Spin is a
Quantum-mechanical property of particles, measuring, as you might guess,
angular momentum.
Spin is measured in terms of
Planck's constant of angular momentum, which is symbolized by
h with a diagonal line through it. It is equivalent to Planck's constant
h divided by 2*
pi (the number of
radians in a full circle). That is,
1.05450*10-34 J*s.
One might think that a basic quantum-mechanical measurement might always be an integer multiple of its basic unit, but spin is measured in multiples of
one-half Planck's constant of angular momentum. Particles are divided into two types based upon their spins:
Bosons, particles with integer spin, and
Fermions, particles with non-integer spin (some integer plus 1/2)
All
Baryons are fermions, since they are made up of three
quarks, each with a spin of 1/2.
The alignment or non-alignment of the spins of any given group of three quarks results in two different types of baryon. For example, the spin-aligned version of the
proton is the
delta+ particle. Both particles consist of two "up" quarks and one "down" quark.
The spin-aligned baryons (with a spin of 1 1/2) have higher energies and masses, and are less stable, than their non-spin-aligned counterparts (with a spin of just 1/2).