On track with CMS

CMS has successfully detected cosmic rays for the first time in a complete sector of muon chambers. These events mark a major milestone in commissioning the CMS Barrel detector.


The CMS barrel seen from its +Z end in the SX5 building at Cessy, where two out of five wheels of the barrel are already equipped with muon chambers. On the right, a snapshot of the online display showing a cosmic muon (white points) through the four DTCs (MB1-MB4) of a complete sector. In each DTC, only eight layers of drift tubes can be seen, those of the transversal plan R-phi.

One of the key elements for the Compact Muon Solenoid (CMS) experiment is the precise detection of muons, as the acronym indicates. Muons are expected to give a clear signature for various physics discoveries, including the Higgs boson. A total of 1400 muon chambers of several types will be used by CMS, including the Drift Tube Chambers (DTCs) for the barrel.

Since the summer of 2004, the team building the DTCs (including institutes in Aachen, Bologna, Madrid, Padova, and Torino) has been engaged in the delicate and complex operation of installing the chambers in the CMS iron yoke.

The DTCs have 12 layers of drift tubes arranged in three groups of four, the one in the middle measuring the coordinate z along the beam direction, and the other two measuring the orthogonal coordinate, known as R-phi. Each layer has up to 60 tubes. Together this information reveals the trajectories of the muons. The image shows muon signals in the R-phi groups in four chambers. Unlike conventional Drift Tube systems, consecutive layers are staggered by a half tube width enabling the DTCs to generate trigger signals for CMS (using a 'mean timer' method).

Chambers from the construction sites are sent to a hall in the former ISR tunnel at CERN, where they are carefully assembled with on-chamber cables and pipes and equipped with the 'Minicrates' containing the readout electronics for the chamber. These 'dressed' chambers require very few external components in order for them to become operational and undergo thorough pre-installation tests.

The chambers are then inserted in the CMS iron yoke in the SX5 building at Cessy and connected to the CMS infrastructure. This operation has to be planned well in advance since chambers are assembled with pre-defined locations in mind. At this point the chambers are ready for final commissioning tests, including long data-taking runs that exploit the chambers' self-triggering capability on cosmic muons. So far two out of the five wheels constituting the CMS barrel yoke have been instrumented, and 82 DTCs commissioned.

The operation went very smoothly; however, this isn't the end of the story! Each chamber needs to be interfaced to the full CMS trigger and data acquisition system, an operation that will take many months. The first sector test marked the beginning of a long series of tests that will culminate in spring of 2006 with the CMS 'Cosmic Challenge'; at this moment the five barrel wheels and the two endcaps will be pushed together in the surface building and the superconducting solenoid operated for the first time. Segments of all sub-detectors will be present and cosmic muons will be detected and measured through a complete slice of CMS, including the data acquisition and triggering systems.

The spectacular operation of lowering the enormous sections of CMS into their underground cavern will begin in summer this year.