The CMS Magnetic Field Map (III): Development of the CMS Magnetic Field Map

This chapter is the third part in a series of publications on the Compact Muon Solenoid (CMS) detector magnetic field map creation. The chapter focuses on pioneering work on the performance of the three-dimensional (3D) magnetic field map in the entire volume of the CMS detector at the Large Hadron Collider at CERN. In the CMS detector the magnetic field deflects the charged particles produced in the proton–proton collisions at the center-of-mass energy of 13.6 TeV. The curvatures of the charged particles allow the measurements of the particle momenta with help of the silicon tracking detectors located in the solenoidal magnetic flux density of 3.81 T. The magnetic system of the CMS detector is of a heterogeneous type, where the magnetic flux is created by a superconducting solenoid coil enclosed in a steel flux-return yoke. The 10,000-ton steel yoke of the magnet is used as a series of magnetized layers up to 620 mm thick which are penetrated only by muons, making it possible to identify them and measure their momenta in a muon spectrometer. The programs for simulation and reconstruction of the momenta of the charged particles emerging from collision events require the knowledge of the value of the magnetic flux density components at the coordinates of space points along the trajectories of the particles. To describe the CMS magnetic flux distribution in the entire CMS detector volume, a system of the primitive 3D volumes containing the values of the magnetic flux density measured inside the superconducting coil inner volume and modelled outside the coil across a special mesh of reference nodes was developed. This system, called the CMS magnetic field map, follows the geometric features of the yoke and allows the interpolation of the magnetic flux density between the nodes to obtain the magnetic field values at any spatial point inside a cylinder of 18 m in diameter and 48 m in length, where all the CMS sub-detectors are located. The geometry of the volumes is described inside one 30° azimuthal sector of the CMS magnet. To obtain the values of the magnetic flux density components across the entire azimuth angle of the CMS detector, rotational symmetry is applied. Volumes are organized in a hierarchical structure optimized for fast global searching, and caching techniques allow simulation and track extrapolation algorithms to minimize the number of global volume searches.