Background from Neutral Current Events

The occurance of neutral current events in the sample can be demonstrated by the $({{E_{\mbox{\em tot}}}\, - \, {P_z}})$-distribution in figure 5.20. The entries in the peak around correspond to neutral current events while the peak at low $({{E_{\mbox{\em tot}}}\, - \, {P_z}})$ corresponds to charged current and photo-production events.

Neutral current events can fake large for several reasons:

• High momentum particles of the proton remnant jet can escape through the beam pipe hole, carrying substantial ${P_t}$. Most of the NC-background events are in fact of this type and they cause the peak around in figure 5.20. For these events $({{E_{\mbox{\em tot}}}\, - \, {P_z}})$ is well measured because the escaped particle carried only little $({{E_{\mbox{\em tot}}}\, - \, {P_z}})$.
• The energy of the scattered positron might not be measured correctly because the positron enters the calorimeter in a crack between modules, because the scattered positron might hit an inactive region of the calorimeter (for example the chimney region in the RCAL) or because the scattered positron might hit the region between BCAL and RCAL or BCAL and FCAL.
• The struck quark jet energy might be measured inaccurately.
• Final state muon or neutrinos from the struck quark jet can escape the detector.

Neutral current events are removed from the sample by requiring that . Through this cut events are rejected and remain.

Figure: for the events which pass all the cuts so far for data (upper histogram) and Monte Carlo (lower histogram). The entries in the peak around correspond to neutral current events while the entries at lower correspond to charged current and photoproduction events.