The first part of the CMS endcap calorimetry, as seen by particles coming from interactions in the centre of the detector, is the "ECAL Preshower" or "ES" for short. "ES−" and "ES+" are used to distinguish the two ends of this subsystem. The ES is used for particle identification in the endcap regions of CMS. Each ES is made of two orthogonal layers of silicon sensors, interspersed with lead layers that serve to generate electromagnetic showers. In order to withstand the high-radiation environment in CMS, the ES silicon sensors need to be cooled; until now they have operated at around 8–10 °C, but when CMS restarts in 2015 it will need to go down to about −15 °C. At the same time, the exterior of the ES needs to be maintained at a constant +18 °C. The ES thus contains sophisticated cooling and heating systems.
In November 2013 a problem was detected with one section of the ES− heating system. Upon investigation, it was traced to a connector at the exterior of the ES− disc. It was promptly decided to replace all connectors of this type (two on each disc), a process necessitating the removal of the ES discs from CMS.
The removal of ES− from CMS was performed on 29 November 2013 (see video above and photos below) and it was then lifted to the surface hall for repair. This operation was relatively straightforward, with the ES− first being lowered to the cavern floor and placed on a transport frame after which the frame and disc were lifted straight to the surface hall via the large access shaft above the detector.
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The removal of ES+ was more complicated as it is on the opposite side of CMS to the access shaft, and had to be lifted over CMS – and there is not much space! The procedure started on 9 December 2013 and indeed the attachment of the ES+ disc to the crane was too long; the disc could not pass over CMS to be lifted to the surface. It was thus lowered to the cavern floor and attached to a transport frame. The attachment was shortened (by about half a metre) and, the following day, the disc and its transport frame passed safely over CMS before being lifted to the surface.
Both ES discs will be repaired early in 2014 before being reinstalled in the spring.