The Bea m Line is published quarterly by t heStanford Linear Accelerator CenterBox 4349, Stanford, CA 94309.Telephone: (650) 926-2585EMAIL: beamline@slac.s tanford.eduFAX: (650) 926-4500

Issues of t he Bea m Line are accessibleelectronically on t he World Wide Web ath t tp:/ / www.slac.stanford.edu /pubs/beamline. SLACis operated by Stanford U niversity under con tractwith the U.S. Depart men t of Energy. The opinionsof the au t hors do not necessarily reflect t he policiesof the Stanford Linear Accelerator Cen ter..

Cover: The Super-Kamiokande detector during fillingin 1996. Physicists in a rubber raft are polishing the

20-inch photomultipliers asthe water rises slowly. SeeJohn Learneds article onpage 8. (Courtesy ICRR,University of Tokyo)

EditorsRENE DO NALDSO N , BILL KIRK

Contributing EditorsMICHAEL RIORDAN , GORDO N FRASER

JU DY JACKSO N , AKIHIRO MAKIPEDRO WALOSCHEK

Editorial Advisory BoardPATRICIA BURCHAT, DAVID BURKE

LA N CE DIXO N , GEORGE SMO OTGEORGE TRILLIN G, KARL VAN BIBBER

HERMA N WINICK

IllustrationsTERRY AN DERSO N

DistributionCRYSTAL TILGHMA N

A PERIODICAL OF PARTICLE PHYSICS

WIN TER 1999 VOL. 29, N U M BER 3

Printed on recycled paper

FEAT U RES

2 GOLDEN STARDUSTThe ISOLDE facility at CER N is being usedto study how ligh ter ele m en ts are forgedin to heavier ones in the furnaces of the stars.

James Gillies

8 NEUTRIN OS HAVE MASS!The Super-Ka m iokande detector has found adeficit of one flavor of neutrino co m ingt hrough the Earth, wit h the li kelyi mplication that neutrinos possess m ass.

John G. Learned

16 IS SUPERSYMMETRY THE NEXTLAYER OF STRUCTURE?Despite its i m pressive successes, theoreticalphysicists believe t hat the Standard Model isinco m plete. Supersy m m etry m ight providethe answer to the puzzles of the Higgs boson.

Michael Dine

22 CER N s Lo w Energy Antiproton RingLEARCER Ns Low Energy A n tiproton Ring isre m e m bered for its transformations,experi m ental results, and re markablem achine physics perform ance.

Gordon Fraser

DEPA RTMEN TS

28 THE U NIVERSE AT LARGEAstroph ysics Faces the Millenniu mVirginia Trimble

37 FROM THE EDIT ORS DESK

38 CO N TRIBUTORS

40 DATES TO REMEMBER

d

ggm

t

d~

co1m

~t

C O N TEN TS

HERE DO GOLD earrings come from?A simple answer is the local jewelryshop, but if you really want to know in

depth, youll have to dig a lot further. Gold is, quite literally,stardust. Abou t half of it is forged in stars t hat burn nor mal-ly, while the rest comes from large stars at the ends of t heirlivesin the cataclysmic explosions called supernovae. Thatm uch we know. Bu t exactly how gold and all other elemen tsheavier t han iron are for med is still unclear. A new series ofexperimen ts at t he ISOLDE facility at t he European Labora-tory for Particle Physics, CERN , in Geneva aims to find out.

The history of the elemen ts is as old as the Universe itself.In t he beginning, a t the Big Bang, only the very ligh testelementshydrogen, heliu m, and a lit tle li thiu mwerefor med. Since then, so lit tle heavier material has beencreated that even today hydrogen and heliu m make up over99 percen t of all t he mat ter in the Universe. Everything elseam ounts to just a tiny fraction of 1 percen t.

After the Big Bang, a billion years passed before any heav-ier elemen ts appeared. They had to wait until the for mationof stars, when gravity squeezed the light ele men ts so t ightlythat t hey fused, igniting the stellar furnaces that forge heav-ier elemen ts from ligh ter ones. In t he nor mally burning par tof t heir lives, these stars build elemen ts as heavy as iron,producing energy from fusion as they do so. Bu t t hen theprocess stops, because anything heavier than iron takes moreenergy to make than fusion gives out. That doesnt meansuch elem en ts cant be made in starsthe fusion process justuses some of t he energy released by light-elemen t fusion

by JA MES GILLIES

2 WIN TER 1999

WIt is the stars, The starsabove us, govern our

conditions;(King Lear, Act IV, Scene 3)

but perhaps

The fault, dear Brutus,

is not in our stars, But

in ourselves.(Julius Caesar, Act I, Scene 2)

[Reprinted from Synthesis of theElements in Stars by E. M. Bur-

bidge, G. R. Burbidge, W. A. Fowler,and F. Hoyle, Reviews of Modern

Physics 29, 547 (1957)]

BEA M LINE 3

but t he Universe simply hasnt been around long enough for all the heavyelements we observe to have been produced that way. Another process m ustbe at work.

In 1957 the husband and wife team of Margaret and Geoffrey Burbidgeworking with Willy Fowler and the maverick British astronomer Fred Hoylefigured out what it could be. T hey pub-lished a paper which has since becomelegendary in the field of theoretical astro-physics and is known to aficionados sim-ply as B2FH. In it, the Burbidges, Fowler,and Hoyle show how neu trons could pro-vide the rou te to the heavier elemen ts.B2FH describes the so-called s- and r-processes t hrough which slow neutronabsorption in stars could generate abou thalf t he present abundance of heavier-than-iron elemen ts, with rapid neu tronabsorption, though t to occur in super-novae, making up the balance.

The reason why neu trons can takeover where fusion leaves off is that t hey are u ncharged. There is no electricalrepulsion resisting their en try in to nuclei, and they can slip in more-or-lessunnoticed. But only up to a point. When a nucleus becomes too neu tron-richit also becomes unstable and decaysnuclei tend to rearrange themselvesinto more energy-efficient configurations. Beta-decay turns a neu tron into aproton, throwing ou t an electron in the process. T he result is a nucleus withthe same total nu mber of constituen t particles, but wit h one more proton andone fewer neu tron.

In nor mally burning stars neutrons are released when heliu m nuclei fusewith other elemen ts. There are relatively few of t hem arou nd, and t he proba-bili ty that a nucleus will encounter one is consequen tly small. Thats whyB2FH named t he neutron-capture process in stars the slow, or s-process:heavier-than-iron elemen ts are built up slowly. What happens is that t he

Left to right, Margaret and Geoffrey Burbidge, William Fowler, andFred Hoyle, authors of the famous 1957 paper, Synthesis of Ele-ments in Stars. (Courtesy Astronomical Society of the Pacific)

4 WIN TER 1999

T he s-process is responsible fora lot of heavy elements, but it cantaccount for them all. There are manys table heavy e le m e nt s w hich arehighly neu tron rich. To reach theminvolves passing t h rough uns tableisotopes on the way. That means thatneutrons have to be so abundant thatan unstable nucleus can absorb sev-eral before it gets a chance to decay,and t hat is where the rapid r-processcomes in. R-process elemen t gener-at ion happens in places w here t heneu tron density is staggeringt hesort of places, in fact, which are onlyfound in cer tain s t ars w hen theyreach t he ends of t heir lives in t hem ost violen t explosions k now n inthe Universesupernovae.

Most stars finish their careers inunspectacular fashion, retiring peace-fully from energy product ion beforeslowly fading away in to dark ness.Our own Sun is one of these. It hasenough fuel to burn its way up to car-bon, and in a few billion years fromnow it will end its days as a slowlycooling lu m p of ash. Heavier st arsdont all go so quietly, and so me oft he m, t he Ja m es Deans of the cos-m os, ins tead go ou t in spectacu larstyle. A supernovae happens when aheavy star has completely burned upits insides. With i ts fuel source ex-hausted, there is not hing left to sup-port it and the star collapses in on it-self. Pro tons i n t he s t ar resis t t hecollapse because of the repulsive elec-tric force between them, but the grav-itational pull of all the mat ter in t hedead star is stronger and the chargeis li terally squeezed ou t of the pro-tons in the form of positive electrons(positrons), turning t he m in to neu-trons. The stars collapse generates as h oc k-w ave t r ave l i ng o u t w ards

n e u t ro n-c a p t u r e chain m archessteadily through the stable neutron-rich versions of an element un til i treaches an u ns table one. T hat n u-cleus t hen decays before i t has achance to absorb ano ther neu t ronand the march towards heavier ele-ments resu mes in the elemen t wit hone more proton.

Ei41.11 eV

A++ eSelective

Ionization

Present: Ag, Mn, Be, Cu

w3

w2

w1

Right, electrons orbit nuclei at well-defined energies which are unique toeach element. The laser ion source(below) works by firing three laserpulses at a cloud of atoms in quick suc-cession. The pulses are tuned so thatthe first lifts an electron from one orbit toanother, the second lifts it again, and thethird knocks it out completely. The com-bination of pulses is unique to the ele-ment required. Below, Michine Viatch-eslav and Ulli Kster adjust the lightbench for CERNs laser ion source.

BEA M LINE 5

which blows the outer layers of t hes tar ou t in to space in an explosionaccompanied by copious neu trons.In this extremely neu tron-rich envi-ronment, an unstable nucleus has agood chance of ca tch ing anot herneutron before it decays. Rapid neu-t ron capt ure ensues, generat ing awide range of u ns t able heavy iso-topes. When this explosive burningis over, these unstable isotopes cas-cade through a chain of beta-decaysending up as stable neutron-rich iso-topes. T his all takes place in jus t afew seconds, and when it is over, thenewly for med elements are sprayedou t in to the U niverse w here even-t ually gravity, th