We continue to discuss material from The Particle Adventure. Key points are summarized below. We also add some thoughts about the material that you will not find on this website.

The rules of quantum mechanics and Einstein's theory of special relativity are the foundation of the modern theory which is called the Standard Model of particles and interactions. Quantum mechanics, as we saw, describes the fact that some things at the atomic and subatomic level come in descrete bundles, or quanta. We already discussed examples of quantization including electric charge, angular momentum, energy and momentum. At the quark level, there are other properties that come in descrete amounts.

In addition to the above properties, quarks come in different so-called flavors, i.e., up, down, charm, strange, top and bottom. The flavors of leptons are associated with the electron, muon or tau nature of the lepton. Quarks also have three different color charges, and the gluons which bind the quarks actually come in color pairs. The rules of the game get beyond the scope of this course and there is no point in giving all of rules at this point. The value of going this far is to reveal some of the deeper levels in our understanding of the subatomic world and to indicate that there are more quantum rules besides the ones mentioned already when we discussed atoms and the periodic table.

Another important quantum-mechanical effect which determines properties of aggregates of particles is related to the "spin" quantum number of particles. There are two spin categories, those particles with half-integer spin (1/2, 3/2, 5/2, etc.) called "Fermions", and those particles with integer spin (0, 1, 2, 3, etc.) called "Bosons." Fermions obey the Paul Exclusion Principle, while Bosons do not. Examples of Fermions include: quarks, leptons, neutrons and protons (and many others). Bosons include force carrier particles, mesons, and any other composite particle with integer spin. Because of obeying the Pauli Exclusion principle, electrons in atoms form shells and give atoms their chemical properties. Likewise, there is a shell structure in the atom's nucleus due the fact that the Pauli Principle applies to neutrons and to protons. Also, the electronic behavior of metals is dependent on the electrons obeying the Pauli Exclusion Principle.

While fermions behave the Pauli Principle, and not more than one fermion can occupy a given quantum state at the same time, just the opposite is true for bosons. (Bosons are named after one of the two originators of the rule, concerning integer spin particles, i.e., Bose and Einstein. It is not clear why Einstein's name does not appear in the commonly used name for this kind of particle...maybe its because "boson" is short and simple.)

Composite "particles" such as the nucleus of a Helium atom may behave like a boson because its net spin, i.e., after adding up the spins of its four composite nucleons, is an integer. Because an unlimited number of bosons can occupy the same quantum state, we get the property of superconductivity (zero electrical resistance) and superfluidity (zero viscosity).

The Particle Adventure website has an amusing analogy to illustrate the difference between fermions and bosons. They show two boarding houses. The "fermions" refuse to share a room, while the "bosons" freely croud into a single room.

Some of this is what we already covered in our discussion of nuclear physics. However, you will now see how nuclear decays can be viewed in terms of what happens at a more fundamental level...the decay of quarks themselves. In particular, note what is behind the beta decay of a neutron. In nuclear physics language, n ---> p + e- + nu. However, viewed in terms of the quark constituents inside the neutron, we see that a d-quark undergoes a weak transition and becomes a u-quark plus a "virtual" W-boson. The W-boson is the force carrier in this weak decay. (Virtual particles exist for a very short time...and they are allowed to exist even though their existence appears to violate energy conservation. It turns out to be ok because the Heisenberg Uncertainty principle permits an energy imbalance for extremely short times. ) The virtual W materializes as a electron-neutrino pair, and the newly-formed u-quark combines with the u-quark and d-quark already in the original neutron to form a proton. So we get the same result as before, but now we see how it happens at the more fundamental level.

For matter to annihilate antimatter, a particle must meet its antiparticle. For example, an electron and a positron will annihilate each other, but an electron and an antiproton will not annihilate.

We will see when we review particle accelerators again that one of the most effective ways to produce new particles is by colliding electrons with positrons at high energy. After the electron positron pair annihilates, pairs of particles can be spontaneously produced such that quantum rules are obeyed. For example, a b-quark can be produced along with an anti-b-quark. There was no "b-flavor" to begin with, so the net amount of b-flavor must remain zero. This happens because the b-quark and the anti-b-quark possess opposite flavors which cancel. The net charge must also be zero, but the charges of the b- and anti-b-quarks are -1/3 and +1/3, respectively, so again, they cancel.

Proton antiproton colliders are also important, but because protons and antiprotons are composite particles made of quarks, it is necessary for all of the quarks in the proton to annihilate all of the antiquarks in the antiproton for complete annihilation to take place. Hence, most of the time when a high energy beam of protons collides with a high energy beam of antiprotons, only a part of the proton interacts with a part of the antiproton. Normally, it is the gluons from within one of the beam particles with a gluon from the other that collide with each other and produce other particles. Collisions between composite particles like protons and antiprotons has often been described as being like smashing together two Swiss watches, each with its internal parts colliding with the other's internal parts.

Einstein said "the most incomprehensible thing about the universe is that it is comprehensible." The fact that we have a Standard Model showing the fabric of the material world is likely to be what Einstein found so incomprehensible, although he died in 1955 when much of what we now know hadn't yet been discovered. As Einstein knew that there was much to still be discovered and understood, he could still marvel at the fact that human consciousness was able to examine and understand so much of its essence.

If we are all made of atoms, as indeed we are, where within these atoms does our awareness of ourselves come? Some think that the brain, the seat of consciousness, is nothing more than a sophisticated computer, and that within the next several decades computers will actually take over the world as they achieve a level of consciousness equivalent or beyond that of humans. Others, like myself, think that is nonsense, but its a hunch, not something we can prove one way or the other by standard scientific inquiry.

Hence ones sees where the boundaries of science overlap with philosophy and religion. Yet there are puzzles that remain which can be addressed and will be answered within the next several decades. Some of these are mentioned below.

Some of the unanswered puzzles that will be discussed next time include the following:

• Why Three Generations?
• What is the Origin of Mass - Why is the Photon Light and the Z/W Heavy?
• What Caused the Universe to Be Made of Matter? Where's the Antimatter?
• Is There a Deeper Level Beyond the Quarks, Leptons and Force Carrier Particles? A Candidate Theory of Everything: Superstrings