institute for Cosmic Ray Research/University of Tokyo

Hyper-Okay, which will be greater however cheaper than DUNE, represents the subsequent in a collection of ever bigger neutrino detectors of the identical basic design developed over 40 years by Japanese physicists. It is all however sure to work as anticipated, says Masato Shiozawa, a particle physicist on the University of Tokyo and co-spokesperson for the 500-member Hyper-Okay collaboration. “Hyper-Okay is a extra established know-how than DUNE,” he says. “That is why I proposed it.”

DUNE will make use of a comparatively new know-how that guarantees to reveal neutrino interactions in gorgeous element and permit physicists to take a look at their understanding of the particles with unprecedented rigor. “Without bragging an excessive amount of, we’re greatest at school,” says Sergio Bertolucci, a particle physicist on the University of Bologna and Italy’s National Institute for Nuclear Physics and co-spokesperson for the 1300-member DUNE collaboration. However, that technological edge comes with a hefty price ticket and, Bertolucci acknowledges, extra threat.

How the rivalry performs out will rely upon elements as mundane as the price of underground excavation and as thrilling as the likelihood that neutrinos, all the time quirky, maintain some shock that will rework physicists’ understanding of nature.

The most typical particles within the universe moreover photons, neutrinos exert no impact on the on a regular basis objects round us. Yet they may carry clues to deep mysteries. Neutrinos and their antimatter counterparts each are available three varieties or flavors—electron, muon, and tau—relying on how they’re generated. For instance, electron neutrinos emerge from the radioactive decay of some atomic nuclei. Muon neutrinos fly from the decays of fleeting particles known as pi-plus mesons, which may be produced by smashing a beam of protons right into a goal. These identities aren’t fastened: A neutrino of 1 sort can become one other, chameleonlike, because it zips alongside at close to–light-speed.

Weirdly, a neutrino of a particular taste has no particular mass. Rather, it’s a quantum mechanical mixture of three totally different “mass states.” For instance, a decaying pi-plus spits out the mix of mass states that makes a muon neutrino. However, like gears turning at totally different speeds, the mass states evolve at totally different charges, altering that mixture. So, a particle that started as an electron neutrino would possibly later seem as a tau neutrino—a phenomenon often called neutrino oscillation.

Theorists can clarify all of this with a mathematical clockwork often called the three-flavor mannequin. It has only a handful of parameters: roughly talking, the possibilities with which one taste will oscillate into one other and the variations among the many three mass states. The image has gaps. Experiments present two mass states are shut, however not whether or not the 2 related states are lighter or heavier than the third—a puzzle often called the hierarchy drawback.

Moreover, neutrinos and antineutrinos would possibly oscillate by totally different quantities, an asymmetry known as charge-parity (CP) violation. Measuring that asymmetry is the prize physicists search, because it may assist clarify how the soup of fundamental particles within the early universe generated extra matter than antimatter.

Neutrinos in high-def

Comprising two rectangular tanks, every full of 17,000 tons of liquid argon, DUNE will exactly monitor all of the charged particles produced when a high-energy neutrino fired from a distant laboratory strikes an argon nucleus.

science Heading: Tracing tracks. The charged particles will liberate electrons, which an electric field will push to a grid of wires. By timing when each wire fires, physicists can infer the particles’ tracks and, ultimately, the incoming neutrino’s energy. Distance between Fermi National Accelerator Laboratory (Batavia, Illinois) and Sanford Underground Research Facility (Lead, South Dakota): 1300km.

Ring round a neutrino

An underground tank full of 260,000 tons of water and lined with photodetectors, Hyper-Kamiokande will detect neutrinos shot from a whole lot of kilometers away by exploiting the optical equal of a sonic increase.

science Heading: Shocking! Striking a nucleus, a neutrino can convert into an electron or muon that travels faster than light in water and generates a shock wave of light, casting a ring on the side of the tank. Distance between Kamioka mine (Kamioka, Japan) and J-PARC (Tokai, Japan): 295km.

V. Altounian/Science

The wispy neutrinos themselves didn’t tilt the matter-antimatter steadiness. Rather, in accordance to some theories, the acquainted neutrinos are mirrored by vastly heavier “sterile” neutrinos that may work together with nothing besides neutrinos. If sterile neutrinos and antineutrinos additionally behave asymmetrically, then within the early universe their decays may have generated extra electrons than antielectrons (additionally known as positrons), seeding the dominance of matter.

Seeing CP violation in unusual neutrinos wouldn’t show this state of affairs performed out, notes Patrick Huber, a theorist at Virginia Polytechnic Institute and State University. But not seeing CP violation amongst unusual neutrinos would render it a lot much less seemingly that the hypothetical heavyweights possess the important thing asymmetry, Huber says. “It’s not inconceivable, nevertheless it’s implausible,” he says.

But, first, scientists should decide whether or not neutrinos actually exhibit this asymmetry. The groups in Japan and the United States will each make use of a well-established approach to probe neutrino conduct. By smashing energetic protons from a particle accelerator right into a goal to produce pipluses, they will generate a beam of muon neutrinos and shoot it towards a distant underground detector. There, researchers will depend the surviving muon neutrinos and the electron neutrinos which have emerged alongside the best way. Then they will change to producing a beam of muon antineutrinos, by gathering pi-minuses as a substitute of pi-pluses from the goal. They’ll repeat the measurements, in search of any variations.

science An output image from DUNE.

In MicroBOONE, a small liquid-argon detector at Fermilab, an brisk neutrino spawns charged particles, together with an electron (lengthy monitor).MicroBOONE Collaboration

The experiment is way tougher than it sounds, as a number of different elements may create a spurious asymmetry. For instance, the neutrino and antineutrino beams will inevitably differ barely, each in depth and of their power spectrum. To account for such variations, the researchers should pattern the particles as they begin their journey by inserting a small detector, ideally with a design as related as attainable to the distant detector, in entrance of the beam supply.

The physics of neutrinos themselves may additionally skew the outcomes. For instance, both neutrinos or antineutrinos will be absorbed extra strongly by the matter they traverse on their flight to the detector. The path of that impact relies on the answer to the hierarchy drawback. So, to spot CP violation, physicists will almost certainly have to remedy the hierarchy drawback, too.

The greatest barrier to sorting all of this out, nonetheless, has been the measly harvest of neutrinos from even the most important experiments. Like their counterparts in Japan, U.S. physicists have already got a neutrino-oscillation experiment, NOνA, which shoots neutrinos from Fermilab to a detector 810 kilometers north in Minnesota. Like T2K, it has netted simply a number of hundred neutrinos.

Hyper-Okay will deal with the shortage primarily by offering a a lot greater goal for the neutrinos to hit. Proposed a decade in the past, it’s a scaled-up model of the storied Super-Okay detector and will encompass a cylindrical stainless-steel tank 78 meters tall and 74 meters vast, holding 260,000 tons of ultrapure water—5 instances as a lot as Super-Okay.

To spot neutrinos, the detector will depend on the optical equal of a sonic increase. Rarely, a muon neutrino zipping by the water will knock a neutron out of an oxygen atom and alter it right into a proton, whereas the neutrino itself morphs right into a high-energy muon. The fleeing muon will truly exceed the pace of sunshine in water, which is 25% slower than in a vacuum, and generate a shock wave of so-called Cherenkov gentle, simply as a supersonic jet creates a shock wave of sound. That conelike shock wave will forged a hoop of sunshine on the tank’s facet, which is lined with photodetectors.

Similarly, an electron neutrino can strike a neutron to produce a high-speed electron, which is lighter than a muon and will be buffeted extra by the water molecules. The consequence will be a fuzzier gentle ring. Muon and electron antineutrinos can spawn detectable antimuons and antielectrons by placing protons, though with about half the effectivity of the neutrino interactions.

science Output image from Super-K data. The amount of light detected by the photosensors is represented as colored squares, which form a ring.

science Output image from Super-K data. The colored squares form another ring, this one with fuzzier edges.

In Super-Okay, a muon neutrino turns right into a muon, which radiates a tidy gentle ring (first picture). An electron neutrino spawns an electron and a fuzzier ring (second picture). Kamioka Observatory/Institute for Cosmic Ray Research/University of Tokyo

Hyper-Okay will be Japan’s third nice detector, all in the identical mining space. From 1983 to 1995, the Kamioka Nucleon Decay Experiment (Kamiokande), a 3000-ton detector, tried to spot the ultrarare decays of protons that some theories predict. Instead, in 1987, it glimpsed neutrinos from a supernova—an advance that gained a share of the Nobel Prize in Physics in 2002. In 1996, Super-Okay got here on-line. It proved neutrinos oscillate by learning muon neutrinos generated when cosmic rays strike the environment. Fewer come up from the bottom than down from the sky, displaying that these traversing Earth change taste alongside the best way. The discovery shared the Nobel in 2015. “It’s spectacular what [Japanese physicists] have achieved,” says Erin O’Sullivan, a neutrino astrophysicist at Uppsala University and a Hyper-Okay member who was drawn by “the dynasty of Super-Okay.”

Hyper-Okay will reuse J-PARC’s neutrino beam, which is now being upgraded to improve its energy by an element of two.5. Overall, it ought to gather information at 20 instances the speed of T2K, says Stephen Playfer, a particle physicist on the University of Edinburgh and the University of Tokyo and the challenge’s lead technical coordinator. Before becoming a member of Hyper-Okay in 2014, he and his Edinburgh colleagues additionally thought-about becoming a member of DUNE. “When it got here to evaluating who was going to be first to see one thing, we thought Hyper-Okay was in a superb place, simply because it could have the statistics and it had a widely known know-how,” he says.

Hyper-Okay will have limitations. In explicit, it gained’t measure the neutrinos’ energies exactly. That issues as a result of the speed at which a neutrino oscillates relies on its power, and a beam incorporates neutrinos with a spread or spectrum of energies. Without a approach to pinpoint every neutrino’s power, the experiment could be unable to make sense of the oscillation charges.

To keep away from this drawback, Hyper-Okay, like present experiments, will depend on a trick. A neutrino beam naturally diverges, with decrease power neutrinos spreading greater than larger power ones. Thus, if a detector sits barely to the facet of the beam’s path, it will see neutrinos with a narrower vary of energies that ought to oscillate at roughly the identical charge. So, like Super-Okay, Hyper-Okay will sit off the beam axis by an angle of two.5°.

Physicists can then tune the power of the beam so the neutrinos attain the detector when the oscillation is at its most. With the neutrinos’ power constrained, physicists principally depend the variety of arriving muon neutrinos, electron neutrinos, and their antimatter counterparts. Hyper-Okay’s CP measurement comes down to evaluating two ratios: electron neutrinos with muon neutrinos and electron antineutrinos with muon antineutrinos.

Workers have already begun the excavation for Hyper-Okay, which ought to take 2 years, Shiozawa says. The entire challenge will value Japan about $600 million, with worldwide companions chipping in a further $100 million to $200 million, he says. The detector will be full in 2027, Shiozawa says, and will begin taking information a 12 months later. So assured are Hyper-Okay researchers of their know-how that they are saying the trickiest a part of the challenge is the digging. “We want to assemble in all probability the biggest underground cavern” on the planet, Shiozawa says. “In phrases of know-how and likewise value, that is the most important problem.”

If, technologically, Hyper-Okay quantities to way more of the identical, DUNE goals to be one thing nearly utterly totally different. It will make use of a know-how that, till just lately, was utilized in just one different massive experiment however that ought to allow physicists to see neutrino interactions as by no means earlier than. “To me, the draw of DUNE is its precision,” says Chris Marshall, a particle physicist on the University of Rochester and DUNE’s physics coordinator. “This is an experiment that will be world main in nearly every part that it measures.”

science A person stands inside the rectangular DUNE tank. The walls, floor, and ceiling are all an identical golden grid.

Since 2015, DUNE researchers have constructed prototypes on the European particle physics laboratory, CERN, which have carried out even higher than anticipated. Brice Maximillien/CERN

Hunkering 1480 meters deep in a repurposed gold mine, DUNE will encompass two rectangular tanks 66 meters lengthy, 19 meters vast, and 18 meters tall. Each will include 17,000 tons of frigid liquid argon cooled to under –186°C. Just as in a water-filled detector, a neutrino can blast a neutron—on this case in an argon nucleus—to create a muon or an electron. But the neutrinos reaching DUNE from Fermilab will pack up to 10 instances extra power than these flowing to Hyper-Okay. So, as well as to the muon or electron, a collision will usually produce a spurt of different particles comparable to pions, kaons, protons, and neutrons.

DUNE goals to monitor all these particles—or at the very least the charged ones—with a know-how known as a liquid argon time projection chamber. As a charged particle streaks by the argon, it will ionize a number of the atoms, liberating their electrons. A powerful electrical area will push the electrons sideways till they hit three intently spaced planes of parallel wires, every aircraft oriented in a unique path. By noting when the electrons strike the wires, physicists can reconstruct with millimeter precision the unique particle’s 3D trajectory. And from the quantity of ionization it produces, they’ll decide its sort and power.

The particulars are mind-boggling. The electrons will have to drift so far as 3.5 meters, pushed by a voltage of 180 kilovolts. And not like Hyper-Okay, DUNE will sit straight within the beam from Fermilab. So, it will seize a much bigger however messier harvest of neutrinos, with energies starting from lower than 1 giga-electron volt to greater than 5 GeV.

DUNE’s means to exactly monitor all of the particles ought to allow it to do one thing unprecedented in neutrino physics: Measure the power of every incoming neutrino to assemble power spectra for every taste of neutrino and antineutrino. Because of the flavour altering, a plot of every spectrum ought to itself exhibit a definite wiggle or oscillation. By analyzing the entire spectra, physicists ought to give you the chance to nail down all the three-flavor mannequin, together with the quantity of CP violation and the hierarchy, in a single fell swoop, Bertolucci says. “It can measure all of the parameters in the identical experiment,” he says.

Until now, the know-how has by no means been absolutely developed. Italian Nobel laureate Carlo Rubbia dreamed up the liquid argon detector in 1977. But it wasn’t till 2010 that one known as ICARUS in Italy’s subterranean Gran Sasso National Laboratory snared a number of neutrinos shot from the European particle physics laboratory, CERN, close to Geneva, Switzerland. Researchers at Fermilab and CERN have launched into a crash program to construct prototypes, which have labored even higher than anticipated, says Kate Scholberg, a neutrino physicist and a DUNE workforce member at Duke University. “It’s form of an entrancing factor to have a look at the occasion shows” coming in, she says. “It’s simply unbelievable element.”

science A researcher holds up a piece of equipment to the side of the DUNE tank. Their headlamp lights up an overlapping group of wires.

Light glints off the planes of intently spaced, electron-catching wires inside a DUNE prototype. The wires are 150 micrometers thick, like heavy hair.CERN

That precision comes at a value. For accounting functions, the U.S. Department of Energy (DOE) splits the challenge in two. One piece, the Long Baseline Neutrino Facility (LBNF), consists of the brand new neutrino beam at Fermilab and all of the infrastructure. The second, DUNE, is a global collaboration that will construct simply the center of the detectors. In 2015, DOE estimated LBNF/DUNE would value $1.5 billion and are available on-line in 2027. Last 12 months, nonetheless, DOE reported that sudden building prices had raised the invoice to $3.1 billion. The detector needs to be accomplished in 2028, says Christopher Mossey, challenge director for LBNF/DUNE-U.S. But the beam will lag till early 2031, probably giving Hyper-Okay a head begin of greater than 2 years.

With contracts in hand and building underway, DUNE builders are assured that the brand new value and timeline will maintain. Excavation has handed 40% and needs to be accomplished in May 2023, Mossey says. “We actually are undertaking huge, tangible issues.” Still, DUNE physicists acknowledge that the challenge is riskier than Hyper-Okay. “It’s a leap into the unknown, and that’s the trade-off you make,” Scholberg says. “Something that’s extra transformative goes to actually entail extra scariness.”

Both experiments produce other scientific objectives, comparable to looking for proton decay. There, Hyper-Okay has a bonus, Huber says, as it’s merely greater and incorporates numerous lone protons within the hearts of the hydrogen atoms in water molecules. Another tantalizing payoff may come if a large star collapses and explodes as a supernova close to our Galaxy, as one did in 1987. The experiments would offer complementary observations, Scholberg says, as DUNE would see the electron neutrinos produced simply because the core implodes and Hyper-Okay would see primarily electron antineutrinos launched later within the explosion.

Nevertheless, the raison d’être for each experiments stays deciphering neutrino oscillations and looking for CP violation. So, how would a gambler handicap this race?

Given their head begin, Hyper-Okay physicists may rating main discoveries earlier than DUNE even finds its toes. If CP violation is as huge as it might probably be, “then we might uncover it in 3 years,” Shiozawa says. “And additionally, we might uncover proton decay in 3 years.” But, he says, “It actually relies on nature.”

Hyper-Okay is optimized to measure CP violation assuming it’s huge and the three-flavor mannequin is the ultimate phrase on neutrino oscillations, Huber notes. Neither assumption might maintain. And with its easier counting approach and shorter baseline, the experiment might wrestle to distinguish CP violation from the matter impact except another experiment independently solves the hierarchy drawback. “Hyper-Okay actually requires extra exterior inputs,” Huber says.

science A researcher stands by a grid of six Hyper-K phototubes, which resemble large golden lightbulbs half a meter in diameter. Each is partially enclosed in a black box, so that just the dome of the bulb is exposed.

Hyper-Kamiokande will deploy new and improved phototubes, which should face up to pressures up to six atmospheres on the backside of the tank.Kamioka Observatory/Institute for Cosmic Ray Research/University of Tokyo

DUNE, in distinction, ought to give you the chance to disentangle the entire mess by itself. Shiozawa, for one, just isn’t counting out his rival. The Japanese challenge has skilled rising pains of its personal, he notes, together with being scaled again from an preliminary 1-million-ton design. And the Japanese authorities gained’t countenance any value improve, placing challenge leaders in fixed stress with contractors, he says. “The state of affairs just isn’t so totally different between the 2 initiatives.”

Ultimately, the rivalry between Hyper-Okay and DUNE could also be much less a splash for glory than a decadeslong slog by uncertainty. If so, the 2 groups may find yourself collaborating as a lot as they compete, at the very least informally. “We’ll have a very long time the place essentially the most correct outcomes will come from a mixture of the 2 [experiments],” Huber predicts.

Most tantalizing, as a substitute of finishing the present concept, the outcomes may upend it. They would possibly reveal deviations from the three-flavor mannequin that would trace at new particles and phenomena lurking within the vacuum. After all, neutrinos have repeatedly shocked physicists, who as soon as assumed that the particles got here in just one sort and have been utterly massless and inert. “Previously, neutrino experiments have taught us that we very hardly ever take information in a neutrino beam and get precisely what we anticipate,” Marshall says.

The sudden could also be an extended shot value betting on.

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