DUNE experiment prepares for supernova watch

• This article was first published in Symmetry and can be found here:  https://www.symmetrymagazine.org/article/dune-experiment-prepares-for-supernova-watch?language_content_entity=und

At this very moment, somewhere in the universe, a star is about to die.

Well, not “die,” exactly. As PhD student David Sweeney and astronomy professor Peter Tuthill of the University of Sydney point out, dying is a term astronomers have borrowed from biology. Stars that cease to produce energy are considered dead.

But if a star is massive enough—at least about eight times as massive as our sun—it releases one last blast of energy as it dies, in an explosion known as a core-collapse supernova. Researchers see these supernovae as high-energy laboratories that could help us unlock cosmic mysteries. 

While these spectacular events happen regularly throughout the universe, scientists estimate that in our own galaxy, the Milky Way, they happen only around twice per century. Because researchers who witness a nearby supernova are unlikely to get a second chance, it’s important for them to be ready to gather as much data from the event as possible, as quickly as possible.

In the near future, astronomers will have a new tool to help them spot a nearby supernova: the Deep Underground Neutrino Experiment. And with help from fellow physicists on experiments at the Large Hadron Collider, DUNE scientists hope to disperse that information fast enough for astronomers around the globe to see the explosive event in action.

Laboratories in the sky

Scientists will use the massive detectors of DUNE—located at the Sanford Underground Research Facility in South Dakota and hosted by the US Department of Energy’s Fermi National Accelerator Laboratory—to capture data from fundamental particles called neutrinos. Neutrinos are ubiquitous—they’re released as a part of natural processes on our planet and throughout the universe—but they’re also difficult to detect.

For the most part, DUNE scientists will study neutrinos from a powerful beam produced at Fermilab and sent to South Dakota through the Earth. But DUNE detectors will begin collecting neutrino data as soon as they are turned on, with or without the beam. So they will also catch neutrinos produced in the surrounding rock, Earth’s atmosphere, the sun—and potentially a nearby supernova. 

A huge amount of energy is released during a supernova event, and though some of that energy is released as a fantastic burst of light that can shine for months, about 99% of the energy is released in the form of neutrinos and their antimatter counterparts, antineutrinos. 

Because neutrinos interact so rarely with matter, they are able to quickly escape the collapsing core of a supernova. Light, on the other hand, goes through multiple rounds of being absorbed and reemitted by the matter around it and thus takes longer to get out. That means the neutrinos zipping away from the supernova will arrive on Earth well before the light that is emitted from the explosion. So, for astronomers wishing to capture the light, the neutrinos serve as an early warning signal.

In a paper published this summer, researchers demonstrated that they will be able to use the interactions of supernova neutrinos in the DUNE detector to determine the location of a supernova in the sky.

Beyond using the particles to find the light, physicists can also learn from the neutrinos themselves. “We always say neutrinos are like the black-box recorders,” says Fermilab physicist Sam Zeller. “They come back with information telling us about the inner workings of stars.”

Irene Tamborra, an astrophysicist with the Niels Bohr Institute, says neutrinos give scientists a unique view of physics under extreme conditions. “The densities, the temperatures, the timescales,” she says—“nowhere in the world can we recreate it.”

Capturing neutrinos from a dying star could also help scientists learn about another of the universe’s greatest mysteries: what happens in the birth of a black hole. 

Most galaxies, including ours, have gigantic black holes at their centers. The Milky Way galaxy’s is called Sagittarius A*, and is 4 million times the Sun’s mass. While physicists are studying existing black holes, they have never actually witnessed the birth of one. 

Scientists think some black holes may form through the merger of multiple, smaller black holes, and others may be created following the death of a single, massive star.

“Not every one of those core collapses turns into a supernova,” says Kate Scholberg, a neutrino researcher with Duke University. “If it makes a black hole, there may not be a supernova. We would see the neutrinos suddenly stop. That would be pretty exciting.”

To catch a dying star

Each neutrino that emerges from a supernova carries only a tiny fraction of the energy of the star’s explosion. And the lower a neutrino’s energy, the harder it is to detect. 

Compared to the neutrinos produced in the beam at Fermilab, supernova neutrinos will be more difficult to find. Neutrinos from supernovae can be thought of as “grapefruit-sized as opposed to car-sized,” Scholberg says. “You don’t have that much energy, and it’s not as clean a signature. It’s easier to lose them in noise.” 

The trick to using DUNE to help scientists study a supernova will be recognizing these very small bursts of energy quickly enough—often in just minutes. “All the energy of the supernova is confined to the first 100 seconds of the event,” says Fermilab scientist Jennifer Ngadiuba. 

Researchers working on DUNE are actively developing what are called "triggers"—algorithms aimed at quickly sorting through millions of recorded particle interactions and saving only the most interesting for physics. This is a common technique used in HEP experiments, including big experiments such as ATLAS and CMS at the Large Hadron Collider at CERN, where hundreds of millions of particle collisions occur within seconds. 

The trigger algorithms will enable researchers in DUNE to quickly identify signs of a nearby supernova and send out an alert. The alert will be sent to astronomers worldwide, who could then orient their telescopes in time to capture the light as it catches up to the neutrinos.

DUNE’s detectors will be unique in the sense that while previous experiments—such as Kamiokande, IMB and Baksan—have been able to detect antineutrinos from astronomical events, DUNE will be able to detect neutrinos. This will allow researchers to compare knowledge gained from both matter and antimatter.