5 - Spin of the Proton

The proton has an intrinsic spin; classically one could think of the analogy of a rotating top. How can we understand this spin in terms of the quarks inside the proton? The simplest idea is to trace the proton spin directly back to the spin of the quarks (the quarks also have a spin; this is a fundamental and well understood property of quarks).

Naive model of the proton spin. According to this model, the proton spin is the result of the spins of the three quarks.

However, measurements at SLAC and at CERN showed that this naive picture is wrong. The spin of the proton must have a more complicated explanation. The HERMES experiment at HERA is designed to find out what really produces the spin of the proton.

Interior of the HERMES detector.

In the HERMES experiment, polarized electrons, i.e. electrons whose spins all point in one direction, are collided with polarized protons. The details of the observed reactions allows one to draw conclusions on the mechanism responsible for the spin of the proton. An important question is, how the different quark species (up, down, sea) contribute. This question has been answered by the HERMES experiment. The figure shows this important result.

Polarization of u(up) quarks, d(down) quarks and sea quarks as a function of x, the momentum fraction of quarks inside the proton (HERMES, Phys.Lett. B464 (1999) 123)

Previous | Overview | Next