HCAL Digis, Rechits, and Local Reconstruction

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Introduction

The standard sequence to invoke the local reconstruction in HCAL is hcalLocalRecoSequence. The HcalSimpleReconstructor module reconstructs rechits starting from digis.

Usage of the algorithm

Software architecture

The hcalLocalRecoSequence in CMSSW simply invokes the "HcalSimpleReconstructor" module for all subdetectors in HCAL. The HcalSimpleReconstructor takes as input a collection of Digis from the event and produces a collection of Rechits appropriate to the subdetector in question (HBHE, HO, and HF).

The data content of a single Digi is as follows:

  • HcalElectronicsId - a tag of where in electronics space the Digi came from; i.e., VME crate W, slot X, spigot Y, channel Z, etc.
  • HcalDetId - a tag of where the electronics Id maps in subdetector space; i.e., HB ieta=xx, iphi=yy, depth=zz.
  • an array of at most ten QIE samples; each QIE sample is a raw ADC count plus ancillary information such as the capacitor ID for the sample, data
valid and error bits, and fiber and fiber channel numbers.
  • the number of samples in the array
  • the number of presamples - meaning the number of time bins within the Digi window that precede the triggered bunch crossing sample.

The HcalSimpleReconstructor takes this information and converts it into a Rechit. Rechits for each subdetector (HBHERecHit, HORecHit, HFRecHit) inherit from the class CaloRecHit and have the following data content:

  • HcalDetId as above
  • energy of the hit in GeV
  • time of the hit in nanoseconds.

The HcalSimpleReconstructor performs the conversion from ADC to GeV in stages, applying calibrations from the Conditions Database at each stage.

The first stage is to convert ADCs to femtoCoulombs (fC). Each sample is converted via lookup tables stored in the Conditions DB; these tables represent the coefficients of a piecewise linear QIE gain function. The function is dependent on the capacitor ID.

The second stage is to convert fC to GeV. A subset of the fC samples is selected for inclusion into the Rechit. This subset is determined via configurable parameters, the first sample to start with and the number of samples to include. Each included sample has pedestal subtracted, and then a gain factor applied. These pedestals and gains are also retrieved from the Conditions DB. The amplitude of each included sample so converted is summed into the total Rechit energy. The ".cfi" files used by the hcalLocalRecoSequence are currently configured to sum four samples in HBHE and HO, and one sample in HF.

Depending on how many samples are configured for inclusion in the Rechit, some of the energy of the pulse may fall outside the summed region, and so an optional final correction to the Rechit energy can be applied based on the phase containment of the pulse. (This correction is not applicable to HF.) The ".cfi" files used by the hcalLocalRecoSequence currently enable this correction.

The time of the Rechit is determined in stages as well. A first order time is arrived at by finding the peak sample in the summed window, and then calculating an amplitude-weighted sum of the peak and the two samples on either side of the peak. Then two additional corrections are applied to the time.

The first time correction derives from the asymmetric nature of the pulse; since the pulse in HBHE and HO has a fast rise and a long tail, a linear weighting scheme suffers from the pulse asymmetry. A correction function calculated from the simulation pulse shape corrects for this asymmetry. The pulse shape in HF is similarly asymmetric, but much faster, and is mostly contained within one sample. Nevertheless the Rechit time in HF is calculated in the same way.

The second time correction is optional and derives from the energy-dependent time response of the QIE: the lower the energy, the slower the response. this correction was determined by QIE bench tests and is termed the "time slew correction". (This correction is not applicable to HF.) The ".cfi" files used by the hcalLocalRecoSequence currently enable this correction.

More details about the precise hit time reconstruction for HBHE can be found in the CMS note CMS-IN 2008/011.

The HcalSimpleReconstructor is also capable of reconstructing "hits" in in the HCAL calibration channels and in ZDC, and a ".cfi" file exists for Rechit reconstruction in the ZDC. However, the sequence does not currently include these.

Testing

Review status

Reviewer/Editor and Date (copy from screen) Comments
CMSUserSupport - 04 Jul 2008 created the page with Sharon's material

Responsible: SaharonHagopian
Last reviewed by: Most recent reviewer

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Topic revision: r1 - 2008-07-04 - CMSUserSupport
 
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