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Cosmic Rays, Fast-Moving Particles That Arrive from Deep Space, Also Produce Muons, Which Might Be Confused with Muons Produced by Neutrino Interac- Tions

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Cosmic Rays, Fast-Moving Particles That Arrive from Deep Space, Also Produce Muons, Which Might Be Confused with Muons Produced by Neutrino Interac- Tions 8 NEUTRINO HUNTERS A sketch of the IceCube neutrino observatory ( J. Yang/NSF) unrelated “noise” poses a challenge: cosmic rays, fast- moving particles that arrive from deep space, also produce muons, which might be confused with muons produced by neutrino interac- tions. Neutrino hunters place their equipment deep underground, or under a thick layer of ice, so that cosmic ray muons cannot get through. As Janet Conrad of the Massachusetts Institute of Technology explains, “If you’re trying to listen to a whisper, you don’t want a lot of noise around.” Neutrinos are hard to catch, but they are also among the “most wanted” of all cosmic messengers for the secrets they hold about the nature of matter, the pyrotechnics of exploding stars, and the structure of the universe itself. Besides, in the words of theorist Boris Kayser of the Fermi National Accelera- tor Laboratory (Fermilab) near Chicago, which is home to sev- 042-54883_ch01_6P.indd 8 10/10/13 7:14 AM SEEDS OF A REVOLUTION 173 The main spectrometer of the KATRIN arrives in the village of Leopold- shafen, near Karlsruhe in northwestern Germany, after a 5,600- mile jour- ney (Karlsruhe Institute of Technology) transferred the spectrometer onto a pontoon for the next leg. Later they used a crane to offl oad the device onto a truck, which squeaked it through the village of Leopoldshafen with great fanfare to its destination, where it arrived after a 5,600- mile circuitous journey that took two months. Once technicians complete assembling and testing the equipment, KATRIN is expected to begin taking data by 2015. One other reason for the skyrocketing interest in neutrinos has to do with their crucial role in bridging disparate fi elds of science. Besides the promise of advancing fundamental physics and hinting at the need for a theory beyond the standard model, neutrinos are pivotal for cosmology. They could reveal how the universe came to be dominated by matter over antimatter, as we discussed in the last chapter, and could also help us understand 042-54883_ch01_6P.indd 173 10/10/13 7:15 AM SEEDS OF A REVOLUTION 169 against Gordon Kane of the University of Michigan, a propo- nent of Higgs, that the particle would not be found. Finding the Higgs boson, or ruling out its existence, was a top priority for the LHC, built over a decade at a cost of nearly $9 billion with the help of thousands of scientists and engi- neers. Not surprisingly, when CERN scheduled a press confer- ence on July 4, 2012, many people expected it to announce the discovery of this long- sought particle. Hundreds of people lined up overnight to get into the auditorium. Journalists reported that the atmosphere at the laboratory was reminiscent of a rock concert. At the event, scientists presented results from two LHC experiments, showing signifi cant peaks above the noise in each data set. The peaks appeared at an energy of about 125 GeV (giga– electron volts), corresponding to a particle about 130 times more massive than the proton. The researchers had The ATLAS detector, one of the two experiments at the Large Hadron Collider that found evidence for the Higgs boson (CERN) 042-54883_ch01_6P.indd 169 10/10/13 7:15 AM 156 NEUTRINO HUNTERS Ettore Majorana (E. Recami and F. Majorana) with Niels Bohr. While working in Germany, he developed a serious case of gastritis, and his illness affected him for years, even after his return to Rome. Perhaps as a result, he published hardly any scientifi c papers, often dismissing his own work as humdrum research not worthy of publication. Despite Fermi’s urging, he even failed to publish his prediction of the neutron’s existence, and so he was denied any credit when other scien- tists found it in de pen dently. With Fermi’s backing, which counted for a lot in Italy at the time, Majorana was offered a chair in theoretical physics at the University of Naples in late 1937. He moved into a hotel there and began teaching at the university the following January. For the fi rst couple of months, his work seemed to go well. And then, on March 23, he took the night boat from Naples to Pal- 042-54883_ch01_6P.indd 156 10/10/13 7:15 AM VANISHING ACTS 151 tral to physics, and so are laws of conservation. Back in 1915, the remarkable German mathematician Emmy Noether real- ized that the two concepts of symmetry and conservation are intimately linked. Despite coming from a family of mathemati- cians, as a woman Noether had to fi ght against the odds to pursue her mathematical interests. Women were not allowed to enroll formally at the University of Erlangen at the time, so she audited the courses instead and aced the fi nal exams anyhow. Later the university lifted restrictions on female students, but despite earning a doctorate with the highest honors, Noether had trouble fi nding a teaching position. When Noether was being considered for appointment as a Privatdozent (the equivalent of an associate professor) at the Emmy Noether (Science Photo Library) 042-54883_ch01_6P.indd 151 10/10/13 7:15 AM Inside the Kamiokande neutrino detector (Institute for Cosmic Ray Research, University of Tokyo) 042-54883_ch01_6P.indd 122 10/10/13 7:15 AM COSMIC CHAMELEONS 105 neutron replaced hydrogen atoms with a lone proton. They pu- rifi ed 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 ves- sel, keeping a constant vigil for neutrino interactions. The whole apparatus was buried in a cathedral- size cavity deep in- side 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 Can- ada Limited lent to the experiment. There were several snags View of the SNO detector during installation (Lawrence Berkeley National Laboratory) 042-54883_ch01_6P.indd 105 10/10/13 7:15 AM COSMIC CHAMELEONS 99 How neutrinos oscillate (Reproduced with permission. Copyright © 2003 by Scientifi c American, Inc. All rights reserved.) neutrino particle is a hybrid of all three fl avors. In the ice cream analogy, it is as if we had swirls of chocolate, vanilla, and strawberry fl avors blending together. As a neutrino travels through space, the waves associated with the three fl avors ad- vance at different rates. The waves mingle with each other along the way, so at different points in space you get a different mix- ture of fl avors. Sometimes you would taste mostly chocolate, while at other times vanilla or strawberry would dominate. So, a neutrino that was born as an electron neutrino could appear as a tau neutrino after some distance. That’s why, as Pontecorvo and Gribov suggested, neutrinos could oscillate between fl a- vors as they travel from the Sun to the Earth. The problem was that their suggestion ran counter to the 042-54883_ch01_6P.indd 99 10/10/13 7:15 AM SUN UNDERGROUND 89 Construction of the tank that Ray Davis used for his solar neutrino experi- ment in the Homestake Gold Mine (Brookhaven National Laboratory) should produce a few dozen argon atoms every few weeks. Davis was confi dent he could fi sh out almost every one of them. He wasn’t inclined to make grandiose proclamations, however, and described his work modestly as “just plumbing,” meaning that the all- important hunt for solar neutrinos came down to the mundane task of building and operating the detector tank and the associated pipes, taking extreme care to leave no room for leaks. But, as Bahcall wrote, “as a nonchemist I am awed by the magnitude of his task and the accuracy with which he can accomplish it . He is able to fi nd and extract from the tank the few dozen atoms of [radioactive argon] that may be produced inside by the capture of solar neutrinos. This makes looking for a needle in a haystack seem easy.” 042-54883_ch01_6P.indd 89 10/10/13 7:15 AM SUN UNDERGROUND 81 The p-p chain of nuclear reactions that convert hydrogen into helium in the Sun’s core and release neutrinos and gamma rays in the pro cess calculations into better agreement with observations, and de- cided to examine all the different routes for converting hydro- gen into helium inside stars. Using just pen and paper, Bethe discovered an alternate chain reaction that we now call the CNO cycle—“CNO” stand- ing for carbon, nitrogen, and oxygen— that would also fi t the bill. As Bethe recalled, “I did not, contrary to legend, fi gure out the carbon cycle on the train home from Washington. I did, however, start thinking about energy production in massive stars upon my return to Ithaca.” Within two weeks of the Washington meeting, he worked out details of the cycle, which began with a 042-54883_ch01_6P.indd 81 10/10/13 7:15 AM GHOST CHASING 65 Frederick Reines (AIP Emilio Segrè Visual Archives) life’s work.” As he thought about interesting projects to pursue, it occurred to him that a nuclear bomb should produce a pro- digious burst of neutrinos, and could therefore come in handy for detecting these spooky particles. He knew that fi ssion of atoms through a chain reaction produces a multitude of unstable nu- clei, which decay in turn through the beta pro cess and emit neutrinos.
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