# “Double-counting” in particle detectors

Apparently, when analysing events from particle detectors, one may incur in double-counting, which happens when

a physics object appears as a single object of its own type, but it may also be part of a jet)

(which I found on a CERN website).

I still don't really get it though, and I couldn't find anything online that properly explains it.

Can anyone shed some light on this, possible with a clear example? And how this issue could be dealt with?

• Anyone trying to answer your question would be helped if they knew (1) if you understand what a jet is and (2) how it appears in how they are identified and characterized in the data-stream. The answer really turns on the latter, but that is driven by the former. – dmckee --- ex-moderator kitten Jan 21 '15 at 19:55

Imagine that you store in your event the energies of $\pi^0$ and the energies of photons. For ex, in the first event you find:
$~~E_{\pi^0}=1.1~~$ $~~E_{\gamma_1}=0.5~~$ $~~E_{\gamma_2}=0.6~~$
$E_{tot} = E_{\pi^0} + E_{\gamma_1} + E_{\gamma_2}$
but if the pion decayed into these 2 photons $\pi^0 \to \gamma_1+\gamma_2$, you're going to count twice the energy of $\pi^0$! With this simple event you should only count $E_{tot} = E_{\pi^0} = E_{\gamma_1} + E_{\gamma_2}$. That was a simple example of double counting. This example is obvious but sometimes the double counting could be much more subtle.