10 - H1 - Desy

34 Theoretical predictions
rejection, its selection rate is of the order 10 −5 . In order to avoid processing of not
needed events, only events with an isolated neutral particle with transverse momentum
p T > 4 GeV are accepted for simulation already on the generator level. To avoid double
counting, events with a hard radiated photon are removed from the sample. Both requirements
remove 98% of all generated events. In the remaining sample, the final selection
rate for direct events amounts to ∼ 1.0%, while for resolved events it drops to ∼ 0.5%.
From the originally generated 0.8 × 10 9 direct background events and 1.5 × 10 9 resolved
background events only approximately 170×10 3 direct and 140×10 3 resolved events were
used for the discrimination process described in chapter 6.
The DIS background was studied using three different MC samples: prompt photon sample
produced in low Q 2 DIS events (MC set XVII), photons radiated from an electron line
in low Q 2 DIS events (MC set XVIII) and high Q 2 events (MC set XIX). The low Q 2 DIS
prompt photon events were studied in [43] and the same signal and background samples
were used in this analysis. DIS events may migrate into the selection if scattered electron
in not correctly identified, or in case of high Q 2 events, it is misidentified as photon.
The shower shape analysis, described in chapter 6, requires additional statistically independent
signal and background samples. Since only the LAr calorimeter signal initiated
by a single photon and photons coming from decayed isolated neutral hadrons is needed,
single particle (SP) MC was produced. SP photons (MC set XX) and SP hadrons (MC
set XXI-XXX) were generated flat both in transverse energy and pseudorapidity and
simulated for 2005 running period. Hadron samples were mixed according to the study
presented in [66]. Table 2.5 summarises all the SP MC samples.