"In proton collisions at the LHC, the composite nature of protons creates an additional challenge for particle bookkeeping. A proton is like a tiny bag of quarks and gluons. In any individual collision, the identities and energies of the particular colliding quarks or gluons are not known. While it is still possible to observe missing momentum, there is a fundamental gap in particle bookkeeping at any proton collider."
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