Really short question, but I cannot find anything on the internet.

What is meant proton polarisation?

Is it to do with the spin of the proton? I guess the spin of the proton is obtained from the vector addition of the 3 quarks' individual spins, so it can't be 0... Can we have unpolarised protons then?



2 Answers 2


Protons and their spin 1/2 were measured and used in experiments long before quarks were a gleam in theorists' eyes.

Spin was studied with the Stern Gerlach experiment

The Stern–Gerlach experiment involves sending a beam of particles through an inhomogeneous magnetic field and observing their deflection. The results show that particles possess an intrinsic angular momentum that is closely analogous to the angular momentum of a classically spinning object, but that takes only certain quantized values.

Polarized protons are protons in a beam with the spins oriented in one direction, up for example.

There are experiments that try to explore how the spin of the proton is built up by the spins of the constituent quarks and gluons, and these need polarized proton beams.


It means that the spins of a collection of protons are correlated. In the ideal case (100% polarization) they would be all in a single direction. In the case of a proton beam, the polarization is generally along the direction of flight (positive helicity) or against the direction of flight (negative helicity).

In most matter the spins are randomly aligned so the material is said to be "unpolarized".

For the most part it does not make sense to talk about the polarization of a single particle, but if you are going to do it anyway, you must have preferred axis against which to measure it.

I guess the spin of the proton is obtained from the vector addition of the 3 quarks' individual spins, so it can't be 0

That is a reasonable guess, but most of the proton's angular momentum comes from sources other than the spin of the valance quarks.

  • $\begingroup$ But a single proton cannot have spin 0 (unpolarised) right? $\endgroup$
    – SuperCiocia
    Commented Feb 2, 2014 at 17:01
  • $\begingroup$ No, it has spin 1/2. It just isn't right to attribute that to the valance quarks except in the crudest models. $\endgroup$ Commented Feb 2, 2014 at 17:02

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