BGV handle

The BGV is expected to help in distinguishing $b_1 \rightarrow 5\gamma$ decays reconstructed as $3\gamma$ from the real $\phi$ in $\eta \gamma$. In order to explore this possibility, it was decided for initial studies to use direct information from the MC generator rather than reconstructed hits in the BGV to avoid dependence upon the primitive BGV clustering algorithm in our current analysis package. Final-state photons with momentum $momF$ that have lab polar angle $30^0<\theta<90^0$ are classified as barrel photons. The distribution of total photon energy in the barrel from $b_1$ and $\phi$ channels are shown on Fig. 5 ($\eta \gamma$ events and $E_{\mbox{thresh}}>4.0$ GeV). Although energy distributions from the $b_1$ and the $\phi$ differ in shape and magnitude, it seems that at this point they cannot be used effectively to isolate the $\phi$. It is found that 1/3 of events from $b_1$ decays that pass the $\eta \gamma$ selection criteria do not produce photons in the barrel. In addition, half of events have just one barrel photon and the remaining 1/6 of events have two photons in the barrel.

Figure 5: The distribution of the total photon energy from the $b_1$ and $\phi$ in the barrel.

In order to see the effect of cutting on the energy in the BGV, in Fig. 6 (left) are shown mass distributions from the $b_1$ (black) and $\phi$ (red) that were shown in Fig 1. The blue histogram in this plot represents the $b_1$ contribution when total photon energy in the barrel is $E_{BGV}>50$ MeV. The right plot in the same figure shows mass distributions of the $b_1$ and $\phi$ with no photons in the barrel. According to this study, cutting on total barrel photon energy, or hard barrel photons above some threshold, does not clean the $b_1$ from under the $\phi$ peak. About 1/3 of events from the $b_1$ that were reconstructed as $\eta \gamma$ in the $\phi$ region have no photons in the barrel, and so cannot be eliminated by a BGV cut. This is to be compared with the factor of 2 reduction in the $\phi$ signal that is suffered using a barrel gamma veto (mainly from suppression of the $\phi(\eta\gamma)\Delta^+$ channel). The question arises, where is are the missing photons in $3\gamma$ reconstructions of $5\gamma$ $b_1$ decays?

Figure 6: Mass distributions dependence on total energy in the BGV.