CLAS detector

Introduction

CEBAF Large Acceptance Spectrometer (CLAS) is a nuclear and particle physics detector located in the experimental Hall B at Jefferson Laboratory in Newport News, Virginia, USA. It is used to study the properties of the nuclear matter by the collaboration of over 150 physicist (CLAS Collaboration) from many countries all around the world.

Powerful magnets steer the electron beam from the accelerator of Jefferson Laboratory 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 are measured. Particle physicists 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.

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 electron-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 composed of a single proton) or deuterium (with a nucleus consisting of a neutron and a proton).

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.

Detector Description

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.

Collaborating Institutions

• Catholic University of America - Washington, DC
• Florida International University - Miami, FL
• George Washington University - Washington, DC
• James Madison University - Harrisonburg, VA
• Moscow State University - Moscow, Russia
• Old Dominion University - Norfolk, VA
• Rensselaer Polytechnic Institute - Troy, NY
• The College of William and Mary - Williamsburg, VA
• University of Connecticut - Storr, CT
• University of New Hampshire - Durham, NH
• University of Richmond - Richmond, VA
• University of Virginia - Charlottesville, VA
• Yerevan Physics Institute - Yerevan, Armenia

See also

• LEPS
• HERA
• HERMES
• SVD

External links

• Official Hall-B web page at Jefferson Laboratory