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. Electrons or photons in the incoming beam collide with the nuclei of atoms in the physics "target" located at the center of CLAS. These collisions generally produce new particles, often after the target nucleons (protons and neutrons) are briefly excited to heavier-mass versions of the familiar protons and neutrons. A whole variety of intermediate-mass short-lived particles called "mesons" can be created. Scattered electron as well as the longer-lived produced particles travel through the CLAS detector, where they are measured. Particle physicists use these measurements to deduce the underlying structure of protons and neutrons and to better understand the interactions that create these new particles.
The CLAS detector system was operational from 1998 until May 2012. From that time onward, analysis of archived data continued for some years, as can be traced in the publications. Since 2012, a similar but new system called CLAS12 was constructed, which is scheduled to start operations with particle beams in 2017.
Overview of Detector Function
The CLAS detector was notable among devices in the area of hadronic particle physics in that it had a very large acceptance; in other words, it measured the momentum and angles of almost all of the particles produced in the electron-proton collisions. Roughly spherical, the detector measured 30 feet across. It surrounded the physics target, which was 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).
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
A diagram of the CLAS detector is shown, as well as a photograph of the detector when it was partially pulled open for maintenance. The CLAS detector was built in a nested, roughly spherical form, with successive layers of different types of particle detectors. As scattered or produced particles leave the the target at the center, charged particle paths are bent by a toroidal magnet field in arcs either toward or away from the beamline. The particles traverse three packages of drift chambers which are used 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.
Physics Program
Two categories of experiments were carried out with CLAS: using electrons in the beam and using so-called real photons created using the electron beam. Experiments using electron scattering primarily probe the structure of protons and their excitations at various sub-nuclear "length scales". Experiments using real photon beams primarily probe the production and decay of mesons and excited baryons.
A list of the scientific and technical papers resulting from the CLAS program is linked at the bottom of this article.
Collaborating Institutions (Cumulative since 1989)
- 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
See also