Abstract
The pixel tracker of the Compact Muon Solenoid (CMS) experiment is the innermost sub-detector, located close to the collision point, and is used for reconstruction of the tracks and vertices of charged particles. The present pixel detector was designed to work efficiently with the maximum instantaneous luminosity of 1 × 1034 cm−2 s−1. In 2017 the Large Hadron Collider (LHC) is expected to deliver a peak luminosity reaching up to 2 × 1034 cm−2 s−1, increasing the mean number of primary vertices to 50. Due to the radiation damage and significant data losses due to high occupancy in the readout chip of the pixel detector, the present system must be replaced by a new one in an extended end-of-year shutdown during winter 2016/2017 in order to maintain the excellent tracking and other physics performances. The main new features of the upgraded pixel detector are a ultra-light mechanical design with four barrel layers and three end-cap disks, digital readout chip with higher rate capability and a new cooling system. In this document, we discuss the motivations for the upgrade, the design, and technological choices made, the status of the construction of the new detector and the future plans for the installation and commissioning.
Highlights
The pixel tracker of the Compact Muon Solenoid (CMS) experiment is the innermost sub-detector, located close to the collision point, and is used for reconstruction of the tracks and vertices of charged particles
Due to the radiation damage and significant data losses due to high occupancy in the readout chip of the pixel detector, the present system must be replaced by a new one in an extended end-of-year shutdown during winter 2016/2017 in order to maintain the excellent tracking and other physics performances
The addition of one tracking layer and the increase of the outer radius covered by the pixel detector result in a doubling of the total number of channels, from 48M to 79M for BPIX, and from 18M to 45M for FPIX
Summary
The new detector features new digital readout chips with larger buffers and increased data transmission bandwidth to overcome the main limitation of the current detector. The geometrical layout of the upgrade system consists of four cylindrical barrel layers placed at radii of 29, 68, 109, 160 mm and three disks in each of the forward regions placed at a distance from the nominal interaction point of 291, 396 and 516 mm This layout is optimised in order to offer full 4-hit tracking coverage up to pseudorapidities of 2.5, with an increased redundancy compared to the present system. The amount of material of the upgraded detector is comparable to the one of the present detector in the central region, despite the addition of an extra detector layer, and it is significantly reduced at higher pseudorapidities This is achieved due to the usage of the new light-weight support structures and to a new CO2 evaporative cooling system, replacing the present C6F14 monophase cooling system, and optimised layout of the services. The sensor is bump-bonded to an array of 2 × 8 ROCs comprising 66 560 pixels
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