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STIC Bhabha events

To calculate the probability that an off-energy electron is recorded together with a physics event, the same basic selection of events as in the VSAT study was used, i.e., the events had to have one shower in each calorimeter with 2.5^o < $\theta$ < 8^o and 0.97 < E_e/E_beam < 1.05. The angle between the two showers had to be larger than 179.85^o. A third shower was required in the event and this shower has to be separated by at least 45^o in azimuth from the closest shower. The angular requirements meant that radiative Bhabha events with a photon energy of at most $\sim$8 GeV could survive the selection; the final sample of Bhahbas + an off-energy electron was selected by requiring the third shower to have an energy larger than 10 GeV.

Figure 13: Left: Energy distribution of the third shower in the selected STIC events. The unshaded histogram is 1998 data and the shaded histogram 1999 data. Right: The probability that an off-energy electron shower will be found in STIC as a function of an energy cut.

The energy distribution of the third shower in the selected events are shown in Figure 13 for both 1998 and 1999 high energy data. The probability of an off-energy electron in STIC as a function of an energy-cut is also shown in Figure 13. The probabilities in 1998 and 1999 are very similar.

The most effective way of removing showers from off-energy electrons is not by an energy-cut but by a cut on the polar angle. By requiring $\theta$ > 3^o, most of the background in the horizontal plane is rejected. The probability of having an electron after this $\theta$-cut is given in Table 7 and Figure 13 and it can be seen that the cut reduces the background by at least one order of magnitude.

Table: 7 The probability of an off-energy electron with energy higher than E_min in STIC. The measurement was done with STIC Bhabha events.

	189 GeV		192-196 GeV
Emin [GeV]	$\cal {P}$[%]	$\cal {P}$$\theta$ > 3o[%]	$\cal {P}$[%]	$\cal {P}$$\theta$ > 3o[%]
10	0.449$\pm$0.013	0.0425$\pm$0.0039	0.434$\pm$0.021	0.0257$\pm$0.0051
15	0.360$\pm$0.011	0.0341$\pm$0.0035	0.334$\pm$0.019	0.0195$\pm$0.0045
20	0.318$\pm$0.011	0.0295$\pm$0.0032	0.292$\pm$0.017	0.0174$\pm$0.0042
25	0.292$\pm$0.010	0.0249$\pm$0.0030	0.270$\pm$0.017	0.0133$\pm$0.0037
30	0.267$\pm$0.010	0.0193$\pm$0.0026	0.246$\pm$0.016	0.0092$\pm$0.0031
40	0.201$\pm$0.008	0.0081$\pm$0.0017	0.198$\pm$0.014	0.0072$\pm$0.0027
50	0.110$\pm$0.006	0.0025$\pm$0.0009	0.142$\pm$0.012	0.0021$\pm$0.0015
60	0.025$\pm$0.003	-	0.040$\pm$0.006	-

One surprising observation is that more than a third of the events (36% in 1998D and 43% in 1999B) are accompanied by charged tracks. This is in contrast to Bhabha events without off-energy electrons, where less than 1% of the events are accompanied by charged tracks. Most of the tracks in the off-energy electron sample survive the standard cuts on impact parameters and momentum error and are not concentrated in the forward region. They are, however, short (average length = 27 cm) and have a low momentum (average p = 0.4 GeV). A study of the detectors used in the reconstruction of the tracks (Figure 14) showed that most of the tracks are seen in the VD only.

Figure 14: The different detectors used in track reconstruction for all tracks (left) and tracks with only one detector used in the reconstruction (right). 1999B data was used in this study.