Implementing Online Calibration Feed Back Loops in the Alice High Level Trigger

Transcription

1 Implementing Online Calibration Feed Back Loops in the Alice High Level Trigger Oliver Berroteran Supervisor: Markus Fasel, CERN

2 Abstract The High Level Trigger (HLT) is a computing farm consisting of around 200 nodes for on-line processing of collisions recorded by A Large Ion CollidEr (ALICE), performing initial processing and analyses of events in real time. Because several of ALICE s sub detectors are sensitive to environmental conditions such as ambient pressure and temperature, precise reconstruction of these events - by the HLT - requires the o -line and on-line calibration of these detectors. O ine calibration, however, can only be accomplished after the data have been taken, a process that can last for over a month. We, therefore attempt to utilize an Online Calibration Feedback Loop to improve on-line reconstruction of the data. Using a bad channel masking framework, one which was applied to o -line data for calibration, the feedback loop will continuously mark Electro-Magnetic Calorimeter (EMCAL) Cells not suitable for event reconstruction in real-time. 1

3 1 Background 1.1 The Electro-Magnetic Calorimeter The ALICE Electro-Magnetic Calorimeter can perform measurements in the high multiplicity environment of heavy ion collisions, with excellent momentum resolution for charged particles from 100 MeV/c to 100 GeV/c [1]. With an e cient and unbiased fast level 0/1 trigger - used for high energy jets - and the ability to measure neutral jet energies, the EMCAL is able to perform measurements of large fractions of jet energies, reducing sensitivity of jet reconstruction to specific jet structure. Additionally, the use of the EMCAL has improved specific jet structure, and has enhanced ALICE s capability to measure high p t photons, neutral hadrons and electrons. For the on-line calibration loop, we will be analyzing the individual cells of the EMCAL in order to sift out cells which may adversely a ect the reconstruction. 1.2 The High Level Trigger The High Level Trigger is an online computing farm designed to perform event analysis of heavy ion and proton-proton collisions, calibration calculations, as well as data reduction on-line, receiving raw event data from the Front End Electronics (FEE) [2] - the readout electronics of the EMCAL PHOton Spectrometer (PHOS) - as direct copies of the event data by the Data Acquisition Read-Out Recorder Cards (D-RORC) [3] during runs. The processing rate of the HLT is between 1 and 3 khz, however, limiting the time alloted for calibration components to perform calibrations on the data. As a result, the tasks are only carried out on a subset of events asynchronously, such that the start of a task is not contingent on another one finishing. Calibration of the detectors is necessitated by their sensitivity to ambient conditions such as ambient pressure and temperature. The feedback loop we implement will be run in the HLT, performing such a calibration whereby bad cells are identified and recorded during each analysis. The initial data-gathering component will then receive the list of bad cells, masking their read-outs, and perform the analysis again cyclically. 1.3 Event Reconstruction Figure 1: Plots of EMCAL Cell ID Versus Amplitude and Cell Time, Respectively for LHC16h. The thin horizontal smears denote cells firing constantly through time, and at increasing energies. 2

4 The reconstruction of event data during the initial data taking procedure will provide the first calibration results - in our case, a list of bad cells that need masking - with which to produce a new reconstruction. This cycle is repeated for as long as bad cells can be identified, and masked. The EMCAL cells are neither supposed to be firing constantly, nor at consistently high rates at higher energies; cells exhibiting such traits can be seen around 10000, 12000, and in Figure one. These cells will be identified individually and considered bad for the purposes of our calibration; their readouts will be masked in the feedback loop. 2 Implementing the Feedback Loop. 2.1 Initial o -line analysis of prerecorded run data. Before we implemented the feedback loop directly to the on-line calibration framework, an analysis task object was created to analyze the data read out by the EMCAL cells for run LHC13c. The data on which we performed the analysis were segmented however, requiring the analysis task to be run on each part, consolidating the results of the analyses after the fact. In this way, we mimic the HLT s asynchronous component processing as a way of testing the implementation of our C++ program. Figure 2: Plots of EMCAL Cell ID versus Amplitude Cell Time, respectively, for LHC13c. The lack of smears suggest most of the cells involved were working as intended. The analysis task yielded data that looked less problematic than those in Figure one. There are seemingly no smears in the Energy or Time Axes, so a calibration task on this data set would likely yield very little cells which need masking. The feedback loop would then leave the reconstruction fairly unchanged. 2.2 What to expect from the feedback loop on on-line data. Unfortunately, a lack of time limited our ability to implement the components necessary to cyclically receive the run data as input, perform the reconstruction and calibration, then feed back the result as input. 3

5 Figure 3: Bad and Dead EMCAL cell plots from o -line calibration of LHC16h data. The plots in Figure 3, generated from fit distributions of average hit occupancy per cell, represent the cells whose inputs were masked in the o -line calibration. They are responsible for the smearing present in the plots in Figure 1. In the context of on-line calibration, these plots would be generated and populated dynamically as the feedback loop ran continuously and masked their input. It s important to note that the set of cells whose readings require masking di er across data-taking periods; di erent cells may react adversely toward ambient conditions for example, potentially yielding an entirely di erent data set for bad and dead cells. 3 Conclusion The o -line calibration of the data is limited by the time required to take collect the data from the EMCAL. Had we applied the on-line calibration feedback loop, the time taken to perform cell masking process would be significantly reduced. Figure 4: Good EMCAL cell plots from o -line calibration of LHC16h data. Figure 4 illustrates those cells whose input - having already been collected from the HLT - indicate they were firing correctly. In the o -line calibration, it is only the data from these cells which are used to make plots such as those for LHC13c (Figure 2). For the on-line calibration, we hope to produce data as clean - free of smears - as those in Figure 2 in real-time, thereby providing clean data as it is being collected and processed, instead of after the data collection is complete. 4

6 References [1] P. Cortese, et al., Alice Electromagnetic Calorimeter Technical Design Report. 2008, CERN-LHCC [2] Krzewicki, Mikolaj, et al The ALICE High Level Trigger: status and plans Journal of Physics: Conference Series. Vol No. 8. IOP Publishing, [3] S R Bablok, et al. High Level Trigger Online Calibration Framework in Alice. Journal of Physics: Conference Series, Vol No 8. IOP Publishing,