Commissioning and first performance studies of the new CMS pixel detector

In the previous months the new CMS pixel detector was brought into operation. The detector was moved from PSI to CERN and installed in February, followed by an intensive period of commissioning and calibration. This process mostly involved the adjustment of many operational parameters which influence the detector performance, e.g. the individual pixel thresholds had to be optimized for each pixel in order to achieve the best possible detector resolution – a challenging task, as there are 80 million pixels which operate at a readout speed of 40 MHz and have to cope with rates of up to 500 MHz/cm² in the inner part of the detector. The detector consists of 4 cylindrical barrel layers located at radii between 3 cm and 16 cm around the LHC collision point. The inner most layer is equipped with a newly developed readout chip (PROC600) which has a very different readout architecture compared to the readout chip used in the other layers. As the inner layer has to deal with the highest rates it took a considerable time to understand and learn how to efficiently operate this part of the detector.

After this intense phase of commissioning and calibration the pixel detector has been included in the CMS data acquisition and first performance studies were done. The two most important performance parameters are the hit efficiency and the position resolution.

The efficiency is higher than in the previous pixel detector: It is above 99%, except for the inner layer, where high-rate related data losses limit the efficiency to 97% at the highest LHC collision rates. The achieved position resolutions are very good, too, and illustrated in the two plots: The detector can measure the position of charged particles with an accuracy of ~12 μm in the r-phi direction and ~22 μm in the z-direction.