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06/17/2020 - A Startling Excess of Particle Detections by the XENON1T Could Point to New Physics

by Adam Hadhazy

scientists in full so-called bunny suits, head to toe white coverings, work on the array of the detector, lots of small cylinders with wiresExperts construct the top PMT array. Image courtsey of XENON collaboration.

A strange thing happened while running the most sensitive dark matter detector built to date, known as XENON1T. Having painstakingly accounted for all known sources of particles that could trigger the exquisitely sensitive apparatus, researchers confidently expected 232 triggering events—no more, no less. Yet XENON1T racked up a surprising excess of 285 particle detections. Researchers are cautiously elated by the findings, announced earlier this week, which could point to brand-new physics.  

To be clear, the eyebrow-raising excess does not match the signal for dark matter—XENON1T's primary quarry, a theoretical substance that constitutes as much as 85 percent of the matter in the cosmos. But on the short list of three conceivable candidates behind the excess, two would represent breakthroughs of their own in physics. The pedestrian candidate is a miniscule trace of tritium, a radioactive form of hydrogen, inside the detector. More likely, however, is a never-before-seen type of particle, called a solar axion, pumped out by the Sun. The final possibility: an undiscovered property of neutrinos, the ubiquitous and ghostly particles that pass through every square centimeter of Earth—including our bodies—by the trillions every second. 

However the excess shakes out, it's a big moment for the XENON1T collaboration, which involves more than 160 scientists from 28 institutions in 11 countries. Six university research groups are based in the United States, including one at the University of Chicago, home to the Kavli Institute for Cosmological Physics. KICP has helped support the involvement in XENON1T of Luca Grandi, Associate Professor of Physics at UChicago, and his graduate student Evan Shockley, one of the analysis leads behind the new results.  

"We have been very cautions and paranoid and have been sitting on this data for a very long period to try to find flukes in our analysis that could have artificially produced the bump," says Luca Grandi, a member of KICP. "We hammered down all potential sources of systematic error that we could think of, but the excess turned out to be very solid and significant."

XENON1T accumulated 278 days of data during runs from October 2016 to February 2018. It represents the latest in an increasingly powerful line of experiments operated at the National Institute for Nuclear Physics' Laboratori Nazionali del Gran Sasso, located in central Italy. Shielded under 1400 meters of mountain rock to avoid contamination from cosmic rays raining down from space, the facility is a premier location for highly sensitive traps for elusive particles. 

The XENON1T experiment itself consists of a giant tank filled with 3.2 tonnes of the element xenon, kept super-chilled in a liquid form at nearly -100 degrees Celsius. The xenon is ultra-purified to be free of radioactive elements, whose decay would trigger the instrument's sensors. When particles do enter XENON1T and undergo rare interactions with its xenon dragnet, the interaction produces tiny light signals that researchers analyze. In most cases, the blips are attributable to expected, ho-hum sources—a so-called background. Finding any excess, then, above and beyond this deeply studied background is grounds for excitement.

The best bet for the newly announced excess, in terms of matching observed signal to theoretical predictions, is an extremely lightweight particle called an axion. These particles were put forth in the late 1970s to work out a kink in the strong force, which holds matter together at the subatomic level and is one of the four fundamental forces of nature. If the Sun does produce XENON1T-detectable versions of these particles, that would boost the case for axions having been produced during the Big Bang 13.8 billion years ago. Such primordial axions should have been cranked out in mind-bogglingly prodigious amounts—enough, in fact, to constitute the universe's long-sought dark matter. So while the recently observed excess is not dark matter proper, it could point the way toward at last tracking down the mysterious substance.

The other compelling candidate for the excess is neutrinos (also produced by the Sun) possessing a larger-than-expected, so-called magnetic moment. All particles have this property, though just what it is for neutrinos has yet to be pinned down (as with so much else involving these enigmatic motes of matter). Neutrinos are already the bad boys and girls in the Standard Model, the encompassing framework for particle physics and three out of nature's four fundamental forces. Discovering an anomalous magnetic moment for the particles would only further blaze trails into new physics. 

"If the excess had to come from solar axions or neutrino anomalous magnetic moment, then this would have big implication on our present understanding of particle physics," says Grandi.

The least heart-stopping candidate for the excess, tritium, would still be important to firmly nail down in order to advance the search for dark matter and other novel particles. The tritium background contamination within XENON1T required to yield the excess would be infinitesimal—just a single tritium atom for every 1025 xenon atoms. (1025 is 10 septillion, but you already knew that, of course).  

"The detector is sensitive enough to see this excess, but not enough to discriminate among the few potential sources that we have considered and that might cause it, some including exciting new physics and some foreseeing the existence of a new type of background that was not accounted for before," says Grandi. "When you push your technology to the edge to be sensitive to these elusive particles, you sometimes bump into unexpected background sources that nobody had thought about before."

The jury likely won't remain out long, thanks to the next generation of the XENON1T experiment, dubbed XENONnT. The upgrade will deliver a xenon mass that is three times larger than XENON1T's and have even more precise components, lowering the background still further, thus increasing events while honing their possible origins. Major progress took place with readying XENONnT earlier this year before the novel coronavirus pandemic brought much of the world to a standstill, and at present, the next-gen detector's start-up is anticipated in late 2020.  

"Given our estimates, we expect that XENONnT will be able to distinguish among the various hypotheses in a few months of data taking," Grandi added in a statement. "This makes even more worth the big effort made, early in the year, to seal the new detector before the lockdown kicked in."

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