Located next to the proton-proton interaction point, the ATLAS vertex detector is instrumental in reconstructing the trajectories of charged particles and allows to measure precisely the position of the decay vertex of some long-lived particles such as the hadrons containing a b quark.

This detector uses silicon sensors divided in small pixels (50 × 400 μm²), each pixel being connected to its own read-out electronics. The pixel detector consists of three cylinders (at 5, 9 and 12 cm in radius) and three disks on each end, providing a coverage up to |η|=2.5. Overall, the detector comprises more than 80 million pixels.

This innovative technology was proposed for the ATLAS detector by our group in 1992. The group played a major role in several aspects of the R&D phase, in particular in radiation-hard micro-electronics and in precision mechanics. Among its contributions to the final design on can note the pixel analog cell for signal amplification and discrimination and the carbon fiber support structure for the modules. Several simulation studies to optimize the layout were conducted at the lab, notably by assessing the impact on b-tagging performance. The group was also involved in beam-tests of the prototypes and developed data acquisition, vizualisation and control software systems. Finally, the evaporative cooling system used for the complete ATLAS silicon tracker was originally designed at CPPM.

In September 2006 the group finished its contribution to the final detector construction by delivering more than a third (42/112) of the ladders of the pixel barrel system, each ladder being equipped with 13 modules of 46080 pixels each. The complete pixel detector is installed in ATLAS since June 2007 and is operational. The CPPM group is now involved in its commissioning, for the time being using cosmic rays while waiting for the first LHC collisions.

Even though the active pixel technology is highly radiation-hard, the innermost pixel layer, dubbed the "b-layer" because of the crucial role it plays for b-tagging, must be replaced in 2014 (SLHC project phase I). The CPPM group is involved in the Insertable B-Layer project and contributes to several topics: the design of the front-end electronics, the ladder mechanics, the cooling and numerical simulations of the physics performance. The group is also exploring 3D electronics for the full replacement of the tracker in the SLHC phase II (2017).

More information is available on our activities and about the ATLAS pixel detector.