Standing on tiptoes
An event in which a photon and a Z boson was created is shown with a single (red) energy deposition in the electromagnetic calorimeter and (yellow) missing energy, which indicates neutrinos from the Z boson decay.
Recent particle physics research studies the rare and difficult to find. With the low hanging fruit picked clean, the time has come to stand on our tiptoes.
DZero physicists have done just that and announced the results of a search for a very rare process. They searched for events in which a Z boson and a photon are produced without anything else observed. While events like this have been observed before, these scientists searched for a special case, specifically when the Z boson decayed into two neutrinos. In earlier studies, the Z boson decayed into electrons or muons. Because neutrinos can traverse many billions of miles of solid lead without interacting, they pass through the detector entirely undetected. Thus, the scientists were looking for collisions in which a single photon was observed and nothing else.
Events like these are clean and very rare. They are easily mimicked by a detector malfunction or a stray cosmic ray. It is only because of the amazing amount of beam delivered by the Tevatron that scientists had any chance to view enough of these events to be certain of what they were observing. The certainty is 99.99997 percent.
While simply seeing something never before observed at a hadron collider is a triumph, DZero physicists went further and used this information to look for even rarer types of physics: an unusually heavy "virtual" photon or Z boson decaying into a Z boson and a photon. The Standard Model predicts that this process, called trilinear gauge boson couplings, is not possible. If some new physical phenomena allowed for this kind of couplings, DZero physicists would have seen more events than expected. Since they observed the number of events predicted by the Standard Model, these scientists were able to set some of the most stringent limits on this unexpected process occurring. Sometimes, standing on tiptoes is all you need to do to reach the prize.
-- Don Lincoln
These physicists played a leading role in this analysis.
|DZero Trigger Board: A good trigger is crucial to any particle physics experiment. It decides what data is recorded and what is discarded. This board studies and sets the parameters that govern the current DZero suite of triggers.