On 11 November 2008 the conclusion of a month-long, major data-taking run by the CMS collaboration brought a two-year commissioning phase to a successful close. The aim of the Cosmic Run At Four Tesla (CRAFT) was to run CMS continuously as a complete experiment, 24 hours a day, to gain further operational experience even without LHC beams. Data from 300 million cosmic muons were recorded with the solenoid at its operating point of 3.8 T for detailed detector studies. By the end of the exercise more than 7 million tracks in the strip tracker and around 75,000 tracks in the pixel tracker were available for alignment and other studies. The data volume totalled an impressive 400 TB. Runs were reconstructed at the Tier-0 centre with a typical latency of six hours before shipping to several Tier-1 and Tier-2 centres.
The CMS data flow was stressed during CRAFT in a way similar to what is foreseen for LHC operations, with calibration and/or alignment sequences performed for the electromagnetic calorimeter, the tracker and the muon systems during the run. Random triggers added on top of the cosmic-muon triggers emulated the trigger rates that will be experienced at the LHC. The high-level trigger ran a menu similar to the one used for the LHC start-up, with the installed complement of nearly 4500 filter processors for the CMS filter farm (around 40% of the final number) being deployed for the first time at the end of the run. Along with the main cosmic data, special raw-data streams created for specific calibration and alignment purposes were shipped to the Tier-0 centre. Teams residing at the CMS analysis facility in Meyrin and at remote centres including DESY and Fermilab checked the data quality offline and validated the online quality-assignments of the data-quality monitoring system.
The precision of tracker alignment previously obtained with data recorded without a magnetic field is now improving significantly with the data collected during CRAFT because the momentum measurement enables better control of the uncertainty that arises from multiple scattering. The run also allowed an initial alignment of the modules comprising the barrel pixel detector. The collaboration completed a first reprocessing of the CRAFT data, incorporating these newly determined calibration and alignment constants, in early December 2008. Several analysis teams will use these data to perform some basic physics measurements, including measurements of the charge ratio and momentum distribution of cosmic muons.
The success of the continuous operation of CMS in “LHC-like” conditions marks the end of a commissioning phase that started two years ago: in November 2006 both the underground experimental cavern and the adjoining service cavern (which is now buzzing with all of the off-detector readout electronics) were empty. The CMS teams are looking back with understandable pride at what has been achieved since then and are looking forward to the challenges of operation with colliding LHC beams in 2009. The commissioning programme to improve the readiness for LHC physics will resume after the annual cooling maintenance, which is expected to take place in late January 2009.