Charm cross section agrees with theory

This schematic diagram shows how a W and a charmed jet are produced and then decay.

The CDF experiment has observed for the first time collisions that produce a W boson and a charm quark jet.

Protons and antiprotons are made up of quarks and gluons. The CDF experiment observes rare processes in which a quark, usually a strange quark, and a gluon fuse together. The quark and gluon decay into a W boson and a single jet. The W boson is inferred from the observation of a subsequent decay into a highly energetic lepton (muon or electron) and missing transverse energy (neutrino). As to the single jet, here we are looking for a special type, called a
c-jet or charmed jet. The charmed jet decay chain sometimes includes the production of another lepton (see above figure), one of lower energy, that is inside the jet. Finding low-energy leptons inside a jet is complicated, but CDF has been doing this now for quite a long time – since the discovery of the top quark in 1995.

The experiment is further complicated by having large backgrounds. Similar to the signals we are looking for, the main background also consists of a W plus an ordinary jet. To help us distinguish the background W + jets from the signal W + jets, we use what we know about the signals’ electrical charges.

The electrical charges of the signal’s W and c-jet have opposite signs. For example, if the W has a positive charge (the charge of the high-energy lepton that decays from the W), the c-jet will have negative charge (the charge of the low-energy lepton located inside the jet). Since we seek events with opposite charge, we subtract all events of like charge, leaving only events from the (oppositely charged) signal.

In 2008, we reported seeing such W +
c-jet events, but this is the first measurement in which a signal is observed at a level that will convince everyone (a level larger than 5 sigma). The measured cross section for the process we observe (W plus a single charmed jet) is 13.6 +3.4/-3.1 picobarns and is in agreement with a value of 11.4 ± 1.3 picobarns calculated by John Campbell and Keith Ellis of the Fermilab Theory Department in 1999.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis. Top row from left: Lucio Cerrito (University College London), John Paul Chou (Harvard, now at Rutgers). Bottom row from left: Melissa Franklin (Harvard), Shulamit Moed (Fermilab).