Quantum Chromodynamics

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Discussion

Basic info

quarks exist only in groups

Color is just a metaphor.

QCD personalities

Rutherford-style scattering experiments showed a three part structure for the proton.

http://www.nobel.se/physics/laureates/1990/press.html

Murray Gell-Mann

In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been "kwork." Then, in one of my occasional perusals of Finnegans Wake, by James Joyce, I came across the word "quark" in the phrase "Three quarks for Muster Mark." Since "quark" (meaning, for one thing, the cry of a gull) was clearly intended to rhyme with "Mark," as well as "bark" and other such words, I had to find an excuse to pronounce it as "kwork." But the book represents the dreams of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the "portmanteau words" in Through the Looking Glass. From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry "Three quarks for Muster Mark" might be "Three quarts for Mister Mark," in which case the pronunciation "kwork" would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.

James Joyce. Finnegan's Wake. Book 2, Episode 4, Page 383
Three quarks for Muster Mark!
Sure he hasn't got much of a bark
And sure any he has it's all beside the mark.
But O, Wreneagle Almighty, wouldn't un be a sky of a lark
To see that old buzzard whooping about for uns shirt in the dark
And he hunting round for uns speckled trousers around by Palmerstown Park?
Hohohoho, moulty Mark!
You're the rummest old rooster ever flopped out of a Noah's ark
And you think you're cock of the wark.
Fowls, up! Tristy's the spry young spark
That'll tread her and wed her and bed her and red her
Without ever winking the tail of a feather
And that's how that chap's going to make his money and mark!

Timeline

  1. The first three quarks are hypothesized: up, down, and strange.
  2. Evidence for a fourth quark is found in November of 1974. Two experiments simultaneously announced the discovery of a meson with a mass of about 3.1 GeV/c2. Called the J meson by one group and the ψ meson by another it was later determined to be a combination of charm and anticharm quarks. Since neither group had priority on the discovery, the meson is now called J/ψ. Like many particles discovered in the Twentieth Century, it was also given a whimsical name: charmonium.
  3. Unexpected discovery of the bottom quark
  4. Mass of the top quark finally determined. The top quark is more massive than many atoms. and it is so unstable that is does not live long enough to combine with other quarks to form a hadron.

18 quarks

  red green blue
up √ √ √
down √ √ √
strange √ √ √
charm √ √ √
top √ √ √
bottom √ √ √

8 gluons

  green-antired blue-antired
red-antigreen   blue-antigreen
red-antiblue green-antiblue  

 

(red-antired) – (green-antigreen)
√2

 

(red-antired) + (green-antigreen) – 2(blue-antiblue)
√6

 

Summary

Problems

practice

  1. The mass of the top quark is approximately 175 GeV/c2; heavier than some atoms. Identify the heaviest element that is lighter than the top quark.

    Solution …

    The mass of subatomic particles is measured in electron volts while chemical elements are weighed in atomic mass units. This is a question about unit conversion.

    mtop =  175 GeV/c2   103 MeV   1 u  = 187.870 u
    1 GeV 931.494 MeV/c2

    Now we need a periodic table. I have a nice one in this book. Find the smallest atomic mass greater than 187.87 u. Osmium satisfies this condition with a mass of 190.23 u. The next lighter element is rhenium at 186.21 u. This must be the answer. The top quark is heavier than an atom of every element up to element 75 -- rhenium.


    [magnify]

    Well, that's almost right. We need to qualify this answer a bit. You may recall that the masses of the elements stated on a periodic table are averages. Atoms of any element can be found with a variety of masses. These variations are called isotopes. Here's a fragment of the table of isotopes for rhenium and osmium.

    selected isotopes of rhenium (z = 75)   selected isotopes of osmium (z = 76)
      mass (u)  half life abundance     mass (u)  half life abundance
    184Re 183.952530 38.0 d     184Os 183.952488 5.6e13 y 0.02%
    185Re 184.952951 stable 37.40%   185Os 184.954041 93.6 d  
    186Re 185.954984 3.7183 d     186Os 185.953830 2.0e15 y 1.6%
    187Re 186.955744 4.35e10 y 62.60%   187Os 186.955741 stable 1.6%
    mass of the top quark = 187.870 u
    188Re 187.958106 17.005 h     188Os 187.955860 stable 13.3%
    189Re 188.959219 24.3 h     189Os 188.958137 stable 16.1%
    190Re 189.961850 3.1 m     190Os 189.958436 stable 26.4%
    191Re 190.963112 9.8 m     191Os 190.960920 15.4 d  
    192Re 191.965870 16 s     192Os 191.961467 stable 41.0%

    There are isotopes of rhenium that are heavier than the top quark, but they are all unstable and have very short half lives. You probably wouldn't find them anywhere on earth outside a research reactor. There are also isotopes of osmium that are lighter than the top quark. These are more significant since they can be found in measurable quantities on earth. The mass of the top quark is more than a typical rhenium atom but less than a typical osmium atom.

  2. Write something.
    • Answer it.
  3. Write something.
    • Answer it.
  4. Write something.
    • Answer it.

combinatorics

  1. A quark-antiquark pair is called a meson. For example, a positive pion is an up-antidown quark pair (π+ = ud).
    1. How many mesons can be made from the first generation quarks only?
    2. How many mesons can be made from all six flavors of quark?
    3. What charges are possible?
  2. A triplet of quarks is called a baryon. For example, a proton is two up quarks and one down quark (p+ = uud) while a neutron is one up quark and two down quarks (n0 = udd). Permutations of quarks are considered identical (for example, uds = usd = sud = … ).
    1. How many baryons can be made from the first generation quarks only?
    2. How many baryons can be made from all six flavors of quark?
    3. What charges are possible?
  3. According to some theoreticians, groups of six quarks may be moderately stable.
    1. How many six-quark particles can be made from the first generation quarks only?
    2. How many six-quark particles can be made from all six flavors of quark?
    3. What charges are possible?
    4. Invent an appropriate name for this class of particles.

investigative

  1. top-quark.pdf
    The purpose of this activity is to determine the mass of the top quark from proton-antiproton collision data gathered at Fermilab in 1995.

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