protons collide head to head or head to tail (IMAGE)
Caption
To investigate gluons' contribution to spin, scientists compared the number of direct photons emitted from two different types of longitudinally polarized proton-proton collisions: Ones where the proton spins were pointing in opposite directions (in this case, at each other, top) and ones where the two proton spins were pointing in the same direction, colliding head to tail (bottom). More direct photons emerged from the collisions of oppositely aligned protons, revealing that gluons' spins are indeed aligned in the same direction as the proton they are in.
Credit
Brookhaven National Laboratory
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