|San José State University|
& Tornado Alley
and the Quarkic
Structure of Nucleons
The magnetic moment μ of a point charge of q rotating about an axis at distance r away from it at a rate of ω radians per second is qr²ω. For a charge of Q uniformly distributed over a surface or volume the magnetic moment must be computed by integration over that surface or volume in exactly the same way the moment of inertia is computed for a mass M distributed over a surface or volume. This means the formula for the magnetic moment of an object is of the form
where R is in the nature of an average distance of the charge from the spin axis and k is a coefficient dependent upon the shape of the distribution of charge. For example if the charge is uniformly distributed over a spherical shell then k=(2/3). For a solid sphere it is (2/5).
The structures to be considered are nucleons, both as distributions of charge and mass and as compositions of quarks. The net magnetic dipole moments of a proton and a neutron, measured in magneton units, are 2.79285 and −1.9130. respectively. For an electrically neutral neutron to have a magnetic moment might be something of a surprise, but the radial distribution of charge easily explains it.
The average radius of the negative charge of the neutron is so much greater than the average radius of its positive charge that the net value is negative. It is notable that direction of the spin of a neutron is the same as that of a proton.
A proton is composed of two up quarks and one down quark. For a neutron its composition is two down quarks and one up quark. Let μU and μD be the magneti moments of the Up and Down quarks, respectively. Then
Dividing the second equation by 2 gives
Subtracting this equation from the first gives
The magnetic moment of the Down quark is then
Now remember that the magnetic moment of a particle is of the form
The charge of the Up quark is +2/3 and that of the Down quark is −1/3. Let the average charge radii of the Up and Down quarks be denoted by RU and RD, respectively. Likewise let ωU and ωD be their spin rates and kU and kD the coefficients for the shapes of their charge distributions. Then
Equivalently these are
Note that the ratio is
The radius and spin rate of a quark are related to its angular momentum
The moment of inertia J of a mass M uniformly distributed over a spherical shell of radius R is (2/3)MR². For a spherical ball it is (2/5)MR². The general form is then
where jS is a coefficient determined by the shape of the distribution of mass. Angular momentum L is given by
The magnetic moment μ is given by
where K is a moment of charge analogous to the moment of inertia and having the form
Generally the magnetic moment of a charged particle is related to its angular momentum by
where α is a coefficient depending upon any difference in the shapes of the distribution of mass and chrge. If the shapes are the same then α=1.
Angular momentum is generally quantized. This would make μ directly proportional to Q and inversely proportional to M. This explains why the magnetic moment of an electron is so much larger than those of the nucleons. Its mass is roughly one two thousandth of their masses. But Q and M can be expected to be proportional to each other. That means that if L is quantized then μ is quantized. Thus μ should approximately be a constant independent of the scale of the structure. In symbols
Usually when angular momentum is said to be quantized this is taken to mean that the minimum angular moment is
equal to the reduced Planck's Constant,
h. But this not true. If an object has n modes of vibration or oscillation
then each of those modes has a minimum of h so the overall minimum is n h. These
modes are also called degrees of freedom.
It is well known that the masses of nucleons are not simply the sum of the masses of its quarks. The mass of the positive pion meson, which consists of an Up quark and the anti-quark Down quark is a small fraction of the masses of the nucleons. Nevertheless consider what the masses of the nucleons implies about the masses of the quarks. Let mU and mD be the masses of the Up and Down quarks, respectively. Then
Thus mD/mU=1.00526. So essentially mD =mU.
Thus if the shapes of the charge distributions of the Up and Down quarks are the same then the ratio of the angular momenta of the Up and Down quarks should be
a ratio of integers. Since L=jSMR² then if jS=kS for each each quark
Note that 9/16 = 0.5625 and 0.566466=(1.007)(9/16). Thus
So indeed the ratio of the angular momenta of the Up and Down quarks is a ratio of whole numbers. The fact that those whole numbers are greater than unity indicates that quarks have complicated shapes with multiple degrees of freedom.
It also may be significant that (9/16)=(3/4)²
(To be continued.)
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