CLAS detector
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 200 physicist (CLAS Collaboration) from many countries all around the world.
The 0.5 to 12.0 GeV electron beam from the accelerator of Jefferson Laboratory is brought into "Hall B", the experimental hall that houses the CLAS system. The beam's individual electrons collide with the protons and neutrons inside the nuclei of atoms in the physics "target" located at the center of CLAS. 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 fly out into the detector, where they are measured. Particle physicists use these measurements to deduce the underlying structure of protons and neutrons and to better understand the forces that create these particles.
The CLAS detector is special among devices in this branch of particle physics in that it has a very large acceptance; in other words, it 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 cylinder of liquid hydrogen (hydrogen's nucleus is composed of a single proton) or deuterium (with a nucleus consisting of a neutron and a proton).
Two categories of experiments were carried out with CLAS: using electrons in the beam and using photons in the beam. Experiments using electron scattering primarily probe the structure of protons and their excitations at various "length scales". Experiments using real photon beams primarily probe the production and decay of mesons and excited baryons.
Each particle-target collision is called an "event". An elaborate data acquisition system records each event measured by the particle detectors, up to several thousand events per second on average. This data is then transferred to a "farm" of computing processors. Teams of physicists analyze the events, looking for new kinds of particles or information related to 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
- Arizona State University - Tempe, AZ
- California State University - Dominguez Hills, CA
- Carnegie Mellon University - Pittsburgh, PA
- Catholic University of America - Washington, DC
- CEA-Saclay - Gif-sur-Yvette, France
- Christopher Newport University, Newport News, VA
- Edinburgh University - Edinburgh, UK
- Florida International University - Miami, FL
- Florida State University - Tallahassee, FL
- George Washington University - Washington, DC
- Idaho State University - Pocatello, ID
- INFN, Laboratori Nazionali di Frascati - Frascati, Italy
- INFN, Sezione di Genova - Genova, Italy
- Institut de Physique Nucléaire - Orsay, France
- ITEP - Moscow, Russia
- James Madison University - Harrisonburg, VA
- Kyungpook University - Daegu, South Korea
- Moscow State University - Moscow, Russia
- Norfolk State University - Norfolk, VA
- Ohio University - Athens, OH
- Old Dominion University - Norfolk, VA
- Rensselaer Polytechnic Institute - Troy, NY
- Rice University - Houston, TX
- The College of William and Mary - Williamsburg, VA
- Thomas Jefferson National Accelerator Facility - Newport News, VA
- Union College - Schenectady, NY
- Universidad Técnica Federico Santa María - Valparaíso, Chile
- University of California Los Angeles - Los Angeles, CA
- University of Connecticut - Storrs, CT
- University of Glasgow - Glasgow, Scotland
- University of Massachusetts - Amherst, MA
- University of New Hampshire - Durham, NH
- Université Paris-Sud 11 - Orsay, France
- University of Richmond - Richmond, VA
- University of South Carolina - Columbia, SC
- University of Virginia - Charlottesville, VA
- Virginia Polytechnic Institute - Blacksburg, VA
- Yerevan Physics Institute - Yerevan, Armenia