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Chapter 46

Particle Physics

and Cosmology

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Atoms as Elementary

Particles Atoms

From the Greek for indivisible

Were once thought to be the elementaryparticles

Atom constituents

Proton, neutron, and electron After 1932 these were viewed as

elementary All matter was made up of these particles

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Discovery of New Particles New particles

Beginning in the 1940s, many new

particles were discovered in experimentsinvolving high-energy collisions

Characteristically unstable with shortlifetimes

Over 300 have been catalogued A pattern was needed to understand all

these new particles

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Elementary Particles Quarks Physicists recognize that most particles are

made up ofquarks

Exceptions include photons, electrons and a fewothers

The quark model has reduced the array of

particles to a manageable few

The quark model has successfully predicted

new quark combinations that were

subsequently found in many experiments

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Fundamental Forces All particles in nature are subject to four

fundamental forces: Nuclear force

Electromagnetic force

Weak force

Gravitational force This list is in order of decreasing strength

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Nuclear Force Attractive force between nucleons

Strongest of all the fundamental forces

Very short-ranged Less than 10-15 m

Negligible for separations greater than this

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Electromagnetic Force Is responsible for the binding of atoms

and molecules

About 10-2 times the strength of the

nuclear force

A long-range force that decreases in

strength as the inverse square of theseparation between interacting particles

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Weak Force Is responsible for instability in certain nuclei

Is responsible for decay processes

Its strength is about 10-5 times that of thestrong force

Scientists now believe the weak and

electromagnetic forces are two

manifestations of a single force, the

electroweak force

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Gravitational Force A familiar force that holds the planets,

stars and galaxies together

Its effect on elementary particles isnegligible

A long-range force

It is about 10-39 times the strength of thestrong force Weakest of the four fundamental forces

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Explanation of Forces Forces between particles are often described

in terms of the actions offield particles orexchange particles Field particles are also called gauge bosons The interacting particles continually emit and

absorb field particles The emission of a field particle by one particle and

its absorption by another manifests itself as aforce between the two interacting particles

The force is mediated, or carried, by the fieldparticles

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Forces and Mediating

Particles

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Paul Adrien Maurice Dirac 1902 1984

Understanding of

antimatter Unification of quantum

mechanics and relativity

Contributions of

quantum physics andcosmology

Nobel Prize in 1933

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Antiparticles For every particle, there is an antiparticle

From Diracs version of quantum mechanics that incorporatedspecial relativity

An antiparticle has the same mass as the particle, butthe opposite charge

The positron (electrons antiparticle) was discovered byAnderson in 1932 Since then, it has been observed in numerous experiments

Practically every known elementary particle has adistinct antiparticle Among the exceptions are the photon and the neutral pi

particles

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Diracs Explanation The solutions to the relativistic quantum

mechanic equations required negative energystates

Dirac postulated that all negative energystates were filled These electrons are collectively called the Dirac

sea

Electrons in the Dirac sea are not directlyobservable because the exclusion principledoes not let them react to external forces

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Diracs Explanation, cont. An interaction may

cause the electronto be excited to apositive energy state

This would leavebehind a hole in the

Dirac sea The hole can reactto external forcesand is observable

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Diracs Explanation, final The hole reacts in a way similar to the

electron, except that it has a positive

charge The hole is the antiparticle of the

electron The electrons antiparticle is now called apositron

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Pair Production A common source of positrons is pair

production

A gamma-ray photon with sufficientenergy interacts with a nucleus and anelectron-positron pair is created fromthe photon

The photon must have a minimumenergy equal to 2mec

2 to create the pair

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Pair Production, cont.

A photograph of pair production produced by 300MeV gamma rays striking a lead sheet

The minimum energy to create the pair is 1.02 MeV The excess energy appears as kinetic energy of the

two particles

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Annihilation The reverse of pair production can also

occur

Under the proper conditions, an

electron and a positron can annihilate

each other to produce two gamma ray

photonse- + e+ 2

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Hideki Yukawa 1907 1981

Nobel Prize in 1949

for predicting theexistence of mesons

Developed the first

theory to explain the

nature of the nuclear

force

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Mesons Developed from a theory to explain the

nuclear force

Yukawa used the idea of forces beingmediated by particles to explain the nuclearforce

A new particle was introduced whose

exchange between nucleons causes thenuclear force It was called a meson

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Mesons, cont. The proposed particle would have a mass

about 200 times that of the electron

Efforts to establish the existence of theparticle were done by studying cosmic rays in

the 1930s

Actually discovered multiple particles pi meson (pion)

muon Not a meson

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Pion There are three varieties of pions

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Muons Two muons exist

- and its antiparticle+

The muon is unstable It has a mean lifetime of 2.2s

It decays into an electron, a neutrino, and

an antineutrino

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Richard Feynman 1918 1988 Developed quantum

electrodynamics Shared the Nobel Prize

in 1965 Worked on Challenger

investigation and

demonstrated theeffects of coldtemperatures on therubber O-rings used

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Feynman Diagrams A graphical representation of the interaction

between two particles

Feynman diagrams are named for RichardFeynman who developed them

A Feynman diagram is a qualitative graph of

time on the vertical axis and space on the

horizontal axis Actual values of time and space are not important

The actual paths of the particles are not shown

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Feynman Diagram Two

Electrons The photon is the field

particle that mediates the

interaction

The photon transfers energyand momentum from one

electron to the other

The photon is called a virtual

photon It can never be detected

directly because it is absorbed

by the second electron very

shortly after being emitted by

the first electron

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The Virtual Photon The existence of the virtual photon

seems to violate the law of conservation

of energy But, due to the uncertainty principle and its

very short lifetime, the photons excessenergy is less than the uncertainty in its

energy The virtual photon can exist for short time

intervals, such that Eh / 2 t

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Feynman Diagram Proton and

Neutron (Yukawas Model) The exchange is via the

nuclear force

The existence of the pion is

allowed in spite of conservationof energy if this energy is

surrendered in a short enough

time

Analysis predicts the rest

energy of the pion to be 130

MeV / c2

This is in close agreement with

experimental results

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Nucleon Interaction More

About Yukawas Model The time interval required for the pion to

transfer from one nucleon to the other is

The distance the pion could travel is ct

Using these pieces of information, therest energy of the pion is about 100MeV

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Nucleon Interaction, final This concept says that a system of two

nucleons can change into two nucleons plus

a pion as long as it returns to its original statein a very short time interval

It is often said that the nucleon undergoes

fluctuations as it emits and absorbs field

particles These fluctuations are a consequence of quantum

mechanics and special relativity

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Feynman Diagram Weak

Interaction An electron and a

neutrino are

interacting via theweak force

The Z0 is the

mediating particle The weak force can also

be mediated by the W

The W and Z0 were

discovered in 1983 at

CERN

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Classification of Particles Two broad categories

Classified by interactions

hadrons interact through strong force leptons interact through weak force

Note on terminology The strong force is reserved for the force between

quarks

The nuclear force is reserved for the force

between nucleons The nuclear force is a secondary result of the strong force

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Hadrons Interact through the strong force Two subclasses distinguished by masses and

spins Mesons Decay finally into electrons, positrons, neutrinos and photons Integer spins (0 or 1)

Baryons Masses equal to or greater than a proton Half integer spin values (1/2 or 3/2) Decay into end products that include a proton (except for the

proton)

Not elementary, but composed of quarks

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Leptons Do not interact through strong force All have spin of 1/2

Leptons appear truly elementary No substructure Point-like particles

Scientists currently believe only six leptons

exist, along with their antiparticles Electron and electron neutrino Muon and its neutrino Tau and its neutrino

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Conservation Laws A number of conservation laws are important

in the study of elementary particles

Already have seen conservation of Energy Linear momentum Angular momentum

Electric charge Two additional laws are

Conservation of Baryon Number Conservation of Lepton Number

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Conservation of Baryon

Number Whenever a baryon is created in a reaction or

a decay, an antibaryon is also created

B is the baryon number B = +1 for baryons B = -1 for antibaryons B = 0 for all other particles

The sum of the baryon numbers before areaction or a decay must equal the sum ofbaryon numbers after the process

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Number and Proton Stability There is a debate over whether the proton

decays or not

If baryon number is absolutely conserved, theproton cannot decay

Some recent theories predict the proton is

unstable and so baryon number would not be

absolutely conserved For now, we can say that the proton has a half-life

of at least 1033 years

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Number, Example Is baryon number conserved in the

following reaction?

Baryon numbers:

Before: 1 + 1 = 2 After: 1 + 1 + 1 + (-1) = 2

Baryon number is conserved The reaction can occur as long as energy

is conserved

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Conservation of Lepton

Number, cont. Assigning electron lepton numbers

Le = 1 for the electron and the electron neutrino

Le = -1 for the positron and the electronantineutrino Le = 0 for all other particles

Similarly, when a process involves muons,

muon lepton number must be conserved andwhen a process involves tau particles, taulepton numbers must be conserved Muon and tau lepton numbers are assigned

similarly to electron lepton numbers

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Conservation of Lepton

Number, Example Is lepton number conserved in the

following reaction?

Check electron lepton numbers:

Before: Le= 0 After: Le = 1 + (-1) + 0 = 0

Electron lepton number is conserved

Check muon lepton numbers: Before: L

= 1 After: L = 0 + 0 + 1 = 1

Muon lepton number is conserved

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Strange Particles Some particles discovered in the 1950s were

found to exhibit unusual properties in theirproduction and decay and were given thename strange particles

Peculiar features include: Always produced in pairs Although produced by the strong interaction, they

do not decay into particles that interact via thestrong interaction, but instead into particles thatinteract via weak interactions They decay much more slowly than particles decaying via

strong interactions

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Strangeness To explain these unusual properties, a new

quantum number S, called strangeness, wasintroduced

A new law, the law of conservation ofstrangeness was also needed It states that the sum of strangeness numbers before

a reaction or a decay must equal the sum of the

strangeness numbers after the process Strong and electromagnetic interactions obey

the law of conservation of strangeness, but theweak interaction does not

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Bubble Chamber

Example of Strange Particles The dashed lines

represent neutral

particles At the bottom,

- + p K0 + 0

Then 0 - + p

and

K0+- +

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Creating Particles Most elementary particles are unstable

and are created in nature only rarely, in

cosmic ray showers In the laboratory, great numbers of

particles can be created in controlled

collisions between high-energy particlesand a suitable target

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Measuring Properties of

Particles A magnetic field causes the charged particles

to curve This allows measurement of their charge and

linear momentum

If the mass and momentum of the incidentparticle are known, the product particlesmass, kinetic energy, and speed can usually

be calculated The particles lifetime can be calculated from

the length of its track and its speed

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Resonance Particles Short-lived particles are known as

resonance particles They exist for times around 10-20 s

They cannot be detected directly

Their properties can be inferred from

data on their decay products

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Experimental Evidence The location of the

peak tells us the mass

of the particle The smaller peaks

indicate the presence

of two other resonance

particles

The width of the peakcan be used to infer

the lifetime of the

particle

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Murray Gell-Mann 1929

Studies dealing with

subatomic particles Named quarks

Developed pattern

known as eightfold

way Nobel Prize in 1969

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The Eightfold Way Many classification schemes have been

proposed to group particles into families These schemes are based on spin, baryon

number, strangeness, etc. The eightfold way is a symmetric pattern

proposed by Gell-Mann and Neeman There are many symmetrical patterns that can be

developed The patterns of the eightfold way have much

in common with the periodic table Including predicting missing particles

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An Eightfold Way for Baryons A hexagonal pattern for

the eight spin 1/2

baryons

Stangeness vs. charge

is plotted on a sloping

coordinate system

Six of the baryons form

a hexagon with theother two particles at its

center

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An Eightfold Way for Mesons The mesons with spins of

0 can be plotted Strangeness vs. charge

on a sloping coordinatesystem is plotted A hexagonal pattern

emerges The particles and their

antiparticles are onopposite sides on theperimeter of the hexagon

The remaining threemesons are at the center

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Eightfold Way for Spin 3/2

Baryons The nine particles

known at the time werearranged as shown

An empty spot occurred Gell-Mann predicted the

missing particle and itsproperties

About three years later,the particle was foundand all its predictedproperties wereconfirmed

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Quarks Hadrons are complex particles with size

and structure

Hadrons decay into other hadrons There are many different hadrons Quarks are proposed as the elementary

particles that constitute the hadrons Originally proposed independently by Gell-

Mann and Zweig

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Original Quark Model Three types orflavors

u up d down s strange

Associated with each quark is an antiquark The antiquark has opposite charge, baryon number

and strangeness

Quarks have fractional electrical charges +1/3 e and 2/3 e

All ordinary matter consists of just u and d

quarks

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Original Quark Model Rules All the hadrons at the time of the

original proposal were explained by

three rules Mesons consist of one quark and one

antiquark This gives them a baryon number of 0

Baryons consist of three quarks

Antibaryons consist of three antiquarks

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Quark Composition of

Particles Examples Mesons are

quark-

antiquark pairs Baryons are

quark triplets

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Active Figure 46.12

(SLIDESHOW MODE ONLY)
http://../Active_Figures/Active%20Figures%20Media/AF_4612.htmlhttp://../Active_Figures/Active%20Figures%20Media/AF_4612.html

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Additions to the Original

Quark Model Charm

Another quark was needed to account forsome discrepancies between predictions ofthe model and experimental results

A new quantum number, C, was assigned tothe property ofcharm

Charm would be conserved in strong andelectromagnetic interactions, but not in weakinteractions

In 1974, a new meson, the J/, wasdiscovered that was shown to be a charm

quark and charm antiquark pair

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More Additions

Top and Bottom

Discovery led to the need for a moreelaborate quark model

This need led to the proposal of two newquarks t top (or truth) b bottom (or beauty)

Added quantum numbers oftopness andbottomness Verification

b quark was found in a Y- meson in 1977

t quark was found in 1995 at Fermilab

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Numbers of Particles

At the present, physicists believe the

building blocks of matter are complete

Six quarks with their antiparticles Six leptons with their antiparticles

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Particle Properties

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More About Quarks

No isolated quark has ever been observed

It is believed that at ordinary temperatures,

quarks are permanently confined insideordinary particles due to the strong force

Current efforts are underway to form a quark-

gluon plasma where quarks would be freed

from neutrons and protons

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Color

It was noted that certain particles hadquark compositions that violated the

exclusion principle Quarks are fermions, with half-integerspins and so should obey the exclusionprinciple

The explanation is an additionalproperty called color charge The color has nothing to do with the visual

sensation from light, it is simply a name

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Colored Quarks

Color charge occurs in red, blue, or green Antiquarks have colors of antired, antiblue, or

antigreen

These are the quantum numbers of color charge

Color obeys the exclusion principle

A combination of quarks of each color

produces white (or colorless) Baryons and mesons are always colorless

Q t Ch d i

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Quantum Chromodynamics

(QCD)

QCD gave a new theory of how quarksinteract with each other by means of colorcharge

The strong force between quarks is oftencalled the color force

The strong force between quarks is mediatedby gluons Gluons are massless particles

When a quark emits or absorbs a gluon, itscolor may change

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More About Color Charge

Particles with like colors repel and those withopposite colors attract Different colors attract, but not as strongly as a

color and its anticolor The color force between color-neutral

hadrons is negligible at large separations The strong color force between the constituent

quarks does not exactly cancel at smallseparations

This residual strong force is the nuclear force thatbinds the protons and neutrons to form nuclei

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Quark Structure of a Meson

A green quark is

attracted to an

antigreen quark The quark

antiquark pair forms

a meson

The resulting mesonis colorless

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Quark Structure of a Baryon

Quarks of different

colors attract each

other

The quark triplet

forms a baryon

Each baryon contains

three quarks withthree different colors

The baryon is

colorless

QCD E l ti f

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QCD Explanation of a

Neutron-Proton Interaction

Each quark within theproton and neutron iscontinually emitting and

absorbing gluons The energy of the gluon

can result in thecreation of quark-antiquark pairs

When close enough,these gluons andquarks can beexchanged, producingthe strong force

El t P ti l

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Elementary Particles

A Current View

Scientists now believe there are three

classifications of truly elementary particles

Leptons Quarks

Field particles

These three particles are further classified as

fermions or bosons Quarks and leptons are fermions

Field particles are bosons

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Weak Force

The weak force is believed to be mediated by

the W+, W-, and Z0 bosons

These particles are said to have weak charge Therefore, each elementary particle can have

Mass

Electric charge

Color charge Weak charge

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Electroweak Theory

The electroweak theoryunifies

electromagnetic and weak interactions

The theory postulates that the weak andelectromagnetic interactions have the

same strength when the particles

involved have very high energies Viewed as two different manifestations of a

single unifying electroweak interaction

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The Standard Model

A combination of the electroweak theory andQCD for the strong interaction form theStandard Model

Essential ingredients of the Standard Model The strong force, mediated by gluons, holds the quarks

together to form composite particles Leptons participate only in electromagnetic and weak

interactions The electromagnetic force is mediated by photons The weak force is mediated by W and Z bosons

The Standard Model does not actually yetinclude the gravitational force

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The Standard Model Chart

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Mediator Masses

Why does the photon have no mass whilethe W and Z bosons do have mass? Not answered by the Standard Model The difference in behavior between low and

high energies is called symmetry breaking The Higgs boson has been proposed to

account for the masses Large colliders are necessary to achieve the energy

needed to find the Higgs boson In a collider, particles with equal masses and equal

kinetic energies, traveling in opposite directions,collide head-on to produce the required reaction

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Particle Paths After a Collision


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with a Gold Nucleus

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The Big Bang

This theory states that the universe had abeginning, and that it was so cataclysmic thatit is impossible to look back beyond it

Also, during the first few minutes after thecreation of the universe, all four interactionswere unified All matter was contained in a quark-gluon plasma

As time increased and temperaturedecreased, the forces broke apart

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A Brief History of the Universe

Cosmic Background Radiation

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Cosmic Background Radiation

(CBR)

CBR represents the

cosmic glow left over

from the Big Bang

The radiation had equalstrengths in all directions

The curve fits a black

body at 2.7K

There are small

irregularities that allowed

for the formation of

galaxies and other objects

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CBR, cont.

The COBE satellite found that the

background radiation had irregularities

that corresponded to temperaturevariations of 0.000 3 K

Including other data, it was concluded

that a peak in fluctuation intensityoccurred 300 000 years after the Big

Bang

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Hubbles Law

The Big Bang theory predicts that the

universe is expanding

Hubble claimed the whole universe isexpanding

Furthermore, the speeds at which galaxies

are receding from the earth is directly

proportional to their distance from us This is called Hubbles law

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Hubbles Law, cont.

Hubbles law can

be written as

v= HR His called the

Hubble constant

H 17 x 10-3 m/s ly

Remaining Questions About

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Remaining Questions About

the Universe

Will the universe expand forever? Today, astronomers and physicists are trying to

determine the rate of expansion

It depends on the average mass density of theuniverse compared to a critical density

Missing mass in the universe The amount of non-luminous (dark) matter seems

to be much greater than what we can see Various particles have been proposed to make up

this dark matter

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Some Questions in Particle

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Some Questions in Particle

Physics

Why so little antimatter in the Universe? Is it possible to unify electroweak and strong forces? Why do quarks and leptons form similar but distinct

families? Are muons the same as electrons apart from their

difference in mass? Why are some particles charged and others not?

Why do quarks carry fractional charge? What determines the masses of fundamental

particles? Can isolated quarks exist?

A New Perspective

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A New Perspective

String Theory

String theory is one current effort at

answering some of the previous

questions It is an effort to unify the four

fundamental forces by modeling all

particles as various vibrational modes ofan incredibly small string

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String Theory, cont.

The typical length of a string is 10-35 m This is called the Planck length

According to the string theory, eachquantized mode of vibration of the

string corresponds to a different

elementary particle in the StandardModel

Complications of the String

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Complications of the String

Theory

It requires space-time to have tendimensions

Four of the ten dimensions are visible tous, the other six are compactified(curled)

Another complication is that it is difficultfor theorists to guide experimentalists

as to what to look for in an experiment Direct experimentation on strings is

impossible

String Theory Prediction

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String Theory Prediction

SUSY

One prediction of string theory is

supersymmetry (SUSY)

It suggests that every elementary particlehas a superpartner that has not yet been

observed

Supersymmetry is a broken symmetry and

the masses of the superpartners are above

our current capabilities to detect

Another Perspective

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Another Perspective

M-Theory

M-theory is an eleven-dimensional

theory based on membranes rather

than strings M-theory is claimed to reduce to string

theory if one compactifies from the

eleven dimensions to ten

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