Abstract
A new pixel detector for the CMS experiment was built in order to cope with the instantaneous luminosities anticipated for the Phase I Upgrade of the LHC . The new CMS pixel detector provides four-hit tracking with a reduced material budget as well as new cooling and powering schemes. A new front-end readout chip mitigates buffering and bandwidth limitations, and allows operation at low comparator thresholds. In this paper, comprehensive test beam studies are presented, which have been conducted to verify the design and to quantify the performance of the new detector assemblies in terms of tracking efficiency and spatial resolution. Under optimal conditions, the tracking efficiency is 99.95 ± 0.05%, while the intrinsic spatial resolutions are 4.80 ± 0.25 μm and 7.99 ± 0.21 μm along the 100 μm and 150 μm pixel pitch, respectively. The findings are compared to a detailed Monte Carlo simulation of the pixel detector and good agreement is found.
Highlights
A new pixel detector for the CMS experiment is being built, owing to the instantaneous luminosities anticipated for the Phase I Upgrade of the LHC
The tracking efficiency of the CMS pixel ROC has been measured as a function of operation time, telescope track multiplicity and track position on the chip at a comparator threshold of 1.7 ke, and is calculated using six-plane telescope tracks that have been tagged by the REF detector as being in the correct 25 ns trigger window
Comprehensive test beam measurements have been performed at the DESY II synchrotron in order to characterize the behavior of the ROC intended for use with layers 2 – 4 of the CMS Phase I pixel detector
Summary
The silicon sensors of the new CMS pixel detector implement an n+-in-n design consisting of ndoped silicon substrate with a nominal thickness of 285 μm and highly doped n+ pixel implants for the electrode segmentation. The pixel pitch of 100 μm × 150 μm is designed for optimal charge sharing between two pixel cells in the presence of the 3.8 T magnetic field of the CMS solenoid [9]. The charge signals collected from the sensor implants are sampled by the on-chip analogto-digital converter (ADC). The assemblies used for the measurements presented in this paper consist of one ROC and a sensor with a physical size of about 10 mm × 10 mm, while the modules produced for the CMS Phase I Pixel Detector feature 16 ROCs on one sensor. The ROCs were bump-bonded to the sensors at DESY using tin-silver alloy solder balls [11]
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