In Experimental Hall B at the Jefferson Laboratory, Newport News, Virginia, the CEBAF Large Acceptance Spectrometer (CLAS) has opened a new “window” on the building blocks of matter: mesons, nuclei and nucleons. CLAS enables detailed studies of the spectrum of nucleon-excited states. This is a source of vital information about the nucleon’s constituents and the forces between them.
The large-acceptance CLAS serves experiments that require the simultaneous detection of several loosely correlated particles in the hadronic final state, and measurements at limited luminosity. It collects data at the unprecedented rate of 3000 events per second, which is substantially higher than its design goal of 1500 events per second. Six superconducting coils generate its toroidal magnetic field.
A seven-year collaboration between 34 institutions in the US, France, Italy, Armenia, Korea, the UK and Russia built CLAS for use in Hall B – the last of three experimental halls to become fully operational at Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF).
The superconducting radiofrequency CEBAF accelerator, which was originally designed for 4 GeV, but which is now delivering up to 5.5 GeV, provides three simultaneous continuous-wave beams of independent current and independent but correlated energies.
With Jefferson Lab seeking to bridge the gap between quark and hadronic descriptions of nuclear matter, CLAS’s operation fits into a three-hall programme of complementary experiments guided by quantum chromodynamics, the fundamental theory of quark interactions. The laboratory’s earliest experiments began in Hall C in late 1995.
CLAS enables particles to be tracked and identified, and their energy, momentum and initial direction to be defined. It does this by using information provided by drift chambers, Cherenkov counters, scintillation counters and electromagnetic calorimeters.
CLAS will be a crucial tool in Jefferson Lab’s investigation of the quark-gluon structure of the nucleon and, in particular, will facilitate the detailed study of its spectrum of excited states. As in atomic physics, the spectrum of this system contains vital information about the nature of the nucleon’s constituents and the forces between them.
It is not clear why the naive constituent quark model is so successful in explaining the particle spectrum discovered so far. CLAS will either support this model by discovering the complete pattern of states that it predicts, or it will reveal the model’s shortcomings.