| The name "quark" was taken by Murray Gell-Mann from Finnegan's Wake by James Joyce. The line "Three quarks for Muster Mark ..." appears in the fanciful book. Gell-Mann received the 1969 Nobel Prize for his work in classifying elementary particles. |
In the 16th and 17th centuries chemists built up a store of knowledge. They described elements and compounds. Mendeleev (and others) made sense of the data by building the Periodic Table of the elements.
Over the last hundred years more than 200 'elementary particles' have been isolated and identified either directly, or as a consequence of their decay products. These 'particles' have peculiar properties, and most have very short lives.
Just as Mendeleev made sense of chemistry by building a table, modern physicists have tried to make sense of the relationships within particle "families" by building tables. The most successful effort so far is the Standard Model, based on patterns that belong to the branch of abstract mathematics called 'group theory'.
Just as Mendeleev found that his table had gaps - which he filled with unknown elements - there are gaps in the elementary particle tables which are filled by as yet hypothetical (undiscovered) particles. It is the crowning joy of the particle physicist to fill a gap.
The Standard Model holds that the hadrons, (protons, neutrons etc.) are composed of combinations of two or three tightly bound subatomic particles called quarks. Quarks come in six flavors (types). Isolated quarks are not observed directly ... we believe we do not yet have enough energy in atomic collisions to produce them. So far, the tables have listed about 200 subatomic particles as combinations of quarks.
The topic is vast and many of the concepts make no sense at all in terms of everyday experience. The editor remembers the occasion at a curriculum writing meeting in New Zealand along with Professor Butler, (a class-mate of his at the University of Canterbury). When deliberating whether or not to include this topic in the New Zealand senior school curriculum one of our company loudly proclaimed that it was easy! She now fully understood the concepts after having read a table of the bosons showing the combinations of quarks. Glances were exchanged across the table and the eyes rolled just a little. Nothing was said. No one, absolutely NO ONE understands any of this at a deeper level. We are in a fascinating, unbelievable bog, with no prospect of escape. Get used to it.
The leptons are fundamental particles which come in three flavors (types), electrons, muons and tau-leptons and their respective neutrinos. Leptons have half integral spin (intrinsic angular momentum) and obey the Pauli exclusion principle. (No two leptons can occupy the same quantum state, which describes how electrons sequentially fill the shells of an atom).
Pair production
The electron anti-particle is the positron. Photons, with a combined mass-equivalent greater than two electron masses, can interact to produce an electron-positron pair. An electron and a positron can annihilate producing two gamma ray photons. Decay to a single gamma ray photon does not occur because it is not possible to conserve both momentum and energy in that process.
In high-energy particle experiments, we use energy and momentum conservation to infer that production of one or more neutrinos occurred. ie. the neutrinos carried off the missing energy (and momentum)... bit like "Ghosts did it!"
The table contains all known leptons. Each lepton has an anti-particle.
Quarks are fundamental particles which come in six flavors (types).
Quarks are fermions with half integral spin. The table contains all known quarks.
Illustrations
Quarks are mathematical symbols. It is not possible to get a good picture of a quark but the editor did manage to find a quirky idea on the web. The cork model is illustrated at right.
The hadrons are influenced by the strong interaction - the
short range force (nuclear glue) that is responsible for
overcoming the coulomb repulsion of protons to hold the nucleus
together. The idea of quarks was first proposed in the 1960's
to describe the many observed hadrons which fall into two classes:
baryons and mesons.
Fermionic hadrons
(baryons) have spin (intrinsic angular momentum) which
is half integral. They obey the Pauli exclusion principle. (No
two baryons can occupy the same quantum state, which describes
how protons sequentially fill the shells of a nucleus.) The
baryons are made from three quarks. The table contains only four
of the dozens of known baryons. Each baryon has an anti-particle
made from three anti-quarks.
Bosonic hadrons (mesons) have integral spin. They do not obey the Pauli exclusion principle. The mesons are each made from one quark and one anti-quark. The only mesons which are long-lived enough to be seen directly by their tracks in a detector are ...
... pions, which contain only u and d quarks and anti-quarks.
... kaons, which contain one u or d quark or anti-quark, and one s quark.
The table contains only four of the dozens of known mesons. Each meson has an anti-particle.
Summary Along with their anti-particles the six quarks and six leptons are the fundamental fermionic particles - that's it?! 12 quarks and 12 leptons. The hadrons are combinations of quarks. That is the Standard Model. |
Note: conservation rules are just rules - they do not explain anything at a deeper level.
All known decays ...
1 ... conserve electric charge.
2 ... conserve *baryon number.
3 ... conserve **lepton number.
4 ... conserve spin.
5 ... *** conserve color charge.
* Baryon number is the number off quarks (+1) plus the number of anti-quarks (-1) divided by 3. eg. A neutron decay to a proton, an electron and an anti-neutrino conserves baryon number.
** Lepton number is the number of leptons (+1) plus anti-leptons (-1). eg. A neutron decay to a proton, an electron and an anti-neutrino conserves lepton number.
*** Color charge is not exactly a joke. For what it is worth at this stage of your reading click here.
Note: the concept of color and the picture of matter as combinations of Gell-Mann's quarks or Feynman's partons is far from simple or complete. To quote stanford.edu....
"The quarks inside a meson or baryon are continually interacting with one another via the strong force field. At any instant in time, they may contain many virtual particles: gluons and additional quark-antiquark pairs."
"The picture of a proton as made of three quarks is a gross simplification. For example, we know from measurements that in a high-momentum proton only about half the momentum is carried by quarks, the rest is carried by gluons."
"The observation of a process that violates one of the conservation rules would be evidence for additional laws of nature beyond the Standard Model. For instance ... there are new experimental results that suggest neutrinos do have mass. If these results are confirmed, the lepton number laws may be downgraded to the status of approximate conservation laws."