Impossible, except…

The Standard Model forbids direct production of a strange quark from the decay of a bottom. Indirect decay of the kind illustrated here is possible and is the focus of this analysis. Since observation of direct production of strange quarks from bottom quarks would indicate a discovery, we need to understand very well the indirect production described here.

Subatomic particles called quarks are some of the building blocks of our universe. The heaviest of the six types of quarks are the top and the bottom quark, and the bottom quark is where we begin our story.

Today’s result explores the decay of a particle called the Λb (pronounced lambda sub b), which contains an up, down and bottom quark, into a regular Λ (lambda) particle, which contains an up, down and strange quark. This decay requires the transmutation of the bottom quark to a strange one.

Historically, the quark type is called flavor, so when a quark decays from one kind to another, we call it flavor changing. If the bottom quark could decay this way, it would do so by emitting a neutral particle, since the bottom, strange and down quarks all have the same electric charge. Because of this, physicists call a possible decay of a bottom quark into a strange quark a flavor-changing neutral current or FCNC.

According to the Standard Model, FCNCs are impossible and to observe one would be a sign of new and interesting physics (and a trip to Stockholm). However, we need to be careful: It is only impossible for a bottom quark to directly decay into a strange quark. The Standard Model allows indirect decays of bottom quarks into strange ones (see above figure). This happens when a bottom quark turns into a charm quark and a W boson, followed by a subsequent decay of the W into a charm quark and a strange quark. This kind of indirect decay is the basis of today’s result. Given that observing direct FCNC would be the sign of a very surprising discovery, we need to understand very well the phenomenon of indirect FCNC so we can account for it in our calculations.

The precision of the measurements in this study are over three times better than all earlier measurements combined. This is a necessary achievement if we ever hope to convincingly observe direct flavor changing neutral currents.

Don Lincoln

These physicists from CINVESTAV in Mexico performed this analysis.
A crucial capability of a modern particle physics experiment is to accurately determine the path of charged particles as they cross the tracking detectors. These physicists form the nucleus of the current DZero tracking algorithm group. Left to right: Michael Cooke, Fermilab; Leah Welty-Rieger, Northwestern University; Mandy Rominsky, Fermilab; Amitabha Das, University of Arizona; Dookee Cho, Brown University; Herb Greenlee, Fermilab; and Guennadi Borissov, Lancaster University.