(r20) SWGuidePixelDigitization < CMSPublic

CMSPublic Web>SWGuide>SWGuideSimulation>SWGuideTrackerDigitization>SWGuidePixelDigitization (revision 20) (raw view)~~EditAttachPDF~~
%TOC{title="Contents:"}%

%ICON{hand}% [[https://twiki.cern.ch/twiki//bin/viewauth/CMS/TrackerDpgSimulation][Link to the DPG Tracker Simulation twiki]]

---++ %OLIVE%Introduction%ENDCOLOR%
The Pixel digitizer reads the collection of !SimHit and writes a collection of Pixel Digi and a collection of links between digi and simhits. The source code of the !PixelDigitizer is available at !SimTracker/SiPixelDigitizer.

From Geant-4 simulations we get for each track so called !SimHits: the entrance and exit points (%$P_{in}$%, %$P_{out}$%) and the deposited energy in keV (&Delta;E). Only the pixel detector geometry is needed to obtain this. To convert !SimHits to something that looks similar to the experimental raw data (Digis) we need the digitizer. The pixel digitizer knows the pixel pitch, signal thresholds, ADC bits,
noise, ....

| <img src="%ATTACHURLPATH%/SimHits.png" alt="SimHits.png" width='500' height='231' /> | <img src="%ATTACHURLPATH%/Track_split.png" alt="Track_split.png" width='500' height='207' /> |

   * The track segment is split into many parts (about 100) each with charge %$q_i$% which is fluctuated according to the G4 dEdx fluctuation formula.
   * Each point charge is drifted to the detector surface under the influence of the B-field (%$2^{nd}$% order Lorentz force used).
   * The point charge is diffused with a Gaussian smearing.
   * All charges within a single pixel limit are collected to give the pixel charge %$Q_n$%.
   * Noise is added. There are two types of noise: detector noise & readout noise. The %$1^{st}$% one determines which pixel is above threshold and is read out. The %$2^{nd}$% one determines the noise contributions to the signal at the ADC input.
   * A threshold is applied and the charge is converted to ADC counts (integer).
   * Inefficiencies and miss-calibration are applied.

---++ %OLIVE%Steps in the pixel digitizer%ENDCOLOR%
(from Danek, November 28, 2009)
---+++ %PURPLE%Digitize%ENDCOLOR%
   * Smear pixel thresholds with a Gaussian, given the sigma. Separate for bpix and fpix.
   * Generate pixel charge using: primary_ionization(), drift() and induce_signal(). Pixel signal is in electrons.
   * Delete pixels in dead modules. Use either the config files or the DB (The list of official dead pixel modules accessible via the Database can be found [[https://twiki.cern.ch/twiki/bin/viewauth/CMS/SiPixelQualityHistory][here]]). Presently the list from the config files is used.
   * Apply the smearing of the signal due to the VCAL variation.  The smearing is a Gaussian where the sigma depends on the pixel charge.
   * Add noise to the hit pixels and add new "noisy" pixels.
   * Delete random hit pixels to simulate the effect of dynamical efficiency losses. Important only at higher luminosities.
   * Delete hits in known "dead" pixels. It is not used for the moment. It uses the !SiPixelGainCalibrationOfflineSimService() object to call ->isDead().

---+++ %PURPLE%Make_digis()%ENDCOLOR%
   * Check threshold, keep only pixels above the threshold. Separate for bpix and fpix.
   * Apply the signal response. Convert electrons to !VcalLow and than use the TANH function to obtain ADC values. The same parameter are used for all pixels (no pixel2pixel variation). Separate for bpix and fpix.
   * Saturate the ADC count at the maximum value (255).
   * Construct the sim-links for the MC association.

---++ %OLIVE%Signal response in the simulation%ENDCOLOR%
A more realistic parameterization of the signal response has been implemented. The analog response is simulated (with distinction between Fpix and Bpix) as:

p3 + p2 * tanh(p0*signal - p1) with signal(in VCAL units)= (signalInElectrons - electronsPerVCAL_Offset) / electronsPerVCAL

where signalInElectrons is the signal in electrons for a hit pixel, electronsPerVCAL = 65.5 and electronsPerVCAL_Offset = -414. 

The two sets of parameters (from Danek's results) to be used for the new parameterization are shown in the following table. These are average of all ROCs. There are variations of course but we decided a while ago to ignore them. 
| %PURPLE%Bpix_analog_signal = 126.5 + 97.4 * tanh(0.0035*signal - 1.23)%ENDCOLOR% |      | %PURPLE%Fpix_analog_signal = 134.6 + 93.6 * tanh(0.0043*signal - 1.31)%ENDCOLOR% |
| <img src="%ATTACHURLPATH%/bpix_vs_elec_lin.gif" alt="bpix_vs_elec_lin.gif" width='309' height='210' /> |      | <img src="%ATTACHURLPATH%/fpix_vs_elec_lin.gif" alt="fpix_vs_elec_lin.gif" width='309' height='210' /> |

We also update the gain calibrations in MC as well. We update the DB payload with the following input numbers: bpix_gain= 3.501, bpix_ped=25.520, bpix_rmsgain=0.513, bpix_rmsped=8.149 and fpix_gain=2.915, fpix_ped=28.030, fpix_rmsgain=0.475, fpix_rmsped=9.534. We don't need to update the clusterizer to read the separate numbers for bpix and fpix because the differentiation is already in the payload. The clusterizer just reads it from the DB.

| %BLUE%Result of the new parameterization in the pixel digitizer: V02-17-08 (Nov. 20th 2009)%ENDCOLOR%|
| <img src="%ATTACHURLPATH%/PixelSignalResponse.png" alt="PixelSignalResponse.png" width='464' height='315' /> |

---++ %OLIVE%Pixel thresholds in electrons%ENDCOLOR%
The official 2009 pixel threshold in electrons (obtained from Cosmic data) are: 
   * !ThresholdInElectrons_FPix is 2483e (RMS 163e)
   * !ThresholdInElectrons_BPix is 2733e (RMS 196e)
   * !All is 2666 (RMS 218)

---++ %OLIVE%Contributions that smear the pixel charge%ENDCOLOR%
   * The readout noise from the electronics is a "permanent" contribution. (The upper limit would be 500e)
   * The VCAL contribution is a "permanent" contribution. Even if we find a way to correct each ROC the pixel2pixel contribution is still there. Following the [[http://indico.cern.ch/getFile.py/access?contribId=1&sessionId=4&resId=0&materialId=slides&confId=47919][study]] made by D. Kotlinski on the VCAL calibration, we implemented a smearing of the pixel charge using an RMS value (obtained from that study) which is a function of the pixel charge value as shown in the plot below:<br />
| <img src="%ATTACHURLPATH%/VCal_smearing_pixelCharge.png" alt="VCal_smearing_pixelCharge.png" width='450' height='250' /> | <verbatim>
The pixel charge smearing is defined in 3 regions: 
charge < 3000           RMS = 543.6 – 0.093*charge
3000 < charge < 6000    RMS = 307.6 – 0.01*charge
charge > 6000           RMS= -432.4 + 0.123*charge
</verbatim>|

---++ %OLIVE%Time of flight of simhits%ENDCOLOR%
The !ToF of simhits was measured in a cosmic sample (look at the pixel simhit !TOF in a !SimHits root file). They peak around 31nsec. For the cosmics we use 31 %$\pm$% 12.5 nsec window. For collisions the !TOF peak is at zero so the window is %$\pm$% 12.5.
   * Cosmics:  process.simSiPixelDigis.TofLowerCut = 18.5 and process.simSiPixelDigis.TofUpperCut = 43.5
   * Collisions: process.simSiPixelDigis.TofLowerCut = -12.5 and process.simSiPixelDigis.TofUpperCut = 12.5

---++ %OLIVE%Pixel inefficiency (mostly due to Luminosity)%ENDCOLOR%
The pixel inefficiency is defined below.  Basically we can consider that the efficiency is 100%. Or we can include only the static inefficiency: static means that no readout related luminosity dependent losses are considered. But, we can include the readout losses for low-luminosity  (2 10^33) or high luminosity (10^34). All this is needed because the MC productions will be done most likely not for the so called "low" luminosity (2 10^33) but for some very low lumi where there will be no readout losses, however some "static" losses comming from bad pixels, bad bump-bonds etc. will be present.

The three inefficiency are: =PixelEfficiency= (pixels), =PixelColEfficiency= (columns) and =PixelChipEfficiency= (readout chips):
   | thePixelEfficiency[i] | thePixelColEfficiency[i] | thePixelChipEfficiency[i] | where the first 3 settings [0], [1], [2] are for the pixel barrel layer 1, 2, 3 and the next 3 settings [3], [4], [5] are for the endcaps. Different scenarios have been implemented: 
   * If !AddPixelInefficiency = -1: All is 100% efficient. 
<verbatim>
for (int i=0; i<6;i++) {
      thePixelEfficiency[i]     = 1.;  // pixels = 100%
      thePixelColEfficiency[i]  = 1.;  // columns = 100%
      thePixelChipEfficiency[i] = 1.; // chips = 100%
}
</verbatim>
   * If !AddPixelInefficiency = 0: We include only the static (non rate depedent) inefficiency. This is useful for very low rates (luminosity). For both barrel and endcaps, we will assume the following settings:
<verbatim>
for (int i=0; i<6;i++) {
      thePixelEfficiency[i]     = 1.-0.001;  // pixels = 99.9% (Assume 1% inefficiency that is given by faulty bump-bonding and seus.)
      thePixelColEfficiency[i]  = 1.-0.001;  // columns = 99.9% (Make 0.1% default)
      thePixelChipEfficiency[i] = 1.-0.001; // chips = 99.9% (A flat 0.1% inefficiency due to lost ROCs)
}
</verbatim>
   * If !AddPixelInefficiency > 0: We include also luminosity rate dependent inefficiency. For both barrel and endcaps, we will assume the following settings:
<verbatim>
for (int i=0; i<6;i++) {
      thePixelEfficiency[i]     = 1.-0.01;  // pixels = 99% (Assume 1% inefficiency that is given by faulty bump-bonding and seus.)
      thePixelColEfficiency[i]  = 1.-0.01;  // columns = 99% (Make 0.1% default)
      thePixelChipEfficiency[i] = 1.-0.0025; // chips = 99.75% (A flat 0.25% inefficiency due to lost data packets from TBM)
}
</verbatim>
   * If !AddPixelInefficiency = 10: Special cases ( High-lumi for 4cm layer ) where the readout losses are higher. This case is for high luminosity and affects the =Barrel layer1=.
<verbatim>
      thePixelColEfficiency[0] = 1.-0.034; // (Make 3.4% inefficiency for pixel barrel layer 1)
      thePixelEfficiency[0]    = 1.-0.015; // (Make 1.5% inefficiency for pixel barrel layer 1)
</verbatim>

---++ %OLIVE%Pixel dead modules%ENDCOLOR%

---++ %OLIVE%Default pixel digitizer parameters and configuration%ENDCOLOR%

| *Parameter* | *Default value* | *Comment* |
| !ReadoutNoiseInElec | 350 | Readout noise from the electronics in front of the ADC, including all readout chain, relevant for smearing |
| !DeltaProductionCut | 0.03 !GeV | Delta ray cutoff in MeV, has to be same as in OSCAR. Max = 0.030 MeV.  |
| !ThresholdInElectrons_FPix | 2483 | CRAFT09 result | 
| !ThresholdInElectrons_BPix | 2733 | CRAFT09 result | 
| !AddThresholdSmearing | True | Add threshold gaussian smearing |
| !ThresholdSmearing_FPix | 160 | CRAFT09 result |
| !ThresholdSmearing_BPix | 200 | CRAFT09 result |
| !MissCalibrate | True | Enable miss-calibration (pixel gain) |
| !GainSmearing | 0 | Get the constant for the miss-calibration studies: Sigma of the gain smearing.|
| !OffsetSmearing | 0 | Get the constant for the miss-calibration studies: Sigma of the offset smearing.|
| !ElectronsPerVcal | 65.5 | Calibration electrons/adc: electrons to VCAL conversion needed in misscalibrate() |
| !ElectronsPerVcal_Offset | -414 | electrons to VCAL conversion Offset needed in misscalibrate() |
| !ElectronPerAdc | 135 | ADC calibration  |
| !TofUpperCut | 12.5 | Time of flight cut in nanoseconds |
| !TofLowerCut | -12.5 | Time of flight cut in nanoseconds |
| !AdcFullScale | 255 | ADC saturation value is 255 = 8bit adc |
| !TanLorentzAnglePerTesla_FPix | 0.106 | Lorentz angle per unit B field |
| !TanLorentzAnglePerTesla_BPix | 0.106 | Lorentz angle per unit B field |
| !AddNoisyPixels | True | Add pixels generated by noise |
| !NoiseInElectrons | 175 | Pixel cell noise in electrons, relevant for generating noisy pixels |
| !Alpha2Order | True | |
| !AddPixelInefficiency | 0 | Controls the pixel inefficiency |
| !AddNoise | True | Add noise to the pixel signal |
| !ChargeVCALSmearing | True | Smear the pixel charge with a gaussian which RMS is a function of the pixel charge (Danek's study) |
| !GeometryType | 'idealForDigi' | |                     
| !useDB | True | |
| !LorentzAngle_DB | True | |
| !DeadModules_DB | True | |
| !killModules | True | |
| !DeadModules | VPSet() | |

Still hard-coded:
Fluctuate generated charge along track subsegments:  fluctuateCharge=conf_.getUntrackedParameter<bool>("FluctuateCharge",%BLUE%true%ENDCOLOR%); =(SimTracker/SiPixelDigitizer/src/SiPixelDigitizerAlgorithm.cc)=

More details are given in the !CMSSW reference manual for the [[http://cmsdoc.cern.ch/Releases/CMSSW/latest_nightly/doc/html/SimTracker_SiPixelDigitizer.html][SiPixelDigitizer]] package.

---++ %OLIVE%How to run the pixel digitizer%ENDCOLOR%
You will need a file of !SimHits. Usually you can choose a file containing !SimHits on [[https://cmsweb.cern.ch/dbs_discovery/][DBS]], e.g =/store/relval/CMSSW_3_3_3/RelValSingleMuPt10/GEN-SIM-RECO=. In <verbatim>SimTracker/SiPixelDigitizer/test</verbatim> there is a python script named =testPixelDigitizer.py=. This scripts has been implemented ( tested with CMSSW_3_3_3) with several process that you can run depending on what you want. You can choose among the following process: 
|%PURPLE%Run the digitizer%ENDCOLOR%|<verbatim>process.p1 = cms.Path(process.mix*process.simSiPixelDigis)</verbatim>|
|%PURPLE%Run the digis validation%ENDCOLOR%|<verbatim>process.p1 = cms.Path(process.mix*process.simSiPixelDigis*process.pixelDigisValid)</verbatim>|
|%PURPLE%Look at cluster charge%ENDCOLOR%|<verbatim>process.p1 = cms.Path(process.mix*process.digis*process.siPixelRawData*process.siPixelDigis*process.pixeltrackerlocalreco)</verbatim>|
|%PURPLE%Make !DQM occupancy maps%ENDCOLOR%|<verbatim>process.p1 = cms.Path(process.mix*process.simSiPixelDigis*process.siPixelRawData*process.siPixelDigis)</verbatim>  followed by <verbatim>cmsRun DQM_Pixel_digi.py</verbatim>|
|%PURPLE%Look at residuals and pulls%ENDCOLOR%|<verbatim>process.p1 = cms.Path(process.mix*process.digis*process.siPixelRawData*process.siPixelDigis*process.rechits*process.tracks</verbatim>|
| |<verbatim>*process.trackinghits)</verbatim>|

with the following path for process.simhits, process.digis, process.rechits, process.tracks and process.trackinghits:

|%PURPLE%Simulation of hits in the tracker%ENDCOLOR%|<verbatim>process.simhits = cms.Sequence(process.g4SimHits*process.trackerHitsValidation)</verbatim>|
|%PURPLE%Pixel digitization%ENDCOLOR%|<verbatim>process.digis = cms.Sequence(process.simSiPixelDigis*process.pixelDigisValid)</verbatim>|
|%PURPLE%Pixel reconstruction-clusterisation%ENDCOLOR%|<verbatim>process.rechits = cms.Sequence(process.pixeltrackerlocalreco*process.siStripMatchedRecHits*process.pixRecHitsValid)</verbatim>| 
|%PURPLE%Tracker reconstructed hits%ENDCOLOR%|<verbatim>process.tracks = cms.Sequence(process.offlineBeamSpot*process.recopixelvertexing*process.trackingParticles</verbatim>|
| |<verbatim>*process.trackingTruthValid*process.ckftracks*process.trackerRecHitsValidation)</verbatim>|
|%PURPLE%Tracking%ENDCOLOR%|<verbatim>process.trackinghits = cms.Sequence(process.TrackRefitter*process.trackingRecHitsValid)</verbatim>|

---++ %OLIVE%Validation plots%ENDCOLOR%
Configuration parameters: [[http://www.fisica.unipg.it/~pioppi/pixeldigi/phase1/][results]] (by M. Pioppi - Feb. 2006)

%AQUA%Created on 21-Oct-2009 by Main.VesnaCuplov, Merged all informations from the old pixeldigitizer twiki page on 05-Dec-2009. %ENDCOLOR%
  


   * [[%ATTACHURL%/MCIssues_35x.pdf][MCIssues_35x.pdf]]: MCIssues_35x.pdf