TABLETOP PHYSICS Researchers adapt atomic- physics tricks to look for PUSHED TO THE EDGE evidence of new particles. BY GABRIEL POPKIN
t’s possible that no one knows the electron in the shadow of high-energy facilities such as them to solve some of the thorniest remain as well as physicist Gerald Gabrielse. He the Large Hadron Collider (LHC), the 27-kilo ing problems in physics. But six years’ worth NATURE once held one in a trap for ten months to metre-circumference, US$5-billion particle of LHC data have failed to produce a definitive measure the size of its internal magnet. accelerator near Geneva, Switzerland. “There detection of anything unexpected. IWhen it disappeared, he searched for two days was a time in my career when there weren’t More physicists are moving in Gabrielse’s before accepting that it was gone. “You get kind very many people doing this kind of thing, and direction, with modest set-ups that can fit in of fond of your particles after a while,” he says. I wondered if it was the right choice,” he says. standard university laboratories. Instead of And Gabrielse has had ample time to become Now, he’s suddenly moving from the fringes brute-force methods such as smashing particles, SCHUKAR FOR ALYSSA fond of the electron. For more than 30 years, of physics to the limelight. Northwestern these low-energy experimentalists use precision he has been putting sophisticated electro University in Evanston, Illinois, is about to open techniques to look for extraordinarily subtle magnetic traps and lasers to work to reveal the a first-of-its-kind research institute dedicated to deviations in some of nature’s most fundamen particle’s secrets, hoping to find the first hints just his sort of small-scale particle physics, and tal parameters. The slightest discrepancy could of what’s beyond the standard model of par Gabrielse will be its founding director. point the way to the field’s future. ticle physics — the field’s long-standing, but The move signals a shift in the search for Even researchers long associated with high- incomplete, foundational theory. Yet for many new physics. Researchers have dreamed of energy physics are starting to look to low- of those years, it seemed as if he was working finding subatomic particles that could help energy experiments for glimpses beyond the
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Gerald Gabrielse in his standard model. If decade has been pinning down the shape of the experiments was conducted at Yale University low-energy-physics such hints emerge, electron. Although it is usually seen as a simple in New Haven, Connecticut, by physicist David lab at Northwestern they could point the point with negative charge, the electron could DeMille and his colleagues, using thallium University in Evanston, way to explaining the have hidden complexity. If certain symmetries ions2. But DeMille was running out of ideas Illinois, with postdoc mysteries of dark mat of nature — rules that say the Universe behaves for teasing more accuracy from his experi Wayne Huang. ter and dark energy, the same under various reversals — are vio ment, which was demanding an increasingly which collectively lated, the electron’s charge won’t have a perfectly byzantine arrangement of highly calibrated constitute some 95% of the Universe. “This is spherical distribution. Instead, virtual particles lasers, vacuum chambers and cryogenics. sort of a tectonic shift in the way we think of that constantly wink in and out of existence will A breakthrough came in 2008, when two doing physics,” says Savas Dimopoulos, a theo skew the overall distribution of charge, squash theorists at JILA, a research institute in Boulder, rist at Stanford University in California. ing it slightly out of shape and giving it what Colorado, reported3 that the molecule thorium physicists call an electric dipole moment, or oxide had an internal electric field roughly 1,000 SQUASHED SPHERE EDM (see ‘Searching the particle sea’). times the strength of thallium’s, which would In some ways, these small-scale experiments The standard model predicts a tiny squashing make a precession effect in its electrons much are a return to how particle physics was once — so small, Gabrielse says, that “there’s essen easier to see. Around the same time, Gabrielse done. Gabrielse drew particular inspiration tially no hope to measure it in my lifetime”. But — who was then at Harvard University in from a 1956 experiment by physicist Chien- Cambridge, Massachusetts — had wrapped Shiung Wu. In a laboratory at what is now the up a long-running study and decided that he US National Institute of Standards and Technol wanted to get into the electric-dipole game. He ogy in Gaithersburg, Maryland, Wu found an talked to John Doyle, also a physicist at Harvard, asymmetrical spatial pattern in how radioac “THIS IS SORT OF A who had invented a new way to make focused tive cobalt-60 atoms emit electrons. The find beams of cold, slow-moving molecules. DeMille ing, along with theoretical work, confirmed that also contacted Doyle, and the three decided to two particles discovered almost a decade before TECTONIC SHIFT IN join forces. In 2009, the trio’s experiment, called were actually one and the same. It also helped Advanced Cold Molecule Electron EDM, or to solidify faith in the burgeoning theoretical ACME, received a 5-year, $6.2-million grant framework for the Universe’s fundamental par THE WAY WE THINK OF from the US National Science Foundation. ticles and most of its fundamental forces, which would soon evolve into the standard model. PRECESSION PROCESSION But physics was already moving towards big DOING PHYSICS.” The group set up shop at Harvard. Gabrielse ger and more-expensive experimental machin worked on making the team’s lasers — eight in ery. Buoyed by a flush of post-Second World total — more stable and accurate. Doyle focused War cash and prestige, and by predictions that some theories posit as-yet-undetected particles on producing high-quality beams of thousands new particles would emerge in high-energy that could make the electron’s EDM roughly one of thorium oxide molecules. And DeMille collisions, physicists proposed increasingly billion times larger. Many of those theories fall designed a system to align the molecules and powerful and expensive particle accelerators. into a class called supersymmetry, an extension shield them from outside interference. And they got them: facilities sprung up at Stan of the standard model that could explain why In the experiment, a lab-made electric field ford; at Fermilab near Batavia, Illinois; at CERN the Higgs boson’s mass is smaller than expected, orients the thorium oxide molecules. A pair of near Geneva; and elsewhere. Quarks, muons, and that could unify the electromagnetic, weak lasers then sets the spin direction of an electron neutrinos and, finally, the Higgs boson were and strong forces in the early Universe. It might inside each molecule to be perpendicular to the discovered. The standard model was complete. also reveal the nature of dark matter. molecule’s internal electric field, and a magnetic And yet, as a description of the Universe, Attempts to measure the electron’s EDM go field is used to make the particle’s spin precess. it is incomplete. The standard model doesn’t back more than four decades. Physicists have If the electron has an EDM, it will slightly add explain, for example, why antimatter and mat taken advantage of the fact that an electron to or subtract from that rotation. After about ter were not created in equal parts at the start with an EDM can rotate, or precess, around an one millisecond, polarized laser light bouncing of the Universe. If they had been, they would electric field, tracing out a loop. The stronger off the molecules reveals how far their electrons have annihilated each other, leaving behind a the electric field, the faster — and more easily have precessed. The experiment is then repeated featureless void. The standard model also says detectable — the precession. with the molecules’ orientations reversed, which nothing about dark matter, which seems to bind But complications abound. Experimental should reverse the direction of precession due to galaxies together, or about the dark energy that ists can’t work with solitary electrons, because an EDM. The larger the difference in precession is pushing the Universe apart at an accelerating a strong electric field would cause them to angle, the larger the EDM. rate. “I like to call the standard model the great skitter away. Luckily, atoms and molecules In early 2014, the researchers reported4 that triumph and the great frustration of modern effectively lock electrons in place — and can they had not seen evidence for an EDM in their physics,” says Gabrielse. On the one hand, he produce internal electric fields stronger than set-up, which was sensitive to an angular differ says, it lets physicists predict some quantities the strongest laboratory-made field. Because ence of about 100-millionths of a degree. That “to ridiculous accuracy. On the other hand, we atoms and molecules absorb light at specific drove the upper limit of the electron EDM down have a hole we can drive the Universe through.” frequencies, researchers can use lasers to trap by more than a factor of 10, to 8.7 × 10−29 in units Gabrielse’s work trapping and probing and cool them — and nudge their internal of centimetres multiplied by electron charge. If particles at very low energies has taken him to a electrons into different configurations. an electron were the size of Earth — and Earth smaller facility at CERN, home of the LHC, to By the mid 2000s, several generations of a perfect sphere — the limit would correspond hunt for differences between matter and anti experiments building on these techniques had to moving a patch of material roughly 20 nano matter (see Nature 548, 20–23; 2017). He and ratcheted down the upper limit on the size of metres thick from one pole to the other. his colleagues have produced the most precise the electron’s EDM, but not quite to the level The ACME team argued that the result measurement yet of a physical quantity — the that would reveal the influence of particles has big implications for theories beyond the size of the electron’s internal magnet, or spin1. predicted by supersymmetry or other exten standard model, nixing many hypothetical But one of his biggest focuses in the past sions of the standard model. One of those supersymmetric particles that would exist in an
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NEWS FEATURE energy range probed by the LHC. But one second over the age of the Uni some theorists counter that plenty of verse. Researchers have since used SEARCHING THE PARTICLE SEA NATURE remaining theories — supersym Physicists are hunting for evidence that the electron’s charge cloud data from such clocks to search for metric and otherwise — predict an might be not be perfectly round, which could indicate the presence changes in the ratio between the electron EDM smaller than those of new particles. electron’s and proton’s masses and in ruled out by the ACME team. Gabri the fine-structure constant — a fun NIK SPENCER/ The electron moves through a sea of virtual particles that are constantly else finds the surviving theories more popping into and out of existence. According to many theories, these damental parameter that governs and more contrived. “Theorists are should distort the electron’s charge cloud, creating a corresponding the strength of the electromagnetic wily,” he says. “Every time we exclude property called an electric dipole moment (EDM). force. Others, following a proposal7 something, they try to wiggle out.” by Asimina Arvanitaki, a theorist at ACME is not alone in this effort. An EDM would arise the Perimeter Institute for Theoreti After earning a Nobel prize in 2001 along the same axis as cal Physics in Waterloo, Canada, are for creating a new phase of matter the electron’s spin. using clocks to look for subtle oscil called a Bose–Einstein condensate, Virtual particles lations that might be created by a JILA physicist Eric Cornell teamed up hypothesized dark-matter candidate with Jun Ye, also at JILA, to look for an called the axion, or a related particle. EDM Electron EDM. Rather than manipulate mol charge cloud So far, these investigations have ecules as they pass by in a beam, as yielded no new physics. But they The charge cloud would ACME does, Cornell and Ye decided be distorted, making show how a younger generation of to use a rotating electric field to trap one side slightly more physicists is infusing the field with molecular ions with large internal negative than the other. new ideas, says Dimopoulos, who was fields, giving electron precessions Arvanitaki’s PhD adviser. “There’s a longer to reveal themselves. DeMille lot of theoretical ideas that have been, calls the idea “brilliant and far from To measure the size of in a sense, overlooked because every o b v i o u s”. the EDM, physicists The electron’s intrinsic body was focusing on the LHC and expose electrons to Spin spin (and EDM, if any) can Cornell faced a setback when he strong electric elds. be aligned perpendicular the previous colliders,” he says. lost an arm to necrotizing fasciitis to the electric eld. No one expects such tabletop in 2004. But it led to a joke he likes experiments to replace particle col to tell when he gives talks: “His left liders. Rather, they could guide sleeve is empty, and he’ll say, ‘If any physicists to the right energy range body should know about asymmetry, for more detailed study. Right now, it’s me’,” says former lab mate Chris Electric eld the collider community suspects that Monroe, now a physicist at the Uni it needs more energy than the LHC versity of Maryland in College Park. is designed to reach, but it’s unclear After a decade building and refining how much will be sufficient. Find If the electron has an EDM, the Other theories predict a much what Cornell calls a “two-tabletop particle will rotate, or precess, larger EDM, with a faster ings from low-energy experiments experiment” (because it occupies around the direction of the precession. Measuring such a might influence a multibillion-dollar electric eld. The standard model precession could indicate that two tables in his lab), he and his co- of particle physics predicts an the EDM is in uenced by decision about the next big collider, authors finally published their first immeasurably small EDM e ect. as-yet-undiscovered particles. and that has put added pressure on results last year5, coming within a researchers working in this tabletop factor of 1.5 of ACME’s 2014 limit. “I realm. “We have to do almost every might not have started if I had realized thing with more care than is typical in how hard it would be,” says Cornell. the standard atomic-physics experi Now, researchers are closing in on Small Large ment,” says DeMille. EDM new EDM results. The ACME physi EDM Gabrielse has high hopes for the cists have increased the number of team’s next experiment — and for the molecules they can send into their work at his centre at Northwestern, experimental apparatus by a factor which is set to open this year. But he of 400. They expect this and other can make no promises. “We’re fishing improvements to sharpen the experi for a fish whose shape and colour and ment’s precision by a factor of ten — allowing technology in service of fundamental physics: speed and equipment for biting are completely them to hunt for effects beyond the energy atomic clocks. The frequencies of radiation unknown.” ■ range of the LHC. The JILA team is also gear absorbed and emitted by the atoms that make ing up for experiments set to push beyond the up these clocks depend only on certain funda Gabriel Popkin is a freelance journalist based LHC’s reach. And researchers at Imperial Col mental constants of nature. A slight deviation in in Mount Rainier, Maryland. lege London who held a former electron-EDM those frequencies could lend support to theories 6 1. Hanneke, D., Fogwell, S. & Gabrielse, G. Phys. Rev. measurement record have plans for experi that attempt to explain why gravity is so much Lett. 100, 120801 (2008). ments with laser-cooled ytterbium mono weaker than the Universe’s other forces. 2. Regan, B. C., Commins, E. D., Schmidt, C. J. & fluoride molecules; they hope their test will be The ability to test this idea was out of reach DeMille, D. Phys. Rev. Lett. 88, 071805 (2002). 1,000 times more precise than ACME’s first run. until the early 2000s, when researchers devel 3. Meyer, E. R. & Bohn, J. L. Phys. Rev. A 78, 010502(R) (2008). The electron isn’t the only low-energy peep oped atomic clocks that operate in the optical 4. The ACME Collaboration. Science 343, 269–272 hole into the world beyond the standard model. range of the electromagnetic spectrum instead (2014). Some physicists are searching for EDMs in neu of in the microwave. Their higher frequencies 5. Cairncross, W. B. et al. Phys. Rev. Lett. 119, 153001 (2017). trons or atoms, which, like the electron, could meant that time could be sampled at a much 6. Hudson, J. J. et al. Nature 473, 493–496 (2011). reveal a violation of one of nature’s symme higher rate, enabling the creation of clocks so 7. Arvanitaki, A., Huang, J. & Van Tilburg, K. Phys. tries. Others are adapting an entirely different precise that they would lose or gain less than Rev. D 91, 015015 (2015).
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