Nathan Whitehorn was not in a good place. It was 2012 and he had just finished his PhD analyzing data from the IceCube Neutrino Observatory in Antarctica. He’d been trying to find neutrinos (weakly interacting fundamental particles that are almost massless) coming from gamma ray bursts in distant galaxies, and he had drawn a blank.
“Everything was always zero, and had been zero from when we turned on the instrument,” he recalls. “It was a little bit depressing. ” But just months later his luck turned.
As his computer at the University of Wisconsin–Madison started churning through a couple of years of IceCube data—using a new way of hunting high-energy neutrinos Whitehorn and his colleague Claudio Kopper had cooked up—alerts signaling a potential detection started pinging up onscreen. The pair quickly corralled their colleagues from down the hall into a small conference room to watch it all unfold. As each alert sounded, the researchers did some rapid checks to ensure the signal wasn’t garbage.
“By the time we finished looking at one event, another would pop up,” says Whitehorn. “It was something else. ” Eventually, the count got up to 28 and stopped.
They had confirmed the detection (made a few months earlier by Japanese colleagues) of the first two high-energy neutrinos known to come from outside our galaxy, and spotted 26 more for good measure. Within a week, the young postdoc found himself presenting his findings over the phone to most of the IceCube collaboration. Without wanting to blurt out the results before they were sure, the team went through roughly a year of cloak-and-dagger confirmation before finally, in late November 2013, letting the whole world know .
But the job wasn’t quite done yet. The IceCube researchers knew the neutrinos came from outside the galaxy. But they didn’t know what was producing them or exactly where they were being made.
If they could identify the sources of extragalactic neutrinos, it would open a new window into the cosmos. Unfortunately, that proved a tough nut to crack. Frustrated, Whitehorn left IceCube in 2014 to work on other projects.
But his self-imposed exile didn’t last. “I came back because it kept bothering me,” he says. His timing was perfect.
Weeks after his return, on September 22, 2017, IceCube captured a neutrino the team subsequently traced back to its origin : a type of supermassive black hole shooting plasma jets straight at Earth, called a blazar. Combined with the first direct observation of gravitational waves in 2015, this one neutrino seemed to herald a new era of astronomy—one no longer solely reliant on using the spectrum of light to observe the universe. However, though gravitational wave astronomy has kicked on—these ripples in spacetime have been recorded 90 times since 2015—back at IceCube, cosmic neutrinos remain stubbornly elusive.
No other high-energy neutrino sources have been reported to the same confidence level as the 2017 blazar neutrino. Until an even bigger detector can be constructed, neutrino-hunting will remain a slow slog. IceCube is an example of how big science, and particularly particle physics, now often works on generational time scales.
Getting from the idea of IceCube to actually drilling its neutrino sensors into a cubic kilometer of Antarctic ice to pinpointing a high-energy neutrino source took 30 years. In that time, key personnel retired, passed away, or moved on to projects offering more instant gratification. Whitehorn’s experience is the exception, not the rule—many scientists have devoted years, decades, or even entire careers to seeking results that never came.
The discovery of the Higgs boson took even longer than extragalactic neutrinos: 36 years from initial discussions about building the world’s biggest and highest-energy particle collider—the Large Hadron Collider (LHC)—to the now famous announcement of the particle’s discovery in 2012. For Peter Higgs, then aged 83, the detection of his eponymous particle was a satisfying epilogue to his career. He shed a tear in the auditorium during the announcement—a full 48 years after he and others first proposed the Higgs field and its associated elementary particle back in 1964.
For Clara Nellist, who was a PhD student working on the LHC’s ATLAS experiment in 2012, it marked a thrilling beginning to her life as a physicist. Nellist and a friend turned up at midnight before the announcement with pillows, blankets, and popcorn and camped outside the auditorium hoping to get a seat. “I did that for festivals,” she says.
“So why wouldn’t I do it for possibly the biggest physics announcement of my career?” Her determination paid off. “To hear the words ‘I think we have it!’ and the cheer in the room was just such an amazing experience. ” The Higgs particle was the last piece of the puzzle that is our best description of what makes up the universe at the smallest scales: the Standard Model of particle physics.
But this description can’t be the final word. It doesn’t explain why neutrinos have mass or why there’s more matter than antimatter in the universe. It doesn’t include gravity.
And there’s the small matter of it having nothing to say about 95 percent of the universe: dark matter and dark energy. “We’re at a really interesting time because when we started, we knew the LHC would either discover the Higgs or rule it out completely,” says Nellist. “Now we have many unanswered questions, and yet we don’t have a direct road map saying that if we just follow these steps, we’ll find something.
” Ten years on from the Higgs discovery, how does she cope with the possibility that the LHC might not answer any more of these fundamental questions? “I’m very pragmatic,” she says. “It’s a bit frustrating, but as an experimental physicist I believe the data, and so if we do an analysis and get a null result, then we move on and look in a different place—we’re just measuring what nature provides. ” The LHC isn’t the only big science facility hunting for answers to these existential questions.
ADMX might be the garage band to LHC’s stadium rockers in terms of size, funding, and personnel, but it happens to also be one of the world’s best shots at uncovering the hypothetical axion particle—a leading candidate for dark matter . And unlike at the LHC, ADMX researchers have set out a clear path to finding what they seek. Theory suggests one of the few ways to spot axions—which could be constantly showering Earth without our knowing—is with strong magnetic fields, which should change axions into photons.
Once they’re photons, researchers would then measure the light’s frequency, which would directly relate to the axion mass. ADMX aims to do just that. “It’s really a glorified AM radio,” says Gianpaolo Carosi, ADMX co-spokesperson.
If axions do exist and the instrument is tuned to precisely the right wavelength, its cavity will resonate, amplifying their signal so that ultra-sensitive quantum electronic detectors can pick it up. “Every 100 seconds or so we just sit at one frequency and get noise like that hiss that you hear on your radio when you don’t have signal,” says Carosi. “Then we’ll move just a small amount, about a kilohertz, and we’ll do another 100 seconds.
” First constructed in 1995, ADMX only achieved the full sensitivity needed to probe whether the axion might be the dark matter particle in 2018. Since then, researchers have been slowly turning the dial through the frequencies. They will complete the current search around 2025.
Though work to optimize the axion hunt is unending, and random fake signals injected into the detector keep the team on their toes, Carosi needs little extra motivation to keep going—even with the very real prospect of potentially having to listen to seven years of static. “I would love for the axion to show up, but if we find dark matter elsewhere, or the axion is ruled out as a candidate, I’m fine with that,” he says. “We’ve already kind of drunk the Kool-Aid.
” Carosi, Whitehorn, Nellist, and thousands of others working on these big science projects aren’t looking for fame or glory. They aren’t even particularly motivated by proving one theory over another. They just love fundamental physics and building cool instruments—and hope they’re standing under the right branch of the physics tree when the next fruit falls.
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From: wired
URL: https://www.wired.com/story/particle-hunters-physics-icecube-lhc/
