• 14.1 Early Discoveries• 14.2 The Fundamental Interactions• 14.3 Classification of Elementary Particles• 14.4 Conservation Laws and Symmetries• 14.5 Quarks• 14.6 The Families of Matter• 14.7 Beyond the Standard Model• 14.8 Accelerators

CHAPTER 14Elementary ParticlesElementary Particles

If I could remember the names of all these particles, I’d be a botanist.

- Enrico Fermi

Elementary Particles• We began our study of subatomic physics in Chapter

12. We investigated the nucleus in Chapters 12 and 13. We now delve deeper, because finding answers to some of the basic questions about nature is a foremost goal of science:

– What are the basic building blocks of matter? – What is inside the nucleus? – What are the forces that hold matter together? – How did the universe begin? – Will the universe end, and if so, how and when?

The Building Blocks of Matter• We have thought of electrons, neutrons, and

protons as elementary particles, because we believe they are basic building blocks of matter.

• However, in this chapter the term elementary particle is used loosely to refer to hundreds of particles, most of which are unstable.

14.1: Early Discoveries

• In 1930 the known elementary particles were the proton, the electron, and the photon.

• Thomson identified the electron in 1897, and Einstein’s work on the photoelectric effect can be said to have defined the photon (originally called a quantum) in 1905. The proton is the nucleus of the hydrogen atom.

• Despite the rapid progress of physics in the first couple of decades of the twentieth century, no more elementary particles were discovered until 1932, when Chadwick proved the existence of the neutron, and Carl Anderson identified the positron in cosmic rays.

The Positron

• Dirac in 1928 introduced the relativistic theory of the electron when he combined quantum mechanics with relativity.

• He found that his wave equation had negative, as well as positive, energy solutions.

• His theory can be interpreted as a vacuum being filled with an infinite sea of electrons with negative energies.

• If enough energy is transferred to the “sea”, an electron can be ejected with positive energy leaving behind a hole that is the positron, denoted by e+.

Antiparticles

• Dirac’s theory, along with refinements made by others opened the possibility of antiparticles which:– Have the same mass and lifetime as their associated

particles– Have the same magnitude but are opposite in sign for

such physical quantities as electric charge and various quantum numbers

• All particles, even neutral ones (with some notable exceptions like the neutral pion), have antiparticles.

Cosmic Rays• Cosmic rays are highly energetic particles,

mostly protons, that cross interstellar space and enter the Earth’s atmosphere, where their interaction with particles creates cosmic “showers” of many distinct particles.

Positron-Electron Interaction• The ultimate fate of positrons (antielectrons) is

annihilation with electrons.

• After a positron slows down by passing through matter, it is attracted by the Coulomb force to an electron, where it annihilates through the reaction

Feynman Diagram

• Feynman presented a particularly simple graphical technique to describe interactions.

• For example, when two electrons approach each other, according to the quantum theory of fields, they exchange a series of photons called virtual photons, because they cannot be directly observed.

• The action of the electromagnetic field (for example, the Coulomb force) can be interpreted as the exchange of photons. In this case we say that the photons are the carriers or mediators of the electromagnetic force.

Figure 14.2: Example of a Feynman spacetime diagram. Electrons interact through mediation of a photon. The axes are normally omitted.

Yukawa’s Meson

• The Japanese physicist Hideki Yukawa had the idea of developing a quantum field theory that would describe the force between nucleons analogous to the electromagnetic force.

• To do this, he had to determine the carrier or mediator of the nuclear strong force analogous to the photon in the electromagnetic force which he called a meson (derived from the Greek word meso meaning “middle” due to its mass being between the electron and proton masses).

• Yukawa’s meson, called a pion (or pi-meson or π-meson), was identified in 1947 by C. F. Powell (1903–1969) and G. P. Occhialini (1907–1993)

• Charged pions have masses of 140 MeV/c2, and a neutral pion π0 was later discovered that has a mass of 135 MeV/c2, a neutron and a proton.

Yukawa’s Meson

Figure 14.3: A Feynman diagram indicating the exchange of a pion (Yukawa’s meson) between a neutron and a proton.

14.2: The Fundamental Interactions

• The fundamental forces in nature responsible for all interactions:

1) Gravitation2) Electroweak (electromagnetic and weak)3) Strong

• The electroweak is sometimes treated separately as the electromagnetic and the weak force thus creating four fundamental forces.

The Fundamental Interactions• We have learned that the fundamental forces act

through the exchange or mediation of particles according to the quantum theory of fields. The exchanged particle in the electromagnetic interaction is the photon. All particles having either electric charge or a magnetic moment (and also the photon) interact with the electromagnetic interaction. The electromagnetic interaction has very long range.

The Fundamental Interactions• In the 1960s Sheldon Glashow, Steven Weinberg,

and Abdus Salam (Nobel Prize for Physics, 1979) predicted that particles, which they called W (for weak) and Z, should exist that are responsible for the weak interaction.

• This theory, called the electroweak theory, unified the electromagnetic and weak interactions much as Maxwell had unified electricity and magnetism into the electromagnetic theory a hundred years earlier.

Other Mesons

• We previously saw that Yukawa’s pion is responsible for the nuclear force. Now we know there are other mesons that interact with the strong force. Later we will see that the nucleons and mesons are part of a general group of particles formed from even more fundamental particles quarks. The particle that mediates the strong interaction between quarks is called a gluon (for the “glue” that holds the quarks together); it is massless and has spin 1, just like the photon.

• Particles that interact by the strong interaction are called hadrons; examples include the neutron, proton, and mesons.

The Graviton

• It has been suggested that the particle responsible for the gravitational interaction be called a graviton.

• The graviton is the mediator of gravity in quantum field theory and has been postulated because of the success of the photon in quantum electrodynamics theory.

• It must be massless, travel at the speed of light, have spin 2, and interact with all particles that have mass-energy.

• The graviton has never been observed because of its extremely weak interaction with objects.

The Fundamental Interactions

The Standard Model

• The most widely accepted theory of elementary particle physics at present is the Standard Model.

• It is a simple, comprehensive theory that explains hundreds of particles and complex interactions with six quarks, six leptons, and three force-mediating particles.

• It is a combination of the electroweak theory and quantum chromodynamics (QCD), but does not include gravity.

14.3: Classification of Elementary Particles

• We discussed in Chapter 9 that articles with half-integral spin are called fermions and those with integral spin are called bosons.

• This is a particularly useful way to classify elementary particles because all stable matter in the universe appears to be composed, at some level, of constituent fermions.

Bosons and Fermions

• Photons, gluons, W ±, and the Z are called gauge bosons and are responsible for the strong and electroweak interactions.

• Gravitons are also bosons, having spin 2. • Fermions exert attractive or repulsive forces on

each other by exchanging gauge bosons, which are the force carriers.

The Higgs Boson• One other boson that has been predicted, but

not yet detected, is necessary in quantum field theory to explain why the W± and Z have such large masses, yet the photon has no mass.

• This missing boson is called the Higgs particle (or Higgs boson) after Peter Higgs, who first proposed it.

The Higgs Boson• The Standard Model proposes that there is a field called

the Higgs field that permeates space.

• By interacting with this field, particles acquire mass.

• Particles that interact strongly with the Higgs field have heavy mass; particles that interact weakly have small mass.

• The Higgs field has at least one particle associated with it, and that is the Higgs particle (or Higgs boson). The properties of the gauge and Higgs bosons, as well as the graviton, are given in the next slide.

• The search for the Higgs boson is of the highest priority in elementary particle physics.

Boson Properties

Leptons

• The leptons are perhaps the simplest of the elementary particles.

• They appear to be pointlike, that is, with no apparent internal structure, and seem to be truly elementary.

• Thus far there has been no plausible suggestion they are formed from some more fundamental particles.

• There are only six leptons plus their six antiparticles.

The Electron and the Muon• Each of the charged particles has an associated

neutrino, named after its charged partner (for example, muon neutrino).

• The muon decays into an electron, and the tau can decay into an electron, a muon, or even hadrons (which is most probable).

• The muon decay (by the weak interaction) is:

Neutrinos• We are already familiar with the electron neutrino that

occurs in the beta decay of the neutron (Chapter 12). • Neutrinos have zero charge. • Their masses are known to be very small. The precise

mass of neutrinos may have a bearing on current cosmological theories of the universe because of the gravitational attraction of mass.

• All leptons have spin 1/2, and all three neutrinos have been identified experimentally.

• Neutrinos are particularly difficult to detect because they have no charge and little mass, and they interact very weakly.

Hadrons

• These are particles that act through the strong force.

• Two classes of hadrons: mesons and baryons.• Mesons are particles with integral spin having

masses greater than that of the muon (106 MeV/c2; note that the muon is a lepton and not a meson).

• All baryons have masses at least as large as the proton and have half-integral spins.

Mesons

• Mesons are bosons because of their integral spin. • The meson family is rather large and consists of many

variations, distinguished according to their composition of quarks.

• The pion (π-meson) is a meson that can either have charge or be neutral.

• In addition to the pion there is also a K meson, which exists in both charged (K±) and neutral forms (K0). The K− meson is the antiparticle of the K+, and their common decay mode is into muons or pions.

• All mesons are unstable and not abundant in nature.

Baryons• The neutron and proton are the best-known

baryons.

• The proton is the only stable baryon, but some theories predict that it is also unstable with a lifetime greater than 1030 years.

• All baryons except the proton eventually decay into protons.

The Hadrons

Particles and Lifetimes• The lifetimes of particles are also indications of their

force interactions.

• Particles that decay through the strong interaction are usually the shortest-lived, normally decaying in less than 10−20 s.

• The decays caused by the electromagnetic interaction generally have lifetimes on the order of 10−16 s, and

• The weak interaction decays are even slower, longer than 10−10 s.

Fundamental and Composite Particles

• We call certain particles fundamental; this means that they are not composed of other, smaller particles. We believe leptons, quarks, and gauge bosons are fundamental particles.

• Although the Z and W bosons have very short lifetimes, they are regarded as particles, so a definition of particles dependent only on lifetimes is too restrictive.

• Other particles are composites, made from the fundamental particles.

14.4: Conservation Laws and Symmetries

• Physicists like to have clear rules or laws that determine whether a certain process can occur or not.

• It seems that everything occurs in nature that is not forbidden.

• Certain conservation laws are already familiar from our study of classical physics. These include mass-energy, charge, linear momentum, and angular momentum.

• These are absolute conservation laws: they are always obeyed.

Additional Conservation Laws

• These are helpful in understanding the many possibilities of elementary particle interactions.

• Some of these laws are absolute, but others may be valid for only one or two of the fundamental interactions.

Baryon Conservation

• In low-energy nuclear reactions, the number of nucleons is always conserved.

• Empirically this is part of a more general conservation law for what is assigned a new quantum number called baryon number that has the value B = +1 for baryons and −1 for antibaryons, and 0 for all other particles.

• The conservation of baryon number requires the same total baryon number before and after the reaction.

• Although there are no known violations of baryon conservation, there are theoretical indications that it was violated sometime in the beginning of the universe when temperatures were quite high. This is thought to account for the preponderance of matter over antimatter in the universe today.

Lepton Conservation

• The leptons are all fundamental particles, and there is a conservation of leptons for each of the three kinds (families) of leptons.

• The number of leptons from each family is the same both before and after a reaction.

• We let Le = +1 for the electron and the electron neutrino; Le = −1 for their antiparticles; and Le = 0 for all other particles.

• We assign the quantum numbers Lμ for the muon and its neutrino and Lτ for the tau and its neutrino similarly.

• Thus three additional conservation laws.

Strangeness• In the early 1950s physicists had considerable difficulty

understanding the myriad of observed reactions and decays. For example, the behavior of the K mesons seemed very odd.

• There is no conservation law for the production of mesons, but it appeared that K mesons, as well as the Λ and Σ baryons, were always produced in pairs in the proton reaction studied most often, namely the p + p reaction.

• In addition, the very fast decay of the π0 meson into two photons (10−16 s) is the preferred mode of decay.

• One would expect the K0 meson to also decay into two photons very quickly, but it does not. The long and short decay lifetimes of the K0 are 10−8 and 10−10 s, respectively.

The New Quantum Number: Strangeness

• Strangeness, S, is conserved in the strong and electromagnetic interactions, but not in the weak interaction.

• The kaons have S = +1, lambda and sigmas have S = −1, the xi has S = −2, and the omega has S = −3.

• When the strange particles are produced by the p + p strong interaction, they must be produced in pairs to conserve strangeness.

Further…

• π0 can decay into two photons by the strong interaction, it is not possible for K0 to decay at all by the strong interaction. The K0 is the lightest S = 0 particle, and there is no other strange particle to which it can decay. It can decay only by the weak interaction, which violates strangeness conservation.

• Because the typical decay times of the weak interaction are on the order of 10−10 s, this explains the longer decay time for K0.

• Only ΔS = ±1 violations are allowed by the weak interaction.

Hypercharge

• One more quantity, called hypercharge, has also become widely used as a quantum number.

• The hypercharge quantum number Y is defined by Y = S + B.

• Hypercharge, the sum of the strangeness and baryon quantum numbers, is conserved in strong interactions.

• The hypercharge and strangeness conservation laws hold for the strong and electromagnetic interactions, but are violated for the weak interaction.

Symmetries• Symmetries lead directly to conservation

laws.

• Three symmetry operators called parity, charge conjugation, and time reversal are considered.

The Conservation of Parity P

• The conservation of parity P describes the inversion symmetry of space,

Inversion, if valid, does not change the laws of physics.

• The conservation of parity is valid for the strong and electromagnetic interactions, but not in the weak interaction (experimentally).

Charge conjugation C

• Charge conjugation C reverses the sign of the particle’s charge and magnetic moment.

• It has the effect of interchanging every particle with its antiparticle.

• Charge conjugation is not conserved in the weak interactions, but it is valid for the strong and electromagnetic interactions.

• Even though both C and P are violated for the weak interaction it was believed that when both charge conjugation and parity operations are performed (called CP), conservation was still valid.

Time Reversal T • Here time t is replaced with –t.

• When all three operations are performed (CPT), where T is the time reversal symmetry, conservation holds.

• We speak of the invariance of the symmetry operators, such as T, CP, and CPT.

14.5: Quarks• We are now prepared to discuss quarks and how they form

the many baryons and mesons that have been discovered experimentally.

• In 1961 Murray Gell-Mann and Yuval Ne’eman independently proposed a classification system called the eightfold way that separated the known particles into multiplets based on charge, hypercharge, and another quantum number called isospin, which we have not previously discussed. Isospin is a characteristic that can be used to classify different charged particles that have similar mass and interaction properties.

• The neutron and proton are members of an isospin multiplet we call the nucleon. In this case the isospin quantum number (I) has the value ½, with the proton having the substate value +½ (“spin up”) and the neutron having −½ (“spin down”). Isospin is conserved in strong interactions, but not in electromagnetic interactions.

Quarks• After the eightfold way was developed, it was

noticed that some members of the multiplets were missing. Because of physicists’ strong belief in symmetry, experimentalists set to work to find them, a task made easier because many of the particles’ properties were predicted by the theoretical model.

• The Ω− was detected in 1964 at Brookhaven National Laboratory (see Figure 14.10 and Example 14.5) in this manner, a discovery that confirmed the usefulness of the eightfold way.

Quarks• However, as other particles were discovered it soon

became clear that the eightfold way was not the final answer. In 1963 Gell-Mann and, independently, George Zweig proposed that hadrons were formed from fractionally charged particles called quarks. The quark theory was unusually successful in describing properties of the particles and in understanding particle reactions and decay.

• Three quarks were proposed, named the up (u), down (d), and strange (s), with the charges +2e/3, −e/3, and −e/3, respectively. The strange quark has the strangeness value of −1, whereas the other two quarks have S = 0.

• Quarks are believed to be essentially pointlike, just like leptons.

Quarks, Antiquarks, and Charm

• With these three quarks, all the known hadrons could be specified by some combination of quarks and antiquarks.

• A fourth quark called the charmed quark (c) was proposed to explain some additional discrepancies in the lifetimes of some of the known particles.

• A new quantum number called charm C was introduced so that the new quark would have C = +1 while its antiquark would have C = −1 and particles without the charmed quark have C = 0.

• Charm is similar to strangeness in that it is conserved in the strong and electromagnetic interactions, but not in the weak interactions. This behavior was sufficient to explain the particle lifetime difficulties.

Quark Properties We can now present the given quark properties and see how they are used to make up the hadrons. In Table 14.5 we give the name, symbol, mass, charge, and the quantum numbers for strangeness, charm, bottomness, and topness. The spin of all quarks (and antiquarks) is 1/2.

Quark Description of Particles• A meson consists of a quark-antiquark pair,

which gives the required baryon number of 0. Baryons normally consist of three quarks.

• We present the quark content of several mesons and baryons in Table 14.6. The structure is quite simple. For example, a π − consists of , which gives a charge of (−2e/3) + (−e/3) = −e, and the two spins couple to give 0 (−1/2 + 1/2 = 0).

• A proton is uud, which gives a charge of (2e/3) + (2e/3) + (−e/3) = +e; its baryon number is 1/3 + 1/3 + 1/3 = 1; and two of the quarks’ spins couple to zero, leaving a spin 1/2 for the proton (1/2 + 1/2 − 1/2 = 1/2).

Quark Description of Particles

• What about the quark composition of the Ω−, which has a strangeness of S = −3? We look in Table 14.6 and find that its quark composition is sss. According to the properties in Table 14.5 its charge must be 3(−e/3) = −e, and its spin is due to three quark spins aligned, 3(1/2) = 3/2. Both of these values are correct. There is no other possibility for a stable omega (lifetime ~10−10 s) in agreement with Table 14.4.

Quantum Chromodynamics (QCD)• Because quarks have spin 1/2, they are all

fermions and according to the Pauli exclusion principle, no two fermions can exist in the same state. Yet we have three strange quarks in the Ω−.

• This is not possible unless some other quantum number distinguishes each of these quarks in one particle.

• A new quantum number called color circumvents this problem and its properties establish quantum chromodynamics (QCD).

Color

• There are three colors for quarks we call red (R), green (G), and blue (B) with antiquark color antired ( ); antigreen ( ) and antiblue ( ).– (A “bar” above the symbol is usually used to describe the

“anti-color”).

• Color is the “charge” of the strong nuclear force, analogous to electric charge for electromagnetism.

R GB

Color• The two theories, quantum electrodynamics and

quantum chromodynamics, are similar in structure; color is often called color charge and the force between quarks is sometimes referred to as color force.

• Earlier we saw that gluons are the particles that hold the quarks together. We show a Feynman diagram of two quarks interacting in Figure 14.11. A red quark comes in from the left and interacts with a blue quark coming in from the right. They exchange a gluon, changing the blue quark into a red one and the red quark into a blue one.

Fig 14.11

Color• A color and its anticolor cancel out. We call this colorless

(or white). All hadrons are colorless. In Figure 14.11 the gluon itself must have the color in order for the diagram to work. Quarks change color when they emit or absorb a gluon, and quarks of the same color repel, whereas quarks of different color attract.

Fig 14.11

Color• To finish the story we should mention that the six different kinds of

quarks are referred to as flavors. There are six flavors of quarks (u, d, s, c, b, t). Each flavor has three colors. Finally, how many different gluons are possible? Using the three colors red, blue, and green, there are nine possible combinations for a gluon. They are

• Note in Figure 14.11 that the gluon is and not . The combination does not have any net color change and cannot be

independent. Therefore, there are only eight independent gluons, and that is what quantum chromodynamics predicts.

• Gluons can interact with each other because each gluon carries a color charge. Note that in this case gluons, as the mediator of the strong force, are much different from photons, the mediator of the electromagnetic force.

Figure 14.12: When a high-energy gamma ray is scattered from a neutron, a free quark cannot escape because of confinement. For high enough energies, an antiquark-quark pair is created (for example, ), and a pion and proton are the final particles.

Confinement

• Physicists now believe that free quarks cannot be observed; they can only exist within hadrons. This is called confinement.

14.6: The Families of Matter

• We now have a brief review of the particle classifications and have learned how the hadrons are made from the quarks.

• In summary:– We presently believe that the two varieties of fermions,

called leptons and quarks, are fundamental particles.

– These fundamental particles can be divided into three simple families or generations.

– Each generation consists of two leptons and two quarks. The two leptons are a charged lepton and its associated neutrino. The quarks are combined by twos or threes to make up the hadrons.

The Families of Matter

Figure 14.14: The three generations (or families) of matter. Note that both quarks and leptons exist in three distinct sets. One of each charge type of quark and lepton make up a generation. All visible matter in the universe is made from the first generation; second- and third-generation particles are unstable and decay into first-generation particles.

The Families of Matter• Most of the mass in the universe is made from the components

in the first generation (electrons and u and d quarks).

• The second generation consists of the muon, its neutrino,

and the charmed and strange quarks. The members of this generation are found in certain astrophysical objects of high energy and in cosmic rays, and are produced in high-energy accelerators.

• The third generation consists of the tau and its neutrino and two more quarks, the bottom (or beauty) and top (or truth). The members of this third generation existed in the early moments of the creation of the universe and can be created with very high energy accelerators.

The Families of Matter• Leptons are essentially pointlike, because they have no

internal structure.• There are three leptons with mass and three others with

little mass (the neutrinos). • Quarks and antiquarks make up the hadrons (mesons

and baryons). Quarks may also be pointlike (< 10−18 m) and are confined together, never being in a free state.

• There are six flavors of quarks (up, down, strange, charmed, bottom, and top) and there are three colors (green, red, and blue) for each flavor.

• Rules for combining the colored quarks allow us to represent all known hadrons.

The Families of Matter• Bosons mediate the four fundamental forces of nature:

gluons are responsible for the strong interaction, photons for the electromagnetic interaction, W± and Z for the weak interaction, and the as yet unobserved graviton for the gravitational interaction.

• In our study of nuclear physics we discussed the pion as the mediator of the strong force. At a more fundamental level, we can now say that the gluon is responsible.

• The gluon is responsible for the attraction between the antiquark and quark that make up the pion, and the gluon is responsible for the attraction between the quarks that make up the nucleons.

14.7: Beyond the Standard Model

• Although the Standard Model has been successful in particle physics, it doesn’t answer all the questions. For example, it is not by itself able to predict the particle masses.

• Why are there only three generations or families of fundamental particles?

• Do quarks and/or leptons actually consist of more fundamental particles?

Neutrino Oscillations

• One of the most perplexing problems over the last three decades has been the solar neutrino problem where the number of neutrinos reaching Earth from the sun is a factor of 2–3 too small if our understanding of the energy-producing (nuclear fusion) is correct.

• Suggestions were made that other processes were going on. Neutrinos come in three varieties or flavors: electron, muon, and tau. Researchers had seen neutrinos generated in the Earth’s atmosphere (from cosmic rays) changing or “oscillating” into another flavor. This could only happen if neutrinos have mass.

• Physicists have seen various oscillations between the three flavors of neutrinos.

Matter-Antimatter

• According to the Big Bang theory, matter and antimatter should have been created in exactly equal quantities. It appears that matter dominates over antimatter now in our universe, and the reason for this has concerned physicists and cosmologists for years.

• The tiny violation of CP symmetry in the kaon decay tilts the scales in terms of matter over antimatter; however, the Standard Model indicates that this violation is too small to account for the predominance of matter.

• B meson decays may yield more about CP violations than with kaons and physicists are exploring theories going beyond the Standard Model.

Grand Unifying Theories

• There have been several attempts toward a grand unified theory (GUT) to combine the weak, electromagnetic, and strong interactions.

Predictions

1) The proton is unstable with a lifetime of 1029 to 1031 years. Current experimental measurements have shown the lifetime to be greater than 1032 years.

2) Neutrinos may have a small, but finite, mass. This has been confirmed.

3) Massive magnetic monopoles may exist. There is presently no confirmed experimental evidence for magnetic monopoles.

4) The proton and electron electric charges should have the same magnitude.

String Theory

• For the last two decades there has been a tremendous amount of effort by theorists in string theory, which has had several variations. The addition of supersymmetry resulted in the name theory of superstrings.

• In superstring theory elementary particles do not exist as points, but rather as tiny, wiggling loops that are only 10−35 m in length.

• Further work has revealed that they describe not just strings, but other objects including membranes and higher-dimensional objects. The addition of membranes has resulted in “brane” theories.

• Presently superstring theory is a promising approach to unify the four fundamental forces, including gravity.

Supersymmetry

• Supersymmetry is a necessary ingredient in many of the theories trying to unify the forces of nature.

• The symmetry relates fermions and bosons. All fermions will have a superpartner that is a boson of equal mass, and vice versa.

• The superpartner spins differ by ħ / 2.• Presently, none of the known leptons, quarks, or

gauge bosons can be identified with a superpartner of any other particle type.

M-theory

• Recently theorists have proposed a successor to superstring theory called M-theory.

• M-theory has 11 dimensions and predicts that strings coexist with membranes, called “branes” for short.

• The number of particles that have been predicted from a variety of different theories include the fancifully named sleptons, squarks, axions, winos, photinos, zinos, gluinos, and preons.

• Only through experiments will scientists be able to wade through the vast number of unifying theories.

14.8: Accelerators

• Particle physics was not able to develop fully until particle accelerators were constructed with high enough energies to create particles with a mass of about 1 GeV/c2 or greater.

• There are three main types of accelerators used presently in particle physics experiments: synchrotrons, linear accelerators, and colliders.

Synchrotron Radiation

• One difficulty with cyclic accelerators is that when charged particles are accelerated, they radiate electromagnetic energy called synchrotron radiation. This problem is particularly severe when electrons, moving very close to the speed of light, move in curved paths. If the radius of curvature is small, electrons can radiate as much energy as they gain.

• Physicists have learned to take advantage of these synchrotron radiation losses and now build special electron accelerators (called light sources) that produce copious amounts of photon radiation used for both basic and applied research.

Linear Accelerators

• Linear accelerators or linacs typically have straight electric-field-free regions between gaps of RF voltage boosts. The particles gain speed with each boost, and the voltage boost is on for a fixed period of time, and thus the distance between gaps becomes increasingly larger as the particles accelerate.

• Linacs are sometimes used as preacceleration device for large circular accelerators.

Colliders

• Because of the limited energy available for reactions like that found for the Tevatron, physicists decided they had to resort to colliding beam experiments, in which the particles meet head-on.

• If the colliding particles have equal masses and kinetic energies, the total momentum is zero and all the energy is available for the reaction and the creation of new particles.