Tracking Physics Object Vector Hits


A new type of hits so-called vector hits were introduced with the aim of answering the question whether they bring tangible benefits in track reconstruction at the HL-LHC.

Vector hits (~offline stubs) are short track segments reconstructed from two clusters in stacked sensors upper and lower clusters. A vector hit contains position and direction information also solve the pattern recognition problem in the tracker.

A vector hit can be mathematically represented in the local system of reference by a 4 component vector:

 <br />(x, y, dx/dz, dy/dz)

where x and y are the most and least precise coordinate, respectively, while dx/dz and dy/dz are the respective direction components on the two axes. The covariance matrix associated to the vector hit is symmetric.



The algorithm used to build vector hits starts from all the clusters found in the event that belong to the lower and the upper sub-module of a particular stack module. The x- and y-coordinate of the vector hit and the corresponding errors are the ones associated to the lower cluster. The dx/dz and dy/dz are computed by a linear fit using the positions of the lower and upper clusters in the x−z and y −z plane. The linear fit gives also a first estimate of the variances of the direction parameters and their covariance with respect to the local position.

The existing tracking algorithms we adapted to the new tracker and the performance of tracking and vertexing for different event types and detector geometries were evaluated. They were found to be similar to the current ones despite the harsher pileup conditions at HL-LHC.

KF was extended to make use of the full information contained in the vector hits, and a new iteration in the Phase-2 tracking, in which seeds are built using only the vector hits was added.

The results show a reduction of about an order of magnitude of the number of mis-identified tracks for high pileup environments and a significant improvement in the reconstruction of tracks coming from displaced vertices.

for more information Erica Brondolin Thesis

TODO: Decribe the performance

Recipe (In progress)

# Tested on lxplus7

export SCRAM_ARCH=slc7_amd64_gcc700
cmsrel CMSSW_11_0_0_pre5
cd CMSSW_11_0_0_pre5/src/
git cms-merge-topic JanFSchulte:VectorHits_1100pre5
scram b -j4 # this will take a while -l 20434.1 -ne -w upgrade 

mkdir VectorHits
cd VectorHits/
voms-proxy-init -voms cms
xrdcp root:// ./step3_DIGI2RAW.root step3  --conditions auto:phase2_realistic -n 10 --era Phase2C8_timing_layer_bar --eventcontent RECOSIM,DQM --runUnscheduled  -s RAW2DIGI,RECO:reconstruction_trackingOnly,VALIDATION:@trackingOnlyValidation,DQM:@trackingOnlyDQM --datatier GEN-SIM-RECO,DQMIO --geometry Extended2026D41

Samples without pileup, reference (D41) geometry

DAS query: dataset=/RelValTTbar_14TeV/CMSSW_10_6_0-106X_upgrade2023_realistic_v2_2023D41noPU-v1/GEN-SIM-DIGI-RAW

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JPEGjpg vectorhit1.jpg r1 manage 23.5 K 2019-08-24 - 22:48 DogukanKizbay  
JPEGjpg vectorhit2.jpg r1 manage 18.3 K 2019-08-24 - 22:48 DogukanKizbay  
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Topic revision: r5 - 2019-09-05 - DogukanKizbay
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