Knocking on Heaven's Door - Lisa Randall [122]
Momentum (which is velocity times mass when particles move slowly but is more like energy moving in a particular direction when the particle travels near the speed of light) is conserved in all directions. As with energy, we have never found any evidence that momentum can be lost. So if the momentum of the particles measured in the detector is less than the momentum that went in, some other particle (or particles) must have escaped, carrying away the missing momentum in the process. This type of logic led Pauli to deduce the existence of neutrinos in the first place (in his case in nuclear beta decay), and to this day it’s how we learn of the existence of weakly interacting particles that seem to be invisible.
At hadron colliders, experimenters measure all the momentum transverse to the beam and calculate if something is missing. They focus on momentum transverse to the beam since a lot of momentum is carried away by particles that head down the beam pipe and is therefore too difficult to keep track of. The momentum perpendicular to the initial protons is much simpler to measure and account for.
Since the initial collision has essentially zero total momentum transverse to the beam, so too should the final state. So if measurements don’t agree with expectations, experimenters can “detect” that something is missing. The only remaining question is how to distinguish which of the many potential noninteracting particles it was. For Standard Model processes, we know neutrinos will be among the undetected elements. Based on the neutrino’s known weak force interactions that we will get to shortly, physicists calculate and predict the rate at which neutrinos should be produced. In addition, physicists already know what the decay of a W boson should look like—for example, an isolated electron or muon whose transverse momentum carries energy comparable to half the W mass is fairly unique. So using momentum conservation and theoretical input, neutrinos can be “found.” Clearly, there are fewer identifying tags on these particles than ones we see directly. Only a combination of theoretical considerations and missing energy measurements can tell us what was there.
It’s important to keep such ideas in mind when we consider new discoveries. Similar considerations apply for other novel particles without any charges, or with charges so weak that they can’t be directly detected. Only a combination of missing energy and theoretical input can be used in those cases to deduce what was there. That’s why hermeticity—detecting as much momentum as possible—is so important.
FINDING HADRONS
We’ve now considered leptons (electrons, muons, taus, and their associated neutrinos). The remaining category of particles in the Standard Model have the name hadrons—particles that interact through the strong nuclear force. This category includes all particles made from quarks and gluons, such as protons and neutrons and other particles called pions. Hadrons have internal structure—they are bound states of quarks and gluons held together by the strong nuclear force.
However, the Standard Model doesn’t list the many possible bound states. It lists the more fundamental particles that get bound together into hadronic states—namely, the quarks and gluons. In addition to the up and down quarks that sit inside protons and neutrons, heavier quarks called charm and strange and top and bottom exist as well. As with the charged and neutral leptons, the heavier quarks have charges identical to their lighter counterparts—the up and down quarks. The heavier quarks are also not readily found in nature. Colliders are needed to study them too.
Hadrons (which interact via the strong force) look very different from leptons (which don’t) in particle collisions. That is primarily because quarks and gluons have such strong interactions