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.