CMS Bulletin

RUN COORDINATION

With the analysis of the first 5 fb^–1 culminating in the announcement of the observation of a new particle with mass of around 126 GeV/c², the CERN directorate decided to extend the LHC run until February 2013. This adds three months to the original schedule. Since then the LHC has continued to perform extremely well, and the total luminosity delivered so far this year is 22 fb^–1. CMS also continues to perform excellently, recording data with efficiency higher than 95% for fills with the magnetic field at nominal value.

The highest instantaneous luminosity achieved by LHC to date is 7.6x10³³ cm^–2s^–1, which translates into 35 interactions per crossing. On the CMS side there has been a lot of work to handle these extreme conditions, such as a new DAQ computer farm and trigger menus to handle the pile-up, automation of recovery procedures to minimise the lost luminosity, better training for the shift crews, etc. We did suffer from a couple of infrastructure issues that resulted in the magnet being at values lower than nominal for short periods of time.

Some time was spent performing Van der Meer scans to get a precise measurement of the luminosity. A couple of special runs were taken with large β* (90 m and 1000 m) for diffractive physics, during which CMS and TOTEM were able to successfully exchange triggers to read out the same collisions in both experiments. A last Van der Meer scan with more Gaussian beams was performed in November to compare the results with the previous one and try to reduce the systematic error on the absolute luminosity determination.

At the end of this year the LHC will explore operation with a bunch spacing of 25 ns. They will first spend some days scrubbing, then study the beam-beam effects in “machine development” mode, and finally deliver collisions in a 25ns pilot run. The number of bunches and pile-up for the physics fill will depend on the outcome of the scrubbing and machine development periods. All this will be very useful to understand possible issues for operating at 25 ns after LS1.

The proton-proton data-collection period is scheduled to end on 17 December 2012. On 7 January 2013, only three weeks later, the machine will begin preparations for the proton-lead run that will last until mid-February. CMS will start powering on again on 3 January to allow sub-detectors to be ready to take cosmic rays by 9 January, just before the beams are expected to be reinjected into the LHC. Christmas will be short this year for the operations crew.

After the proton-lead run we will hold a Run Coordination workshop in Torino (13–15 February). Goals for the workshop include a review of the operational developments, experience, and system performance during 2012. We will plan the CMS commissioning activities during LS1, and examine the challenges and identify the limitations for operation in the different possible running scenarios (e.g. 50 ns vs. 25 ns) after LS1.

We are also preparing to get cosmics data in different magnet (ON/OFF) and trigger (all, bottom-only) configurations to get a snapshot of CMS before the Long Shutdown. During the length of the shutdown we will only be able to take cosmics data.