B mesons are short-lived and decay inside the beam pipe, which is about 2.5 cm (1 inch) in diameter. Physicists project the tracks seen in the detector elements outside the beampipe back to where the particles must have traveled inside the beam pipe.
We call a point where particles collide or decay a vertex. A way to identify a B meson is to look for two vertices with a gap between them.
On the left is a standard event picture. On the right is a blowup of what happens close to the collision point inside the beam pipe. The vertex is where the B meson along with other particles was created and the secondary vertex is where it decayed.
The solid green lines are the actual tracks of the decay particles outside the beam pipe. The dotted lines are the projection of the tracks into the beam pipe. Where they intersect are the vertices. The B travels between the first vertex and the secondary vertex along the black dotted line before it decays. Thus, the gap between the two vertices is a measure of the lifetime of the B meson. We will be looking for this decay length in our data. We will find a minimum or "threshold" value that will tell us to save events for further analysis.
Assignments: Identifying B