Working at CERN

Between nine thirty and seven or eight o’clock, you’ll find me at CERN Prevessin where I’m testing new electronics for the Muon Endcap Cathode Strip Chambers. The CSCs are part of the CMS detector at CERN.

A Cathode Strip Chamber operates by maintaining an electrical potential difference between two orthogonal sets of cathode strips and anode wires. A Muon, essentially a heavier (i.e. more massive) electron, ionizes a mixture of argon, carbon tetrafluoride, and carbon dioxide. The ionization induces a current which the on-detector electrons record and report to the high-level CMS control. The induced current integrated over time provides charge which should be roughly equal to e, the elementary charge.

If you’re really psyched about the physics of Cathode Strip Chambers, you might be interested in this monograph from Brookhaven National Lab which, in great detail, discusses the induced charge distribution.

The cross-hatching of the strips and wires produces a vaguely Cartesian coordinate grid. Each chamber has seven layers of the gas-wire-strip contraption and each muon endcap has four layers of chambers, providing a third spatial coordinate.

The momentum, and thus the energy, of a muon can be calculated from its trajectory through the detector, as recorded by the cathode strip chambers. The calculation is familiar to all students of physics as the Lorentz force law:

F = q(v × B)

Where () is 4 T, (q) is the elementary charge. The only hitch is that we’re really looking for momentum. Momentum is given by

p = mv = qBr

which follows when the Lorentz force law is equated with the expression for a centripetal force.

$$ q v B = F = \frac{m v^2}{r} $$