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San José State University

Thayer Watkins
Silicon Valley
& Tornado Alley

Magnetic Moments
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

μ = QkR²ω

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 of its magnetic moment is negative. It is notable that direction of the spin of a neutron is the same as that of a proton.

The Magnetic Moments of Nucleons
in Relation to that of their Composite Quarks

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 magnetic moments of the Up and Down quarks, respectively. Then

U + μD = 2.79285
μU + 2μD = −1.9130

Dividing the second equation by 2 gives

½μU + μD = −0.9565

Subtracting this equation from the first gives

(3/2)μU = 3.74935
and hence
μU = 2.49957 magnetons

The magnetic moment of the Down quark is then

μD = 2.79285 − 2μU = −2.20628 magnetons.

Now remember that the magnetic moment of a particle is of the form

μ = QkR²ω

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

(2/3)kURU²ωU = 2.49957
(−1/3)kDRD²ωD = −2.20628

Equivalently these are

kURU²ωU = 3.749355
kDRD²ωD = 6.61885

Note that the ratio is

3.749355/6.61885 ≅ 9/16 = (3/4)²

The radius and spin rate of a quark are related to its angular momentum

Moments of Inertia and Angular Momenta

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

J = jSMR²

where jS is a coefficient determined by the shape of the distribution of mass. Angular momentum L is given by

L = Jω

The magnetic moment μ is given by

μ = Kω

where K is a moment of charge analogous to the moment of inertia and having the form

K = kSQR²

Generally the magnetic moment of a charged particle is related to its angular momentum by

μ = αL(Q/M)

where α is a coefficient depending upon any difference in the shapes of the distribution of mass and charge. 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

μ(M/Q) = L

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 strictly 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 nh. These modes are also called degrees of freedom.

The Mass of the Up and Down Quarks

It is well known that the masses of nucleons cannot be 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 imply about the masses of the quarks. Let mU and mD be the masses of the Up and Down quarks, respectively. Then

2mU + mD = 1.6726231×10-27 kg
mU + 2mD = 1.6749286×10-27 kg


mU = 0.556777×10-27 kg
mD = 0.559708×10-27 kg

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

LU/LD = nU/nD

a ratio of integers. Since L=jSMR² then if jS=kS for each quark

= 3.749355/6.61885 = 0.566466

Note that 9/16 = 0.5625 and 0.566466=(1.007)(9/16). Thus

LU/LD ≅ (9/16)

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 may have complicated shapes with multiple degrees of freedom.

It is quite plausible that an Up and an Down quark have the same shape so kU=kD. Furthermore there is a rationale for ωUD. See Sensible Model of Quarkic Structure of Nucleons.

These equalities would mean


But quark radii is likely to be quantized; i.e.

R = ρμ

where μ is an integer and ρ is a constant.

This would mean

LU/LD = μU²/μD² =(μUD

The implication of the above is that

RU/RD = 3/4

Thus it is significant that (9/16)=(3/4)².

For more on the quarkic spatial structure of nucleons see Sensible Model of Quarkic Structure of Nucleons.

(To be continued.)

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