1 Introduction

The first aim of the design of the CMS detector [1] was the construction of a moderate size detector optimized for muon measurement. The collaboration adopted a solution with a 4T superconducting solenoid having the calorimeters confined inside the coil and the iron yoke used for muon detection as shown in Figure 1.

This kind of solution imposes several important constraints (mainly on the dimensions and on the usable materials) for the design of the detectors inside the coil, leaving instead an almost absolute engineering freedom for the muon chamber project.

In the chosen layout four muon stations are interleaved with the iron of the yoke to make full use of the magnetic return flux (~1.8T) for a muon momentum standalone measurement in order to simplify the matching of the muon reconstructed track to its image inside the inner tracker.

The main drawback of a scheme with muon chambers packed close to the iron is the presence of an important electromagnetic background due to showering in the iron induced by muon bremsstrahlung, that complicates the track reconstruction.

Together with the unavoidable cracks introduced for the supports and the cabling of large detectors, this background is the most important reason for the choice of an highly redundant design.

Figure 1: Longitudinal view of CMS detector ( Very Forward Calorimeter not shown).