The 4-Percent Universe

by Richard Panek

  Zero.

  A cheer went up—not unlike the spontaneous applause that greeted a team member later that month at a UCLA symposium on dark matter when he stood before a hundred or so colleagues from around the world and re-created, via PowerPoint, the revelation of non-detection. CDMS II had leapfrogged back into the lead, leaving a XENON10 team member to interrupt his own PowerPoint presentation later that morning to sigh, "I guess this graph is about forty-five minutes out of date."

  It was some indication of just how difficult the WIMP problem was that even a null result was cause for celebration. Later that day, one of the team leaders graciously accepted congratulations on his team's work as he boarded an elevator. "Of course," he added, softly, as the doors closed, "a detection would have been better."

  Nineteen months later, he got his wish. The next "unblinding party" for CDMS II was also its last. In the interim, that incarnation of the experiment—five towers of six detectors each—had been decommissioned to make way for an upgrade: SuperCDMS. When the team "opened the box" on that last round of data, they expected the result to be more of the same: plenty of nothing. Instead, they got two "somethings": one from August 5, 2007, the other from October 27, 2007.

  A null result would have made a definitive statement, excluding one more phase space for future experiments to investigate. Two detections, however, occupied a particle physics purgatory. Statistically, that number wasn't enough even to claim "evidence for," let alone the "discovery" that five events would have justified. If both events were due to background noise such as cosmic rays or radiation from within the mine, then you were unlucky. If both events were indeed the "edge of the signal," and a competing collaboration such as XENON100 (the successor to XENON10, already up and running when CDMS II opened the box) wound up seeing a statistically satisfying number of events and got to claim the discovery ... then you were still unlucky. As one graduate student said, expressing his disappointment at not getting a null result, "We would have totally dominated!"

  "We're actually in the game to see something," Jodi Cooley had to remind him. The coordinator of data analysis for the experiment, she had joined the collaboration as a postdoc at Stanford five and a half years earlier, and she had secured her first faculty position, as an assistant professor at Southern Methodist University, two months earlier. By the standards of a Bernard Sadoulet, she was a newcomer to the dark-matter game. But she was also enough of a veteran that she had tired of celebrating the sighting of nothing.

  Still, she knew what that grad student meant. In a way, Cooley told herself (though not the grad student), a total of two detections was "the worst-case scenario."

  The collaboration spent the next few weeks running the results through data quality checks. Were the detections well inside the detector, where stray radiation was less likely to reach? Yes. Did a detection come during a time when the instruments had been behaving smoothly? Yes. Did a detection come at the same time as another detection—a double-WIMP detection that would have defied belief? No. Did the two detections occur on the same detector? No. In the end, the team subjected the results to more than fifty checks, and both detections passed every test.

  The quality of the results was strong. It was the quantity that was the problem. The collaboration just didn't have enough events to let them know what they'd seen.

  Still, they'd seen something. That fact alone made the result more worth reporting to the community than a null result would have been. The collaboration would have published a paper on the results no matter what the outcome, but this something—these two somethings—merited a more direct interaction with the community. The collaboration scheduled simultaneous presentations at Fermilab and Stanford the following month, as well as smaller educational sessions at other institutions in the collaboration. The subject of dark matter was tantalizing enough that the talks were sure to attract some attention.

  They had no idea.

  Within days, rumors about the result were dominating the particle physics slice of the blogosphere. "Dark matter discovered?" "Has Dark Matter Finally Been Detected on Earth?" "Rumor has it that the first dark matter particle has been found!" "¿Se ha descubierto la materia oscura en el CDMS?" "Pátrání po supersymetrické skryté hmotě." " dark matter ."

  The team realized that by scheduling all the sessions for one day they had inadvertently given the impression that there was about to be a before-and-after moment in science. There wasn't. At best there would be a sort-of-before-and-sort-of-after-but-we-won't-really-know-until-some-other-experiment-reinforces-our-results-and-even-then-today's-announcement-would-be-seen-in-retrospect-asat-best-a-hint-of-detections-to-come moment. They "pre-poned" the Fermilab and Stanford announcements, moving them up a day, separating them from the more casual sessions, hoping to lower expectations.

  Too late. If they did announce a null result, wrote one blogger, "the Thursday speakers will be torn to pieces by an angry mob, and their bones will be thrown to undergrads." To which "Anonymous" added in the Comments section on the same website, "Independent of the rumors,* I have it from a very well-known physicist that CDMS will in fact announce that they have discovered dark matter tomorrow." Discover magazine live-blogged Cooley's standing-room-only presentation, prefacing the tick-tock with: "Personally, I have heard rumors that they have either 0, 1, 3, or 4 signal events."

  Well, no, no, no, and no. "THE NUMBER IS TWO!!!!"

  Or as Cooley carefully explained, "The results of this experiment cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as a signal." Theirs wasn't a detection. It wasn't a null result. It was a neither-nor conclusion that taught the portion of the world that cared about such things a lesson that Jodi Cooley, Bernard Sadoulet, and a certain graduate student had already learned the hard way:

  If you let it, dark matter will break your heart.

  "I'm in love with the axion!"

  Les Rosenberg didn't care who knew it. When the mood struck, he wasn't afraid to declare his affection to the world. Karl van Bibber—fit; never took the elevator when stairs were an option; looked like Leonard Nimoy, in a good way—was a bit more discreet; the word he repeatedly used about his relationship to the axion was "smitten." Like the Red Sox—and unlike the WIMP—the axion seemed to inspire a certain kind of blind devotion and underdog identification.

  The natural mathematical match between the neutralino and dark matter—how many neutralinos would have survived the primordial conditions, multiplied by the predicted mass of a neutralino, equaling the best estimates of the current density of dark matter—had always made it the favorite candidate among physicists. The longer it remained undetected, however, the more the community was willing to consider alternatives. The axion might not have been as obvious a match, but it was a match nonetheless.

  Like WIMPs, the axion was a hypothetical particle that fell out of an adjustment to the standard model. In 1964 physicists discovered the violation of a certain kind of symmetry in nature—in part, that the laws of physics wouldn't hold if a particle and its antiparticle traded places. In 1975 the physicists Frank Wilczek and Steven Weinberg independently realized that a particle with certain properties could solve the problem. "I called this particle the axion, after the laundry detergent," Wilczek once explained, "because that was a nice catchy name that sounded like a particle and because this particular particle solved a problem involving axial currents."*

  Unlike WIMPs, however, the axion was not a massive particle. The neutralino would be fifty to five hundred times the mass of a proton; the axion would be one-trillionth the mass of an electron, which itself was 1/1,836th the mass of a proton. If axions existed, they would be a trillion times lighter than an electron, making the chance of their interacting—or coupling—with baryonic matter, as van Bibber said, "vanishingly small." But in 1983 the physicist Pierre Sikivie realized that while the axion, unlike the neutralino, couldn't couple with matter, it could interact with magnetism. Under the influence o
f a strong enough magnetic field, an axion could disintegrate into a photon—and that's what a detector could detect.

  In 1989 van Bibber attended a meeting at Brookhaven National Laboratory, on Long Island, where Adrian Melissinos, of the University of Rochester, asked a couple of dozen physicists whether they wanted to participate in the construction of such a detector. He went around the table: "Are you in or out?" Van Bibber was in. When Melissinos had finished surveying the scientists, van Bibber pointed out that Melissinos hadn't polled himself. Was he in or out?

  "Oh, this is too much like hard work," Melissinos said. "This is for you young guys."

  And so van Bibber found himself leading an experiment that might outlive his professional life. ADMX was a highly magnetized resonant cavity. If axions were entering it, then they would interact with the magnetism and disintegrate into photons. If they disintegrated into photons, then they wouldn't be able to pass back through the casing of the cavity. Instead, they would remain inside, bouncing off the walls, emitting a faint microwave signal. That signal was what ADMX should be able to detect. In other words, ADMX was a radio receiver.

  By 1997 he and Rosenberg had a prototype up and running. The following year they published a paper that put the community on notice: You could actually do Sikivie's strong-magnet, resonant-cavity axion experiment. Their impression was that the success of the prototype in 1998 surprised the community; it stunned them, anyway. That instrument—a waist-high copper cylinder—still sat in a corner of the shed. Once, the public relations department at Livermore contacted van Bibber about displaying it in the visitors' center, and they asked him to send a photo. He did, and he never heard back.

  But it remained a thing of beauty to him. As a kid, van Bibber wanted to be a cartoonist, just like his father, Max, who drew the Winnie Winkle comic strip. Karl's drawings, however, were disturbing, and his parents thought he might be emotionally troubled, until they realized he was colorblind. His father did nonetheless influence his choice of profession. One day he brought home from Manhattan a science textbook, and Karl, then in his early teens, performed experiment after experiment until he'd exhausted the book. He was, well, "smitten."

  For van Bibber, ADMX was a low-energy physics version of one of those experiments. It was usually a ten- to fifteen-person collaboration among friends, including the half-dozen kids who were doing the heavy lifting, literally. In collider physics, thousands of students could spend their entire educational careers writing the software for an experiment they might never be able to touch. "Cannon fodder," van Bibber called them. But here in the shed, the kids could spend their summers in T-shirts and shorts, griping about the heat. (Once, just for fun, the group moved a thermometer up a ladder one step at a time, and at each rung the temperature rose one degree, until it peaked at 118°F.) And as they worked, they could find out whether they, like van Bibber, nonetheless got a kick out of building a detector that could find a signal equivalent to the cosmic microwave background ... plus one photon.

  ADMX was, in a real sense, a labor of love. Van Bibber loved that the axion was a high-risk career move. He loved that Rosenberg had the kind of "crazy streak" that allowed him to take that same risk. (For his part, Rosenberg called van Bibber the Mick Jagger of axions: the leader of the band, the salesman for the brand. And in 2006 the two of them even made the cover of Rolling Stone—or, at least, wrote a major article on ADMX for Physics Today.) Van Bibber loved that his collaboration was basically the only one in the world looking for the axion. He loved that the annual cost of the experiment was maybe 1 or 2 percent of the nearly one hundred million dollars spent on the two or three dozen WIMP experiments underway around the world at any one time.

  But most of all, van Bibber loved that the axion signal would be so unfathomably faint. It meant that he was performing a seemingly paradoxical feat: "macroscopic quantum mechanics." He loved that if the axion was there, the instrument would detect it. You wouldn't know the frequency in advance, so the search of the microwave spectrum would have to be numbingly methodical. But when Phase II was over—Phase I ended in 2004—he would know: The axion exists, or the axion doesn't exist.

  That certainty, van Bibber recognized, was something WIMP hunters could only envy—one of them a friend of his at the University of Chicago. Juan Collar was part of a generation that had joined the search for neutralinos in the 1990s. Since then he had abandoned the CDMS prototype. The acronym for his experiment, the Chicago Observatory for Underground Particle Physics (COUPP), which resided at a depth of a thousand feet in a tunnel at Fermilab, was significant: The p's were silent, as in "coup." Shaking his fist at an imaginary enemy, Collar would say, "It has the connotation of a terrible blow to the system"—the system being the whole cryogenic approach.

  COUPP was less a technological advance than a throwback to an earlier era of physics: a bubble chamber. The chamber was filled with a superheated heavy liquid and outfitted with a camera; unlike other dark-matter experimenters, the COUPP team would have the thrill of seeing an actual visual result: a bubble. And bubbles they got: muons, again. Collar worried whether his generation—the particle physicists who had started out with such optimism in the 1990s, sure that they would be the ones to find the WIMP and win the race to discover dark matter—would stick around long enough to see the right kind of bubble, to hear the right kind of ping. He had his doubts. Sometimes at conferences Collar and his no-longer-on-the-prodigy-side-of-forty colleagues would convene at the hotel bar and "howl at the moon." And sometimes he would retreat to a downstairs lab at Chicago to play with a detector that had nothing to do with WIMPs, if only because when he put it next to a reactor, he would actually see a signal.

  When Collar talked about his generation of researchers, he would say, "It gets kind of old, to look for a particle that might be there or not and always getting a negative result." A negative result from an experiment, after all, didn't mean that the neutralino didn't exist. It might mean only that theorists hadn't thought hard enough or that observers hadn't looked deep enough. Collar kept a graph taped to a wall in his office that showed the range where he and other researchers hoped the neutralino might reside, and sometimes he would find himself looking below the sheet of paper, at the blank wall. "If the neutralino is way down there," he would think, "we should retreat and worship Mother Nature. These particles maybe exist, but we will not see them, our sons will not see them, and their sons won't see them." And then he would think of his friend in the California desert. "Karl," Collar would tell himself, "knows he's going to get the job done, dammit."

  But van Bibber, a generation older than Collar, had experienced a different kind of frustration: For years he and his fellow dark-matter hunters thought they owned the universe, if only they could find it. After 1998, they realized they owned maybe a quarter of the universe. Not bad, but van Bibber thought it was "sort of a rude demotion."

  Still, he remained sanguine, as someone still in love, long into a marriage, does. When he reached his mid-fifties and thought about the possibility that ADMX would take another ten years, and that he might wind up with nothing, and that it might be the experiment that would close out his career—that he would have spent the latter half of his professional life in one way or another looking for the axion—he thought it still would have been a worthwhile pursuit. He hoped, of course, that his experiment would be the one that detected dark matter. But sometimes when he and his old friend and longtime colleague Les Rosenberg got to talking, they had to admit that after the Red Sox won the World Series, baseball was never the same.

  11. The Thing

  THEY KNEW WHERE they were going. Or at least they knew where they hoped they were going, and they were pretty sure they were headed in roughly the right direction. Once in a while the wind would ease and the veil of snow would part and they would glimpse, in the distance, the distinctive silhouette of the Dark Sector. But then the wind would gather again, and the white would envelop them, and the summer crew for the South Pole Telescope would
lower their heads and withdraw behind the fur lining of their hoods, trusting that they would soon be climbing the metal stairs to the laboratory and resuming their search for clues about dark energy, a mission that had now taken science to the ends of the Earth, literally.

  Welcome to the most benign environment on the planet. Or so William L. Holzapfel, a UC Berkeley astrophysicist and veteran of several stays at the South Pole, liked to say, and not just because the whiteout was the exception and the recent weather had been unseasonably mild. Other days that week—the week between Christmas and New Year's, early summer in the Southern Hemisphere and midway through the six months that the Sun is continually up at the Pole—the temperatures were barely in the minus single digits Fahrenheit (and one day even broke zero to set a record high for the date), and the wind was mostly calm. Holzapfel routinely made the walk from the Amundsen-Scott Station (literally a snowball's throw from the Pole itself, which is marked with, yes, a metal pole) to the telescope wearing jeans and running shoes. One afternoon the lab's heating system went a little haywire and the crew had to prop a door open to cool off. And for those of the two hundred fifty or so working at the Pole who remembered to pack their swim trunks—including Holzapfel—the traditional makeshift New Year's Eve outdoor sauna was as bracing as ever.