Dec 6 2013 6:00pm
Neutrino Hunters (Excerpt)
Detective thriller meets astrophysics in Ray Jayawardhana's Neutrino Hunters: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe, available December 10th from FSG.
The incredibly small bits of matter we call neutrinos may hold the secret to why antimatter is so rare, how mighty stars explode as supernovae, what the universe was like just seconds after the big bang, and even the inner workings of our own planet.
For more than eighty years, adventurous minds from around the world have been chasing these ghostly particles, trillions of which pass through our bodies every second. Extremely elusive and difficult to pin down, neutrinos are not unlike the brilliant and eccentric scientists who doggedly pursue them.
One late November afternoon in 2010, I drove north for four hours, through intermittent snow flurries, from Toronto to Sudbury, Ontario. The next morning, in the predawn darkness without a GPS device to depend on, I nearly got lost driving from the B&B where I had stayed to the site of the Sudbury Neutrino Observatory, or SNOLAB, the world’s deepest underground laboratory, which exists inside an active nickel mine. SNOLAB’s director, Nigel Smith, had agreed to give me a tour, and I managed to arrive just in time to catch the last elevator that went down at 7:00 a.m.
Inside a locker room at the ground level, donning blue overalls and steel-toed boots, Nigel Smith fastened a light on his hard hat and a battery pack on his safety belt, and asked me to do the same. After placing two tags—one for Smith and the other for a “visitor”—on a peg wall so that it would be easier to take a tally in case of an emergency, we stepped into a dark, creaky elevator suspended by a cable almost as thick as my arm. Two dozen miners packed into the open cage with us. Our drop down to the pits of the Earth began slowly, but soon picked up speed. The headlamps provided just enough light for me to make out the rocky walls of the mine shaft rushing past in front of us. The cage made several stops on its way down to let out groups of miners, and I caught glimpses of lighted tunnels receding into the distance at each level. About halfway down, my eardrums could feel the pressure change, so I worked my jaws and forced a yawn. At the final stop, just over a mile and a quarter below the surface, Smith and I stepped out, along with the few remaining miners. Our descent, including the stops along the way, had taken about ten minutes.
Our journey was far from over, however, since we still had more than a mile-long trek through a muddy tunnel ahead of us to reach SNOLAB. Thankfully, a combination of concrete props, roof bolts, and steel screens held off the rock overhead from crumbling under pressure, and a ventilation system produced a cool breeze, without which we’d be sweating buckets. The miners veered off to side tunnels in search of nickel, while Smith and I kept on going straight, walking along rail tracks laid for trolleys. At last we reached a sign that declared SNOLAB: mining for knowledge, signaling that we had arrived. We washed the mud off our boots with a hose and pulled open a bright-blue door. I was immediately struck by the contrast between the pristine laboratory compound inside, with spotless floors, shiny walls, and dust-free air, and the grimy mine we had just walked through. Before going farther, we took showers and changed into a new set of overalls, boots, and hairnets. As the last step of the elaborate cleaning ritual before we entered the inner sanctum, we passed through an air shower to clear off any remaining dirt or dust particles so that we would preserve the integrity of the sensitive experiments housed at SNOLAB. The entire laboratory is operated as a clean room, with the air filtered continuously; everyone and everything that enters it must be thoroughly cleaned to remove any traces of radioactive elements, which are plentiful in the mine dust and would otherwise interfere with measuring neutrino signals.
The Italian physicist Bruno Pontecorvo had two crucial insights over a half century ago that contained the keys to solving the mystery of why experimenters were detecting fewer neutrinos from the sun than the astrophysicist John Bahcall’s solar model predicted. Pontecorvo’s first insight was that there was more than one variety of neutrino. He came to this conclusion while examining the decay of an unstable particle called a muon, which belongs to the lepton family, along with the electron and the neutrino, all fundamental building blocks of matter. Like the electron, the muon is negatively charged, but about two hundred times more massive, and it lives for just over two-millionths of a second before breaking up. Pontecorvo proposed that the muon and the electron each had a distinct variety of neutrino associated with it.
Three physicists at Columbia University—Leon Lederman, Melvin Schwartz, and Jack Steinberger—confirmed the existence of two neutrino varieties while experimenting with a particle collider in 1962, and proved Pontecorvo right on this score. When Martin Perl of Stanford University and his colleagues identified a third, even more massive, member of the lepton family, called the tau particle, researchers expected that there should be a third type of neutrino associated with it. Physicists at Fermilab near Chicago finally observed tau neutrinos in the year 2000. We use the whimsical term “flavors” to describe the three neutrino types.
Pontecorvo’s second insight was that neutrinos could be fickle. He found that the laws of quantum mechanics allowed neutrinos to morph, or “oscillate,” between types, but this could only happen if they had some mass. Soon after a deficit of solar neutrinos was first reported in 1968, Pontecorvo and his Russian colleague Vladimir Gribov proposed that neutrinos oscillating from one flavor to another on their way from the Sun could account for the shortfall. It was as if they had suggested that chocolate ice cream could turn into vanilla, but as weird as the theory may sound, their suggestion offered a simple and elegant explanation for the missing solar neutrinos: two-thirds of the electron neutrinos produced in the Sun could turn into other varieties during their long journey to Earth, and thus escape detection.
So, many researchers were excited when clear-cut experimental evidence of neutrinos morphing between flavors came to light in the 1990s. By then, Japanese neutrino hunters had a powerful, upgraded detector called Super-Kamiokande or Super-K, which could record not only solar neutrinos but also neutrinos produced by cosmic rays hitting the Earth’s upper atmosphere. These so-called atmospheric neutrinos are hundreds or even thousands of times more energetic than those coming from the Sun, so they are easier to trap. Scientists estimated that muon neutrinos should be twice as common as electron neutrinos among the cosmic ray debris. Fortunately, the Super-K detector was able to distinguish between these two neutrino types: an electron neutrino hitting the detector’s water would produce a fuzzy circle of light, whereas a muon neutrino interaction would lead to a sharp ring. After observing atmospheric neutrinos of both types for nearly two years, the Super-K team reported a surprising result: instead of twice as many of the muon variety, they found roughly equal numbers of the two types. One possibility, they reasoned, was that half the muon neutrinos were morphing into the third type, tau neutrinos, which Super-K could not identify easily.
The most intriguing clue had to do with the direction from which neutrinos arrived. Roughly equal numbers of cosmic rays should hit the Earth’s atmosphere from all directions, so the number of neutrinos produced by these particle collisions should also be the same all around the globe. Sure enough, the Super-K researchers found equal numbers of electron neutrinos coming down from the sky and coming up through the ground, from the other side of the Earth. But that wasn’t true for muon neutrinos: only half as many were coming up from below as coming down from overhead. It seemed to the Super-K team that muon neutrinos were somehow disappearing during their journey through the Earth. “That was the smoking gun,” as Ed Kearns of Boston University, a member of the Super-K collaboration, put it. Most likely, they concluded, the muon neutrinos were changing identity, morphing into tau neutrinos that Super-K couldn’t detect readily. Thanks to these findings, by the late 1990s many more physicists were willing to accept that oscillating neutrinos could be responsible for the atmospheric neutrino anomaly as well as for the solar neutrino deficit.
However, showing that some muon neutrinos disappear mid-flight wasn’t direct proof of their metamorphosis into a different variety. To be sure this interpretation was correct, physicists needed to measure what the electron neutrinos from the Sun turned into, or at least measure the electron neutrinos separately from the other flavors. That was the primary goal of SNOLAB—to solve the solar neutrino riddle once and for all.
Once inside, walking by the racks of flickering electronics or having a snack in the lunchroom with a couple of scientists, it was easy to forget that there was more than a mile of rock above your head. Even if you felt claustrophobic in the elevator cage or the tunnel, you probably wouldn’t here. But you might notice that there are no windows to let in sunlight. So it’s perhaps ironic that this laboratory was built in the first place to peer at the Sun. Sixteen scientists came together in the mid-1980s to propose the construction of SNO to catch a handful of the neutrinos that stream out of the Sun and pass through rock more easily than sunlight through a windowpane.
Art McDonald, then a professor at Princeton University, was among them. Growing up near the eastern edge of Cape Breton Island in Nova Scotia, McDonald was always interested in how things worked. As a kid, he enjoyed taking clocks apart and trying to put them back together. Later, as a physicist, he took pleasure in applying mathematics to understand how nature worked. He returned to Canada in 1989, to take up a professorship at Queen’s University and to lead the SNO project. Two years later, he and his colleagues secured sufficient funding to turn their dreams of a powerful underground neutrino observatory into reality.
The centerpiece of the SNO neutrino detector was a giant spherical vessel made of transparent acrylic. Instead of ordinary water, researchers filled it with a thousand tons of heavy water, in which deuterium atoms containing a proton and a neutron replaced hydrogen atoms with a lone proton. They purified the heavy water to remove not only dust but also any vestiges of radioactive gases. A geodesic sphere with 9,600 light sensors mounted on its inside walls surrounded the acrylic vessel, keeping a constant vigil for neutrino interactions. The whole apparatus was buried in a cathedral-size cavity deep inside the mine. When I visited the site, I could peek at it from a platform above. Building the SNO took more than nine years and over $70 million in Canadian dollars, not counting the $200 million value of the heavy water, which Atomic Energy of Canada Limited lent to the experiment. There were several snags along the way, but SNO began taking data in the summer of 1999.
Two years later, Art McDonald announced the first results of their experiment after it had recorded interactions between neutrinos and the heavy water for 241 days. Comparing the number of neutrinos detected at SNO and at Super-K, his team confirmed that some must have changed their flavor. “We’ve solved a thirty-year-old puzzle of the missing neutrinos of the Sun,” he told the media at the time. “We now have high confidence that the discrepancy is not caused by problems with the models of the Sun but by changes in the neutrinos themselves as they travel from the core of the Sun to the Earth.” Their results bolstered the case for neutrino oscillations and for neutrinos having at least a smidgen of mass.
This was a significant step, to be sure, but it didn’t quite close the book on the problem. The cleanest test would be for SNO itself to measure all three flavors of neutrinos, without having to combine and compare with the measurements from Super-K—and that’s just what the researchers set out to do next. Among other upgrades, they added two tons of sodium chloride (otherwise known as pure salt) to the heavy water. They knew that the chlorine in the salt would improve the chances of capturing neutrinos and distinguishing between the different varieties. Their clever trick paid off. Already in 2002 the team announced that the interim SNO results alone confirmed that solar neutrinos change from one type to another during their journey. The following year they reported definitive results on the neutrino numbers. The total matched what John Bahcall’s solar model had predicted. Sure enough, only a third of the solar neutrinos arriving on Earth were of the electron variety. The other two-thirds were of the muon and tau types. Here was proof that electron neutrinos produced in the Sun morphed into other flavors midflight.
Several profound consequences ensued from the discovery of neutrino oscillations. For one, it showed that neutrinos were not massless, contrary to the expectations of the standard model. Thus it constituted the first bit of definitive evidence that the standard model may not be the whole story. For another, measuring those oscillations offered a way to explore “new physics,” a term physicists use to describe phenomena that aren’t accounted for by the standard model. As Karsten Heeger, a physicist at the University of Wisconsin-Madison, told me, “Traditional particle physics only confirmed the standard model. Neutrino oscillations were the first sign that there is something beyond the standard model. That discovery gave a huge boost to the field.”
The discovery that neutrinos have mass is also of interest to cosmologists. Since neutrinos are the second most numerous particles in the universe after photons, even if each one has only a smidgen of mass, the total could add up to a lot. So some cosmologists had hoped that neutrinos would account for much of the mysterious dark matter, whose presence is only “seen” through its gravitational influence on galaxies and galaxy clusters. But the neutrino’s mass has turned out to be way too tiny to explain dark matter. That means some other particle or particles, hitherto unknown to physics, must exist.
The hunt is on.
Excerpted from NEUTRINO HUNTERS: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe by Ray Jayawardhana, to be published next week by Scientific American/Farrar, Straus and Giroux, LLC. Copyright © 2013 by Ray Jayawardhana. All rights reserved.