The BABAR Detector
This page provides background information on HEP detectors in general and the BaBar detector in particular.
A Technical Introduction to Particle-Physics Experiments
A particle physics experiment has two basic components: an accelerator and a detector.
The particle accelerator's job is to produce the high-energy particles. It does this by taking a particle, speeding it up using electromagnetic fields, and crashing it into another particle. At first, only one or two high-energy particles are produced, but these soon decay to many more lower-energy particles, so you end up with lots of particles shooting out from the collision point.
The detector's job is to record information about the particles. A typical particle detector consists of several subdetectors, each of which performs a different type of measurement. Particles from the collision pass through and interact with each subdetector, and the results are recorded.
Most of the particles produced in a collision event are very short-lived, and decay before they make it to the detector. So in general, the detector observes only the most stable end products - the final state particles. These are electrons, muons, photons, pions, charged kaons, or protons. The original decay must be reconstructed based on the measurements from these particles.
Most particle detectors follow the same basic design. Tracking devices in a magnetic field provide measurements of position, charge and momentum for charged particles. Calorimeters provide energy and position measurements. Both subsystems contribute to particle identification. Some experiments also include other subdetectors for particle identification.The CERN Particle Detector BriefBook, written by Bock and Vasilescu, is an very useful online encyclopedia of terms relevant to high energy physics experiments. The reader is encouraged to consult it to learn more about any topics that are unfamiliar.
The BaBar experiment uses two accelerators: the SLAC linac (linear accelerator) and the PEP-II storage ring facility.
The SLAC linac serves as an injector: it accelerates the electron or positron beams to the required high energies, and the injects them into one of PEP-II's storage rings.
PEP-II consists of two storage rings, a High Energy Ring (HER) for the 9.0 GeV electron beam, and a Low Energy Ring (LER) for the 3.1 GeV positron beam. The two beams move in opposite directions and collide at the interaction point, where the BaBar detector is located.
In the center-of-mass frame, the collision energy is equal to the resonance energy of the Upsilon(4S) particle. This means that Upsilon(4S) are produced at a very high rate. The highly unstable Upsilon(4S) decays almost instantly to two B mesons, the particles of interest at BaBar. The mass of the Upsilon(4S) is twice the mass of a B meson, so in the center-of-mass frame the B mesons are prouduced at rest. However, because the electron and positron beams have different energies, the laboratory frame is not the center-of-mass frame, and the B mesons have nonzero momentum in the laboratory frame. This makes it possible for the B mesons to travel a (barely) measurable distance before they decay. The ability to measure this distance is very important for CP violation studies at BaBar.
The BaBar detector
The BaBar detector is made up of five subdetectors. From the inside out, they are:
(Source: The BABAR physics book)
The following sections describe these subdetectors in more detail. The focus is on BaBar; for more information about the subdetector types in general, the reader is encouraged to refer to the CERN BriefBook.
Silicon Vertex Tracker (SVT)
A vertex tracker is a detector in collider experiments positioned as close as possible to the collision point. The goal of a vertex tracker is to measure particle tracks very close to the interaction point. Most vertex detectors are made of (usually silicon) semiconductor devices, because semiconductor devices have very good energy, spatial resolution, and response time.
The Silicon Vertex Tracker (SVT) is BaBar's innermost subdetector, and the only tracking device inside the support tube. (The support tube is a 20-cm-radius structure that supports the beam pipe.) It consists of five concentric cylindrical layers of double-sided silicon microstrip detectors. The SVT's job is to obtian precise measurements of charged track position (z,r,phi). In addition, the SVT provides the only tracking measurements for low-momentum particles that decay before they reach the DCH. Finally, the SVT and DCH work together to provide measurements of track production angles, and dE/dx mesurements for particle identification.
An SVT measurement of particular importance is the distance between the decay positions of the two B mesons, This distance is directly related to the CP violation parameter sin2beta, a measure of CP violation. Measurement of this parameter was the primary goal of the BaBar and Belle experiments. Both experiments succeeded in 1999.
The Drift Chamber (DCH)
A drift chamber is the standard tracking device in most particle detectors. The gas-filled chamber contains field wires to maintain and electric field, and sense wires to detect ionization electrons. When a charged track traverses the chamber, it ionizes the gas, and the resulting electrons drift toward the sense wires. The position of the original ionizing track can be determined from the time it takes the ionization electrons to travel to the sense wire. (The drift velocity is determined from the known electric field.)
The Drift Chamber (DCH) is BaBar's main tracking device. Its most important task is to obtain the best possible momentum resolution for charged tracks. It also supplies dE/dx measurements for particle identification. The DCH consists of 40 concentric cylindrical layers, each made of thousands of drift cells. It uses a helium-based gas mixture with low mass wires to mimimize multiple scattering. The magnetic field of 1.5T gives very good momentum resolution.
Detector of Internally Reflected Cerenkov radiation: DIRC
A Cerenkov detector is a particle identification device. It uses the Cerenkov angle of a charged track to determine the track velocity. Combined with momentum measurements from tracking devices, the velocity is used to determine the mass of the particle. Since each particle has its own unique mass, this tells you the identiy of the particle.
In BaBar's Detector of Internally Reflected Cerenkov radiation (DIRC), charged particles traverse quartz bars, generating Cerenkov radiation. The photons are transferred by total internal reflection (which preserves the angle) to a large water tank. The light is observed by an array of photomultiplier tubes at the outside of the tank The Cerenkov angle is determined from the photon position and the original track position.
The primary task of the DIRC is to distinguish between charged pions and charged kaons at high momentum. (At low momentum, pion/kaon separation is based on dE/dx measurements in the SVT and DCH.) It can also help with particle identification for other charged particles (muons, electrons) as well.
The Electromagnetic Calorimeter (EMC)
A calorimeter is a device that measures the energy and position of a particle by absorbing it. Absorption of the particle generates showers of new particles, and the energy and momentum of the original particle is distributed among the shower particles. If the original particle is fully absorbed, then all of its energy goes to the shower particle. If the original particle generates showers but is not absorbed, then it will keep some of its energy for itself, and give the rest to the showers. Whether or not a particle is fully absorbed depends on the type of material used in the calorimeter. Electromagnetic calorimeters are best at absorbing electromagnetic particles - that is, charged particles and photons. Hadronic calorimeters are best at absorbing hadrons.
In an electromagnetic calorimeter, particles are distinguished based on how much they are absorbed, and their different shower shapes. Electrons and photons are fully absorbed and have short and narrow showers. Hadrons, on the other hand, are only partially absorbed and generate wide and scattered showers. Muons (despite being electromagnetic particles) are not absorbed and do not shower. Whether the particle is charged or neutral can be inferred from whether it is associated with a charged track.
The main purpose of BaBar's electromagnetic calorimeter (EMC) is to determine the position, energy and identity of electrons, photons, and neutral pions (which decay to two photons). The EMC is built from CsI(TI) crystals, which provide excellent energy and angular resolution even at very low photon energies.
Instrumented Flux Return
Muons are unusual: Although they are charged and therefore electromagnetic paricles, they do not shower in an electromagnetic calorimter. Therefore, experiments in which muons are important often have a muon detector as the outermost subdetector. Most particles decay long before they get to the muon detector. But muons and high-momentum pions often reach it.
The IFR is BaBar's outermost subdetector. It is used to detect muons and long-lived neutral hadrons. BaBar's IFR does double duty --- as the flux return for the magnetic solenoid, and as a muon and neutral hadron detector. A flux return by itself is not a particle detector. But at BaBar the flux return is made of layers of iron and steel, with active detectors between each layer to detect the passage of particles (or of showers generated in the IFR layers). Muons are generatlly able to penetrate more layers of iron or steel than pions, and this serves as the basis for muon/pion discrimination.
At the beginning of the experiment, all of the layers were made of iron, and all of the active detectors were Resistive Plate Chambers (RPCs). But rapid aging and efficiency loss of the original RPCs forced upgrade/replacements in the forward endcap and barrel. In the summer of 2002 the original RPCs were replaced by new RPCs and 2 additional absorption lengths of absorber (brass in 5 IFR gaps and more external steel layers). In 2004, 2 of the IFR RPC sextants were replaced by LSTs and brass absorber. The remaining 4 RPC sextants in the barrel will be replaced by LSTs in the summer of 2006. Each of the LST sextants contains 12 layers of LSTs and 6 layers of brass absorber.
Without a magnetic field, a tracking device could not measure charge or momentum, but only position. But when a magnetic field is present, the charged tracks curve, and the charge and momentum of the particle can be determined from the direction and curvature of the track.
BaBar uses a superconducting solenoid, located between the EMC and the IFR. To achieve good momentum resolution without increasing the tracking volume and therefore calorimeter cost, the magnetic field is set at 1.5T.
BaBar Coordinate SystemThe BaBar coordinate system is defined in BaBar Note 230. It is defined as a right handed system such that:
Although the beams collide head-on they are separated while still inside the detector magnet field. The detector is rotated 20mr relative to the beam direction (around the y-axis) to minimize the resulting orbit distortions. The z direction thus corresponds to the magnetic field direction, and deviates slightly from the boost direction. The coordinate system origin is the nominal collision point, which is offset in the -z direction from the geometrical center of the detector magnet.