Knocking on Heaven's Door - Lisa Randall [149]
Supersymmetric particle decays are distinctive from an experimental vantage point in that the lightest of the neutral supersymmetric particles will remain, even after the decay is complete. Cosmological constraints tell us that the LSP carries no charges, so it won’t interact with any elements of the detector. This means that whenever a supersymmetric particle is produced and decays, momentum and energy will appear to be lost. The LSP will disappear from the detector and carry away momentum and energy to where it can’t be recorded, leaving as its signature missing energy. Missing energy isn’t specific to supersymmetry alone, but since we already know a good deal about the supersymmetric spectrum, we know both what we should and shouldn’t see.
For example, suppose a squark, the supersymmetric partner of a quark, is produced. Which particles it can decay into will depend on which of the particles are lighter. One possible mode of decay will always be a squark turning into a quark and the lightest supersymmetric particle. (See Figure 59.) Recall that because decays can occur essentially immediately, the detector records only the decay products. If such a squark decay occurred, detectors would record the passage of the quark in the tracker and in the hadronic calorimeter that measures energy deposited by a strongly interacting particle. But the experiment will also measure that energy and momentum are missing. Experimenters should be able to tell that momentum is missing in the same way they can when neutrinos are produced. They would measure momentum perpendicular to the beam and find that it doesn’t add to zero. One of the biggest challenges the experimenters face will be to unambiguously identify this missing momentum. After all, anything that is not detected appears to be missing. If something is wrong or mismeasured and even small amounts of energy go undetected, the missing momentum could add up to mimic an escaping supersymmetric particle’s signal, even though nothing exotic was produced.
[ FIGURE 59 ] A squark can decay into a quark and the lightest super-symmetric particle.
In fact, because the squark is never created on its own, but only in conjunction with another strongly interacting object (such as another squark or an antisquark), the experimenters will measure at least two jets (see Figure 60 for an example). If two squarks are created by a proton collision, they would give rise to two quarks that detectors would record. The net missing energy and momentum would escape undetected, but their absence would be noted and provide evidence for new particles.
[ FIGURE 60 ] The LHC might produce two squarks together, both of which decay into quarks and LSPs, leaving a missing energy signature.
One major advantage of all the delays in the LHC schedule was that experimenters had time to fully understand their detectors. They calibrated them so that measurements were very precise from the day the machine went on line, so missing energy measurements should be robust. Theorists, on the other hand, had time to think about alternative search strategies for supersymmetric and other models. For example, together with a theorist from Williams College, Dave Tucker-Smith, I found a different—but related—way to search for the squark decay just described. Our method relies on measuring only the momentum and energy of the quarks emerging from the event, with no need to explicitly measure missing momentum, which can be tricky. The great thing about the recent LHC excitement was that a number of CMS experimenters immediately ran with the idea and not only showed that it worked, but generalized and improved it within a few months. It’s now part of the standard supersymmetry