Welcome to Hall B
The CEBAF accelerator is shaped like a horse race-track. For race-fans, it is 7 furlongs in
length (7/8 of a mile). It has two straight sections in which a beam of electrons is pumped up
in energy by special microwave cavities. The beam from one straight section is then steered
around a curve by guide magnets into the next straight section. After going around the
race-track five times, each electron in the beam has an energy of 6 billion electron-volts.
For comparison, the beam of electrons in a television tube has about 20 thousand electon-volts
of energy. All of this energy is tightly focussed. The beam of electrons is about the width of
a human hair — only about 0.1 mm wide. At this point, special "kicker" magnets send the beam to
the experimental area.
What is Hall B?
There are three experimental areas at JLab; Halls A, B and C (we physicists aren't as creative in naming our equipment as we are at naming new particles, like cascade baryons, strange and charmed quarks and gluons). Hall B is the site of the CEBAF Large Acceptance Spectrometer (CLAS) detector. The detector was built and is run by a collaboration of nearly 150 physicists from more than 30 universities in the U.S., Europe, the former Soviet Union and Korea. It took about ten years and $30 million to build, and it has been taking data since December, 1997.
Powerful magnets steer the electron beam into a target in the experimental hall. The beam's
individual electrons smash into the protons and neutrons inside the nuclei of atoms in the
target. These violent collisions produce new particles; heavier
versions of the familiar protons and neutrons as well a whole variety of
intermediate mass particles called "mesons". The outgoing electron
which collided with the target nucleus as well as the produced particles
go flying out into our detector, where they're measured. Our job as particle physicists
is to use these measurements to try to deduce the underlying structure of protons and
neutrons in the target and to try to understand the forces that create these particles.
CEBAF Large Acceptance Spectrometer (CLAS)
The CLAS detector is unique in that it has a very large acceptance; in other words, we can measure the momentum and angles of almost all of the particles produced in the electon-proton collisions. Roughly spherical, the detector measures 30 feet across. It completely surrounds the target, which is typically a small vial of liquid hydrogen (hydrogen's nucleus is comprised of a single proton) or deuterium (with a nucleus consisting of a neutron and a proton).
The CLAS detector is built like an onion, with successive layers of different types of particle detectors. As the particles that fly out of the target enter the detector, their paths are bent by the detector's magnet. The particles first enter devices called wire chambers which measure the curved paths of these particles to determine the particles' momentum.
Next, a layer of detectors measure the time of arrival of the particles. By dividing the path length of a particle by the time of travel, we get its speed. Now we know the momentum and speed of the particle and can figure out its mass. Since different particles have different masses, we know its identity! The CLAS detector also contains special detectors ("Cherenkov" counters and "electromagnetic calorimeters") whose purpose is to distinguish electrons from other types of particles.
Each electron-proton collision is called an "event". A computer records each event measured by the particle detectors, about 2000 events per second on average. This data is then transferred to a "farm" of computing processors. A team of physicists and students analyze the events, looking for new kinds of particles or evidence for the underlying structure of the proton.