Dark-matter detector observes a rare nuclear decay

Dark matter must exist. Gravitational phenomena such as the rotation of galaxies (see Physics Today, December 2006, page 8 ) and lensing of light from distant stars (see Physics Today, June 2015, page 18 ) consistently show signs of far more mass than ordinary protons, neutrons, and electrons can account for. Unless there’s some large but subtle gap in the theory of general relativity, the universe must be swarming with some mysterious particles that feel the force of gravity but can’t be seen or touched.

What those particles are is anyone’s guess. One possibility—that dark matter is made of weakly interacting massive particles, or WIMPs—is appealing because it arises theoretically out of solutions to unrelated problems in particle physics. WIMPs, if they exist, could interact with ordinary matter not just gravitationally but also via a force on the scale of the weak interaction that neutrinos experience. So in principle, they can be detected the same way neutrinos are: Gather a large quantity of some material that’s expected to produce a distinct scattering signature, put it deep underground to shield it from cosmic rays, surround it with sensors, and wait.

Dozens of would-be WIMP detectors have been built and operated over the years in underground labs around the world. With the exception of disputed results from one group (see Physics Today, July 2016, page 28 ), they’ve all come up empty so far. But even if WIMPs aren’t the solution to the dark-matter puzzle, the effort to observe them needn’t be all for naught. The extraordinarily sensitive detectors, painstakingly rid of almost all background, are in a position to make measurements that no other experiments can, and they potentially can uncover new physics in quarters unrelated to dark matter.

The XENON collaboration, founded in 2002 by Elena Aprile of Columbia University, has just made that potential look a lot more like a reality. ¹ With its detector—which unsurprisingly uses chilled xenon as its target material—at Gran Sasso National Laboratory in Italy, the group has observed a rare form of nuclear decay, two-neutrino double-electron capture (2νECEC), in the neutron-poor isotope ¹²⁴Xe.

Figure 1.

The result itself is not terribly startling. Nuclear theory predicts that ¹²⁴Xe should undergo 2νECEC, and the measured half-life is in line with expectations. But it’s an experimental tour de force. Less than 0.1% of natural Xe is ¹²⁴Xe, and its half-life, (1.8 ± 0.6) × 10²² yr, is the longest ever measured directly. As WIMP detectors continue to improve, they’ll be ready to observe even rarer events. One much-discussed possibility is the neutrinoless version of the same decay, 0νECEC. If detected, 0νECEC would establish that neutrinos are their own antiparticles and that lepton number is not conserved.

Slow decay

A cousin of two-neutrino double-beta decay (2νββ; see Physics Today, December 1987, page 19 ), 2νECEC is also related to single-beta decay and single-electron capture. All are based on the same fundamental weak interaction: Either a neutron decays into a proton, an electron, and an antineutrino, or a proton and electron combine to yield a neutron and a neutrino. (Nuclear-physics parlance isn’t always fussy about distinguishing neutrinos and antineutrinos, but to conserve lepton number, they need to be counted as different particles.)

Nuclides that undergo beta decay include tritium and carbon-14, with half-lives of 12 yr and 5700 yr, respectively. But 2νββ, which requires the weak process to happen twice simultaneously, is correspondingly more infrequent: All known half-lives are greater than 10¹⁸ yr, so it’s detectable only in nuclides for which single-beta decay is forbidden or strongly suppressed.

In general, 2νECEC is rarer still, because it requires not just two simultaneous weak events but also two electrons to be localized in the nucleus at the same time. Before XENON’s results, indications of 2νECEC had been directly observed ² in only one nuclide, krypton-78, with a half-life of around 10²² yr. The decay mode is also known to occur in barium-130, with a half-life on the order of 10²¹ yr, but that was determined by analyzing geological samples for ¹³⁰Ba and its daughter nuclide ¹³⁰Xe, not by catching the atoms decaying in real time. ³

Many other nuclides are expected to undergo 2νECEC, but they haven’t been subjected to observation. It’s just too costly and challenging to amass enough of the isotope of interest, build a detector capable of sensing the decays, and suppress background to the point where ultrarare events become visible. But Xe-based dark-matter detectors were doing all those things anyway. As a by-product of the hunt for WIMPs, they got a 2νECEC measurement essentially for free.

Multipurpose detectors

Efforts to use Xe-based WIMP detectors to look for 2νECEC began almost a decade ago with the XMASS collaboration, whose experiment is based at the Kamioka Observatory in Japan. Because the electrons captured in 2νECEC are almost certainly the core electrons of ¹²⁴Xe, the daughter atom, tellurium-124, is left in a highly excited state. As it relaxes, it produces a cascade of photons and electrons with a total detectable energy of 64 keV. The energetic particles cause the Xe to scintillate, and the emitted light is detected by photomultiplier tubes.

When the XMASS researchers looked for a peak in their energy spectrum at 64 keV, they found none. ⁴ They concluded that at the 90% confidence level, the decay should have a half-life longer than 2.1 × 10²² yr; otherwise, they would have observed it.

XENON also went through the motions of looking for the decay. ⁵ “But the experiment had no chance, since XMASS was more sensitive at that stage,” says XENON team member Alexander Fieguth of Stanford University, who worked on the project while a student at the University of Münster. At the time, XENON was using its XENON100 detector with just 165 kg of Xe, and the XMASS detector had 832 kg.

It’s typical for dark-matter groups to upgrade their detectors every few years, as they learn to overcome experimental challenges and gain more funding to buy more target material. In 2016 XENON unveiled its new XENON1T detector (shown in figure 1) with an immense 3.2 tons of Xe. But the increased mass contributed only part of the sensitivity improvement. Says XENON researcher Christian Wittweg, also a Münster student, “We screened each part of the detector—each screw, each photosensor, each cable—for tiny traces of radioactivity that cause backgrounds, and we selected only the lowest-radioactivity materials to build the detector.”

From the start, the XENON researchers were prepared to look for 2νECEC. So they wouldn’t be biased by their perception of the results, they kept the data between 56 keV and 72 keV hidden, or blinded, until the data taking and background characterization were complete. And they took measures to combat an especially pesky source of background: When ¹²⁴Xe captures a neutron, it transforms into ¹²⁵Xe, then iodine-125, then an excited state of ¹²⁵Te, which emits a signal at 67 keV that’s easily mistaken for the 2νECEC signal at 64 keV. They can’t prevent that sequence of reactions, but they can stop it in its tracks: The half-life of ¹²⁵I is 59 days, and they can purify their detector of all iodine much faster than that.

The results, based on 177.7 days of data from 2017 and 2018, are shown in figure 2. The green and blue peaks at either end are from metastable Xe and Kr nuclides created on purpose to help calibrate the energy scale. The solid brown peak shows what’s expected to be left of the ¹²⁵I background: 10 or so events over the whole data set. And the small bump in the blinded region of the data is the 2νECEC signal.

Figure 2.

It may not look like much—a few hundred events, barely visible above the already low background—but it’s larger than the researchers expected. The corresponding half-life, (1.8 ± 0.6) × 10²² yr, falls in the region XMASS had excluded with 90% confidence (although the error bars extend above the XMASS lower bound). “Nature was kind to us in providing such a large signal,” says Fieguth. “It could easily have been a few times weaker, but it couldn’t have been much stronger than what we saw.”

The new generation

Detecting 2νββ is seen as a natural first step in looking for 0νββ (see Physics Today, January 2010, page 20 ), and 2νECEC is no different. “Certainly, if we hadn’t seen it, it would be a lot less feasible to look for an even more exotic decay,” says Wittweg. The neutrinoless decays might not even be possible—neutrinos might not be their own antiparticles after all. But if they are, it would signal a major deviation from the standard model of particle physics that could unlock a host of new insights, including the secrets of the neutrino masses.

Because neutrinos can change flavor—an electron neutrino might turn into a muon neutrino or a tau neutrino, say—they must have three mass states that aren’t the same as the three flavor states. The three masses are all different, so at least two of them must be nonzero, but it’s not even known which one is the largest. In the so-called normal hierarchy—in which m₃, the mass of the state made up mostly of the muon and tau flavor states, is largest—the half-lives of neutrinoless decays would be roughly 10²⁹ yr. In the inverted hierarchy, in which m₃ is the smallest, they’re around 10²⁷ yr. They’re both far beyond the reach of current Xe-based detectors but could be observable by future ones.

As WIMP experiments become more sensitive, they could also double as real-time neutrino detectors in a uniquely low-energy regime. Right now, neutrinos from the Sun are just one of several sources of background—they appear as the orange curve in figure 2—but that can change as the detectors grow and other background components are comparatively reduced. Whereas water-based neutrino detectors, such as Super-Kamiokande, can see only those rare solar neutrinos produced by beta decay of boron-8 (see Physics Today, December 2015, page 16 ), a Xe-based detector could provide a complementary view by detecting the far more numerous neutrinos from proton–proton fusion.

The XENON1T detector has already been shut down, and the XENON researchers are getting ready for their next upgrade, XENONnT, with 8 tons of Xe. It will be joined in the next few years by two other new detectors: LZ (for LUX–Zeplin, a merging of the Large Underground Xenon and the Zoned Proportional Scintillation in Liquid Noble Gases experiments) in South Dakota and PandaX-4T (Particle and Astrophysical Xenon detector) in Sichuan, China. The main goal is still to look for WIMPs, but it’s now clear that that’s not all the detectors are capable of. “Maybe we’ll stumble upon something totally unexpected along the way,” says Wittweg, “as so often happens in physics.”