Link to the DPG Tracker Simulation twiki

Introduction

From Geant-4 simulations we get for each track so called SimHits: the entrance and exit points (, ) and the deposited energy in keV (ΔE). 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, ....

• The track segment is split into many parts (about 100) each with charge 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 ( 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 .
• Noise is added. There are two types of noise: detector noise & readout noise. The one determines which pixel is above threshold and is read out. The 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.

Steps in the pixel digitizer

Digitize

• 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 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().

Make_digis()

• 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.

Signal response in the simulation

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.

Bpix_analog_signal = 126.5 + 97.4 * tanh(0.0035*signal - 1.23)		Fpix_analog_signal = 134.6 + 93.6 * tanh(0.0043*signal - 1.31)

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.

Result of the new parameterization in the pixel digitizer: V02-17-08 (Nov. 20th 2009)

Pixel thresholds in electrons

• ThresholdInElectrons_FPix is 2483e (RMS 163e)
• ThresholdInElectrons_BPix is 2733e (RMS 196e)
• All is 2666 (RMS 218)

Contributions that smear the pixel charge

• 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 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:
  

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

Time of flight of simhits

• Cosmics: process.simSiPixelDigis.TofLowerCut = 18.5 and process.simSiPixelDigis.TofUpperCut = 43.5
• Collisions: process.simSiPixelDigis.TofLowerCut = -12.5 and process.simSiPixelDigis.TofUpperCut = 12.5

Pixel inefficiency (mostly due to Luminosity)

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.

for (int i=0; i<6;i++) {
      thePixelEfficiency[i]     = 1.;  // pixels = 100%
      thePixelColEfficiency[i]  = 1.;  // columns = 100%
      thePixelChipEfficiency[i] = 1.; // chips = 100%
}

• 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:

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)
}

• If AddPixelInefficiency > 0: We include also luminosity rate dependent inefficiency. For both barrel and endcaps, we will assume the following settings:

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)
}

• 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.

      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)

Pixel dead modules

Default pixel digitizer parameters and configuration

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

More details are given in the CMSSW reference manual for the SiPixelDigitizer package.

How to run the pixel digitizer

DBS

SimTracker/SiPixelDigitizer/test

process.p1 = cms.Path(process.mix*process.simSiPixelDigis)

process.p1 = cms.Path(process.mix*process.simSiPixelDigis*process.pixelDigisValid)

process.p1 = cms.Path(process.mix*process.digis*process.siPixelRawData*process.siPixelDigis*process.pixeltrackerlocalreco)

process.p1 = cms.Path(process.mix*process.simSiPixelDigis*process.siPixelRawData*process.siPixelDigis)

cmsRun DQM_Pixel_digi.py

process.p1 = cms.Path(process.mix*process.digis*process.siPixelRawData*process.siPixelDigis*process.rechits*process.tracks

*process.trackinghits)

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

process.simhits = cms.Sequence(process.g4SimHits*process.trackerHitsValidation)

process.digis = cms.Sequence(process.simSiPixelDigis*process.pixelDigisValid)

process.rechits = cms.Sequence(process.pixeltrackerlocalreco*process.siStripMatchedRecHits*process.pixRecHitsValid)

process.tracks = cms.Sequence(process.offlineBeamSpot*process.recopixelvertexing*process.trackingParticles

*process.trackingTruthValid*process.ckftracks*process.trackerRecHitsValidation)

process.trackinghits = cms.Sequence(process.TrackRefitter*process.trackingRecHitsValid)

Validation plots

results

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

• MCIssues_35x.pdf: MCIssues_35x.pdf