CMS Tracker Performance results in Run 2 Legacy processing

Contact: The CMS Tracker DPG conveners (cms-dpg-conveners-tracker@cernNOSPAMPLEASE.ch)
and the CMS Tracker Performance hypernews (hn-cms-trackerperformance@cernNOSPAMPLEASE.ch).
Public CDS Record: CMS DP-2019/027

Pixel detector aging

For non irradiated, fully depleted detector, the pixel charge profile (normalised average pixel charge as a function of the production depth) is expected to be flat as detector is fully efficient and all charge is collected, while for irradiated detector the losses are expected due to the trapping of carriers. The losses are larger for the charges released further from the readout plane. Due to the increased irradiation of the pixel detector in Run-2, in 2018 the radiation damage effects were incorporated in the CMS pixel detector simulation. The radiation damage effects in the pixel detector are simulated by reweighting the charge of each pixel using the cluster shapes for the non irradiated and irradiated detector obtained from the PIXELAV simulation tuned to the CMS data [1,2].

Figure in PNG format (click on plot to get PDF) Description
The normalised average pixel charge as a function of the production depth in the silicon substrate is shown for Layer 1 of the CMS pixel barrel detector after 30.1 fb-1 (black) and is compared to the profiles obtained using the CMS Pixel detector simulation with incorporated radiation damage effects (red). Apart from the bins close to the sensor edges, the agreement between data and MC is better the 4%.

Tracker alignment Validation with primary vertices

The resolution of the reconstructed vertex position is driven by the pixel detector since it is the closest detector to the interaction point and has the best hit resolution. The primary vertex residual method is based on the study of the distance between the track and the vertex, the latter reconstructed without the track under scrutiny (unbiased track-vertex residual). Selection and reconstruction of the events is the following [3]:
  • Events used in this analysis are selected online using collision events triggers without specific selections
  • The fit of the vertex must have at least 4 degrees of freedom.
  • For each of the vertices, the impact parameters are measured for tracks with:
    • more than 6 hits in the tracker, of which at least two are in the pixel detector,
    • at least one hit in the first layer of the Barrel Pixel or the first disk of the Forward Pixel,
    • $\chi^{2}/ndof$ of the track fit < 5.
  • The vertex position is recalculated excluding the track under scrutiny.
  • A deterministic annealing clustering algorithm [4] is used in order to make the method robust against pileup, as in the default reconstruction sequence.
The distributions of the unbiased track-vertex residuals in the transverse plane, $d_{xy}$ and in the longitudinal direction, $d_{z}$ are studied in bins of track azimuth $\phi$ and pseudo-rapidity $\eta$. Random misalignments of the modules affect only the resolution of the unbiased track-vertex residual, increasing the width of the distributions, but without biasing their mean. Systematic movements of the modules will bias the distributions in a way that depends on the nature and size of the misalignment and of the selected tracks.

Figure in PNG format (click on plot to get PDF) Description
Mean track-vertex impact parameter in the transverse plane $d_{xy}$ as a function of track azimuth $\phi$. The impact parameters are obtained by recalculating the vertex position after removing the track being studied from it, and considering the impact parameter of this removed track. The black points show the results with the alignment constants used during 2017 End-Of-Year reprocessing, the red points show the results with the alignment constants as obtained in the Run-2 Legacy alignment procedure. The alignment constants are time-dependent, those used here were obtained for data taken in July 2017. Modulations visible in this distribution with the alignment constants used in the EOY processing are improved with the Legacy tracker alignment.
Mean track-vertex impact parameter in the longitudinal plane $d_{z}$ as a function of track azimuth $\phi$. The impact parameters are obtained by recalculating the vertex position after removing the track being studied from it, and considering the impact parameter of this removed track. The black points show the results with the alignment constants used during 2017 End-Of-Year reprocessing, the red points show the results with the alignment constants as obtained in the Run-2 Legacy alignment procedure . The alignment constants are time-dependent, those used here were obtained for data taken in August 2017. Modulations visible in this distribution with the alignment constants. Modulations visible in this distribution with the alignment constants used in the EOY processing are improved with the Legacy tracker alignment.

Figure in PNG format (click on plot to get PDF) Description
The mean of the average impact parameter in the transverse plane $d_{xy}$ versus track azimuth $\phi$, as a function of integrated luminosity. The vertical blue lines indicate changes in the calibration of the local hit reconstruction in the pixel tracker. The black points show the results with the alignment constants used during 2017 End-Of-Year reprocessing, the red points show the results with the alignment constants as obtained in the Run-2 Legacy alignment procedure. Aligning the tracker improves the mean of this distribution. Outliers in the trend are understood as degraded tracking performance caused by suboptimal pixel local reconstruction calibration input.
The RMS of the average impact parameter in the transverse plane $d_{xy}$ in bins of the track azimuth $\phi$ , as a function of integrated luminosity. The vertical blue lines indicate changes in the calibration of the local hit reconstruction in the pixel tracker. The black points show the results with the alignment constants used during 2017 End-Of-Year reprocessing, the red points show the results with the alignment constants as obtained in the Run-2 Legacy alignment procedure. Aligning the tracker reduces the RMS.

Track curvature bias investigation with Z$\rightarrow\mu\mu$ decays

Distortions of the tracker geometry can lead to a bias in the reconstructed track curvature $\kappa \propto \pm\frac{1}{p_{T}}$. These are investigated using the reconstructed Z$\rightarrow\mu\mu$ mass as a function of the muon direction and separating $\mu^{+}$ and $\mu^{-}$ as the curvature bias has an opposite effect on their $p_{T}$. The invariant mass distribution is fitted to a Breit-Wigner convolved with a Crystal Ball function, thus taking into account the finite track resolution and the radiative tail, for the signal plus an exponential background. The fit range is 75-105 CMS.GeV/$c^{2}$ and the $Z^{0}$ width is fixed to the PDG value of 2.495 CMS.GeV/$c^{2}$ [5]. Note that this does not show the CMS muon reconstruction and calibration performance. Additional muon momentum calibrations are applied on top of this in physics analyses.

Figure in PNG format (click on plot to get PDF) Description
$Z^{0}$ peak mass as a function of the pseudorapidity difference $\Delta \eta = \eta(\mu^{+}) - \eta(\mu^{-})$. The black points show the results with the alignment constants used in End-Of-Year (EOY) 2017 processing, the red points show the results with the alignment constants as obtained in the 2017 data Legacy alignment procedure. A twist distortion ($\Delta \phi \propto z$) would introduce a slope, as seen in the 2017 EOY alignment. This s fixed in the legacy alignment thanks to the use of $Z\rightarrow\mu^{+}\mu^{-}$ mass constraint in the alignment fit [3].
$Z^{0}$ peak mass as a function of the azimuthal angle of the positive muon $\phi(\mu^{+})$ for all events at all psedorapidities. This distribution is sensitive to distortions of the tracker in the transverse plane (e.g. the so-called "sagitta"). The black points show the results with the alignment constants used in End-Of-Year (EOY) 2017 processing, the red points show the results with the alignment constants as obtained in the Run-2 Legacy alignment procedure. Overall pattern in the Legacy processing is significantly reduced with respect to End-Of-Year reprocessing 2017 data.
$Z^{0}$ peak mass as a function of the azimuthal angle of the negative muon $\phi(\mu^{-})$ for all events at all psedorapidities. Sensitive to distortions of the tracker in the transverse plane (e.g. the so-called "sagitta"). The black points show the results with the alignment constants used in End-Of-Year (EOY) 2017 processing, the red points show the results with the alignment constants as obtained in the Legacy alignment procedure. Overall pattern in the Legacy processing is significantly reduced with respect to End-Of-Year reprocessing 2017 data.
$Z^{0}$ peak mass as a function of the pseudorapidity $\eta(\mu^{+})$ and the azimuthal angle $\phi(\mu^{+})$ of the positive muon for 2017 End-of-Year reprocessing CMS data. The z-axis range is centered at the peak value corresponding to the fitted mass for all events in the 2017 sample (91.08 CMS.GeV/$c^{2}$).
$Z^{0}$ peak mass as a function of the pseudorapidity $\eta(\mu^{+})$ and the azimuthal angle $\phi(\mu^{+})$ of the positive muon for the Legacy processing of 2017 data. The z-axis range is centered at the peak value corresponding to the fitted mass for all events in the 2017 sample (91.08 CMS.GeV/$c^{2}$). Overall pattern is significantly reduced with respect to End-Of-Year reprocessing of 2017 data.

References

[1] M. Swartz, “CMS Pixel Simulation”, Nucl. Inst. Meth. A511 (2003) 88 doi:10.1016/S0168-9002(03)01757-1
[2] V. Chiochia et al., “Simulation of the CMS prototype silicon pixel sensors and comparison with test beam measurements CMS”, IEEE Trans. Nucl. Sci (2005) 1067 Meth. A511 (2003) 88 doi:10.1109/NSSMIC.2004.1462427
[3] CMS Collaboration “Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV” JINST 7 (2012) P10002 doi:10.1088/1748-0221/7/10/P10002
[4] R. Frühwirth, W. Waltenberger, and P. Vanlaer, “Adaptive Vertex Fitting”, CMS NOTE 2007-008, CERN, 2007.
[5] CMS Collaboration "Alignment of the CMS tracker with LHC and cosmic ray data " 2014 JINST 9 P06009 doi:10.1088/1748-0221/9/06/P06009

-- MarcoMusich - 2019-09-03

Topic attachments
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PDFpdf MassVsEtaPhiPlus_file0.pdf r1 manage 21.1 K 2019-09-03 - 12:47 MarcoMusich  
PNGpng MassVsEtaPhiPlus_file0.png r1 manage 37.3 K 2019-09-03 - 11:28 MarcoMusich Z- mm validation
PDFpdf MassVsEtaPhiPlus_file1.pdf r1 manage 21.0 K 2019-09-03 - 12:47 MarcoMusich  
PNGpng MassVsEtaPhiPlus_file1.png r1 manage 38.1 K 2019-09-03 - 11:28 MarcoMusich Z- mm validation
PDFpdf MassVsEtaPlusMinusDiff.pdf r1 manage 15.6 K 2019-09-03 - 12:47 MarcoMusich  
PNGpng MassVsEtaPlusMinusDiff.png r1 manage 23.8 K 2019-09-03 - 11:28 MarcoMusich Z- mm validation
PDFpdf MassVsPhiMinus.pdf r1 manage 16.3 K 2019-09-03 - 12:47 MarcoMusich  
PNGpng MassVsPhiMinus.png r1 manage 25.4 K 2019-09-03 - 11:28 MarcoMusich Z- mm validation
PDFpdf MassVsPhiPlus.pdf r1 manage 16.2 K 2019-09-03 - 12:47 MarcoMusich  
PNGpng MassVsPhiPlus.png r1 manage 25.0 K 2019-09-03 - 11:28 MarcoMusich Z- mm validation
PDFpdf RMS_dxy_phi_vs_lumi.pdf r2 r1 manage 36.5 K 2019-09-17 - 17:58 MarcoMusich PV Validation
PNGpng RMS_dxy_phi_vs_lumi.png r1 manage 79.5 K 2019-09-03 - 11:33 MarcoMusich PV Validation
PDFpdf chargeprofiles_L1_2017UL_approval_norm55_data2017UL_ratio.pdf r1 manage 19.6 K 2019-09-03 - 11:56 MarcoMusich PixelChargeReweighting
PNGpng chargeprofiles_L1_2017UL_approval_norm55_data2017UL_ratio.png r1 manage 23.8 K 2019-09-03 - 11:56 MarcoMusich PixelChargeReweighting
PDFpdf dxyPhiBiasCanvas_July20172017_EOY_processing_vs_2017_Legacy_processing.pdf r1 manage 20.8 K 2019-09-03 - 12:56 MarcoMusich  
PNGpng dxyPhiBiasCanvas_July20172017_EOY_processing_vs_2017_Legacy_processing.png r1 manage 17.7 K 2019-09-03 - 11:37 MarcoMusich  
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PNGpng dzPhiBiasCanvas_August20172017_EOY_processing_vs_2017_Legacy_processing.png r1 manage 17.9 K 2019-09-03 - 11:37 MarcoMusich  
PDFpdf mean_dxy_phi_vs_lumi.pdf r1 manage 35.6 K 2019-09-03 - 11:33 MarcoMusich PV Validation
PNGpng mean_dxy_phi_vs_lumi.png r1 manage 65.3 K 2019-09-03 - 11:33 MarcoMusich PV Validation
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