Sand fairies redux

The top figure shows the normal production and decay processes of top quarks. In the bottom figure, the magenta section shows a FCNC process. This type of event is forbidden in the Standard Model and, if it were observed, would require rewriting of the books.

In an earlier article, I compared the search for something that the Standard Model deems impossible to searching for tiny fairies in the sand on a beach. I also promised that this wouldn’t be the last time you heard about the idea. Today’s result describes another search for the phenomenon using markedly different techniques.

According to the Standard Model, it is exceedingly rare for a quark to change its identity (physicists say flavor) without also changing its charge. In fact, it is so rare that we do not expect to see this happen at the Tevatron. We have a name for this type of process. We call it Flavor Changing Neutral Currents, or FCNC, where the neutral in the title means no change in charge.

Recall that up, charm and top quarks all have the same electrical charge (2/3 that of a proton), while down, strange and bottom quarks all have an electrical charge that is -1/3 that of a proton. When a bottom quark decays, it can only decay into an up or charm quark. Similarly, a top quark could decay into a down, strange or bottom quark, but never an up or charm one.

For other reasons, top quarks decay essentially 100 percent of the time into a W boson and a bottom quark. This provides an ideal situation to search for FCNCs. DZero physicists looked for events in which two top quarks were produced; one of which decayed in the normal way, while the other one underwent FCNC decay and emitted a neutral Z boson and an up or charm quark. As you can see from the figure above, a normal top quark event resulted in two bottom quarks and two W bosons, while a FCNC event resulted in a W boson, a Z boson and two quarks. By selecting specific decay modes of the Z boson, physicists could reliably distinguish these two kinds of events.

The final result of this measurement was that the data is entirely consistent with the Standard Model. No evidence for FCNC was observed and physicists used the data to set the best limit so far achieved. Falling back on our extended, multi-article, metaphor, it’s back to the drawing board until we return with a better sieve.

— Don Lincoln

These physicists were responsible for this analysis.

DZero’s forward preshower detector helps identify electrons, which are W&Z boson decay products. These physicists are responsible for making this detector work.