CMS and the CMS Tracker

This page contains an introduction to the CMS experiment and the CMS tracker.

Introduction to the CMS experiment
The CMS tracker

Introduction to the CMS experiment

The CMS experiment is a particle detector, as big as a multi-storey building, built by an international collaboration and located at CERN, Geneva, Switzerland. Its purpose is to detect the particles that are created during proton-proton collisions delivered by the Large Hadron Collider (LHC). Its scientific mission is to

understand the mechanism behind electroweak symmetry breaking (find the Higgs Boson or prove it is not existing),
precisely measure known particles (like the W Boson or top quark mass),
search for new particles that possibly can explain Dark Matter (find supersymmetric particles)
investigate several theories that explain why gravity is so weak compared to electromagnetism (Large Extra Dimensions, Supersymmetry, ...)
and much, much more.

The CMS experiment consists of several components that closely work together to achieve its mission: To measure the energy, momentum, charge and type of the particles created in the proton-proton collisions, which happen in the very center of the experiment. The components are, going from inside to outside:

A pixel vertex detector. It measures the flight distance of short-lived charged particles from their place of birth to the point where they decay.
A strip tracker. It measures the momentum of charged particles.
An electromagnetic calorimeter. It measures the energy of electrons and photons by absorption. Other particles only deposit a small amount of energy.
A hadronic calorimeter. It measures the energy of particles built from quarks and gluons, regardless whether they are neutral or charged.
A magnet with a solenoidal field. It bends the tracks of charged particles, allowing to measure the momentum by curvature measurements in the tracker and the muon spectrometer.
A muon spectrometer. The only charged particles that are not absorbed in one of the previously mentioned detectors are muons. Similar to the strip tracker, the muon spectrometer measures the curvature of the muon tracks and therefore allows to measure their momentum.
The only particles that leave the detector undetected are neutral particles that interact only weakly. The only known particles are neutrinos, but e.g. supersymmetry predicts new particles dubbed "neutralinos" that would leave the detector undetected as well.

Introduction to the CMS tracker

The CMS tracker consists of two parts, the silicon pixel vertex detector and the silicon strip detector. Their names are derived from the sensor technology that they use to detect the particles. Both use thin silicon detectors. In the pixel detector, the silicon is divided in small two-dimensional cells (like in your digital camera) and thus can measure two coordinates (each pixel has an x and a y coordinate). The silicon strip detector is made of parallel strips and thus can measure only one coordinate (it is insenitive along the strip, it cannot tell you where the particle passed the strip, whether it was at the beginning, in the middle or in the end). The CMS pixel detector is built from 1440 modules. Each module has pixel with a pixel size of 100 μm x 150 μm. In total, 66 million pixels are available for the measurements. The CMS strip tracker consists of 15148 modules. About half of the modules contain two silicon sensors.

Author: Martin.Weber@cern.ch Last modified: Wed Aug 18 14:51:12 2010