The Standard Model of Particles and Interactions Ian
- Slides: 69
The Standard Model of Particles and Interactions Ian Hinchliffe 26 June 2002
What is the World Made of? • Ancient times – 4 elements • 19 th century – atoms • Early 20 th 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, • Neutron can decay to i. e. transform from one electron and a proton to another • Energy appears to be • Most are unstable missing. It is carried off by a neutrino • Proton and electron are stable
Generations • The six quarks and the six leptons are each organized into three generations • The generations are heavier “Xerox” copies • “Who ordered the 2 nd and 3 rd 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 (m) and taus (t) 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 (g) • 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 anticolor 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 forcecarrier 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 shortlived. • 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 p 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 hc 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
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
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
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