Muon Trigger Level 1
Overview of the Level 1 Muon Trigger
There are six Level 1 muon triggers; MU06, MU08, MU10, MU11, MU20, MU40. Each trigger designed to be 90% efficient for a muon of pT equal to its label. For example, MU06 is 90% efficient for 6 GeV muons.
In version 12 of the analysis code, the Level 1 muon triggers are exclusive which means that each muon fires only one trigger. MU06 only accepts muons between 6 and 8 GeV, MU08 accepts events between 8 and 10 GeV, and so on. In version 13 of the analysis code, the muon triggers will be inclusive.
In the TopView ntuples, which were used in the following studies, each trigger decision is represented by a single integer flag for each event. Since an event can have more than one muon, each of which can fire a different muon trigger, it is possible that a single event has more than one muon trigger fired.
Events with no Truth muons which fire the muon trigger
It is possible that events with with no muons at the truth level can still fire the Level 1 muon trigger. The phenomenon was first observed while using the Truth0 tree in the TopView ntuples. The Truth0 tree includes all muons from W decay but only muons with pT > 5 GeV originating from other sources. The TruthAll0 tree, which includes all muons, was then run over and the same effect was observed. The number of events with no Truth and TruthAll muons still passing a muon trigger are shown below. The sample analyzed contained 56,250 ttbar (5200) events. The percentages shown in parenthesis represent the number of events passing a certain triger with no Truth or TruthAll muons with respect to all events passing that trigger.
It can be seen that the percentage of events with no Truth and TruthAll muons passing a trigger decreases as the muon trigger thresholds are raised. For example, 23.6% of the 407 events passing MU06 have no Truth muons while only 0.3% of the 17017 events passing MU40 have 0 Truth muons. The decrease is more dramatic for events passing the trigger with no Truth muons than no TruthAll muons because the sample with no Truth muons does not include the muons with pT < 5 GeV coming from sources other than W decay. These low pT events are much more likely to pass the triggers with low thresholds (it is possible for a muon with pT < 5 GeV
to pass the trigger with thresholds of 6 GeV and higher but this becomes less and less probable the higher the thresold).
It was necessary to investigate events passing a muon trigger having no Truth muons but having TruthAll muons in order to ensure that these muons, which are in not in the Truth0 tree but are in the TruthAll0 tree, do indeed have a pT < 5 Gev.
The plot below on the right shows the multiplicity distribution of TruthAll muons for events passing MU06 and having no Truth muons. There are 407 entries in this histogram, corresponding to the 407 in the first row of the table above. It is also noticable in this plot that a162 events (first bin of the histogram) also have no TruthAll muons. The plot on the right shows the pT distribution of TruthAll muons for events passing MU06 and having no Truth muons. All muons have a pT less that 5 GeV which confirms the prediction that all of these muons originate from sources other than W decay and are not stored in the Truth0 tree because their energy is too low.
Events with no TruthAll muons which fire the muon trigger
We now want to investigate those 162 events in the first bin of the multiplicity histogram shown above, that is, those events which have no TruthAll muons but still pass the muon trigger. It is possible that these muons are produced from jets and therefore it is illustrative to look at jet distributions. Below is shown, from right to left, jet multiplicity, jet pT, and jet eta. In each plot, the black points represent jets in events which pass the MU06 trigger but have no TruthAll muons and the red line is a (normalized) distribution of jets without any cuts applied. It can be seen that the multiplicity distrubtion with no TruthAll muons is shifted slightly to the right compared to the raw distribution. It also has slightly less events at low pT and slightly more events at hight pT. There are also more events with no TruthAll muons at low eta and muons from some of these jets may be responsible for firing of the triggers. There is a 10-15 cm hole around eta = 0 in the muon system which can give a partial explanation of this behavior but the excess seen in the eta distribution extends across a larger eta region than that.
The Level 1 muon trigger is designed to make decisions based upon muons originating at the vertex. Muons from jets interacting with the calorimeter and muons from in-flight particle decays may be responsible for events passing the muon triggers yet having no TruthAll muon. However, there is no way to know for sure because the rate of occurance of these behaviors in the Level 1 muon trigger is not known. In addition, these events may be due to a software bug.
Single Top Trigger Efficiencies
Trigger efficiencies were calculated at both the truth and reconstructed levels. For the truth efficiencies, no selection cuts were applied and the efficiency was simply calculated by taking the ratio of the number of events before and after the trigger. For the reconstructed efficiencies, the common single top event selection was applied before applying the trigger and the efficiency was calculated from the ration of number of events after the trigger to the number of events after the event selection but before the trigger.
The common Single Top event selection cuts are:
- Lepton Selection
- At least 1 high-pT lepton (e or μ)
- Electron pT > 25 GeV
- Muon pT > 20 GeV
- No secondary lepton (e or μ) with both
- pT > 10 GeV
- -1.475 < η < 1.475
- Jet Selection
- NJet ≥ 2, where the Jet 1 has pT > 30 GeV and Jet 2 has pT > 25 GeV
- NB-Jet ≥ 1; where the 1 B-jet has pT > 35 GeV
- Other Selection Criteria
The exclusive muon slots were combined using a logical OR. For example, the efficiency for MU11 was calculated with the following formula: (MU11 || MU20 || MU40) / (Total Number of Events). It is correct to add the triggers such as (MU11 + MU20 + MU40)/(Total Number of Events) because this leads to double counting since it's possible that a single event may have more than one muon trigger fired.
The sample of events analyzed for this study were procssed using TopView
version 1212 and consisted of the MuidTauRec
Reproduction of ttbar (5200) Results
The results of the ttbar group's Level 1 muon trigger efficiencies were reproduced to ensure the accuracy of this study. In the plot below, the black points are the truth efficiencies from the current study, the blue squares are the truth efficiencies from the ttbar group's studies, and the red squares are the reconstructed efficiencies from the current study. It can be seen that the current study agrees almost exactly with the ttbar group's study and that there is about a 20% difference between the truth and reconstructed efficiencies.
Wt (5500) Efficiencies
The Wt Single Top efficiencies are shown in the plot below. The black points show the truth efficiencies and the red squares show the reconstructed efficiencies. Both efficiencies have the same shape but the reconstructed efficiencies are about 15% higher than the truth efficiencies. The reconstructed efficiencies are lower than the 90% naively expected. Also, there is only about an 8% drop in efficiency when moving from MU06 to MU40, which is not very significant. This channel has the highest efficiency of all three single top channels.
s-channel (5501) Efficiencies
The s-channel efficiencies are shown below. Once again, the truth and reconstructed efficiencies have the same shapes but in this case, the reconstructed efficiency is about 25% higher than the truth efficiency. There is about a 10% efficiency drop moving from MU06 to MU40 and this channel has the lowest efficiency of all three single top channels.
t-channel (5501) Efficiencies
The t-channel efficiencies are shown below. The truth and reconstructed efficiencies have the same shapes but in this case, the reconstructed efficiency is about 20% higher than the truth efficiency. There is about a 10% efficiency drop moving from MU06 to MU40.
- 10 May 2007