Z bosons more charming than expected

The first dedicated study of Z boson plus charm quark production shows that the Z boson has more charm than predicted.

Ask someone how often he’s seen a physicist with charm and you might get a wild response, but ask a physicist how often a Z boson is found with charm and—thanks to a new result from DZero—they can give you a quantitative answer.

Sometimes a Z boson, the neutral weak-force carrier, is produced in association with quarks. This process is important to understand because it is a background to studies of the Higgs boson. In particular, it is crucial to know how often the quark that accompanies the Z boson is a charm quark or a bottom quark since that can mimic the Higgs boson’s decay signature.

Interactions involving the strong force are difficult to predict directly in the Standard Model and are instead estimated from models that are constrained by data. While previous studies have measured how often a bottom quark accompanies a Z boson, this is the first study focused on Z boson-plus-charm quark (Z+c) production.

The difficulty in making this measurement lies in discriminating between the different flavors of quarks. When a quark is produced after a particle collision, strong-force interactions quickly begin to generate more quarks and gluons around it, creating a spray of particles called a jet. The jets initiated by charm quarks and bottom quarks may be separated by a few millimeters from the other particles made in a collision. By looking for this signature, DZero scientists were able to create a data sample enhanced with charm and bottom quarks. The analyzers then exploited the slight differences between the jets from charm quarks and those from bottom quarks in order to measure the number of Z+c events. By comparing the Z+c events to the total number of Z boson-plus-jet events or Z boson-plus-bottom quark events, the analyzers were able to reduce the uncertainty on their measured rate.

None of the tested models predicted a ratio of Z+c events to total Z boson-plus-jet events as high as was observed in the data, falling on average a factor of two-and-a-half too low. The enhancement is consistent, however, with that observed in recent measurements of photon-plus-charm quark production at the Tevatron. While finding out that Z bosons come with more charm than expected may not be pleasant news for the physicists studying the Higgs boson, it is important information that will feed into future models of Z boson-plus-quark production.

Mike Cooke

These physicists made major contributions to this analysis.
As chair of the Speakers’ Bureau at DZero, Horst Wahl, Florida State University, leads the efforts to arrange speakers to fulfill presentation requests from conference organizers and to archive public talks and proceedings.