CMS Bulletin

TRACKER

Pixel Tracker

At the beginning of May, the Pixel detector was successfully extracted from inside CMS. The operation lasted one and a half days each for the forward and barrel Pixel detectors. Everything went smoothly: new people were trained during the exercise and care was taken to minimise radiation exposure – see Image 3.  Lessons learned were noted in an updated written extraction procedure.  Care was also taken to prepare for reinsertion around the new beam pipe next year, with new alignment targets placed on the barrel Pixel detector. All pieces were lifted to the surface and are now safely stored at low temperatures in the dedicated Pixel laboratory at Point 5 (see Image 4 and previous Bulletin).

Image 3 (a) and (b): Extracted FPIX and BPIX detector

The subsequent maintenance of the forward Pixel detector started on 27 May.  Since then one of four half cylinders has been repaired and, even more importantly, most of the failures have been fully understood. In particular one failure type was related to the loss of signal bandwidth in some of the readout channels, which made the signal impossible to be interpreted by the back-end electronics. The problem was caused by a misalignment between a cable and the analogue high-density interconnector carrying the analogue electrical signals and power to the disks. Another failure type was related to the reliability of programming of some readout chips. Also in this case, the problem was related to the digital high-density interconnector carrying, just as for the analogue one, the digital electrical signals and power to the disks. The team of experts is in the process of searching for means to mitigate future occurrences of such problems.  So far about 50% (corresponding to a detector total of ~3%) of the failures in FPIX have been repaired, and repairs will continue in the coming months.

Image 4: Pixel laboratory at the CMS site in Cessy. The two white boxes contain one forward Pixel half cylinder each. The DAQ/DCS station is located on the left.

The cause of the barrel Pixel detector failures (in total ~2.3% of BPIX channels) are identified to be either broken modules (i.e. broken wire-bonds) or faulty services (e.g. broken analogue-to-optical transducers and disconnected sense-wires). As a conservative approach the team of experts will repair all broken services and all modules, which are easily accessible (i.e. located on the outer-most layer). A thorough risk assessment will be performed in order to understand whether or not to repair modules on inner layers.

The Pixel community is eager to take full advantage of the current LHC long shutdown period (LS1), not only to repair the detector, but also to fully calibrate the detector at different temperatures for a prompt adaptation of the detector performance to any of the possible running conditions that will be faced after LS1. Also Pixel DOC shifts are re-opened – join the fun.

Strip Tracker

CMS is now in the fully open configuration and operation “Tracker Going Cold” is in full swing. The new dry-gas membrane system has been commissioned including connection to the distribution racks underground and installation of new copper pipes up the cryostat. The installation of multilayer pipes up to the bulkhead is imminent. The C₆F₁₄ cooling refurbishment went exceptionally well and reached the final commissioning stage. The humidity-seal inspection inside the vactank has been done and the team is confident to achieve the final sealing by August, with the deployment of new dew-point instrumentation. A complex gas humidity analysis rack, with five chilled mirrors and 26 industrial Vaisala dew-point sensors, has been delivered to Point 5.

The Online/DAQ group has been active at the former Tracker Integration Facility (TIF) in building 186. The Cosmic Rack (C-Rack) detector has been re-commissioned, including its cosmic trigger. The former Magnet and Cosmic Challenge (MTCC) structure is being made operational again to act as an additional and independent development and testing system at the TIF; especially to test failure scenarios within a larger system.

Online software is being developed and tested for various updates scheduled during LS1, such as the transition to Scientific Linux 6 and the new PCI VME interface card allowing a significant reduction in the number of PCs needed.

A first Strip DAQ hands-on tutorial was organised to train future experts on all the commissioning tasks envisioned for the autumn re-comissioning of the Strip Tracker at lower operating temperatures. The participation in the current LS1 activities is a rare opportunity to create the next generation of experts for the coming LHC run.

Studies have commenced on possible improvements for the cluster-finding algorithm in the Front End Drivers (FEDs) to have the firmware ready for occupancies well beyond the initial design specifications.

In the last few months a renewed effort was put into the study of the Strip Tracker detector ageing. The radiation damage corresponding to several integrated luminosity scenarios and its effect on the evolution of Strip modules leakage currents and depletion voltages was modelled. The leakage current modelling was validated against data; while dedicated Noise and Signal Bias scan results are being analysed to validate the depletion voltage evolution model. Strip calibration conditions (bad components, noise and gain) were produced for upgrade luminosity scenarios. These ageing conditions have been the basis for the upgrade studies simulations ongoing for the ECFA meeting. The Strip Calibration and the Tracker Alignment groups are producing improved calibration constants (bad components and alignment) for the legacy reprocessing of the 2011 data. The new, more accurate algorithms are being used to provide the best data quality and both workflows are being included in the Prompt Calibration Loop (PCL) at T0, ready for the next data-taking period. The Tracker alignment framework has been extended to treat position-sensitive calibration parameters, such as Lorentz Angle and Strip backplane corrections, with the MillePede II algorithm.

New people are welcome to join this exciting effort and take the rare opportunity to participate in the re-commissioning of the detector!