CMS Pixel Detector

The CMS Si-pixel tracker, Fig. 1, is located at the center of the CMS detector. It will be used to track charged particle near the interaction region, provide important pattern recognition aide to the Si tracker, provide primary and secondary vertex information to the high level trigger (HLT) for selection of b-jet end τ-jet events, and finally make major contributions to offline analyses.

The pixel detector has the capability of L1 triggering through a FAST OR output on each pixel chip. This feature has not been implemented and is presently under study by the Mississippi group for the forward detectors.


Figure 1: CMS Pixel Detector: 2x2 Configuration.

A single pixel detector, Fig. 2, consists of an array of 150x150 micron2 pixels bump bonded to a pixel readout chip. Each sensor contains a 52x53 array or 2756 pixels. The forward pixel detector alone will use a total of about 6500 sensors or about 18M pixels. After receiving an L1 trigger hit pixels are recorded during a double column scan orchestrated by an external token bit manager readout chip. The data is then piped out through optical links to the DAQ. A slow control system constantly monitors the pixel systems, passing on alarm information. It is also through this slow control link that DAC thresholds are uploaded to the chips. All electronics near the interaction region must be fabricated in radiation hard 1/4 micron CMOS technology.


Figure 2: Pixel detector unit.

The present baseline detector has two barrel and two forward layers, but space is provided for 3 forward and 3 barrel. The pixel hits provide a 25 μm impact parameter resolution when combined with hits in the CMS silicon strip tracker, providing vertex tagging and triggering possibilities. This apparatus is to be installed in the CMS detector at CERN in 2008.

Mississippi has L3 responsibility for the forward pixel detector cooling. Working closely with Northwestern U. and Fermilab, we have performed cooling tests and hydrodynamic calculations to insure safe operation of the detector. This work has ended recently as the design has matured. Cooling system tests at Fermilab are beginning and we are participating in the analysis and design.

Our machine shop has provided the first prototypes of the blade-like mechanical structure and continues to work with engineers at Fermilab on the cooling tube design. Our proposal for brazing of the tubes is the leading solution at this time. We are presently working with Alpha Braze in Freemont CA on the prototyping.

In Nov 2001 we were asked to present a pixel position optimization plan to the CMS tracker establishment. This expertise grew out of our intimate knowledge of the pixel material budget and geometry. We installed the latest materials and geometry in to the CMSIM Monte Carlo package and reported the studies at CMS tracker week in Feb 2002. We continue to manage the materials database and geometry implementation for the foreseeable future.

In Spring 2002 updated CMSIM (CMS GEANT) package was used to generate CMS Monte Carlo for the official Trigger/DAQ TDR just released. Our group used it's experienced in building large disk arrays with software and hardware RAID solutions to quickly provide two 1 TB RAID arrays to this TDR effort.

We have taken a lead role in detector calibration and monitoring issues, by presenting a plan just recently (Oct 2002) at the 3rd CMS Pixel Workshop at PSI. Many of our ideas are under consideration. Special attention was given to the possible use of RAID arrays in the frequent downloading of detector constants, as each pixel is individually trimmed and large storage capability is needed.

L. Cremaldi gave the overall CMS Pixel presentation at the Vertex 2002 Conference in November 02.