doctoral thesis

6 CHAPTER 1. INTRODUCTION
more than 0.5 mm from the collision point before they decay. In the case of a colliding
beam experiment they do not even have time to escape from the beam pipe. They can
only be detected by extrapolating the very precisely measured tracks of the more stable
decay products to the secondary vertex (displaced vertex), where they decayed, close
to the collision point 1 . From the remaining 15 particles the following eight (plus the
corresponding antiparticles) are by far the most frequent ones:
electrons, muons, photons, charged pions,
charged kaons, neutral kaons, protons and neutrons.
This leads us to a very basic insight:
The task of modern high energy physics detector systems is to identify eight different
particles (and the corresponding antiparticles) that are crossing the device and
to measure their momenta and/or energy. The same task is repeatedly implemented in
similar ways in all high energy physics experiments.
Fig. 1.2 shows the basic setup of many modern high energy physics experiments.
The reason that detectors are divided into many components is that each component
tests for a special set of particle properties. These components are stacked such that all
particles will go through the different layers sequentially. We summarize the different
tasks of the detector subsystems:
Tracking Chambers: Directions, momenta, and signs of charged particles have to be
measured. Finely subdivided tracking detectors are used to reconstruct charged
particle trajectories. A magnetic field causes the trajectories to bend in circular
paths: the radius of each circle determines the momentum, and the ’bending
direction’ the sign of charge.
Electromagnetic Calorimeter: The energy carried by electrons and photons is measured
by the electromagnetic calorimeter. It is generally subdivided into segments
that absorb the energy of incident electrons and photons, and produce
signals proportional to that energy.
Hadronic Calorimeter: The energy carried by hadrons (protons, pions, neutrons,
etc.) is measured by the hadronic calorimeter. It detects hadronic showers in a
similar way as the electromagnetic calorimeter detects electromagnetic showers.
The hadronic calorimeter is always downstream (outside) of the electromagnetic
calorimeter, due to the much larger interaction length of hadrons.
1 The identification of such a displaced vertex can be used for the tagging of events (τ-, D- or Btagging)
or even for triggering like in the LHCb experiment at CERN [13, 14].