BaBar Experiment - Detector Description

Detector Description

The BaBar detector is cylindrical with the interaction region at the center. In the interaction region, 9 GeV electrons collide with 3.1 GeV antielectrons (sometimes called positrons) to produce a center-of-mass collision energy of 10.58 GeV, corresponding to the ϒ(4S) resonance. The ϒ(4S) decays immediately into a pair of B mesons – half the time B+B− and half the time B0B0. To detect the particles there are a series of subsystems arranged cylindrically around the interaction region. These subsystems are as follows, in order from inside to outside:

• Silicon Vertex Tracker (SVT)

Made from 5 layers of double-sided silicon strips, the SVT records charged particle tracks very close to the interaction region inside BaBar.

• Drift Chamber (DCH)

Less expensive than silicon, the 40 layers of wires in this gas chamber detect charged particle tracks out to a much larger radius, providing a measurement of their momenta. In addition, the DCH also measures the energy loss of the particles as they pass through matter. See Bethe-Bloch formula.

• Detector of Internally Reflected Cherenkov Light (DIRC)

The DIRC is composed of 144 quartz bars which radiate and focus Cherenkov radiation to differentiate between kaons and pions.

• Electromagnetic Calorimeter (EMC)

Made from 6580 CsI crystals, the EMC identifies electrons and antielectrons, which allows for the reconstruction of the particle tracks of photons (and thus of neutral pions (π0)) and of "long Kaons" (K
L), which are also electrically neutral.

• Magnet

The Magnet produces a 1.5 T field inside the detector, which bends the tracks of charged particles allowing deduction of their momentum.

• Instrumented Flux Return (IFR)

The IFR is designed to return the flux of the 1.5 T magnet, so it is mostly iron but there is also instrumentation to detect muons and long kaons. The IFR is broken into 6 sextants and two endcaps. Each of the sextants has empty spaces which held the 19 layers of Resistive Plate Chambers (RPC), which were replaced in 2004 and 2006 with Limited Streamer Tubes (LST) interleaved with brass. The brass is there to add mass for the interaction length since the LST modules are so much less massive than the RPCs. The LST system is designed to measure all three cylindrical coordinates of a track: which individual tube was hit gives the φ coordinate, which layer the hit was in gives the ρ coordinate, and finally the z-planes atop the LSTs measure the z coordinate.