Some ideas on where the missing spin might be

Last time, we saw an overview of the proton spin crisis (whose historical basis is discussed by Jaffe): Each proton has a spin value of 1/2, but only about 25% of this value comes from its three constituent quarks. So, physicists need to look elsewhere for the building blocks of the other 75% of this spin value.

(Side note: This quest isn't entirely unlike the quest to find the missing matter and energy in the universe!)

Recent experiments have probed two possible sources: gluons and pair production.

Gluons. Remember how a proton is composed of three quarks (two up and one down)? Well, those quarks are held together by the strong force, which is mediated by gluons (similarly to how the electromagnetic force is mediated by photons). Therefore, a proton is really three quarks held together by a sea of gluons. Gluons have a spin value of 1, so it's possible for them to be oriented in such a way as to contribute to the proton's spin. Experiments at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory lend support to this idea.

Anti-quarks. It's also possible to temporarily break up a gluon into a quark and anti-quark. So, a proton's sea of gluons features temporary "flashes" of quark-antiquark pairs. So, these pairs might also be contributing to the proton's spin. However, other experiments at RHIC exploring this pair production showed that their spin contributes very little to the proton spin.

The Proton Spin Crisis - What protons are made of

You may not be aware of it, but there is a crisis going on in physics! Here's the brief summary:

• Each proton has a spin value of 1/2.
• Protons are made of three quarks: two ups and one down.
• The proton's spin value should, therefore, be the sum of the spin values of its quarks: For example, 1/2 + (-1/2) + 1/2 = 1/2.
• When you measure the spin of the quarks inside a proton, you don't come up with the correct total!

You can see why we call it a crisis! (Well, maybe "puzzle" is a bit less alarming and more intriguing. But, this is the internet...) This week, we're going to look at the different pieces to this puzzle, and bring you up to date on some of the latest insights into the problem.

First up: How do we know that the proton is made of these three quarks? We can't see a proton, and the nature of the strong force is such that the quarks don't exist independently!

APS's Physics provides an excellent overview of the original particle collision experiments that first revealed the proton's internal structure. The discovery required a detailed back-and-forth conversation between experimental and theoretical physicists and various mathematical models, ultimately winning the researchers the 1990 Nobel Prize in Physics.