Abstract

A new CMS Tracker is under development for operation at the High Luminosity LHC from 2026 onwards. It includes an outer tracker based on dedicated modules that will reconstruct short track segments, called stubs, using spatially coincident clusters in two closely spaced silicon sensor layers. These modules allow the rejection of low transverse momentum track hits and reduce the data volume before transmission to the first level trigger. The inclusion of tracking information in the trigger decision is essential to limit the first level trigger accept rate. A customized front-end readout chip, the CMS Binary Chip (CBC), containing stub finding logic has been designed for this purpose. A prototype module, equipped with the CBC chip, has been constructed and operated for the first time in a 4 GeV/c positron beam at DESY. The behaviour of the stub finding was studied for different angles of beam incidence on a module, which allows an estimate of the sensitivity to transverse momentum within the future CMS detector. A sharp transverse momentum threshold around 2 GeV/c was demonstrated, which meets the requirement to reject a large fraction of low momentum tracks present in the LHC environment on-detector. This is the first realistic demonstration of a silicon tracking module that is able to select data, based on the particle's transverse momentum, for use in a first level trigger at the LHC . The results from this test are described here.

Highlights

  • The cluster width is in good agreement with geometric predictions, and the fraction of very broad clusters is below 1%

  • The separation from low-pT tracks is excellent; particles with momentum below 1.70 GeV/c could be rejected with a resolution of about 0.07 GeV/c

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Summary

Module hardware

The concept of pT -discrimination with a 2S-pT module and the module design foreseen for the future CMS tracker are introduced. The window size can be adjusted across the module to handle potential variations in the sensor spacing after construction These programmable parameters will be calibrated on real data following installation. Signals from both sensor layers are fed to the CBC readout chips, which are bump-bonded onto one side of a flexible readout hybrid implemented in a technology that provides the necessary high density interconnectivity. The two halves of the pT module are read out independently and as such would normally contribute to a stub finding inefficiency for severely inclined tracks This effect becomes non-negligible for the innermost barrel layers of the tracker where the PS-pT modules are located. Details of the Phase-II tracker layout and design can be found in [4]

The CBC2 chip
Prototype mini-modules
Beam test module construction
Experimental setup
Data acquisition
Gbps GBT optical fibre
Calibration tasks
Recorded data sets
Event reconstruction
Noise suppression
Offline reconstruction
Latency adjustment
TDC phase
Threshold determination
Alignment of the DUT
Beam parameters
Results
Cluster width
Cluster efficiency
Stub efficiency
Summary
Full Text
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