11.4 Particle accelerators and detectors  (Page 4/10)

where $E_1$ and $E_2$ are the total energies of the incoming particles (1 and 2), $p_1$ and $p_2$ are the magnitudes of their momenta, and $m_1$ and $m_2$ are their rest masses.

Higher beam energies require larger accelerators, so modern colliding beam machines are very large. The LHC, for example, is 17 miles in circumference ( [link] ). (In the 1940s, Enrico Fermi envisioned an accelerator that encircled all of Earth!) An important scientific challenge of the twenty-first century is to reduce the size of particle accelerators.

Particle detectors

The purpose of a particle detector    is to accurately measure the outcome of collisions created by a particle accelerator. The detectors are multipurpose. In other words, the detector is divided into many subdetectors, each designed to measure a different aspect of the collision event. For example, one detector might be designed to measure photons and another might be designed to measure muons. To illustrate how subdetectors contribute to an understanding of an entire collision event, we describe the subdetectors of the Compact Muon Solenoid (CMS), which was used to discover the Higgs Boson at the LHC ( [link] ).

The beam pipe of the detector is out of (and into) the page at the left. Particles produced by pp collisions (the “collision fragments”) stream out of the detector in all directions. These particles encounter multiple layers of subdetectors. A subdetector is a particle detector within a larger system of detectors designed to measure certain types of particles. There are several main types of subdetectors. Tracking devices determine the path and therefore momentum of a particle; calorimeters measure a particle’s energy; and particle-identification detectors determine a particle’s identity (mass).