Lectures 14,15,16,17

Class demonstrations

1) Geiger counter - measuring radioctivity, decay rate, half life (see Hewitt page 642-643)
2) Cloud chamber - particle tracks (see Hewitt page 644-645)

Passing high energy particle knocks out electrons from atoms in in gas or vapor, this produces electrons and ions (remaining nucleus and electrons) along the track of the particle.

• In the geiger counter the electrons that are knocked out are attracted to a positively charged rod in the center - this produces a flow of electrons, that is a current, which activates the geiger counter to make noises and record the level of the activity.
• In the cloud chamber the cool gas condenses on the ions along the particle path, and this makes the particle path visible. This is similar to jet trails in the sky that consists of ice-crystal trails formed by condensation along the path.
• more sophisticated tools to study particle tracks include the bubble chamber, the spark chamber, the streamer chamber.

• Radioactivity in Nature.
• Cosmic rays are showers of particles coming from all over the universe. They play a role in mutation and biological evolution.
• There are numerous applications of nuclear science, from smoke detectors to medicine.

Credits for graphics used in this lecture - The Particle Adventure.

What is fundamental?

 
 

The Particle Adventure .  See Site Map
    Particle Physics summary and history, see also The Standard Model  of elementary particles.
    Particle Physics Timeline
        Early atomic understanding
        Scientific revolution and Classical Mechanics timeline
        Quantum Theory timeline
        Standard Model  timeline
 
 

The Standard Model of Particles and Interactions provides a provisional answer:
    Matter particles: 6 quarks, 6 leptons, and their anti-particles,
    Force carrier particles: photon, gluons, W,Z, graviton.
 

Matter particles:  6 quarks, 6 leptons, and their anti-particles Force carrier particles:  graviton, photon, gluons, W+,W-,Z0

• electric charges:   u,c,t = 2/3;   d,s,b= -1/3;   neutrinos= 0;   e,mu,tau = -1
• graviton, photon, gluons, Z⁰ are neutral; W⁺,W^- have +,- charges.
• color charges: quarks r,g,b; anti-quarks opposite, gluons qqcombinations;
• leptons, graviton, photon, W, Z are color neutral
• antiparticles have opposite electric or color charges

mesons :  quark + antiquark different combinations of spin uu ud uc us ut ub du dd dc ds dt db cu cd  cc cs ct  cb su sd sc ss st sb tu  td  tc  ts  tt   tb bu bd bc bs bt bb pion+ = ud 2/3+1/3=1 kaon+ = us 2/3+1/3=1

baryons: quark + quark + quark antibaryons: antiquark + antiquark +antiquark  uuu, uud, uuc, ..... udd, ddd, ddc, .... etc. proton=uud,    2/3+2/3-1/3=1  neutron=udd,  2/3-1/3-1/3=0

Particle decay (heavier ones decay to lighter ones - doublet pairs couple to intermediate W)
    neutron decay (the inside story)
    quark decay in action
    bubble chamber photograph

Collisions of particles at high energies
    Electron-positron annihilation and creation of mesons
    Proton-antiproton annihilate and produce matter that contains  top+antitop quarks

 

FOUR FORCES (and beyond)

At a fundamental level, a force  arises due to a thing which is passed between two particles.

• The four fundamental forces
• see lecture on forces.

• Electromagnetism acts on electric or magnetic charges. This force holds together atoms and molecules and is responsible for all chemistry. The force carrier is the photon. Molecular forces are residual electromagnetic forces.  Also responsible for all macroscopic electric and magnetic phenomena, TV, radio, light, communication, lasers, etc.. The quantum theory of electromagnetism is called quantum electrodynamics or QED for short.
• Strong force, acts on  color charges. This force confines quarks inside particles. The force carriers are 8 gluons. The nuclear force that hold the nucleus together is residual strong force. The nuclear energy radiated by stars mainly through the nuclear fission processes is energy stored by the strong force. The quantum theory of the strong force is called quantum chromodynamics or QCD for short.
• Weak force acts on "weak" charges. This force is responsible for decay of particles and change of "flavor". The decay process also contributes to the energy radiated by stars, including the Earth. The force carriers are W⁺,W^-,Z^0. The Weak and Electromagnetic forces are unified at 10^-18 meters (one thousandth the size of a proton). This is explained by the Standard Model and is called the unified electroweak force, and its quantum theory is called quantum flavordynamics of QFD for short. QFD contains QCD.
• Gravitational force acts on mass or energy. This force holds people on Earth, planets around the Sun, stars in galaxies, governs the expansion of the universe as a whole, is responsible for quasars, supernovae, black holes, bending of light around stars, ...
• The Standard Model is the theory that encompasses QFD and QCD in the same framework, while grand unified theories (GUTs) are attempts to explain these as coming from the same origin.
• GUT goes beyong the four forces. The additional forces may be responsible for proton decay, and mater-antimatter assymetry in the universe.
• Gravity remains outside of the scope of the Standard Model, therefore the unification idea remains incomplete. Superstrings is the theory that has the structure to unify GUTs and Gravity (this will be explained after we discuss two other topics: quantum mechanics and general relativity) ....

Accelerators are used to smash particles at high energies and study their interactions.
Quarks, heavier leptons, W,Z, gluons all were discovered in the major accelerators during the past 26 years.
You can take virtual tours of major accelerators at these links: CERN,  Fermilab ,  SLAC ,
Acceleration of particles
How do you "see" particles?
 

What Questions Remain?

The Standard Model, with its six types of quarks, six leptons, and the four force types, explains everything we have observed so far about the structure of matter and how matter interacts in all possible ways. The Standard Model is in remarkable agreement with all experiments performed so far.

The SM may even explain where mass comes from. This will be understood in the next round of experiments that will discover the Higgs particle either at Fermilab starting in 2002, or at CERN starting in 2006.

But the Standard Model leaves many other questions unanswered:

• Why do quarks and leptons come in repetitions, in the form of the 3 generations?
• How do we understand the their masses, are there patterns of masses?
• Are there more generations, or other types of particles and forces to be discovered at yet higher-energy accelerators?
• Are the quarks and leptons really fundamental, or do they, too, have substructure?
• How can the gravitational interactions be included?
• Is the dark matter in the universe made up of some of the particles discovered already (such as neutrinos) of some other stuff yet to be discovered?

Fundamentally Speaking
The Rules of the Game

Reading assignment:
Go through the chapters of the The Particle Adventure . Take all quizzes, try to answer all questions (do not turn in).

For more information:
Particle physics education sites

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