CPV handle

Before it was realized that major background in the $\eta \gamma$ sample comes from neutral $b_1$ decay, we tried to use the CPV as a handle to suppress the background. In Fig. 3 is shown the energy loss distribution in the CPV without any cuts (left) and with standard cuts: $N_{phot}=3$, $N_{pixel}=1$, $E_{\mbox{thresh}}>4.0$ GeV (right) for all 79 reactions. The minimum-ionization peak (MIP) is very pronounced in both cases. The second peak that comes from multiple tracks is more visible for the high energy threshold. The MIP, calculated from the Bethe-Bloch formula for the energy loss of charged $e^-$ in a 0.4 cm scintillator [4], $(dE/dx)_{min}=0.79$ MeV is very close to the one observed. Thus, requiring no hits in the CPV with energy above the MIP threshold, say 0.5 MeV, can veto charged particle reconstruction in the LGD. Fig. 4 shows the effects of this requirement on mass distributions in the case of the 4.0 GeV energy threshold. It should be noted that with the CP veto turned on, increasing the energy threshold does not play as significant a role as before. Although it is clear that the CPV cannot be used to distinguish the $b_1$ from the $\phi$ in $\eta \gamma$, it might be used to enrich the $\omega$ in $\pi ^0\gamma$ or the $b_1$ in $\pi ^0\pi ^0\gamma$ events.

Figure 3: The energy loss distribution in the CPV from all 79 reactions with no cuts (left) and with standard cuts: $N_{phot}=3$, $N_{pixel}=1$, $E_{\mbox{thresh}}>4.0$ GeV (right).

Figure 4: The invariant mass distributions after CP veto has been applied.