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Chapter 29 Lecture

Particle Physics

Prepared by

Dedra Demaree,

Georgetown University

© 2014 Pearson Education, Inc.

Particle Physics

• What is antimatter?

• What are the fundamental particles and

interactions in nature?

• What was the Big Bang, and how has the

universe evolved since? © 2014 Pearson Education, Inc.

Be sure you know how to:

• Use the right-hand rule to determine the

direction of the magnetic force exerted by a

magnetic field on a moving charged particle

(Section 17.4).

• Explain beta decay (Section 28.6).

• Write an expression for the rest energy of a

particle (Section 25.8).


What's new in this chapter

• Beta decay can produce antineutrinos, a form of

antimatter.

– Every known particle has a corresponding

antiparticle.

– In this chapter we investigate elementary

particles such as the positron and their

fundamental interactions.

– This area of physics is called particle physics.


Antiparticles

• By 1930, physicists had identified four particles:

the electron, the proton, the neutron, and the

photon.

• At that time, these were the only known truly

elementary particles—a description used to

indicate the simplest and most basic particles.

• This view changed with the proposal and

discovery of so-called antiparticles.


Antielectrons predicted

• Dirac predicted that free electrons would have

an infinite number of possible quantum states

with negative total energy.

– A free electron in a positive energy state

should be able to transition to one of these

negative energy states.

– How could a free electron have negative total

energy?

– These negative energy states are occupied

by an infinite number of positrons (then called

antielectrons), a new type of particle that had

not yet been observed. © 2014 Pearson Education, Inc.

Antielectrons detected


Pair production

• Under the right conditions, a photon can

produce an electron and a positron:


Pair annihilation

• If an electron and a positron meet, it is possible

for them to annihilate and produce a photon.


Conceptual Exercise 29.1

• Imagine that an electron and a positron meet

and annihilate each other. Assume that they are

moving directly toward each other at constant

speed.

A. Will one or two photons be produced? Write

a reaction equation for this process.

B. In which directions do these photons move

relative to each other after the process?


Beta-plus decay: Transforming a proton into

a neutron

• If a proton captures a gamma-ray photon, the

energy of the excited-state proton may be great

enough to produce a neutron and the other

particles.

• The proton absorbs the photon and then decays

into a neutron, a positron, and a neutrino.

– This process is called beta-plus decay to

indicate that a positron (not an electron) is

produced.


Positron emission tomography

• Positron emission tomography (PET) is a process for

imaging the brain.

– Fluorine-18 isotopes undergo beta-plus decay

continually, producing positrons.

– The positrons meet electrons and annihilate each

other, producing a pair of gamma-ray photons that

move in opposite directions.


Other antiparticles

• The positively charged proton that is part of all

nuclei has a negatively charged antiproton of the

same mass but opposite electric charge.

• Even though the neutron has zero electric

charge, it also has an antimatter counterpart;

other properties besides charge differentiate the

neutron from the antineutron.

• Occasionally, a particle is its own antiparticle;

the photon is an example.


Fundamental interactions

• Fundamental interactions are the most basic

interactions known, such as the electromagnetic

interaction between charged particles.

• Nonfundamental interactions, such as friction,

can be understood in terms of fundamental

interactions.

– Friction is a macroscopic manifestation of the

electromagnetic interaction between the

electrons of two surfaces that are in contact.


Fundamental interactions: Gravitational

interaction

• All objects in the universe participate in

gravitational interactions due to their mass.

– This interaction is important for very massive

(mega-) objects.

– It is much less important for objects in our

daily lives and extremely insignificant for

microscopic objects.


Fundamental interactions: Electromagnetic

interaction

• Electrically charged objects participate in

electromagnetic interactions.

– The interaction is electric if the objects are at

rest or in motion with respect to each other.

– The interaction is magnetic only if the objects

are moving with respect to each other.

– The electromagnetic interactions between

nuclei and electrons are important in

understanding the structure of atoms.


Comparing the electromagnetic interaction

to the gravitational interaction

• The electrostatic force that an electron and a

proton exert on each other in an atom is about

1039 times greater than the gravitational force

that they exert on each other.


Fundamental interactions: Strong

interaction

• The binding of protons and neutrons together

into a nucleus is a residual interaction of the

strong interaction.

– The strong interaction is a very short-range

interaction, exerted by protons and neutrons

only on their nearest neighbors within the

nucleus.


Fundamental interactions: Weak interaction

• The weak interaction is responsible for beta

decay.

– Protons, neutrons, electrons, and neutrinos all

participate in it.

– The weak interaction is significantly weaker

than the strong interaction and has a

significantly shorter range.


Mechanisms of fundamental interactions

• This particle exchange mechanism has been

successful in describing the weak and strong

interactions.

• It has also had some success in describing the

gravitational interaction.

– The emitted and absorbed particle is called a

mediator.

– For the electromagnetic interaction, the

mediator is the photon.


Interaction mediators

• Photons: electromagnetic interaction

• Gluons: strong interaction

• W and Z bosons: weak interaction

• Gravitons: gravitational interaction

– The mediators of the electromagnetic, strong,

and weak interactions have all been

discovered.

– The hypothetical mediator for the gravitational

interaction, the so-called graviton, has not.


Fundamental interactions


Quantitative Exercise 29.2

• Convert the masses of the W ± and Z

0 particles

into electron volts.


Elementary particles and the Standard

Model

• Particle accelerators facilitate collisions between

particles with total energy significantly greater

than the rest energies, allowing for additional

particles to be produced.

– The properties of these additional particles

can be determined using elaborate detectors.

– Most of the particles produced are not stable.


Leptons

• Leptons interact only through weak,

electromagnetic, and gravitational interactions, but

not through strong interactions.

– The electron and the electron neutrino are

examples of leptons.

– These two particles form a generation (or

family) of leptons.


Lepton generations (families)


Hadrons

• Two different types of hadrons can be

distinguished: baryons and mesons.

– The proton and the neutron are baryons.

– In 1935, Hideki Yukawa suggested the

existence of new particles that mediated the

strong interaction—the first example of a

meson.

– In 1947, physicists discovered a meson in

cosmic rays that participated in strong

interactions and had the correct properties to

be Yukawa's meson.


Properties of hadrons


Particle


Quarks

• Hadrons are made up of smaller, more

fundamental particles that have fractional

electric charge, known as quarks.

– Six different quarks have been discovered

experimentally.

– These different quark types are known in the

physics community as flavors. © 2014 Pearson Education, Inc.

Quarks


The proton and quark charge

• The total electric charge adds to e, and the total

color is neutral. © 2014 Pearson Education, Inc.


• Which combination of quarks will combine to

have the correct properties to be a neutron?


Particles (matter) and their interactions


Confinement

• No experiment has ever produced a quark in

isolation.

– Every quark and antiquark ever produced have

always been part of a hadron.

– This phenomenon, called confinement, is an

indication of a feature of the strong interaction.

– The strong interaction between quarks is

weakest when they are close together and

gets stronger the farther apart the quarks are.


Development of the Standard Model

• The Standard Model is the combined theory of

the building blocks of matter and their

interactions.

– In the late 1940s, physicists Feynman,

Schwinger, and Tomonaga independently

combined the ideas of special relativity and

quantum mechanics into a single model that

explains all electromagnetic phenomena.

– Their model, known as quantum

electrodynamics (QED), is a cornerstone of the

Standard Model.


Standard Model


Higgs particle

• In 1967, Glashow, Salam, and Weinberg

independently put forth a model that unified the

electromagnetic and weak interactions into a

single interaction, which they called the

electroweak model.

• This model predicted the existence of a particle,

which became known as the Higgs particle after

physicist Peter Higgs.


Predictions of the electroweak model

• In the very distant past when the universe was

much smaller and very much hotter, all particles

were massless.

• This situation led to the existence of the Higgs

particle.

• As the universe cooled, the Higgs particle began

interacting significantly with other elementary

particles, reconfiguring them into the familiar

forms they have today.

– This is known as the Higgs mechanism.


Quantum chromodynamics

• In 1973, using Yang's and Mills' mathematical

framework, Fritzsch and Gell-Mann formulated

quantum chromodynamics (QCD), a

mathematical model of the strong interaction that

plays a role in the exchange of gluons between

quarks.

– Between 1976 and 1979, scientists

discovered the tau lepton and bottom quark

and found direct evidence for gluons.


Additional particle discovery timeline

• The 1980s brought the discoveries of the

predicted weak interaction mediators W and Z.

• The 1990s gave us the top quark.

• In 2000, the tau neutrino was discovered.

• In July 2012, CERN announced the discovery of

a particle that may be the long-sought-after

Higgs particle.


Unanswered questions of the Standard

Model

1. Can the strong interaction be unified with

the electroweak interaction?

2. Why are there only three families of

quarks/leptons?

3. Are the Standard Model particles truly

fundamental?

4. Are there additional particles beyond those

predicted by the Standard Model?


Summary of the Standard Model

• Quarks and leptons, which make up the matter of

the universe

• The theory of strong interactions (QCD) mediated

by gluons

• The theory of electromagnetic (QED) and weak

interactions mediated by photons and the W and

Z particles

• The Higgs particle, which explains, through the

Higgs mechanism, why some of the fundamental

particles have nonzero mass


Cosmology

• Why is our universe not filled with equal

numbers of particles and antiparticles? Why is

there an imbalance?

– These questions are answered in part by

particle physics and by cosmology—a branch

of physics that studies the composition and

evolution of the universe as a whole.


Big Bang


Standard Model


Inflation

• When the universe first became "cold" enough

that quarks and leptons emerged as

distinguishable particles, a fundamental change

in the structure of the universe occurred,

resulting in an extremely rapid exponential

expansion known as cosmic inflation.

– During inflation, small fluctuations in the

density of the universe decreased.

– Areas where the density was slightly above

average would later act as the seeds of

galaxy formation.


Nucleosynthesis

• A few minutes after the Big Bang, the

temperature had dropped to about 1 billion K, and

the average density of the universe was close to

the density of air at sea level on Earth today.

– For the first time, protons and neutrons were

able to combine to form the simplest nuclei:

deuterium, helium, and trace amounts of

lithium.

– This process is known as Big Bang

nucleosynthesis.


Atoms, stars, and galaxies

• When the universe had cooled enough,

gravitational interactions became the dominant

driver of its further evolution.

– Density fluctuations led to the formation of the

first galaxies and stars just 500,000 years

after the Big Bang.

– These early stars went through their life

cycles, with some ending in a violent collapse

and explosion known as a supernova, which

created heavier elements such as carbon,

oxygen, iron, and gold.


Dark matter and dark energy

• When astronomers measure the mass of all the

stars and gas that they can see, they find that

the total mass is only about one-tenth of the

mass needed to account for the speed of the

solar system around the center of the galaxy.

– The universe is "missing" about 90% of the

mass needed to account for the observed

motion of stars and galaxies.

– How can this contradiction be resolved?


Dark matter

• In 1933, astrophysicist Fritz Zwicky speculated

that there must be some unseen dark matter

present in the Coma cluster; for about 40 years,

his observation was the only evidence for its

existence.

• In the 1970s, Vera Rubin presented further

evidence.

• It was at this point that the dark matter

explanation started to become more widely

accepted.


Dark matter

• Dark matter does not emit photons or otherwise

participate in the electromagnetic interaction

(this is why it is called "dark").

• Dark matter cannot be some sort of dark cloud

of protons or gaseous atoms, because these

could be detected by the scattering of radiation

passing through them.


MACHOs: Massive compact halo objects

• These objects could be black holes, neutron

stars, or brown dwarfs.

• Astronomers have detected MACHOs through

their gravitational effects on the light from distant

objects.

• The small number of detected events translates

into MACHOs accounting for at most 20% of the

dark matter in our galaxy.

– There must be another (or an additional)

explanation.


WIMPs: Weakly interacting massive

particles

• WIMPs are "weakly interacting": they can pass

through ordinary matter with almost no interaction,

and they neither absorb nor emit light.

• WIMPs are "massive": their mass is not zero.

– Prime candidates for WIMPs include neutrinos,

axions, and neutralinos.

– Axions and neutralinos are not Standard Model

particles and require the Standard Model to be

extended to accommodate their existence.


Grand unified theories

• Grand unified theories combine the strong,

weak, and electromagnetic interactions into a

single interaction.

• These theories predict the existence of "sterile

neutrinos," which could have even fewer

interactions and be far more massive than

Standard Model neutrinos.

– Physicists do not know how to detect such a

particle.

– If it exists in sufficient abundance, it could

account for dark matter.


Supersymmetry

• Supersymmetry is an extension of the Standard

Model:

– It effectively doubles the number of

elementary particles.

– It gives insight into the cosmological constant

problem.

– It allows for a more precise understanding of

the unification of interactions in grand unified

theories.

– It gives a potential candidate for dark matter.


Explaining the accelerating expansion of the

universe

• Invoke a discarded feature of Einstein's general

theory of relativity (our current best model of the

gravitational interaction) known as the

cosmological constant.

• Suggest the existence of a strange kind of

energy-fluid that fills space and has a repulsive

gravitational effect.

• Propose a modified version of general relativity

that includes a new kind of field that creates this

cosmic acceleration.


The cosmological constant model

• The dominant model of the universe was the

steady-state model, which asserted that the

universe essentially did not change in any major

way as time passed.

– General relativity predicted that a static

universe was unstable.

– Einstein introduced the cosmological constant

into general relativity in an attempt to allow

the theory to accommodate a steady-state

universe.


The dark energy model

• The cosmological constant seems to represent a

type of dark energy that is present at every point

in space with equal density.

– Even as the universe expands, the density

does not decrease because it is a property of

space itself.

– Dark energy has a negative pressure.

– In general relativity, this produces a

gravitationally repulsive effect on space.


Modified general relativity

• Some better theory of the gravitational interaction

would make even better predictions than general

relativity.

– The challenge has been to construct the new

theory in such a way that it does not make

predictions that contradict experiments that

have already been done.

– Thus far physicists have been unsuccessful in

achieving this goal.


The proportion of matter, dark matter, and

dark energy in the universe


Cosmological constant problem

• Dark energy is the sum of the zero-point

energies of all quantum fields in the universe.

– When physicists predict values for the

cosmological constant, they get a result that

is 10120 times the observed value.

– This is the largest disagreement between

prediction and experiment in all of science.

– This so-called cosmological constant problem

is a major unsolved problem in physics.


Tip


Is our pursuit of knowledge worthwhile?

• Our models describe the behavior of only 4% of

the content of our universe.

– The nature of the remaining 96% of our

universe currently remains an unsolved

problem.

• Will our eventual knowledge of the other 96% of

the universe someday make people's lives better?

– It is impossible to say for sure, but history

suggests that it very likely will.


Summary


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