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Welcome to Hall B

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How Experiments Work at JLab

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 four experimental areas at JLab; Halls A, B, C, and D (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 was the site of the CEBAF Large Acceptance Spectrometer (CLAS) detector and is now the site of an upgraded version of that spectrometer called CLAS12 that was built as part of the JLab 12 GeV upgrade project. The detectors were built and run by a collaboration of nearly 150 physicists from more than 30 universities in the U.S., Europe, South America, the former Soviet Union, and Korea.

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 that 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

CEBAF Large Acceptance Spectrometer (CLAS)

The CLAS detector operated in Hall B in the period from 1997 to 2012. This detector was unique in that it had a very large acceptance; in other words, we could measure the momentum and angles of almost all of the particles produced in the electon-proton collisions. Roughly spherical, the detector measured 30 feet across. It completely surrounded the target, which was 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 was built like an onion, with successive layers of different types of particle detectors. As the particles that flew out of the target entered the detector, their paths were bent by the detector's magnet. The particles first entered devices called wire chambers that measured the curved paths of these particles to determine the particles' momentum.

Next, a layer of detectors measured the time of arrival of the particles. By dividing the path length of a particle by the time of travel, we get its speed. From the measured momentum and speed of the particle, we can determine its mass. Since different particles have different masses, we know their identity! The CLAS detector also contained special detectors ("Cherenkov" counters and "electromagnetic calorimeters") whose purpose was to distinguish electrons from other types of particles.

Each electron-proton collision is called an "event". A computer recorded each event measured by the particle detectors, about 2000 events per second on average. This data was then transferred to a "farm" of computing processors. A team of physicists and students analyzed the events, looking for new kinds of particles or evidence for the underlying structure of the proton.

CLAS12 Spectrometer

CLAS12 Spectrometer

The new CEBAF Large Acceptance Spectrometer for 12 GeV (CLAS12) was constructed in Hall B as part of the JLab 12 GeV upgrade project. The construction of the detector took place in the period from 2012 to 2017. This large acceptance detector system is quite similar to the CLAS detector that it replaces, except that it was built to study higher energy reactions and at higher event rates compared to CLAS.

While the CLAS spectrometer resembled a sphere surrounding the beam-target interaction region, the new CLAS12 spectrometer, while still spanning roughly the same angular regions as CLAS, is more forward directed. This is because it was built to detect higher energy particles created in reactions with higher energy beams of electrons.

The CLAS12 detector system consists of both a Forward Detector (FD) and a Central Detector (CD). At the heart of the FD is the new superconducting Torus magnet. As for the Torus magnet in CLAS, the new magnet consists of six superconducting coils arranged symmetrically in azimuth, but unlike CLAS, covers only the range of 5o to 40o in polar angle. The FD system consists of detectors to measure the trajectories of scattered charged particles, and detectors to identify both charged and neutral particles created in the interactions of the beam electrons with the target. The detector system positioned just after the torus magnet are roughly 7 m away from the target.

The CD is based on a compact superconducting Solenoid magnet with a maximum central magnetic field of 5 T. The Solenoid magnet provides momentum analysis for charged tracks at polar angles from 35o to 135o. It also protects the tracking detectors from intense background electrons and acts as a polarizing field for polarized solid-state targets. All three functions require a high magnetic field along the beam axis. The overall size of the Solenoid is restricted to 200 cm in diameter, which allows a maximum opening for placement of detectors of only 100 cm in diameter. The CD consists of tracking systems to measure the trajectories of charged particles created in the beam interactions within the target, as well as systems to detect both charged and neutral particles very similar to those within the FD but much smaller in scale given that they are located within 0.5 m from the target.