The FOCUS experiment, also known as Fermilab Experiment 831 (E831), is a charm photoproduction experiment in the Fermilab fixed target program. Charmed particles are produced by the photon-gluon fusion process in which the virtual charm component of the photon is made real by exchanging a hard gluon with a target hadron (in our case a proton or a neutron). A diagram for this process is shown in intro:pgf.
FOCUS is the extension of Fermilab E687, using the same beamline and many of the same spectrometer components. The primary goal of FOCUS is to reconstruct one million charm decays (the decay modes , , and ), a factor of 10 increase over E687 [21]. This goal was surpassed, enabling FOCUS to perform precision studies of charm decays not possible before.
The experiment took data during the 1996-1997 Fermilab fixed target run. The reconstruction of the data began early in 1998 and was completed by mid-1999.
In order to produce charmed particles with incident photons on a target, high energy photons must first be created. This is a multi-step process which begins with high energy protons. The first section of this chapter describes the proton acceleration process at Fermilab. The second section describes the method used by FOCUS to obtain high energy photons from protons. The final section describes the method used to measure the energy of these photons.