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Dalitz decay of $\pi ^{0}$

The Dalitz decay of the $\pi ^{0}$ ( $\pi^0 \rightarrow e^{+}+e^{-}+\gamma$, BR=1.12%) is another noticeable source of low energy electrons in the data. Unlike photon conversions, Dalitz decays take place at the primary event vertex. This rules out explicit identification via invariant mass reconstruction: combinatorial problems arise due the high multiplicity at the primary vertex. Without explicit identification we can either rely on a full MC simulation, or try to correlate the number of Dalitz decays to other experimental observables using MC simulations. We choose to do the latter: given the simple kinematics of the $\pi ^0$ Dalitz decay it is straightforward to relate the distribution of Dalitz decay electrons to the $\pi ^{0}$ meson distribution. Although the $\pi ^{0}$ meson distribution itself is not directly measured in ZEUS, it is identical to the charged pion distribution if we assume that


\begin{displaymath}
N^{\pi^{+}}(p,\theta) = N^{\pi^{-}}(p,\theta) = N^{\pi^{0}}(p,\theta).
\end{displaymath} (3.9)

This assumption reduces the problem to measuring the momentum and angular distribution of charged pions. Pions are measured well by the CTD, but at momenta larger than 1 GeV we have no means to discriminate between pions and other charged hadrons. JETSET [47] simulations predict however than the ratio $N(\pi^{\pm})/N(h^{\pm})$ is relatively constant at 0.65 for $p_{T}>700$ MeV, except in the very forward region (figure 4.31). Summarising, we calculate the neutral pion distribution as

Figure 4.31: Pion-to-hadron ratio of reconstructed tracks in a RAPGAP MC event sample. The angular distribution (left) includes only tracks with $p_T>0.7$ GeV. The transverse momentum distribution includes only tracks in the angular range 0.65-2.50 rad.
\begin{figure}
\begin{center}
\epsfig {file=plots/fig_dalz_jset.ps,height=8cm}\end{center}\end{figure}


\begin{displaymath}
N_{\pi^0}(p,\theta) = r(\pi/h)_{JETSET} \cdot \eta_{CTD} \cdot N_{track}(p,\theta),
\end{displaymath} (3.10)

where $r(\pi/h)_{JETSET}$ is the charged pion fraction according to JETSET and $\eta_{CTD}$ is the tracking efficiency of the CTD (95%). The distribution is shown in figure 4.32(left). The neutral pion distribution is used to reweight a 50K sample of single $\pi^0 \rightarrow e^+ e^- \gamma$ MC events (figure 4.32 right). Application of the standard electron candidate selection yields the background of Dalitz decays in our sample. Figure 4.33 shows the estimated Dalitz contribution to the inclusive electron sample, which amounts to 6%.

Figure 4.32: Left: $p,\theta $ distribution of $\pi ^{0}$ mesons in the data, obtained from track multiplicity distributions. Right: distribution of electrons from $\pi ^{0}$ Dalitz decay The grey bands indicate the regions where insufficient Monte Carlo $\pi ^0$ events are available.
\begin{figure}
\begin{center}
\epsfig {file=plots/fig_dalz_count.ps,height=8cm}\end{center}\end{figure}

Figure 4.33: Contribution of $\pi ^{0}$ Dalitz decays to the inclusive electron sample.
\begin{figure}
\begin{center}
\epsfig {file=plots/fig_dalz_sig.ps,height=8cm}\end{center}\end{figure}


next up previous contents
Next: Miscellaneous rare decays into Up: Electron backgrounds Previous: Results   Contents
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1999-12-21