longitudinally polarized proton collision (IMAGE)
Caption
Direct photons (γ) are emitted when a quark in one proton interacts with a gluon in the other proton. By comparing the number of direct photons emitted in collisions where proton spins are pointing in opposite directions (as shown) with the number emitted when one proton spin is flipped so both are pointing in the same direction, scientists can determine whether gluon spins are aligned with or against the proton spin direction.
Credit
Brookhaven National Laboratory
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