Summer 1993 Vol
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. CONTENTS Zew 4wee - - - A PERIODICAL OF PARTICLE PHYSICS SUMMER 1993 VOL. 23, NUMBER 2 FEATURES Editors RENE DONALDSON, MICHAEL RIORDAN A PEOPLE'S GUIDE TO THE STANDARD MODEL Executive Editor A non-technicalintroduction BILL KIRK to the dominant theory of particlephysics. Editorial Advisory Board Patricia Burchat JAMES BJORKEN, ROBERT N. CAHN, DAVID HITLIN STEWART C. LOKEN, RONALD RUTH 8 TWO PREVIOUS STANDARD MODELS WINICK MARVIN WEINSTEIN, HERMAN Scientists of Napoleonic France and Victorian England believed Photographic Services in universalphysical theories TOM NAKASHIMA that were the "standardmodels" BETTE REED of their times. Illustrations J. L. Heilbron TERRY ANDERSON, KEVIN JOHNSTON 15 POSITRON EMISSION TOMOGRAPHY SYLVIA MACBRIDE, JIM WAHL A promising medical imaging technique Distribution that borrows heavily from nuclear and particle physics is giving physicians TILGHMAN CRYSTAL a valuable new window on the functions of the human body. Mark Mandelkern DEPARTMENTS The Beam Line is published quarterly by the Stanford Linear Accelerator Center, P.O. Box 4349, Stanford, CA 94309. 24 THE UNIVERSE AT LARGE Telephone: (415) 926-2585 INTERNET: [email protected] Some Faint, ParticularStars: 1 BITNET: BEAMLINE@SLACVM Virginia Trimble FAX: (415) 926-4500 SLAC is operated by Stanford University under contract with the U.S. Department of Energy. The opinions of the 30 DATES TO REMEMBER authors do not necessarily reflect the policy of the Stanford Linear Accelerator Center. 31 TOWARD THE NEXT LINEAR COLLIDER Cover: Image of the brain of an epilepsy patient made using Probing the TeV Energy Scale Positron Emission Tomography (PET). It indicates substantially reduced metabolic activity in the left temporal lobe (blue area Fred Harris and Xerxes Tata at lower right), consistent with an epileptogenic focus, which is the likely source of this patient's seizures. PEOPLE AND EVENTS (This image and that on page 15 courtesy of Peter S. Conti, Director, 34 PET Imaging Service Center, University of Southern California) 39 FROM THE EDITORS' DESK Printed on Recycled Paper Ah 40 CONTRIBUTORS A PEOPLE TO THE S r by PATRICIA BURCHAT A non-technical in to the dominant ti of particle physics. "THE SI commo around theoret journal articles is so ml usually BEAM LINE 1 Over the past 30 years or so, this processes are never seen to occur. In very successful model of the many cases, the absence of a subatomic world-a combination of particular type of process leads to a assumptions and mathematical "conservation rule," which tells us theories-has evolved based on the that some property or quantity is work of many experimental and neither gained nor lost in a given theoretical physicists and on data reaction. A familiar example is con- from particle research laboratories servation of electric charge: the net around the world. It has been electric charge at the beginning of extremely successful at describing every reaction must equal the net the interactions that occur between electric charge at the end. This simple the fundamental particles that make conservation rule eliminates many up matter. Two of the central ideas potential processes; they cannot of this model are that all matter is occur because electric charge would composed of tiny constituents called not be conserved. quarks and leptons and that inter- The Standard Model evolved to actions (or forces) between these explain both the observation of cer- particles occur through the exchange tain processes and the absence of of other particles. The Standard otherprocesses. Experimental results Model is used to make accurate quan- have sometimes led to the discovery titative predictions for many proces- of new, previously unobserved con- ses involving elementary particles, stituents of matter, and at other times from their rates of production to how to the discovery of a new rule for how fast they disintegrate. the constituents interact. The Stan- The Standard Model developed dard Model incorporates both the through a complex interplay between constituents and their interactions theoretical ideas and experimental to give a complete description of all results. It has always been con- known processes involving elemen- strained by the particles and pro- tary particles. cesses that are actually observed. At The shape puzzle (see box at left) certain times during its evolution, gives an example of static interac- the Standard Model predicted par- tions between constituents. In par- ticles or processes that had not been ticle physics we are usually studying observed yet.* Experiments were dynamic systems, in which interac- conducted, motivated in part by these tions between particles lead to a sys- predictions. The outcome of the ex- tem that changes with time. Two periments proved or disproved those particles, such as an electron and a particular aspects of the model. positron, might come together and Important constraints are also annihilate each other to create two placed on the Standard Model by new particles. A heavy particle might what is not observed-it must some- disintegrate into several lighter par- how explain why certain particles or ticles. In the Standard Model, these interactions occur through the ex- change of other particles called the *Two important examples are the weak neutral current and the charm quark, carriers or mediators of the force or both of which are discussed in the text. interaction. 2 SUMMER 1993 To understand how a force can be who recognized the correspondence mediated by a particle, consider the between the elements of these dia- analogy of two rollerbladers, gliding grams and the mathematical terms along on parallel paths, one of them that appear in calculations giving holding a ball (see illustration at the probability of a particular inter- right). When one rollerblader throws action occurring. Each line (or par- the ball at the other, she recoils ticle) in the diagram corresponds to a slightly in the opposite direction from factor in a theoretical calculation. her toss-much like a cannon re- Each point at which two lines (or coils when it fires a cannonball, only particles) meet corresponds to an- on a smaller scale. If she throws a other factor. These diagrams help ball to her left, she will receive a physicists conceptualize subatomic Particle B small kick to her right. When the processes and develop deeper insight other rollerblader catches the ball, into particle interactions. For par- a similar kick to his ticle physicists, Feynman diagrams he will receive Force-carrying for in- left. If we were to watch this interac- provide an indispensable tool Particle tion but could not see the ball being tuition, communication, and calcu- exchanged between the two, we lation. might well conclude that a repulsive Particle A force is acting between them. This QUARKS AND LEPTONS are force was actually "mediated" by particles that appear to be Interactions between subatomic the exchange of a ball, which "car- fundamental. No matter how deeply particles can be compared to what when two rollerbladers toss a force between them. we probe with the experimental happens ried" the ball between them. The effects of the The Standard Model describes the facilities available today, we see no force between the rollerbladers (or various possible interactions be- structure within them. Quarks and particles) are "carried" by the ball (or a tween particles in terms of the ex- leptons do not appear to be composed third particle). (Drawing by Bayard change of several different particles. of smaller particles. In fact, they do Colyear) The rollerblader analogy has limita- not even have a detectable size. With tions (for example, it can't be used to current accelerators, for example, explain attractive forces), but it is a physicists can tell that the diameter useful one to keep in mind. Often as of a quark is less than a millionth of you walk the halls of high-energy a millionth of a millionth of a meter! physics laboratories or glance into To put this in perspective, imagine offices of particle physicists, you will that an atom is blown up to the size see diagrams similar to the one at of the earth. Then the nucleus is bottom right on chalkboards or scraps about the size of a football field, a of paper. Instead of rollerbladers, the neutron or proton inside the nucleus lines represent the paths taken by is about the size of an automobile, quarks or leptons. Instead of balls, and a quark is smaller than a tiny you see symbolic representations of pebble! the particles mediating the interac- The word lepton comes from the tion between them. Greek root lepto, meaning "small" These schematic pictures of par- or "slender." The leptons were so- ticle interactions are called Feynman named because the first of them to diagrams, after the late Richard P. be found were very light compared to Feynman, the theoretical physicist other particles. However, much more BEAM LINE 3 massive leptons have since been two "weight classes." The middle- discovered, leading to the non- weights were called mesons and the sequitur "heavy lepton." The leptons heavyweights baryons. In the early include the first elementary particle 1960s, Gell-Mann proposed that all to show up: the familiar electron, the observed mesons and baryons discovered by the British physicist are made of three kinds of quarks, J. J. Thompson in 1897. The next which he called up, down, and lepton, discovered in 1937 in cosmic strange. He postulated that baryons rays, is a highly penetrating particle are made of three quarks and that called the muon. The third lepton, mesons are made of a quark and an discovered by Martin Perl at the antiquark (the antiparticle of a quark). Stanford Linear Accelerator Center Individual quarks are never observed in 1975, is called the tau lepton. The in Nature; they are always bound muon and the tau lepton both carry inside baryons or mesons.