Mpeople think they know what mass is, but they understand only part of

the story. For instance, an elephant is clearly bulkier and weighs more thanan ant. Even in the absence of gravity, the elephant would have greater mass—

it would be harder to push and set in motion. Obviously the elephant is more massivebecause it is made of many more atoms than the ant is, but what determines the mass-es of the individual atoms? What about the elementary particles that make up the at-oms—what determines their masses? Indeed, why do they even have mass?

We see that the problem of mass has two independent aspects. First, we need tolearn how mass arises at all. It turns out mass results from at least three differentmechanisms, which I will describe below. A key player in physicists’ tentative theoriesabout mass is a new kind of field that permeates all of reality, called the Higgs field.Elementary particle masses are thought to come about from the interaction with the

Riggs field. If the Riggs fieldexists, theory demands that ithave an associated particle,the Riggs boson. Using particle accelerators, scientists arenow hunting for the Riggs.

The second aspect is thatscientists want to know whydifferent species of elementaryparticles have their specificquantities of mass. Their in-trinsic masses span at least 11orders of magnitude, but wedo not yet know why thatshould be so [see illustrationon page 36]. For comparison,an elephant and the smallestof ants differ by about 11 or-ders of magnitude of mass.

What Is Mass?ISAAC NEWTON presentedthe earliest scientific defini

tion ofmass in 1687 in his landmark Principia: “The quantity ofmatter is the measureof the same, arising from its density and bulk conjointly.” That very basic definitionwas good enough for Newton and other scientists for more than 200 years. They understood that science should proceed first by describing how things work and later byunderstanding why. In recent years, however, the why of mass has become a researchtopic in physics. Understanding the meaning and origins of mass will complete andextend the Standard Model of particle physics, the well-established theory that describes the known elementary particles and their interactions. It will also resolve mysteries such as dark matter, which makes up about 25 percent of the universe.

The foundation of our modern understanding of mass is far more intricate thanNewton’s definition and is based on the Standard Model. At the heart of the Standard

www.sciam.com SCIENTIFIC AMERICAN 33

heMysteries of

Physicists are huntingfor an elusive particle that wouldreveal the presence of a new kind offield that permeates allof reality. Finding that Higgsfield will give us a morecomplete understanding about how the universe works

By Gordon Kane

1tI

Model is a mathematical function calleda Lagrangian, which represents how thevarious particles interact. From thatfunction, by following rules known asrelativistic quantum theory, physicistscan calculate the behavior of the elementary particles, including how they cometogether to form compound particles,such as protons. For both the elementaryparticles and the compound ones, wecan then calculate how they will respondto forces, and for a force F, we can writeNewton’s equation F = ma, which relatesthe force, the mass and the resulting acceleration. The Lagrangian tells us whatto use for m here, and that is what ismeant by the mass of the particle.

constituents’ rest mass and also their kinetic energy of motion and potential energy of interactions contribute to theparticle’s total mass. Energy and massare related, as described by Einstein’s famous equation, E = mc2 (energy equalsmass times the speed of light squared).

An example of energy contributingto mass occurs in the most familiar kindof matter in the universe—the protonsand neutrons that make up atomic nucleiin stars, planets, people and all that wesee. These particles amount to 4 to 5 percent of the mass-energy of the universe[see box on page 37]. The StandardModel tells us that protons and neutronsare composed of elementary particles

The Higgs MechanismU N L I K E P R 0 T 0 N S and neutrons, truly elementary particles—such as quarksand electrons—are not made up of smaller pieces. The explanation of how theyacquire their rest masses gets to the veryheart of the problem of the origin ofmass. As I noted above, the account proposed by contemporary theoretical physics is that fundamental particle massesarise from interactions with the Higgsfield. But why is the Higgs field presentthroughout the universe? Why isn’t itsstrength essentially zero on cosmicscales, like the electromagnetic field?What is the Riggs field?

The Riggs field is a quantum field.

But mass, as we ordinarily under-stand it, shows up in more than justF = ma. For example, Einstein’s specialrelativity theory predicts that masslessparticles in a vacuum travel at the speedof light and that particles with masstravel more slowly, in a way that can becalculated if we know their mass. Thelaws of gravity predict that gravity actson mass and energy as well, in a precisemanner. The quantity m deduced fromthe Lagrangian for each particle behavescorrectly in all those ways, just as we expect for a given mass.

Fundamental particles have an intrinsic mass known as their rest mass(those with zero rest mass are calledmassless). For a compound particle, the

called quarks that are bound together bymassless particles called gluons. Al-though the constituents are whirlingaround inside each proton, from outsidewe see a proton as a coherent object withan intrinsic mass, which is given by add-ing up the masses and energies of itsconstituents.

The Standard Model lets us calculatethat nearly all the mass of protons andneutrons is from the kinetic energy oftheir constituent quarks and gluons (theremainder is from the quarks’ rest mass).Thus, about 4 to 5 percent of the entireuniverse—almost all the familiar matteraround us—comes from the energy ofmotion of quarks and gluons in protonsand neutrons.

That may sound mysterious, but the factis that all elementary particles arise asquanta of a corresponding quantumfield. The electromagnetic field is also aquantum field (its corresponding elementary particle is the photon). So in this respect, the Higgs field is no more enigmatic than electrons and light. The Riggsfield does, however, differ from all otherquantum fields in three crucial ways.

The first difference is somewhat tech-nical. All fields have a property calledspin, an intrinsic quantity of angular momentum that is carried by each of theirparticles. Particles such as electrons havespin ½ and most particles associatedwith a force, such as the photon, havespin 1. The Riggs boson (the particle ofthe Riggs field) has spin 0. Raving 0 spinenables the Riggs field to appear in theLagrangian in different ways than theother particles do, which in turn allows—and leads to—its other two distinguishing features.

The second unique property of theRiggs field explains how and why it hasnonzero strength throughout the universe. Any system, including a universe,will tumble into its lowest energy state,like a ball bouncing down to the bottomof a valley. For the familiar fields, suchas the electromagnetic fields that give us

1Zr Why is the Higgs field present throughoutthe universe7What is the Higgs field2

OverviewlHiaas Phusics. Mass is a seemingly everyday property of matter, but it is actually mysterious

to scientists in many ways. How do elementary particles acquire massin the first place, and why do they have the specific masses that they do?

. The answers to those questions will help theorists complete and extend theStandard Model of particle physics, which describes the physics that governsthe universe. The extended Standard Model may also help solve the puzzleofthe invisible dark matterthat accounts for about 25 percent ofthe cosmos.

1 Theories say that elementary particles acquire mass by interactingwitha quantum field that permeates all of reality. Experiments at particleaccelerators may soon detect direct evidence ofthis so-called Higgs field.

34 SCIENTIFIC AMERICAN THE FRONTIERS OF PHYSICS

PERMEATING REALITY

Atypical field, such as the electromagnetic field, has its lowestenergy at zero field strength (left). The universe is akin to a ballthat rolled around and came to rest at the bottom ofthe valley—that is, it has settled at a field strength ofzero. The Higgs, incontrast, has its minimum energy at a nonzero field strength,and the “ball” comes to rest at a nonzero value (right). Thus, theuniverse, in its natural lowest energy state, is permeated by thatnonzero value ofthe Higgs field.

Energy Energy

INTERACTING WITH OTHER PARTICLES

j/ Fggs field

Force diagrams called Feynman diagrams represent how theHiggs particle interacts with other particles. Diagram (a)represents a particle such as a quark or an electron emitting(shown) or absorbing a Higgs particle. Diagram (b) shows thecorresponding process for a WorZ boson. The W andZ can alsointeract simultaneously with two Higgs, as shown in (c), whichalso represents a WorZscattering(roughly speaking,

b WorZ boson

CAUSING TWO PHENOMENA

Two completely different phenomena—theacquisition ofmass by a particle (top) and theproduction ofa Higgs boson (bottom)—arecaused by exactly the same interaction. Thisfact will be of great use in testing the Higgstheory by experiments.

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colliding with) a Higgs particle. The interactions representedby diagrams (a) through (c) are also responsible for generatingparticles’ masses. The Higgs also interacts with itself, asrepresented by diagrams (d) and (e). More complicatedprocesses can be built up byjoiningtogethercopies of theseelementary diagrams. Interactions depicted in (d) and (e) areresponsible for the shape ofthe energy graph (above left).

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www. Sc i a m .com SCIENTIFIC AMERICAN 35

r

HOW THE HIGGS FIELD GENERATES MASS

PROPERTIES OF THE ELUSIVE HIGGS

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*k“Empty” space, which is filled with the A particle crossingthat region ofspace isHiggs field, is like a beach full ofchildren. like an ice cream vendor arriving . . . him down—as if he acquires “mass.”

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radio broadcasts, the lowest energy stateis the one in which the fields have zerovalue (that is, the fields vanish)—if anynonzero field is introduced, the energystored in the fields increases the net en-ergy of the system. But for the Higgsfield, the energy of the universe is lowerif the field is not zero but instead has aconstant nonzero value. In terms of thevalley metaphor, for ordinary fields thevalley floor is at the location ofzero field;for the Higgs, the valley has a hillock atits center (at zero field) and the lowestpoint of the valley forms a circle aroundthe hillock [see box on preceding page].The universe, like a ball, comes to restsomewhere on this circular trench,which corresponds to a nonzero value ofthe field. That is, in its natural, lowestenergy state, the universe is permeatedthroughout by a nonzero Riggs field.

The final distinguishing characteristic of the Riggs field is the form of its interactions with the other particles. Particles that interact with the Riggs fieldbehave as if they have mass, proportional to the strength of the field times thestrength of the interaction. The massesarise from the terms in the Lagrangianthat have the particles interacting withthe Riggs field.

Our understanding of all this is notyet complete, however, and we are notsure how many kinds of Higgs fieldsthere are. Although the Standard Modelrequires only one Riggs field to generateall the elementary particle masses, physicists know that the Standard Modelmust be superseded by a more complete

theory. Leading contenders are extensions of the Standard Model known asSupersymmetric Standard Models(SSMs). In these models, each StandardModel particle has a so-called super-partner (as yet undetected) with closelyrelated properties see “The Dawn ofPhysics beyond the Standard Model,” byGordon Kane, on page 4]. With theSupersymmetric Standard Model, atleast two different kinds of Riggs fieldsare needed. Interactions with those twofields give mass to the Standard Modelparticles. They also give some (but notall) mass to the superpartners. The twoRiggs fields give rise to five species ofRiggs boson: three that are electricallyneutral and two that are charged. Themasses of particls called neutrinos,which are tiny compared with other par-tide masses, could arise rather indirectlyfrom these interactions or from yet athird kind of Higgs field.

Theorists have several reasons forexpecting the SSM picture of the Riggsinteraction to be correct. First, withoutthe Riggs mechanism, the W and Z bosons that mediate the weak force wouldbe massless, just like the photon (whichthey are related to), and the weak interaction would be as strong as the electromagnetic one. Theory holds that theRiggs mechanism confers mass to the Wand Z in a very special manner. Predictions of that approach (such as the ratioof the W and Z masses) have been confirmed experimentally.

Second, essentially all other aspectsof the Standard Model have been well

400.-. Higgs

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— 200

- 100

tested, and with such a detailed, inter-locking theory it is difficult to changeone part (such as the Higgs) without affecting the rest. For example, the analysis of precision measurements of W andz boson properties led to the accurateprediction of the top quark mass beforethe top quark had been directly pro-duced. Changing the Riggs mechanismwould spoil that and other successfulpredictions.

Third, the Standard Model Riggsmechanism works very well for givingmass to all the Standard Model particles,w and Z hosons, as well as quarks andleptons; the alternative proposals usually do not. Next, unlike the other theories, the SSM provides a framework tounify our understanding of the forces ofnature. Finally, the SSM can explainwhy the energy “valley” for the universehas the shape needed by the Riggs mechanism. In the basic Standard Model theshape of the valley has to be put in as apostulate, but in the SSM that shape canbe derived mathematically.

Testing the TheoryNATURALLY, PHYSICISTS want tocarry out direct tests ofthe idea that massarises from the interactions with the different Riggs fields. We can test three keyfeatures. First, we can look for the signature particles called Riggs bosons. Thesequanta must exist, or else the explanation is not right. Physicists are currentlylooking for Riggs bosons at the TevatronCollider at Fermi National AcceleratorLaboratory in Batavia, Ill.

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MASSES OFTHE PARTICLES oftheStandard Model differby atleast 11orders of magnitude and are believed to be generated by interactionswith the Higgs field. At least five Higgs particles are likely to exist.Their masses are not known; possible Higgs masses are indicated.

36 SCIENTIFIC AMERICAN THE FRONTIERS OF PHYSICS

Second, once they are detected wecan observe how Riggs bosons interactwith other particles. The very sameterms in the Lagrangian that determinethe masses of the particles also fix theproperties of such interactions. So wecan conduct experiments to test quantitatively the presence of interaction termsof that type. The strength of the interaction and the amount of particle mass areuniquely connected.

Third, different sets of Higgs fields,as occur in the Standard Model or in thevarious SSMs, imply different sets ofHiggs bosons with various properties, sotests can distinguish these alternatives,too. All that we need to carry out thetests are appropriate particle colliders—ones that have sufficient energy to pro-duce the different Higgs bosons, sufficient intensity to make enough of themand very good detectors to analyze whatis produced.

A practical problem with performingsuch tests is that we do not yet under-stand the theories well enough to calculate what masses the Riggs bosons them-selves should have, which makes search-ing for them more difficult because onemust examine a range of masses. A corn-bination of theoretical reasoning anddata from experiments guides us aboutroughly what masses to expect.

The Large Electron-Positron Collider (LEP) at CERN, the Europeanlaboratory for particle physics near Ge-neva, operated over a mass range thathad a significant chance of including aHiggs boson. It did not find one—al-though there was tantalizing evidencefor one just at the limits of the collider’senergy and intensity—before it was shutdown in 2000 to make room for constructing a newer facility, CERN’sLarge Hadron Collider (LHC). TheRiggs must therefore be heavier thanabout 120 proton masses. Nevertheless,LEP did produce indirect evidence thata Riggs boson exists: experimenters atLEP made a number of precise measurements, which can be combined withsimilar measurements from the Tevatron and the collider at the StanfordLinear Accelerator Center. The entireset of data agrees well with theory only

The theory ofthe Higgs field explains howelementary particles, the smallestbuilding blocks ofthe universe, acquire.their mass. But the Higgs mechanism isnot the only source of mass-energy in theuniverse (“mass-energy” refers to bothmass and energy, which are related byEinstein’s 6 = mc2).

About 70 percent ofthe mass-energy ofthe universe is in the form ofso-called dark energy, which is notdirectly associated with particles. Thechiefsign ofthe existence.ofdark energyis that the universe’s expansion isaccelerating. The precise nature of darkenergy is one ofthe most profound openquestions in physics [see “A CosmicConundrum,” by Lawrence M. Krauss andMichael S. Turner, on page 66].

The remaining 30 percent of theuniverse’s mass-energy comes frommatter, particles with mass. The mostfamiliar kinds of matter are protons,neutrons and electrons, which make upstars, planets, people and all that we see.These particles provide about one sixthof the matter of the universe, or 4 to 5percent of the entire universe. As isexplained in the main text, most of thismass arises from the energy of motion ofquarks and gluons whirling around insideprotons and neutrons.

A smaller contribution to theuniverse’s matter comes from particlescalled neutrinos, which come in three

THE UNIVERSEDark energy

varieties. Neutrinos have mass butsurprisingly little. The absolute masses ofneutrinos are not yet measured, but theexisting data put an upper limit on them—lessthan haifa percent ofthe universe.

Almost all the rest ofthe matter—around 25 percent ofthe universe’s totalmass-energy—is matterwe do not see,called dark matter. We deduce itsexistence from its gravitational effectson what we do see. We do not yet knowwhat this dark matter actually is, butthere are good candidates, andexperiments are underway to testdifferent ideas [see “The Search for DarkMatter,” by David B. Cline; SCIENTIFICAMERICAN, March 2003]. The dark mattershould be composed of massiveparticles because it forms galaxy-size

clumps under the effects of thegravitational force. A variety of argumentshave let us conclude that the dark mattercannot be composed ofany of thenormal Standard Model particles.

The leading candidate particle fordark matter is the lightest superpartner(LSP], which is discussed in greaterdetail in the main text. The lightestsuperpartner occurs in extensions of theStandard Model called SupersymmetricStandard Models. The mass of the LSP isthought to be about 100 proton masses.That the LSP was a good candidate for thedark matter was recognized by theoristsbefore cosmologists knew that a newform of fundamental matterwas neededto explain dark matter. —G.K.

A Cosmic Stocktaking

Proton

‘quark

“Up” quark

MOST VISIBLE MASS is locked up in protonsand neutrons. Each of these consists ofquarks and gluons flying around. Almost all ofthe proton’s or neutron’s mass is from theenergy of motion of the quarks and gluons.

Dark matter

Visible matter

Neutrinos

MASS-ENERGY ofthe universe mainlycomes in four broad types: mysterious darkenergy that causes the universe’s expansionto accelerate; invisible dark matter that wecan detect by its gravitational effects;visible matter; and neutrinos.

www.sciam.com SCIENTIFIC AMERICAN 3?

if certain interactions of particles withthe lightest Higgs boson are includedand only if the lightest Riggs boson isnot heavier than about 200 protonmasses. That provides researchers withan upper limit for the mass of the Riggsboson, which helps to focus the search.

For the next few years, the only collider that could produce direct evidencefor Riggs bosons will be the Tevatron.Its energy is sufficient to discover a Higgsboson in the range of masses implied bythe indirect LEP evidence, if it can consistently achieve the beam intensity it

was expected to have, which so far hasnot been possible. In 2007 the LHC,which is seven times more energetic andis designed to have far more intensity

than the Tevatron, is scheduled to begintaking data. It will be a factory for Higgsbosons (meaning it will produce many ofthe particles a day). Assuming the LHCfunctions as planned, gathering the relevant data and learning how to interpretit should take one to two years. Carryingout the complete tests that show in detailthat the interactions with Riggs fieldsare providing the mass will require anew electron-positron collider in addition to the LRC (which collides protons)and the Tevatron (which collides protonsand antiprotons).

Dark MatterWHAT IS DISCOVERED about Riggsbosons will not only test whether theRiggs mechanism is indeed providingmass, it will also point the way to howthe Standard Model can be extended tosolve problems such as the origin of darkmatter.

With regard to dark matter, a key

particle of the SSM is the lightest super-partner (LSP). Among the superpartnersof the known Standard Model particlespredicted by the SSM, the LSP is the onewith the lowest mass. Most superpartners decay promptly to lower-mass superpartners, a chain of decays that endswith the LSP, which is stable because ithas no lighter particle that it can decayinto. (When a superpartner decays, atleast one of the decay products should beanother superpartner; it. should not decay entirely into Standard Model particles.) Superpartner particles would havebeen created early in the big bang butthen promptly decayed into LSPs. TheLSP is the leading candidate particle fordark matter.

The Higgs bosons may also directlyaffect the amount of dark matter in theuniverse. We know that the amount ofLSPs today should be less than theamount shortly after the big bang, be-cause some would have collided and an-nihilated into quarks and leptons andphotons, and the annihilation rate maybe dominated by LSPs interacting withHiggs bosons.

As mentioned earlier, the two basicSSM Riggs fields give mass to the Standard Model particles and some mass tothe superpartners, such as the LSP. Thesuperpartners acquire more mass via additional interactions, which may be withstill further Riggs fields or with fieldssimilar to the Higgs. We have theoreticalmodels of how these processes can happen, but until we have data on the superpartners themselves we will not knowhow they work in detail. Such data areexpected from the LHC or perhaps evenfrom the Tevatron.

Neutrino masses may also arise frominteractions with additional Riggs orRiggs-like fields, in a very interestingway. Neutrinos were originally assumedto be massless, but since 1979 theoristshave predicted that they have small mass-es, and over the past decade or so severalimpressive experiments have confirmedthe predictions (see “Solving the SolarNeutrino Problem,” by Arthur B. Mc-Donald, Joshua R. Klein and David L.Wark, on page 22] . The neutrino massesare less than a millionth the size of thenext smallest mass, the electron mass.Because neutrinos are electrically neu- Itral, the theoretical description of theirmasses is more subtle than for chargedparticles. Several processes contribute to

the mass of each neutrino species, andfor technical reasons the actual massvalue emerges from solving an equationrather than just adding the terms.

Thus, we have understood the threeways that mass arises: The main form ofmass we are familiar with—that of pro-tons and neutrons and therefore of at-oms—comes from the motion of quarksbound into protons and neutrons. Theproton mass would be about what it iseven without the Riggs field. The massesof the quarks themselves, however, andalso the mass of the electron, are entirelycaused by the Higgs field. Those masseswould vanish without the Higgs. Last,but certainly not least, most of theamount of superpartner masses, andtherefore the mass of the dark matterparticle (if it is indeed the lightest super-partner), comes from additional interactions beyond the basic Higgs one.

Finally,’we consider an issue knownas the family problem. Over the past halfa century physicists have shown that theworld we see, from people to flowers tostars, is constructed from just six particles: three matter particles (up quarks,down quarks and electrons), two forcequanta (photons and gluons), and Riggsbosons—a remarkable and surprisingly

4:\ The LEP collider saw tantalizingJ(’ evidence for the Higgs particle

:bGORDON KANE: a particle theorist, is Victor Weisskopf Collegiate Professor of Physics

atthe University ofMichigan atAnn Arbor. His work explores waysto test and extend the

Standard Model ofparticle physics. In particular, he studies Higgs physics and the Stan--

dard Model’s supersymmetric extension and cosmology, with a focus on relating theory

and experiment. Recently he has emphasized integrating these topics with string

theory and studyingthe implications for collider experiments.

38 SCIENTIFIC AMERICAN THE FRONTIERS OF PHYSICS

simple description. Yet there are fourmore quarks, two more particles similarto the electron, and three neutrinos. Allare very short-lived or barely interactwith the other six particles. They can beclassified into three families: up, down,electron-neutrino, electron; charm,strange, muon-neutrino, muon; and top,bottom, tau-neutrino, tau. The particlesin each family have interactions identicalto those ofthe particles in other families.They differ only in that those n the second family are heavier than thse in thefirst, and those in the third family areheavier still. Because these masses arisefrom interactions with the Higgs field,the particles must have different interactions with the Higgs field.

Hence, the family problem has twoparts: Why are there three families whenit seems only one is needed to describethe world we see? Why do the familiesdiffer in mass and have the masses theydo? Perhaps it is not obvious why physicists are astonished that nature containsthree almost identical families even ifone would do. It is because we want tofully understand the laws of nature andthe basic particles and forces. We expect that every aspect of the basic lawsis a necessary one. The goal is to have atheory in which all the particles andtheir mass ratios emerge inevitably,without making ad hoc assumptionsabout the values of the masses and with-out adjusting parameters. If havingthree families is essential, then it is aclue whose significance is currently notUnderstood.

Tying It All TogetherTHE STANDARD MODEL and theSSM can accommodate the observedfamily structure, but they cannot explain it. This is a strong statement. It isnot that the SSM has not yet explainedthe family structure but that it cannot.For me, the most exciting aspect of stringtheory is not only that it may provide uswith a quantum theory of all the forcesbut also that it may tell us what the elementary particles are and why there arethree families. String theory seems ableto address the question of why the interactions with the Higgs field differ among

the families. In string theory, repeatedfamilies can occur, and they are notidentical. Their differences are describedby properties that do not affect thestrong, weak, electromagnetic or gravitational forces but that do affect the in-teractions with Higgs fields, which fitswith our having three families with different masses. Although string theoristshave not yet fully solved the problem ofhaving three families, the theory seemsto have the right structure to provide asolution. String theory allows many different family structures, and so far noone knows why nature picks the one weobserve rather than some other [see“The String Theory Landscape,” by Raphael Bousso and Joseph Polchinski, onpage 40]. Data on the quark and leptonmasses and on their superpartner masses

may provide major clues to teach usabout string theory.

One can now understand why it tookso long historically to begin to under-stand mass. Without the Standard Mod-el of particle physics and the developmentof quantum field theory to describe par-tides and their interactions, physicistscould not even formulate the right questions. Whereas the origins and values ofmass are not yet fully understood, it islikely that the framework needed to understand them is in place. Mass could nothave been comprehended before theoriessuch as the Standard Model and its supersymmetric extension and string theory existed. Whether they indeed providethe complete answer is not yet clear, butmass is now a routine research topic inparticle physics.

A HIGGS PARTICLE might have been created when a high-energy positron and electron collided inthe L3 detector of the Large Electron-Positron Collider at CERN. The lines represent particletracks. The green and purple blobs and gold histograms depict amounts of energy deposited inlayers of the detector by particles flying away from the reaction. Only by combining many suchevents can physicists conclude whether Higgs particles were present in some of the reactions orif all the data were produced by other reactions that happened to mimic the Higgs signal.

MORE TO EXPLOREThe Particle Garden. Gordon Kane. Perseus Publishing, 1996.

The Little Book ofthe Big Bang: A Cosmic Primer. CraigJ. Hogan. Copernicus Books, 1998.Mass without Mass II: The Medium Is the Mass.age. FrankWilczek in Physics Today, Vol. 53,No. 1, pages 13—14; January 2000.

Supersymmetry: Unveiling the Ultimate Laws of Nature. Gordon Kane. Perseus Publishing, 2001.An excellent collection of particle physics Web sites is listed atparticle adventure. org/particle adventure/other/othersites . html

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