Cutting#
Typical cuts#
In papers from the BESIII Collaboration, you will usually encounter the following cuts. They are also listed here (requires login).
Charged tracks#
Distance of closest approach of the track to the interaction (IP) in \(xy\) plane: \(\left|\text{d}r\right| < 1\text{ cm}\).
Distance of closest approach of the track to the IP in \(z\) direction: \(\left|\text{d}z\right| < 10\text{ cm}\).
Polar angle: \(\left|\cos\theta\right| < 0.93\).
PID: usually making use of MDC and TOF and using a probability of at least \(0.001\).
Sometimes: events with non-zero net charge are rejected.
Neutral tracks#
Neutral tracks are reconstructed from electromagnetic showers in the EMC, which consists of a barrel and an end cap.
Energy for barrel showers |
\(\cos\theta < 0.8\) |
\(E > 25\text{ MeV}\) |
|---|---|---|
Energy for end cap showers |
\(0.86 < \cos\theta < 0.93\) |
\(E > 50\text{ MeV}\) |
If there is more than one charged track, there is a time requirement of \(0 \leq T \leq 14\) (\(50\text{ ns}\)).
Kinematic fits#
\(\chi^2\) of the kinematic fit is often determined in the final event selection with a efficiency scan using a Figure-Of-Merit. To limit the amount of events stored, a cut-off value of \(\chi^2 < 200\) is usually used.
Cut flow#
Cut flow is usually represented in the form of a table that lists the cuts and the corresponding number of events that passed the cut. This gives you insight in how much signal remains after your cuts, but also gives some idea of efficiencies if you make a cut flow table for an exclusive Monte Carlo sample.
A typical example would be (with some thought up numbers):
Total number of events |
\(100,000\) |
\(100\%\) |
|---|---|---|
Number of events with \(m\) number of charged tracks |
\(53,234\) |
\(53\%\) |
Number of events with at least \(n\) neutral tracks |
\(43,156\) |
\(43\%\) |
Number of events with exactly the final state particles |
\(20,543\) |
\(21\%\) |
Number of events with \(\chi^2\) for the kinematic fit |
\(18,163\) |
\(18\%\) |
Number of events that passed reconstructed mass cut |
\(15,045\) |
\(15\%\) |