Use case for calibration of the detector

Outline of use case

The RICH needs to be calibrated before RAW data can be processed for physics. The RICH will also be used in HLT exclusive selections. Many of the calibration constants used on-line in EFF will not be the same as in off-line data reconstruction for physics analysis. Here we concentrate on RICH calibrations for off-line physics analysis.

RICH reconstruction depends crucially on quality of tracking, thus calibration of tracking must be finalized before RICH calibration is performed.

There are several layers of RICH calibration: image distortions in HPDs due to magnetic field, alignment of mirrors and photo-detectors, refractive index (varies with pressure and gas mixture composition), single photon efficiency and resolution per HPD tube, PDFs for likelihood functions used in particle identification. Since they depend on each other, multiple passes through the data may be needed to calibrate all reconstruction steps.

Selection

Most of RICH calibrations can be performed on any type of events which populate the area of the RICH detectors with tracks with saturated Cherenkov rings (v/c around 1). It is desirable to exclude events which were kept via exclusive HLT triggers which use, and therefore also bias, the RICH information.

Calibration/assessment of high level RICH analysis (particle identification) will require D* data, since they provide source of kaons and pions identified without use of the RICH.

Issues

Many details of the calibration of the tracking system and of the RICH detectors are fuzzy at this point. For example, how often various constants will need to be changed? Can any of them be fixed from hardware info or special calibration runs? Which calibrations will run in the monitoring farm, and which will require dedicated processing at TIER-1 centers? In any case, special stream of calibration data will be needed. It could be created already in monitoring farm, where it could undergo initial processing. DSTs from such processing could be saved and transferred to TIER-1 for further calibration steps. Alternatively, all processing of calibration stream could happen in TIER-1 centers, with only RAW data being transferred off the pit.

It would be beneficial if more than one subdetector shared a calibration stream. This is likely to be the case for tracking and lower levels of RICH calibration which can be done on any types of data which contain some high momentum tracks. The muon detector will require a muon enriched sample, which would be also fine for tracking and RICH except that rate of such events may be insufficient especially with lower luminosities.

At least two reconstruction passes through the data will be needed. The first pass will serve mainly to calibrate the tracking system. If it happens in the monitoring farm, information from the other detectors could also be reconstructed for monitoring purposes. The second pass would be done with final tracking constants and saved in DST format as RICH calibrations jobs are likely to reread these data several times.

The amount of data in the calibration stream will be a fraction of the total RAW data output from the experiment and will depend on the requirements of the subdetectors and amount of computing power available for calibration purposes.

The most likely scenario is one where the monitoring farm will be used to check if the alignment and calibration constants in effect are good enough to allow the data to go to reconstruction. This quality check might indeed be formed by redoing the alignment and "subtracting" it from the current one. If any major difference is present the new aligment constants might be applied before the reconstruction. Initially this will always require manual intervention.

To enable debugging of the behaviour in the monitoring farm it should be possible to persist the data used for calibration. This will require a separate low rate stream from the detector.

Timing

Quick processing of calibration stream(s) is essential, since reconstruction of all data awaits the calibration results.

Number of streams from the detector

As outlined above lower levels of RICH calibrations can be performed on any type of RICH unbiased data with some high Pt tracks, while the higher level will require D* events. Two separate selections are likely, since the D* events will be incoming at lower rate.

Information required on trigger, stripping and luminosity

Calibration streams will not require bookkeeping needed to determine integrated luminosity. Information of selections used in stripping of calibration data will be needed only if different types of selections are mixed in together into common calibration stream.

Information stored for each event

Tracking information will need to be saved once the calibration stream is processed with final tracking constants. It is likely that it will be beneficial to save tracks also from preliminary processing of the calibration data.

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Topic revision: r4 - 2006-12-12 - UlrikEgede
 
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