PhD Thesis Chapter VIII
March 3, 2011 Leave a comment
Heavy-Flavor Events in the Regional First Level Trigger
Prior to machine turn-on, uncertainties about the details of many types of events exist. The trigger must be able to achieve high acceptances combined with good beam-gas rejection independent of the details of the final event shape. To this end, it is useful to use different generators to examine the effects of theoretical uncertainties on the trigger efficiency. An important question also concerns the effect that gluon bremsstrahlung will have on measurements in the detector.
Although DIS events are a major aspect of HERA physics, it is necessary to ensure that other important reactions are not removed at the FLT: such a reaction is the generation of bbbar and ccbar pairs by boson-gluon fusion (BGF) at low Q2 and low x (see section 184.108.40.206).
The standard ZEUS Monte Carlo for boson-gluon fusion is HFLGEN 1.3 based on the AROMA generator,. Parton showers, string fragmentation and decays are carried out by JETSET. A second generator HARHEA, working within the framework of the HERWIG 5.0 Monte Carlo, also produces BGF events[84–86]. HARHEA differs from HFLGEN in using a cluster hadronization model and including gluon radiation from the initial state quarks.
A HERWIG ASCII interface was written for ZEUSGeant such that the data could be read by ZGANA. This enabled direct comparison of measured parameters in the CTDFLT and the FTDFLT.
One thousand NC ccbar and bbbar events were generated from each of the two heavy flavor generators. Also four thousand beam-gas events distributed homogeneously along the beam-line from z = -19m to z = +1m were produced using the FRITIOF generator (version 1.5).
An initial comparison of the two generators was achieved by using a parametrization of the CTD and calorimeter FLTs. This aims to provide a simple understanding of the likely response of the whole FLT to a set of events. Its philosophy is based on energy deposition and charged tracks. If tracks are found from the vertex, then only loose energy constraints are applied. On the other hand, if no tracks are found then substantial energy deposition (at high angles) is required.
In fact, if no track pointing to the vertex was found in the CTD, an event was accepted if the calorimeter registered more than 5 GeV/c in transverse momentum; if a vertex track was detected, an event was accepted if the transverse momentum was greater than 12 GeV/c. Section 220.127.116.11 explains how these quantities are measured by the CALFLT. Finally, the events were passed through the standalone CTDFLT and FTDFLT simulations and the RBOX simulation to examine the combined tracking response.
Table 8.1 shows the percentage of events passing the parametrization of the FLT for the four types of events. There is a small difference in the two BGF generators for ccbar events but a major difference is seen for bbbar events. In both cases, it is much easier to trigger on the bottom pair events.
Table 8.1: Percentage of events accepted by the simple parametrization of the tracking and calorimeter first level trigger.
Table 8.2 shows the percentage of ccbar events falling into each of the tracking trigger classes and table 8.3 shows the same figures for bbbar events. These figures may be compared with those for beam-gas leakage, shown in table 6.7. As before, for the CTD standalone mode class 3 events will probably be accepted along with class 2 events so these figures must be summed to produce a final figure.
Table 8.2: FLT classifications for the full FLT simulations for ccbar events.
Table 8.3: FLT classifications for the full FLT simulations for bbbar events.
The parametrization of the GFLT is dependent on track multiplicity and transverse energy deposition. The results obtained for GFLT efficiency are thus entirely explained by figure 8.2 and figure 8.3 which show that high acceptance is related to both high mean track multiplicity and high mean transverse energy. This may be clearly illustrated by plotting the means of the figures against the efficiencies.
This is done in figure 8.1. For comparison, figure 8.4 and figure 8.5 show the distribution of transverse energy and charged multiplicity for beam-gas events.
Figure 8.1: Effect of multiplicity and transverse energy on acceptance.
Figure 8.2: Multiplicity of charged tracks per event with a pt > 0.5 GeV/c for heavy flavor events.
Figure 8.3: Total transverse energy (GeV) per event as measured by the calorimeter for heavy flavor events.
Figure 8.4: Total transverse energy (GeV) per event as measured by the calorimeter for beam-gas events.
Figure 8.5: Multiplicity of charged tracks per event with a pt > 0.5 GeV/c for beam-gas events.
In the tracking detectors, a vertex decision is made in the triggers using essentially tracks with a transverse momentum > 0.5GeV/c. The tracking chamber triggers use the ratio of tracks from the vertex to all tracks. This ratio is therefore affected by changes in track multiplicity and transverse momentum.
The distributions in polar angle explain the event classes found. Figure 8.6 shows the polar angle of Geant tracks (tracks with energy of less than 1 GeV were omitted).
Figure 8.6: Polar angle of Geant tracks for both types of heavy flavor events in full and FTD-only angular ranges. The solid lines are HFLGEN events and the dashed lines are HERWIG events.
It can be seen that across the broad angular range, both generators are in good agreement with both giving higher multiplicities for bbbar events than for ccbar events. This explains the CTDFLT classes found, which showed both generators giving similar acceptances which were higher in the case of bbbar events. But in the FTDFLT, it can be seen that there is a significant deterioration in efficiency in HERWIG ccbar events which is not seen in bbbar events. In order to examine this more closely, figure 8.6 also shows the same plots magnified to show only the angular region covered by the FTD, 0.195 rad to 0.495 rad. It can clearly be seen that the event classes found are reflective of the observed multiplicities.
It has been shown that the effects of gluon bremsstrahlung may be neglected for bbbar events but become more significant in the case of ccbar. The combined FTD and CTD FLT acceptance is excellent for both bbbar and ccbar events with either generator. Higher multiplicities and higher transverse energy for bbbar events mean that they are more likely to pass the tracking trigger. The performance of the tracking triggers would not need to be optimized further in a dedicated sub-trigger. The simple parametrization of the combined calorimeter and tracking trigger indicates that a simple transverse energy cut by the calorimeter reduces bbbar acceptance by at least 10% but eliminates almost 60% of all ccbar events accepted by the tracking trigger alone. A dedicated sub-trigger would need to relax the transverse energy cut and restore beam-gas efficiency to reasonable levels by using information from other components such as collimators, the veto-wall and also timing data.