SMC: How do quarks spin?

In the Spin Muon Collaboration (SMC), 150 physicists are working in an experiment at CERN to determine the spin direction of quarks in protons and neutrons.

In the front the polarised target set-up is visible with the detectors behind it.
All the electronic equipment is in the yellow huts to the right.

Spin is the quantity of rotation of a particle around its axis. A particle rotating to the left has an upward spin, a particle rotating to the right a downward. Electric charge on such a spinning top causes a current and produces a magnetic dipole. Due to this effect, charged particles with spin behave like compass needles in a magnetic field. The amount of spin direction we call polarisation.

Protons and deuterons (nuclei which are made up from a proton and a neutron) behave like very weak dipole magnets; they have spin. By using different microwave, superconducting and cryogenic techniques, the SMC experiment has succeeded in polarising large quantities of protons and deuterons. During the measurements we looked at the quarks in the protons and deuterons. The question we wanted answered was: Do quark spins have a preferential direction in respect to the spin of the proton or neutron in which they are locked? (figure 1)

To investigate this, muons were shot at targets of polarised protons and deuterons. Muons originate together with neutrinos in the decay of short lived particles which are made with CERN's particle accelerators. Thanks to a mysterious property of neutrinos -which always have left hand spin- the muons are polarised. This polarisation plays a role in the collisions with quarks; chances of collision are higher when a quark and a muon have opposing spin.

Figure 2. The target material is surrounded by a solenoid and a dipole coil (a). When the current through these coils change as shown (b), the direction of the magnetic field is reversed (c). The magnetic dipoles (polarised protons and deuterons follow as indicated by the compass needles (d).

We have investigated if there is a difference in collision chances when we reverse the polarisation direction of the target (figure 2). Such a difference, expressed in the asymmetry A, points at a preferential direction in quark spin in the proton and the deuteron. After the collision, the muons were deflected in a magnetic field and their tracks detected in wire chambers (figure 3). It can be established from the reconstruction of the muons tracks whether the quark with which a muon collided represents a proportionally large or a small amount of mass and impulse of the proton (or neutron). This ratio we call the impulse fraction x of the quark.

The results in figure 4a show we found an asymmetry for the proton: quarks with large x are polarised in the proton spin direction. For quarks with small x this preference disappears. For the deuteron (figure 4b) the asymmetry is smaller because the deutron also contains a neutron in which the roles of quarks with 2/3 and -1/3 charge are reversed. In the proton quarks with 2/3 charge are polarised in the proton spin direction, in the neutron they point at the opposite direction.

Figure 4. The experimentally determined asymmetry of the change of collision with polarisation of beam and target and as a function of the impulse fraction of the quark. The asymmtery is zero by a small x. When x increases, so does the asymmetry. The curve is adjusted for the proton data points. The deuteron data points are clearly below this curve.

When we add up all the quark spin contributions for all values of x, we only find half the value which was predicted by theory 20 years ago. This lack of polarisation has now been irrefutably established in the SMC experiment, and also in experiments at SLAC (Stanford) and DESY (Hamburg). The next step is to research gluons, the carrier particles of the strong nuclear force. At this moment it is not clear how large the spin preference of gluons is. If they are strongly polarised, this would explain the lack of quark polarisation in a natural way. Future experiments will tell us more.

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