4.10 Performance and Validation
4.10.3 Cosmics Validation
In the beginning of the ATLAS cosmic data-taking, the HLT was run in a transparent mode, i.e.
no events were rejected in the HLT, although algorithms were running. This has been used to intensively study and debug the full HLT system.
Starting in September 2008, the HLT actively joined the on-line cosmic event selection: LVL2 algorithms scanned the inner detector (ID) to enrich a physics data stream with ID muon tracks.
Fig. 4.15 shows an ATLAS event display of a cosmic ray with hits in the barrel ID, seen and triggered by LVL2 (run90127). The solenoid magnetic field was off in this run.
The use of LVL2 cosmic trackers has been rendered possible by the validation work of an ATLAS task-force which studied and optimised the LVL2 trackers’ efficiencies and fake rates with respect to off-line reconstructed tracks. In the following a brief overview is given.
10From Eq. (4.1), we get1/FPTL2 = (NPTpassed L2−NPSpassed L2)/(Nactive L2−NPSpassed L2).
4.10.3 Cosmics Validation 63
Figure 4.15: ATLAS event display of a cosmic ray with hits in the barrel inner detector, seen and triggered by LVL2 (run90127). The solenoid magnetic field was off.
[GeV]
Offline track pT
0 20 40 60 80 100
L2 event trigger efficiency
0.5 0.6 0.7 0.8 0.9 1
1.1 ATLAS preliminary
Any L2 ID track SiTrack IDScan TRT
offline track d0 [mm]
-400 -300 -200 -100 0 100 200 300 400
L2 event trigger efficiency
0.5 0.6 0.7 0.8 0.9 1
1.1 ATLAS preliminary
Any L2 ID track SiTrack IDScan TRT
Figure 4.16: LVL2 reconstruction efficiency for cosmic ray tracks with respect to off-line reconstruction, as a function of the track transverse momentum pT (left) and of the track impact parameter d0 (right).
Different symbols indicate different LVL2 algorithms as shown in the legend.
LVL1 muon triggers in the barrel and end-caps have a high rate for cosmic muons. The fraction that contains an ID track, however, is low.11 HLT (LVL2) tracking was introduced to exercise the HLT system with the goal of increasing the rate of ID tracks.
Three tracking algorithms are available at LVL2: TrigTRTSegFinder, SiTrack, and IDSCAN. As the names suggest, the first one searches for tracks in the TRT, and the latter two scan the Pixel and SCT detectors respectively for tracks. All three algorithms have been studied in cosmic runs with and without the solenoid magnetic field.
The main aspects covered are:
• Efficiencies: The efficiencies of the three LVL2 tracking algorithms were studied stand-alone as well as in combined mode (running all algorithms, and trigger on the logical-OR).
The efficiency was defined with respect to off-line reconstructed tracks and for various track selections (standard off-line definitions).
• Fake rate: The rate of fake tracks was estimated for the tracking algorithms from a random trigger stream.
• Monte Carlo comparison: preliminary studies compared the data distribution of track pa-rameters with the MC expectations.
• Correlations between the LVL2 ID tracks and LVL2 stand-alone muons were tested and quantified.
As part of these studies, the HLT algorithms’ parameters have been tuned for cosmic data (includ-ing tim(includ-ing, thresholds etc.).
Fig. 4.16shows the obtained LVL2 efficiencies for golden silicon tracks12 as a function of the off-line track transverse momentum and depending on the track impact parameter. Errors were
11Simulations of cosmic data predict a rate of approximately0.5Hz of tracks going through the Pixel detector. The LVL1 muon triggers provided a rate of about0.03Hz due to a relatively low efficiency for tracks going through the ID.
12Golden silicon tracks are defined to contain at least 3 silicon space points in the upper and 3 in the lower ID.
4.10.3 Cosmics Validation 65
Tansverse momentum [GeV]
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Tracks
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103
LVL2 tracking (IDSCAN) HLT on−line monitoring, COSMIC run: 91900
φ
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Tracks
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2500 LVL2 tracking (IDSCAN)
HLT on−line monitoring, COSMIC run: 91900
Figure 4.17: LVL2 IDSCAN track distribution ofpT (left) andφ(right), obtained from cosmic run91900.
Note that the peak atpT '0is due to tracks for which the track-fitter failed (either because of very small or very high momentum).
estimated using the binomial error formulae (p
ε(1−ε)/N) and are therefore zero atε = 1. It was also shown that he LVL2 tracks are well correlated with LVL2 stand-alone muons inηandφ.
Following these findings, ATLAS activated LVL2 tracking in the cosmic runs and started to suc-cessfully select large statistics samples of ID muon tracks. Fig. 4.17shows the pT (left) andφ distribution of tracks found by IDSCAN. Both plots are obtained from HLT on-line monitoring of the cosmic run91900.
The Steering, at the heart of the HLT system, contributed to this first effective HLT on-line event selection of cosmic data. It’s design and implementation were found to be satisfactory: no sub-stantial design flaws have been spotted, no major changes were requested.
Together with the full HLT system, the Steering’s flexibility enabled the employment of the LVL2 cosmic track finder algorithms whose setup differs in some ways from the nominal HLT design.
It is noteworthy to point out these differences, in particular because it indicates which parts of the HLT and Steering were used and hence validated, and which were not.
In terms of the Steering design, the main characteristics of the LVL2 cosmic tracking setup are as follows.
No trigger chain seeding: the cosmic tracking LVL2 trigger chains start from any LVL1 accepted event.13 By default, one LVL2 (EF) trigger chain is only activated if the single preceding LVL1 (LVL2) trigger chain (item) fired.
No ROI mechanism: the LVL2 cosmic tracking algorithms do not start from any LVL1 ROI. In other words, the full ID scan is performed no matter what ROIs are present.14
This shows that the Steering is compliant with the demanding requirements of commissioning and cosmic running. Concurrently, however, it also shows that the LVL1–LVL2 interface, i.e. trigger chain seeding and algorithm seeding through ROIs, is not fully validated yet with real data.
HLT validation studies with cosmic ray data continue. A full validation of the ATLAS trigger
13an HLT chain can be configured without a preceding trigger (lower chain).
14the AllTEAlgo (cf. Section4.5) runs once per event no matter how many input ROIs (TEs) are present.
system, however, will only be possible with LHC collision data.