You collect coins, and you’re on the trail of a legend: According to rumor, a manufacturing defect led to one in every thousand 1939 nickels replacing Thomas Jefferson with a Sasquatch (also known as Bigfoot). But all of these weathered nickels now look about the same. How can you tell that you have found your elusive quarry?
Finding something new in particle physics is much the same. We frequently know roughly what a new particle might look like, but this “signature” is often similar to that of other particles. One of the best ways to aid our search is to paint extremely accurate pictures of known particles and then look for exceptions to that rule.
Heavy particles like dark matter candidates, the Higgs boson or particles predicted by supersymmetry share a common signature: They may decay into particles including a “vector boson,” V (a type of particle that transmits the weak force), and a “charmed meson,” D* (a particle made of two quarks, one of which is a charm quark).
CDF physicists performed a search for these V+D* events — the normal nickels — to make certain that our picture of them is accurate. Models of events such as these are known to be accurate at high energies; however, at lower energies, subtleties in the strong force that binds together fundamental particles become more important, and the models may break down.
This study was the first to test V+D* production at lower energies in hadron collisions. The V particle is either the W boson or the Z boson. The full Tevatron Run II data sample was used (9.7 inverse femtobarns).
The figure shows the data when the V particle is the W particle. The experiment measured 634 ± 39 such events. The W particle is found by looking for an energetic lepton (a muon or an electron) and missing transverse energy (neutrino). The D* particle is observed from its decay into the D0 particle and a low-energy pion. The D0 decays into a negative kaon and a positive pion.
Several sources of systematic uncertainty cancel in calculating the ratio of the decay probabilities for these two processes. We found that V+D* production behaves just as predicted. Providing such a stringent test of these models widens the net that we can cast in future studies. This, in turn, betters our chances of fishing out something new and exciting, perhaps previously undiscovered particles or particle decays.