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Cosmic muon events

The cosmic muon events, rejected in the previous analysis, can also be used to look for off-energy electrons in VSAT. The following cuts were made to select the events:

1. Two muons had to be found, with no other particles in the event. The muons should be identified as ``very loose'' or better.
   
2. The energy of each muon had to fulfil E_$\mu$ > 10 GeV.
   
3. Neither of the two muons should come from the primary vertex ( Q(LPV+4)$\neq$0 ).
   
4. It was required that |$\theta_{n}^{}$ + $\theta_{m}^{}$ - 180⁰| < 1⁰ and ||$\phi_{n}^{}$ - $\phi_{m}^{}$| - 180⁰| < 1⁰ where n and m can be a positive $\mu^{+}_{}$ or a negative $\mu^{-}_{}$ .

After this selection, 5396 events were found, 337 $\mu^{+}_{}$$\mu^{+}_{}$ pairs, 336 $\mu^{-}_{}$$\mu^{-}_{}$ and 4723 $\mu^{+}_{}$$\mu^{-}_{}$ events. Distributions of the total energy of the muon pair (E_tot) and of the azimuthal angle ( $\phi_{\mu }^{}$) and polar angle ( $\phi_{\mu }^{}$ ) of the individual muons are shown in Figure 9.

Of the 5396 events, there was one VSAT off-energy electron in 136 events and two off-energy electrons in 5 events. The probability of an electron in VSAT computed from these events is presented in Table 4.

Figure 10: Left: Energy distribution of the showers in the four VSAT modules. The dotted line shows the uncorrected and the full line the corrected spectrum. Right: The probability that an off-energy electron shower will be found in VSAT, as a function of an energy cut.