# Presentation: 693403

## The Standard Modelof Particles and Interactions

- Ian Hinchliffe
- 26 June 2002

## What is the World Made of?

- Ancient times – 4 elements
- 19th century – atoms
- Early 20th century – electrons, protons, neutrons
- Today – quarks and leptons

## The Atom in 1900...

- Atoms get rearranged in chemical reactions
- More than 100 atoms (H, He, Fe ...)
- Internal structure was not understood – known to have electric charge inside

## Periodic Table

- Elements are grouped into families with similar properties (e.g. Inert gasses He, Ne etc.) led to the Periodic Table
- This suggested an new structure with simpler building blocks

## Models of the Atom

- Experiments broke atoms apart
- Very light negative charged particles (electrons) surrounding a heavy positive nucleus
- Atom, is mostly “empty”

## The Nucleus

- Nucleus is small and dense; it was thought for a while to be fundamental
- But still as many nucleii as atoms
- Simplification – all nucleii are made up of charged **protons** and neutral **neutrons**

## Quarks

- We now know that even protons and neutrons are not fundamental
- They are made up of smaller particles called **quarks**
- So far, quarks appear to be fundamental (“point-like”)

## The Modern Atom

- A cloud of electrons in constant motion around the nucleus
- And protons and neutrons in motion in the nucleus
- And quarks in motion within the protons and neutrons

## Size inside atoms

- The nucleus is 10,000 times smaller than the atom
- Proton and neutron are 10 times smaller than nucleus
- No evidence that quarks have any size at all !

## New Particles

- Collisions of electrons and nucleii in cosmic rays and particle accelerators beginning in the 1930’s led to the discovery of many new particles
- Some were predicted but many others came as surprises
- Muon like a heavy electron: ‘Who ordered that?’
- At first, all of them were thought to be fundamental

## Only a few at first

## These can be explained as made of a few quarks

## What is Fundamental?

- Physicists have discovered hundreds of new particles
- Most, we now know are not fundamental
- We have developed a theory, called **The Standard Model,** which appears to explain what we observe
- This model includes 6 quarks, 6 leptons and 13 force-related particles

## What is the World made of?

- The real world is not made of individual quarks (more on that later)
- Quarks exist only in groups making up what we call hadrons: (proton and neutron are hadrons)
- E.g. a proton is 2 up quarks and 1 down quark
- We are all made from up and down quarks and electrons

## Matter and Antimatter

- For every particle ever found, there is a corresponding antimatter particle or antiparticle
- They look just like matter but have the opposite charge
- Particles are created or destroyed in pairs

## Particles can decay

- Particles may decay, i.e. transform from one to another
- Most are unstable
- Proton and electron are stable

- Neutron can decay to electron and a proton
- Energy appears to be missing.  It is carried off by a neutrino

## Generations

- The six quarks and the six leptons are each organized into three generations
- The generations are heavier “Xerox” copies
- “Who ordered the 2nd and 3rd generations?”
- The quarks have fractional charges (+2/3 and -1/3) The leptons have charge -1 or 0

## What about Leptons?

- There are six leptons, three charged and three neutral
- They appear to be point-like particles with no internal structure
- Electrons are the most common and are the only ones found in ordinary matter
- Muons (and taus ( are heavier and charged like the electron
- Neutrinos have no charge and very little mass

## Matter Summary

- So all the universe is made of First Generation quarks and leptons
- We now turn to how the quarks and leptons interact with each other, stick together and decay

## Four Forces

- There are four fundamental interactions in nature
- All forces can be attributed to these interactions
- Gravity is attractive; others can be repulsive
- Interactions are also responsible for decay

## How do Particles Interact?

- Objects can interact without touching
- How do magnets “feel”  each other to attract or repel?
- How does the sun attract the earth?
- A force is something communicated between objects

## Electromagnetism

- The electromagnetic force causes opposite charges to attract and like charges to repel
- The carrier is called the photon (
- The photon is massless and travels at “the speed of light”

## Residual E-M

- Normally atoms are neutral having the same number of protons and electrons
- The charged parts of one atom can attract the charged parts of another atom
- Can bind atoms into molecules

## Why Doesn’t a Nucleus Explode?

- A heavy nucleus contains many protons, all with positive charge
- These repel each other
- Why does it not blow apart?

## Strong Force

- In addition to their electric charge, quarks also carry a new kind of charge called color charge
- The force between color charged particles is the “strong force”

## The Gluon

- The strong force holds quarks together to form hadrons
- Its carrier particles are called gluons; there are 8 of these
- The strong force only acts on very short distances

## Color and Anti-color

- There are three color charges and three anti-color charges
**But note, these colors have nothing to do with color and visible light, they are only a way describing the physics**

## Colored Quarks and Gluons

- Each quark has one of the three color charges and each antiquark has one of the three anticolor charges
- Baryons and mesons are color-neutral just as red-green-blue makes white light

## Quark Confinement

- Color force (QCD) gets _stronger_ at long distances!!
- Color-charged particles cannot be isolated
- Color-charged quarks are confined in hadrons with other quarks
- The composites are color neutral

## Color Field

- Quarks in a hadron exchange gluons
- If one of the quarks is pulled away from its neighbors, the color field stretches between that quark and its neighbors
- New quark-antiquark pairs are created in the field

## Quarks Emit Gluons

- When a quark emits or absorbs a gluon, the quarks color charge must change to conserve color charge
- A red quark emits a red/antiblue gluon and changes into a blue quark

## Residual Strong Force

- The strong force between the quarks in one proton and the quarks in another proton is strong enough to overwhelm the repulsive electromagnetic force

## Weak Force

- Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons
- Example: neutron to decay into proton + electron + neutrino
- This is why all matter consists of the lightest quarks and leptons (plus neutrinos)

## Electroweak Force

- In the Standard Model, the weak and the electromagnetic forces have been combined into a unified electroweak theory
- At very short distances (~10-18 meters), the weak and electromagnetic interactions have comparable strengths
- Force particles are photon, W and Z

## What about Gravity?

- Gravity is very weak
- Relevant at macroscopic distances
- The gravity force carrier, the graviton, is predicted but has never been seen

## Interaction Summary

## Quantum Mechanics

- Behavior of atoms and particles is described by Quantum Mechanics
- Certain properties such as energy can have only discrete values, not continuous values
- Particle properties are described by these values (quantum numbers) such as:
- Electric Charge
- Color Charge
- Flavor
- Spin

## The Pauli Exclusion Principle

- We can use quantum particle properties to categorize the particles we find
- Some particles, called Fermions, obey the Exclusion Principle while others, called Bosons, do not

## Fermions and Bosons

## What Holds the World Together?

- We have learned that the world is made up of six quarks and six leptons
- Everything we see is a conglomeration of quarks and leptons (and their antiparticles)
- There are four fundamental forces and there are force carrier particles associated with each force

## How does a particle decay?

- The Standard Model explains why some particles decay into other particles
- In nuclear decay, a nucleus can split into smaller nuclei
- When a fundamental particle decays, it has no constituents (by definition) so it must change into totally new particles

## The Unstable Nucleus

- We have seen that the strong force holds the nucleus together despite the electromagnetic repulsion of the protons
- However, not all nuclei live forever
- Some decay

## Nuclear Decay

- The nucleus can split into smaller nuclei
- This is as if the nucleus “boiled off” some of its pieces
- This happens in a nuclear reactor

## Muon Decay

- Muon decay is an example of particle decay
- Here the end products are not pieces of the starting particle but rather are totally new particles

## Missing Mass

- In most decays, the particles or nuclei that remain have a total mass that is less than the mass of the original particle or nucleus
- The missing mass gives kinetic energy to the decay products

## Particle Decay Mediators

- When a fundamental particle decays, it turns itself into a less massive particle and a force-carrier particle (the W boson)
- The force-carrier then emerges as other particles
- A particle can decay if it is heavier than the total mass of its decay products and if there is a force to mediate the decay

## Virtual Particles

- Particles decay via force-carrier particles
- In some cases, a particle may decay via a force-carrier that is more massive than the initial particle
- The force-carrier particle is immediately transformed into lower-mass particles
- The short-lived massive particle appears to violate the law of energy conservation

## The Uncertainty Principle

- A result of the Heisenberg Uncertainty Principle is that these high-mass particles may come into being if they are very short-lived.
- These particles are called **virtual particles**

## Different Interactions

- Strong, electromagnetic, and weak interactions all cause particle decays. However, only weak interactions can cause the decay of fundamental particles into other types of particles.
- Physicists call particle types "flavors." The weak interaction can change a charm quark into a strange quark while emitting a virtual W  boson (charm and strange are flavors).
- Only the weak interaction (via the W boson) can change flavor and allow the decay of a truly fundamental particle.

## Other Interactions

- Electromagnetic Decays:
- The 0 (neutral pion) is a  meson. The quark and antiquark can annihilate; from the annihilation come two  photons. This is an example of an electromagnetic decay.
- Strong Decays:
- The  cparticle is a meson made up of a c and an anti-c. It can undergo a strong decay into two gluons (which emerge as hadrons).

## Annihilations

- These are not decays but they also take place through virtual particles
- Annihilations of light quarks at very high energy can produce very massive quarks in the laboratory

## Antiproton Annihilation

- This bubble chamber shows an antiproton colliding with a proton, annihilating and producing eight pions
- One pion decays into a muon and a neutrino (which leaves no track)

## Fundamental Processes

- With what you have now learned, you can make models of the fundamental processes that physicists study
- These models are the foundation for detailed calculations of what happens at a high energy accelerator

## Neutron Beta Decay

## Electron-Positron Annihilation

## Top Production

## Mysteries and Failures

- The Standard Model is a theory of the universe
- It provides a good description of phenomena observed by experiments
- It is still incomplete in many ways: why 3 generations? What is dark matter?

## Is the Standard Model Wrong?

- We need to go beyond the Standard Model in the same way that Einstein’s Theory of Relativity extended Newton’s laws of mechanics
- We will need to extend the Standard Model with something new to explain mass, gravity, etc.

## Three Generations

- There are three sets of quarks and three sets of leptons
- Why are there exactly three generations of matter?
- Why do we see only one in the real world?

## What About Masses?

- The Standard Model cannot explain why a particle has a certain mass
- Physicists have theorized the existence of a new field, called the Higgs field, which interacts with other particles to give them mass
- So far, the Higgs has not been seen by experiment

## Grand Unified Theory

- We believe that GUT will unify the strong, weak and electromagnetic forces
- All three forces would be different aspects of the same, unified interaction
- The three forces would merge into one at high enough energy

## Supersymmetry

- Some physicists attempting to unify gravity with the other fundamental forces have suggested that every fundamental particle should have a massive “shadow” particle

## Modern physics has theories for quantum mechanics, relativity and gravity but they do not quite work with each other
If we lived in a world of more than three spatial dimensions, these problems can be resolved
String theory suggests that in a world with three ordinary dimensions and some additional very small dimensions, particles are strings and membranes

- If we lived in a world of more than three spatial dimensions, these problems can be resolved
- String theory suggests that in a world with three ordinary dimensions and some additional very small dimensions, particles are strings and membranes

## Extra Dimensions

- String Theory requires more than three space dimensions
- These extra dimensions could be very small so that we do not see them
- Experiments are now searching for evidence of extra dimensions

## Dark Matter

- It appears that the universe is not made of the same kind of matter as our sun and the stars
- The dark matter does exert a gravitational attraction on ordinary matter but has not been detected directly

## The Accelerating Universe

- Recent experiments using Type Ia Supernovae have shown that the universe is still expanding and the rate of expansion is increasing
- This acceleration must be driven by a new mechanism which has been named **dark energy**

## The Expanding Universe

- Studies of the most distant supernova ever detected indicates that the universe did go through a phase where the expansion slowed down
- It is now speeding up

## Slide 68

## Conclusion

- The Standard Model is a powerful synthesis that explains a huge number of observations in a simple framework.  It is to physics what evolution is to biology.
- There are many important questions beyond the Standard Model