Winter 1999 Vol

A PERIODICAL OF PARTICLE PHYSICS WINTER 1999 VOL. 29, NUMBER 3 FEATURES Editors 2 GOLDEN STARDUST RENE DONALDSON, BILL KIRK The ISOLDE facility at CERN is being used to study how lighter elements are forged Contributing Editors into heavier ones in the furnaces of the stars. MICHAEL RIORDAN, GORDON FRASER JUDY JACKSON, AKIHIRO MAKI James Gillies PEDRO WALOSCHEK 8 NEUTRINOS HAVE MASS! Editorial Advisory Board The Super-Kamiokande detector has found a PATRICIA BURCHAT, DAVID BURKE LANCE DIXON, GEORGE SMOOT deficit of one flavor of neutrino coming GEORGE TRILLIN G, KARL VAN BIBBER through the Earth, with the likely HERMAN WINICK implication that neutrinos possess mass. Illustrations John G. Learned TERRY AN DERSON 16 IS SUPERSYMMETRY THE NEXT Distribution LAYER OF STRUCTURE? C RYSTAL TILGHMAN Despite its impressive successes, theoretical physicists believe that the Standard Model is The Beam Line is published quarterly by the incomplete. Supersymmetry might provide Stanford Linear Accelerator Center the answer to the puzzles of the Higgs boson. Box 4349, Stanford, CA 94309. Telephone: (650) 926-2585 Michael Dine EMAIL: [email protected] FAX: (650) 926-4500 Issues of the Beam Line are accessible electronically on the World Wide Web at http://www.slac.stanford.edu/pubs/beamline. SLAC is operated by Stanford University under contract with the U.S. Department of Energy. The opinions of the authors do not necessarily reflect the policies of the Stanford Linear Accelerator Center. Cover: The Super-Kamiokande detector during filling in 1996. Physicists in a rubber raft are polishing the 20-inch photomultipliers as the water rises slowly. See John Learned’s article on page 8. (Courtesy ICRR, University of Tokyo) Printed on recycled paper CONTENTS 22 CERN’s Low Energy Antiproton Ring L•E•A•R CERN’s Low Energy Antiproton Ring is remembered for its transformations, experimental results, and remarkable machine physics performance. ~ Gordon Fraser DEPARTMENTS d d 28 THE UNIVERSE AT LARGE Astrophysics Faces the Millennium Virginia Trimble 37 FROM THE EDITORS’ DESK 38 CONTRIBUTORS 40 DATES TO REMEMBER m m ~ t g t co g 1 by JAMES GILLIES HERE DO GOLD earrings come from? A simple answer is the local jewelry “It is the stars, The stars shop, but if you really want to know in above us, govern our Wdepth, you’ll have to dig a lot further. Gold is, quite literally, stardust. About half of it is forged in stars that burn normal- conditions”; ly, while the rest comes from large stars at the ends of their lives—in the cataclysmic explosions called supernovae. That (King Lear, Act IV, Scene 3) much we know. But exactly how gold and all other elements but perhaps heavier than iron are formed is still unclear. A new series of experiments at the ISOLDE facility at the European Labora- “The fault, dear Brutus, tory for Particle Physics, CERN, in Geneva aims to find out. The history of the elements is as old as the Universe itself. is not in our stars, But In the beginning, at the Big Bang, only the very lightest in ourselves.” elements—hydrogen, helium, and a little lithium—were formed. Since then, so little heavier material has been (Julius Caesar, Act I, Scene 2) created that even today hydrogen and helium make up over 99 percent of all the matter in the Universe. Everything else [Reprinted from “Synthesis of the Elements in Stars” by E. M. Bur- amounts to just a tiny fraction of 1 percent. bidge, G. R. Burbidge, W. A. Fowler, After the Big Bang, a billion years passed before any heav- and F. Hoyle, Reviews of Modern ier elements appeared. They had to wait until the formation Physics 29, 547 (1957)] of stars, when gravity squeezed the light elements so tightly that they fused, igniting the stellar furnaces that forge heavier elements from lighter ones. In the normally burning part of their lives, these stars build elements as heavy as iron, producing energy from fusion as they do so. But then the process stops, because anything heavier than iron takes more energy to make than fusion gives out. That doesn’t mean such elements can’t be made in stars—the fusion process just uses some of the energy released by light-element fusion— 2 WINTER 1999 but the Universe simply hasn’t been around long enough for all the heavy elements we observe to have been produced that way. Another process must be at work. In 1957 the husband and wife team of Margaret and Geoffrey Burbidge working with Willy Fowler and the maverick British astronomer Fred Hoyle figured out what it could be. They published a paper which has since become legendary in the field of theoretical astrophysics and is known to aficionados simply as B2FH. In it, the Burbidges, Fowler, and Hoyle show how neutrons could provide the route to the heavier elements. B2FH describes the so-called s- and r- processes through which slow neutron absorption in stars could generate about half the present abundance of heavier- than-iron elements, with rapid neutron absorption, thought to occur in super- Left to right, Margaret and Geoffrey Burbidge, William Fowler, and Fred Hoyle, authors of the famous 1957 paper, “Synthesis of Ele- novae, making up the balance. ments in Stars.” (Courtesy Astronomical Society of the Pacific) The reason why neutrons can take over where fusion leaves off is that they are uncharged. There is no electrical repulsion resisting their entry into nuclei, and they can slip in more-or-less unnoticed. But only up to a point. When a nucleus becomes too neutron-rich it also becomes unstable and decays—nuclei tend to rearrange themselves into more energy-efficient configurations. Beta-decay turns a neutron into a proton, throwing out an electron in the process. The result is a nucleus with the same total number of constituent particles, but with one more proton and one fewer neutron. In normally burning stars neutrons are released when helium nuclei fuse with other elements. There are relatively few of them around, and the proba- bility that a nucleus will encounter one is consequently small. That’s why B2FH named the neutron-capture process in stars the slow, or s-process: heavier-than-iron elements are built up slowly. What happens is that the BEAMLINE3 neutron-capture chain marches The s-process is responsible for steadily through the stable neutron- a lot of heavy elements, but it can’t rich versions of an element until it account for them all. There are many reaches an unstable one. That nu- stable heavy elements which are cleus then decays before it has a highly neutron rich. To reach them chance to absorb another neutron involves passing through unstable and the march towards heavier ele- isotopes on the way. That means that ments resumes in the element with neutrons have to be so abundant that one more proton. an unstable nucleus can absorb sev- eral before it gets a chance to decay, and that is where the rapid r-process comes in. R-process element gener- Right, electrons orbit nuclei at well- + Ei A + e ation happens in places where the defined energies which are unique to Selective 41.11 eV each element. The laser ion source Ionization neutron density is staggering—the (below) works by firing three laser sort of places, in fact, which are only pulses at a cloud of atoms in quick suc- w3 found in certain stars when they cession. The pulses are tuned so that reach the ends of their lives in the the first lifts an electron from one orbit to most violent explosions known in another, the second lifts it again, and the w2 the Universe—supernovae. third knocks it out completely. The com- bination of pulses is unique to the ele- Most stars finish their careers in ment required. Below, Michine Viatch- unspectacular fashion, retiring peace- eslav and Ulli Köster adjust the light Present: Ag, Mn, Be, Cu w1 fully from energy production before bench for CERN’s laser ion source. slowly fading away into darkness. Our own Sun is one of these. It has enough fuel to burn its way up to car- bon, and in a few billion years from now it will end its days as a slowly cooling lump of ash. Heavier stars don’t all go so quietly, and some of them, the James Deans of the cos- mos, instead go out in spectacular style. A supernovae happens when a heavy star has completely burned up its insides. With its fuel source ex- hausted, there is nothing left to sup- port it and the star collapses in on itself. Protons in the star resist the collapse because of the repulsive elec- tric force between them, but the grav- itational pull of all the matter in the dead star is stronger and the charge is literally squeezed out of the protons in the form of positive electrons (positrons), turning them into neutrons. The star’s collapse generates a shock-wave traveling outwards 4 WINTER 1999 which blows the outer layers of the in a lock by selecting just the ele- star out into space in an explosion ment of interest. At ISOLDE a beam accompanied by copious neutrons. of protons strikes a target. The im- What Makes an In this extremely neutron-rich envi- pact causes a range of unstable atoms Element an Element? ronment, an unstable nucleus has a to be created. These are evaporated good chance of catching another from the target and allowed to find N ATOM IS MADE UP neutron before it decays.

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