Special Report THE COMING REVOLUTIONS IN PARTICLE PHYSICS The current Standard Model of particle physics begins to unravel when probed much beyond the range of current particle accelerators. So no matter what the Large Hadron Collider finds, it is going to take physics into new territory By Chris Quigg hen physicists are forced to give a sin- The Matter at Hand gle-word answer to the question of What physicists call the “Standard Model” of W why we are building the Large Had- particle physics, to indicate that it is still a work ron Collider (LHC), we usually reply “Higgs.” in progress, can explain much about the known The Higgs particle—the last remaining undis- world. The main elements of the Standard Mod- covered piece of our current theory of matter— el fell into place during the heady days of the is the marquee attraction. But the full story is 1970s and 1980s, when waves of landmark much more interesting. The new collider pro- experimental discoveries engaged emerging the- vides the greatest leap in capability of any oretical ideas in productive conversation. Many KEY CONCEPTS instrument in the history of particle physics. We particle physicists look on the past 15 years as do not know what it will find, but the discover- an era of consolidation in contrast to the fer- ■ The Large Hadron Collider ies we make and the new puzzles we encounter ment of earlier decades. Yet even as the Stan- (LHC) is certain to find are certain to change the face of particle phys- dard Model has gained ever more experimental something new and pro- ics and to echo through neighboring sciences. support, a growing list of phenomena lies out- vocative as it presses into In this new world, we expect to learn what side its purview, and new theoretical ideas have unexplored territory. distinguishes two of the forces of nature—elec- expanded our conception of what a richer and ■ The Standard Model of par- tromagnetism and the weak interactions—with more comprehensive worldview might look like. ticle physics requires a par- broad implications for our conception of the ev- Taken together, the continuing progress in ticle known as the Higgs eryday world. We will gain a new understand- experiment and theory point to a very lively boson, or a stand-in to play ing of simple and profound questions: Why are decade ahead. Perhaps we will look back and its role, at energies probed by the LHC. The Higgs, in there atoms? Why chemistry? What makes sta- see that revolution had been brewing all along. turn, poses deep questions ble structures possible? Our current conception of matter comprises of its own, whose answers The search for the Higgs particle is a pivotal two main particle categories, quarks and lep- should be found in the step, but only the first step. Beyond it lie phe- tons, together with three of the four known fun- same energy range. nomena that may clarify why gravity is so much damental forces, electromagnetism and the weaker than the other forces of nature and that strong and weak interactions [see box on page ■ These phenomena revolve around the question of could reveal what the unknown dark matter 48]. Gravity is, for the moment, left to the side. symmetry. Symmetries that fills the universe is. Even deeper lies the Quarks, which make up protons and neutrons, underlie the interactions prospect of insights into the different forms of generate and feel all three forces. Leptons, the of the Standard Model but matter, the unity of outwardly distinct particle best known of which is the electron, are immune are not always reflected in categories and the nature of spacetime. The to the strong force. What distinguishes these the operation of the mod- questions in play all seem linked to one another two categories is a property akin to electric el. Understanding why not and to the knot of problems that motivated the charge, called color. (This name is metaphorical; is a key question. prediction of the Higgs particle to begin with. it has nothing to do with ordinary colors.) —The Editors The LHC will help us refine these questions and Quarks have color, and leptons do not. will set us on the road to answering them. The guiding principle of the Standard Model 46 SCIENTIFIC AMERICAN Februar y 20 0 8 © 2008 SCIENTIFIC AMERICAN, INC. STUDYING THE WORLD with a reso- lution a billion times finer than atomic scales, particle physi- cists seek a deeper understand- ing of the everyday world and of the evolution of the universe. [THE AUTHOR] is that its equations are symmetrical. Just as a function—the interactions among particles— sphere looks the same whatever your viewing that the theory describes. For instance, the strong angle is, the equations remain unchanged even nuclear force follows from the requirement that when you change the perspective from which they the equations describing quarks must be the are defined. Moreover, they remain unchanged same no matter how one chooses to define quark even when the perspective shifts by different colors (and even if this convention is set indepen- amounts at different points in space and time. dently at each point in space and time). The Ensuring the symmetry of a geometric object strong force is carried by eight particles known places very tight constraints on its shape. A as gluons. The other two forces, electromagne- sphere with a bump no longer looks the same tism and the weak nuclear force, fall under the Chris Quigg is a senior scientist at from every angle. Likewise, the symmetry of the rubric of the “electroweak” forces and are based Fermi National Accelerator Labora- tory, where for 10 years he led the ) equations places very tight constraints on them. on a different symmetry. The electroweak forces theoretical physics department. He Quigg These symmetries beget forces that are carried are carried by a quartet of particles: the photon, is the author of a celebrated text- + – by special particles called bosons [see “Gauge Z boson, W boson and W boson. book on the so-called gauge theo- Theories of the Forces between Elementary Par- ries that underlie the Standard ticles,” by Gerard ’t Hooft; Scientific Ameri- Breaking the Mirror Model, as well as the former editor can, June 1980, and “Elementary Particles and The theory of the electroweak forces was formu- of the Annual Review of Nuclear and Particle Science. Quigg’s research Forces,” by Chris Quigg; Scientific Ameri- lated by Sheldon Glashow, Steven Weinberg and on electroweak symmetry breaking );COURTESY OF CHRIS QUIGG ( can, April 1985]. Abdus Salam, who won the 1979 Nobel Prize in and supercollider physics highlight- In this way, the Standard Model inverts Louis Physics for their efforts. The weak force, which ed the importance of the terascale. Sullivan’s architectural dictum: instead of “form is involved in radioactive beta decay, does not act He is a frequent visitor to CERN. Vitruvianman follows function,” function follows form. That on all the quarks and leptons. Each of these par- When not blazing the trail to the deepest workings of nature, he can is, the form of the theory, expressed in the sym- ticles comes in mirror-image varieties, termed be found hiking on one of France’s KENNBROWN ( metry of the equations that define it, dictates the left-handed and right-handed, and the beta-decay Sentiers de Grande Randonnée. www.SciAm.com SCIENTIFIC AMERICAN 47 © 2008 SCIENTIFIC AMERICAN, INC. [THE BASICS OF PARTICLE PHYSICS] What Really Matters If you look deep inside a lump of matter, it is made up of only a few types of elementary particles, drawn from a palette of a dozen flavors. The Standard Model treats the particles as geometrical points; sizes shown here reflect their masses. SUBSTANCE ATOM PARTICLES OF MATTER PARTICLES OF FORCE QUARKS BOSONS These particles make up protons, neutrons and a veritable zoo of lesser-known particles. At the quantum level, each force of They have never been observed in isolation. nature is transmitted by a dedicated particle or set of particles. UP u CHARM c TOP t PHOTON γ 2 2 2 Electric charge: 0 Electric charge: + /3 Electric charge: + /3 Electric charge: + /3 Mass: 0 Mass: 2 MeV Mass: 1.25 GeV Mass: 171 GeV Carrier of electromagnetism, the quantum Constituent of ordinary matter; Unstable heavier cousin of the up; con- Heaviest known particle, of light acts on electrically charged two up quarks, plus a down, stituent of the J/ particle, which helped comparable in mass to an atom particles. It acts over unlimited distances. make up a proton. physicists develop the Standard Model. of osmium. Very short-lived. Z BOSON DOWN d STRANGE s BOTTOM b Z Electric charge: 0 1 1 1 Electric charge: – /3 Electric charge: – /3 Electric charge: – /3 Mass: 91 GeV Mass: 5 MeV Mass: 95 MeV Mass: 4.2 GeV Mediator of weak reactions that do not Constituent of ordinary matter; Unstable heavier cousin Unstable and still heavier change the identity of particles. Its range two down quarks, plus an up, of the down; constituent of the copy of the down; constituent is only about 10–18 meter. compose a neutron. much studied kaon particle. of the much studied B-meson particle. W+/W – BOSONS W LEPTONS Electric charge: +1 or –1 These particles are immune to the strong force and are observed as isolated individuals. Each neutrino shown Mass: 80.4 GeV here is actually a mixture of neutrino species, each of which has a definite mass of no more than a few eV. Mediators of weak reactions that change particle flavor and charge.