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Seeing the Invisible - A Tale of Two Colliders

In particle physics, discovery often depends on meticulous bookkeeping. The fundamental forces in high energy collisions can do their work in a septillionth of a second, creating highly unstable new particles that decay almost immediately into many “daughter” particles. Computers write an elaborate record for each collision event, determining as completely as possible what particles went in, what particles came out, how fast and in what direction each particle was moving. Physicists then reconstruct the most likely explanation for what happened in the collision.

In some events, the numbers don’t add up, and the books don’t balance. For example, the total energy of all the particles produced may be less than the total energy of the original collision; this is a missing energy problem. Another example is a new heavy particle that moves off at right angles to the colliding beams, with nothing to balance it in the opposite direction; this is a missing momentum problem. Missing energy and momentum can be signals of missing particles: particles that interact too weakly for direct detection but that betray their existence by carrying off energy and momentum.

If dark matter particles are produced at colliders, they will pass through the detectors without a trace. To document their fleeting presence, physicists will look for signs of missing energy or momentum. By detecting the other particles produced in the same collisions, physicists can then infer the properties of the dark matter particles. These are the same techniques now employed to deduce the role of neutrinos in high energy collisions.

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.

In electron-positron collisions, though, experimenters know the identities, energies and momenta of the colliding particles, allowing for simple and complete particle bookkeeping and making a linear collider an incisive tool for identifying dark matter.