The LHC and the ATLAS detector are impressive engineering accomplishments that were installed during the last decade at CERN. The LHC is currently carrying out an aggressive commissioning program, delivering an impressive amount of new data for each run.
The ATLAS detector comprises of different layers of sub-detectors that can mea-sure the properties of different elementary particles. ATLAS is currently fully oper-ational and recording the data that is being delivered by the LHC. At the time of writing (August 2011), ATLAS has recorded over 2 fb−1 of proton-proton collision data at √
s = 7 TeV. All sub-detectors as well as the Trigger-DAQ system are func-tional and meeting the design specifications. The maybe largest issue encountered so far is the dying OTx plug-ins for the LAr calorimeter, which cannot be replaced dur-ing the run period and is affectdur-ing the acceptance of the calorimeter. The SCT and Pixel detectors are subject to a similar problem with their TXs. However, this does not affect the current data-taking since they can be accessed and replaced without opening up the detector.
Commissioning of ATLAS
The ATLAS detector was fully installed, prior to the first LHC beam operation, in September 2008. Subsequent to the LHC mal-function that prevented first collisions until November 2009 (see section 3.1.3), the ATLAS detector continued its commis-sioning phase with three more extensive ATLAS combined cosmic ray data-taking periods in 2008 and 2009. During this period, more than five hundred million events were recorded in the whole ATLAS volume and several million tracks with SCT hits reconstructed in the much smaller volume of the Inner Detector. The majority of these tracks originate from cosmic ray muons but a small fraction of the tracks orig-inate fromδ-ray electrons. Identification of electrons in cosmic ray data was studied in great detail by the author and is discussed in section 4.4.
The performance results from the commissioning phase of cosmic data-taking with the SCT are described in section 4.2 and have been presented by the author in [67].
During this commissioning period and more importantly during collision data-taking, the online data quality monitoring (DQM) is extremely important in order to recog-nize any issue with the detector and the data at the time data collection. Section 4.3 describes the technicalities and the development of the SCT DQM as part of the author’s service task during the commissioning and first data period.
4.1 Cosmic ray data collected by ATLAS prior to first collisions
The first extended cosmic ray data-taking periods in ATLAS took place in September and October 2008. The number of total recorded events during this time, as well as the number of events from the individual trigger streams, are shown in Figure 4.1 as a function of run number. A total of 216 million events were recorded during this time, while the Inner Detector trigger stream collected around 7 M events. The
relative cosmic data rate for the ID trigger is low due to the small volume of the ID in comparison with the muon spectrometer. The majority of the events are hence triggered by the muon triggers. The background colors in Fig. 4.1 show the status of the solenoid and toroid magnetic fields, which were turned off during a considerable fraction of the runs. The full ATLAS detector was included in the data-taking for the majority of the time. In some higher rate runs the LAr calorimeter was taken out, allowing trigger accept rates of approximately 500 Hz (corresponding to the steep rises in the curves).
Figure 4.1: Integrated cosmic data rate for ATLAS as a function of run number for the 2008 September/October combined cosmic data-taking period. The total number is approximate, taking bilinear overlaps between the inclusive streams into account for nearly all runs (the overlap information is not available for some runs, where the run ended in a incorrect manner). The background colors in the plot show the status of the solenoid and toroid magnetic field (orange: solenoid on, green: toroid on and blue: both fields on) [42].
The succeeding cosmic data period occurred in the beginning of summer 2009 and the corresponding number of recorded cosmic ray events is shown in Figure 4.2.
During this two week period of data-taking a total of 93 M cosmic ray events were recorded whereas nearly 20 M events were triggered in the Inner Detector. The increase of the tracking rate was due to the enabling of the TRT “Fast-OR” trigger,
which directly sends a signal to the Central Trigger Processor (CTP) from the front-end electronics of the TRT. Also the fraction of runs with the solenoid magnet on is larger than for the 2008 cosmic data-taking, encouraging the pursuit of tracking studies.
Figure 4.2: Equivalent to Figure 4.1, showing number of recorded events during the cosmic ray data-taking period in early summer 2009. The relative increase of events triggered in the Inner Detector is due to the TRT Fast-OR trigger [42].
The third and final combined continuous ATLAS cosmic ray data-taking period began in October 2009 and continued until the first proton-proton collisions were seen in ATLAS in November 2009 (see Figure 4.3). A total of 266 million events were recorded, while about 25 million events were triggered by either the TRT Fast-OR or the ID high-level trigger. The solenoid magnetic field was enabled during most of this period.
Table 4.1 summarizes the number of total cosmic ray events collected during the three extended data-taking periods in ATLAS. The number of events collected with the Inner Detector trigger (or TRT Fast-OR) as well as after requiring the solenoid and toroid magnets to be on are also included in the table. These are the cosmic ray events considered for electron identification, which is discussed in section 4.4. However, due to the gas mixture applied in the TRT, the data collected during summer 2009 cannot be used for this study (for more details see section 4.4).
Figure 4.3: Equivalent to Figure 4.1 and 4.2, showing number of recorded events during the cosmic ray data-taking period in fall 2009 lasting up to first collisions [42].
[million events] Fall 2008 Summer 2009 Fall 2009 All periods
Total nbr of events 216 93 266 575
with ID track 7 20 25 52
and magnets on 4 10 10 24
Table 4.1: Approximate rates of cosmic ray events (in millions) in total, collected with the ID trigger (or TRT Fast-OR) and collected with the ID trigger while the solenoid and toroid magnets were both on. The numbers of events are shown for the different data-taking periods as well as all together.