Long, long ago in a junior high school far, far away, Mrs. Haight asked her students, “How do the planets spin around their axes? Is it clockwise? Or counterclockwise?” To which my younger brother (he’s the smart one in the family) responded, “Well, are you looking at it from the top or from the bottom?”
To specify the direction of a spin, you need a reference direction in space. If the reference direction for Mrs. Haight’s question is assumed to be coming out the North Pole into space, then the answer is that Earth is spinning in what we call a right-handed or positive direction; Maine sees the sun rise before California. Fans of maps that put Australia on the top might enjoy taking their reference direction to be coming out the South Pole into space instead. If you are above the North Pole, looking down, that is the reference direction that you are going in. With that reference direction, the spin of planet Earth is negative, or left-handed, and it is clockwise from that direction.
Top quarks, like planet Earth, spin. When top pairs are created in the Tevatron, there are several reference directions that we can use to determine in which direction they are spinning. You might choose the direction of the incoming proton in the collision that made the pair of top quarks. You might choose the directions in which the top quark flies out of the collision point; or you might choose a direction perpendicular to both of these. For any particular quark the spin is either right-handed or left-handed; but the average spin might be anywhere from 100 percent one right-handed to 100 percent left-handed.
Recently, DZero measured the average direction of spins, using all three of these reference directions, in top pairs produced at the Tevatron using its full data set. These can also be measured at the LHC, but according to the Standard Model, the result should be different, because in the LHC, protons collide with protons, whereas in the Tevatron, protons collide with antiprotons.
The DZero results match the prediction of the Standard Model for the Tevatron very well, which improves our confidence that we understand the processes by which heaviest elementary particle, the top quark, is produced.