A very busy year of shutdown activities comes to an end, bringing us to the half-way-point of LS1.

The most visible change of CMS is an additional fourth shielding disk on the +z end (pointing towards the Jura) of the detector, which is currently parked at the head wall of the cavern. On the outside of the next disk, new CSC and RPC chambers have been mounted, completing the fourth layer of muon detectors, one of the major objectives of LS1. The installation of the shielding disk on the other end will take place in spring and the installation of the remaining muon chambers is foreseen towards the end of LS1.

Figure 1. To replace the central beam pipe in CMS the entire beam pipe has to be removed and reinstalled. As the beam pipe is very fragile and almost always in the way, it has been first, before any other major work started. Here the part called “endcap beam pipe” has been removed and is craned away.

The most important objective of LS1 for CMS is to prepare the detector for cold operation of its Silicon Tracker (strips and pixel) with cooling fluid temperatures around -20⁰C. The Silicon detectors themselves are constructed for these operation temperatures but all cooling lines from the cooling plant to the detector entrance have to be either sealed perfectly or put into an atmosphere of extreme low humidity. In particular inside the vacuum tank of the magnet the space is very limited making the insulation very difficult. To keep the atmosphere dry a special plant has been built that can provide up to

400 Nm³/h dry-air or N₂ from the atmosphere that can be pumped into the vacuum tank and onto the bulkhead of the Tracker where all lines pass through. Once more the modular structure of CMS was a big advantage allowing parallel access inside the vacuum tank and to the muon chambers all over the detector, to the calorimeters and to the periphery. A crucial test of the tremendous work on sealing and instrumenting the tracker cooling lines will take place end of January when the system will be run cold for the first time with the detector completely open and all lines being exposed to be able seeing any ice or condensation developing.

During the same time a major investment into the future of CMS will be made with the installation of vacuum insulated cooling pipes for the next 4-layer Pixel Detector, which will be installed in the Extended-Year-End-Technical-Stop of 2016/17. Also as preparation for this new Pixel Detector, CMS will install a new central beam pipe with only 45mm outer diameter.

Figure 2: The photograph shows the installation of the 4^th layer of CSC chambers on the +z end of CMS

During this first year of LS1 the maintenance and repair work of almost all components on the –z end of CMS (pointing towards the lake of Geneva) has been performed and the installation work on the +z end. In 2014 the same has to be done in the mirror configuration. To make sure the work on the hardware was successful the detector is operated whenever possible and safe. Thanks to the efficient work and the flexibility of the technical groups it was possible to perform the large maintenance program on the infrastructure  (electricity, cooling, HVAC etc.) at the very beginning of LS1. Since begin of October around the clock shift operation has been resumed at PT5 allowing an efficient timely commissioning.

This dense program can only be performed because the up to 1400t heavy elements can be moved around in the hall with a few millimeters precision.

At the beginning of LS1, we were concerned whether the more than 100 movements of heavy elements that were necessary throughout LS1 can be performed safely within the limited time foreseen. After having invested some time and effort into precision of the movement system in spring 2013 and having a well-trained and experienced crew of experts for these operations, this concern vanished.

At the end of the first year of LS1, CMS has completed approximately half of its upgrade program. There is still a lot of work ahead of us. But trusting in the expertise and motivation of the teams working at PT5 and using the experience gained in 2013, CMS is confident to come out of LS1 with a detector well prepared for the challenges ahead of us.