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The BaBar Detector

Provides background on HEP detectors in general and the BaBar detector in particular. Quick links:

General Introduction to Particle Detectors

Experimental particle physicists study the fundamental structure of matter by making fast particles collide with each other, and by analyzing the reaction products. Ideally, one would like to know the four-vectors of all the particles produced in these processes as accurately as possible. For many years the bubble chamber, still unsurpassed in accuracy, was the major experimental tool. The overwhelming majority of the known particles and resonances have been discovered in bubble chamber experiments.

In order to be able to study more rare processes one started using other techniques, based on a digitized recording of experimental data, in order to speed up the analysis, but yet trying to approach as much as possible the accuracy of bubble chambers. In particular charm and beauty studies have taken advantage of this approach. The experimental setup consisted of (usually many) detector planes recording the passage of charged particles, for which a wide variety of techniques were employed. In addition, converters for gammas, an absorber allowing muon identification, and a magnetic field for momentum analysis were standard items. In the last (two) decades, a different class of detectors have gradually become more and more important. We call them calorimeters, or total absorption detectors. [Extracted from Instrumentation in Elementary Particle Physics : Proceedings of the ICFA School on. Trieste, Italy June 1987]

The implementation of any detector varies in accordance with the parameters of the experiment, most profoundly on the energy range. However, there are traditional approaches to achieving these goals. A solenoidal magnetic field is used to create momentum and charge dependent track separation. A detector is divided into systems designed to determine tracking, particle identification, and calorimetry, via measurement of ionization, position and time, energy and momentum. Well developed technologies are reused whenever possible, and thus many detectors share common devices. Some descriptions of these components are given below.

The CERN Particle Detector BriefBook, written by Bock and Vasilescu, is an online encyclopedia of terms relevant to high energy physics experiments. The following definitions are only very brief excerpts. For more information on any given topic refer to the BriefBook itself.

Vertex Detector
A detector in collider experiments positioned as close as possible to the collision point. It is typically made of cylindrical layers, positioned at radii of a few centimetres, the innermost layers preferably with pixel readout. The goal of a vertex detector is to measure particle tracks very close to the interaction point (inner radii of a few cm, close to the beam pipe), thus allowing one to identify those tracks that do not come from the vertex ( a signature for short-lived decaying particles). Most vertex detectors seem to be made of semiconductor detectors, but precise drift chambers have also been used successfully.

Semiconductor Detectors
Semiconductor detectors have been used in high-energy physics applications in the form of pixel detectors, microstrip detectors pads; they are popular due to their unmatched energy and spatial resolution, and have excellent response time. These detectors are manufactured mainly of silicon, traditionally on high-resistivity single crystal float-zone material. GaAs is perhaps a future alternative to silicon; presently, it seems to be an expensive and not fully mastered technology of potentially better radiation hardness.

Drift Chamber
A multiwire chamber in which spatial resolution is achieved by measuring the time electrons need to reach the anode wire, measured from the moment that the ionizing particle traversed the detector. Drift chambers have been built in many different forms and sizes, and they are standard tracking detectors in more or less all experiments. To translate good time resolution into spatial resolution, it is important to have a predictable electron drift velocity in the gas, and a simple relation for tracks passing under different angles; this means that the shape and constancy of the electric field needs more careful adjustment and control than in ordinary multiwire proportional chambers. Planar drift chambers measure the coordinates of the intersection of a particle track with a wire plane, by making the electrons drift in the plane. Hence multiple planes are needed to determine a trajectory; they are typically given several different wire orientations, to get different projections, thus offering the possibility of reconstruction in three dimensions.

Related topics in the BriefBook: Drift Chamber, Drift Tubes, Field Shaping, Jet Chambers, Sense Wires, Time Projection Chambers.

A composite detector using total absorption of particles to measure the energy and position of incident particles or jets. In the course of showering, eventually, most of the incident particle energy will be converted into ``heat'', which explains the name calorimeter (calor = Latin for heat) for this kind of detector; of course, no temperature is measured in practical detectors, but characteristic interactions with matter (e.g. atomic excitation, ionization) are used to generate a detectable effect, via particle charges. Calorimetry is also the only practicable way to measure neutral particles among the secondaries produced in a high-energy collision. Calorimeters are usually composed of different parts, custom-built for optimal performance on different incident particles.
Typically, incident electromagnetic particles, viz. electrons and gammas, are fully absorbed in the electromagnetic calorimeter , which is made of the first (for the particles) layers of a composite calorimeter. Incident hadrons, on the other hand, may start their showering in the electromagnetic calorimeter, but will nearly always be absorbed fully only in later layers, i.e. in the hadronic calorimeter , built precisely for their containment. Discrimination, often at the trigger level, between electromagnetic and hadronic showers is a major criterion for a calorimeter; it is, therefore, important to contain electromagnetic showers over a short distance, without initiating too many hadronic showers. The critical quantity to maximize is the ratio , which is approximately proportional to Z1.3 (see [Fabjan91]); hence the use of high-Z materials like lead, tungsten, or uranium for electromagnetic calorimeters.
Calorimeters can also provide signatures for particles that are not absorbed: muons and neutrinos. Muons do not shower in matter, but their charge leaves an ionization signal, which can be identified in a calorimeter if the particle is sufficiently isolated (and the dynamic range of electronics permits), and then can be associated to a track detected in tracking devices inside the calorimeter, or/and in specific muon chambers (after passing the calorimeter). Neutrinos, on the other hand, leave no signal in a calorimeter, but their existence can sometimes be inferred from energy conservation.

Related topics in the BriefBook: Calorimeter, Compensating Calorimeter, Electromagnetic Calorimeter, Energy Resolution in Calorimeters, Electromagnetic Shower, Hadronic Shower, Homogeneous Shower Counters, Crystal Calorimeter, Heterogeneous Shower Counters (Sampling Calorimeters).

Particle Identification
Certain detectors have as their main objective the identification of particles by their mass or quantum numbers, as opposed to position-sensitive detectors used for tracking, or calorimeters used for measuring particle energy. Particle identification relies on special properties of some particles, like muons which carry charge but do not shower nor interact strongly, or the electromagnetic shower characteristic for electrons and gammas. In other cases, the mass sensitivity of some radiation can be used (Cherenkov or transition radiation), or the mass dependence of ionization loss (dE/dx).

Related topics in the BriefBook: Cherenkov Counter, Transition Radiation, Ionization Sampling.


A solenoid is used to create a constant, time independent magnetic field in the interaction region. Lorentz force causes a particle to bend in a magnetic and/or electric field. In most cases, the electric field is negligible; then the magnetic field is time independent. In this case, the energy E and momentum are constants of motion. The solenoid is a key component of the tracking system.

Related topics in the BriefBook: Lorentz Force, Equations of Motion, Trajectory of a Charged Particle.

The BaBar Detector Systems

The events will result from electron positron collisions at the PEP-II collider at SLAC, with 9.0 and 3.1 GeV respective energy, thus there is an energy boost in the lab frame. The tracking system consists of a silicon vertex detector and a drift chamber, particle identification will be accomplished by a newly developed technology (DIRC), a calorimeter and solenoid. The angular acceptance for the entire experiment is determined by the vertex detector, which is limited by machine components.

The following sections are subsystems of the BaBar detector, ordered from the inside out. Each section has a link to that detector component's home page, other useful links, and introductory descriptions that are excerpts from the BaBar Technical Design Report (TDR).

Vertex Detector

The vertex detector is the only tracking device inside the 20cm radius of the support tube. It is used to measure precisely both impact parameters for charged tracks (z and r - phi); these measurements are used to determine the difference in decay times of the two B^0 mesons. The vertex detector also provides the measurements of production angles, given momentum information from the drift chamber. Finally, charged particles with p_t between ~40MeV/c and ~100MeV/c are tracked only with the vertex, which must therefore provide good pattern recognition.

The vertex detector consists of five layers of double-sided silicon strip detectors. The inner three layers are in a barrel geometry with detectors parallel to the beam pipe. The outer two layers combine barrel detectors in the central region with wedge detectors forward and backward.

Vertex Detector Home Page

Drift Chamber / Central Tracker

The second component of the tracking system is the drift chamber, which is used primarily to achieve excellent momentum resolution and pattern recognition for charged particles with p_t > 100 MeV/c. It also supplies information for the charged track trigger and a measurement of dE/dx for particle identification. The chamber extends in radius from 22.5 cm, just outside the support tube to 80 cm.

For most particles of interest at PEP-II, the optimum momentum resolution is achieved by having a continuous tracking volume with a minimum amount of material to cause multiple scattering. By using a helium-based gas mixture with low mass wires and a magnetic field of 1.5T, very good momentum resolution can be obtained. The forward edge of the chamber is situated 1.66 m from the interaction point, which makes it possible to obtain reasonable momentum resolution down to the limit of forward acceptance, 300mr.

A design of four axial and six stereo superlayers, each consisting of four individual layers, was chosen as the baseline design for the drift chamber.

The chamber is designed to minimize the amount of material in front of the particle identification and calorimeter systems in the heavily populated forward direction. The readout electronics are mounted only on the back end of the chamber and the endplates are designed as truncated cones.

Central Tracker Home Page

Barrel Particle ID: DIRC

There are two primary goals for the particle identification system. One is to identify kaons for tagging beyond the momentum range well-separated by dE/dx. The other is to identify pions from few body decays. A new detector technology is required to meet these goals, and in the barrel region, a DIRC (Detector of Internally Reflected Cherenkov radiation) is used. Cherenkov light produced in 1.75 x 3.5 cm^2 quartz bars is transferred by total internal reflection, while preserving the angle, to a large water tank outside the backward end of the magnet. The light is observed by an array of photomultiplier tubes at the outside of the tank, where images governed by the Cherenkov angle are formed. A mirror at the forward end of the bars reflects the forward-going light, preserving the angle information.

Further details and diagrams are found in the article What is the DIRC? , which is linked to from the DIRC Home Page .


The electromagnetic calorimeter must have superb energy resolution down to very low photon energies. This is provided by a fully projective CsI(TI) crystal calorimeter, which has excellent energy and angular resolution and retains high detection efficiency at the lowest relevant photon energies. The calorimeter consists of a cylindrical barrel section with inner radius of 90.5 cm and a conical forward endcap. The barrel calorimeter contains 5880 trapezoidal crystals; the forward endcap calorimeter contains 900 crystals. Each crystal is readout by two independent silicon photodiodes. Electronic noise and beam-related backgrounds dominate the resolution at low photon energies, while shower leakage from the rear of the crystals dominates at higher energies.

Calorimeter Home Page

Instrumented Flux Return

The IFR is designed to separate pions from muons for momenta > 0.5 GeV/c; it also has the ability to detect and provide coordinate information on neutral hadrons. The magnetic flux return iron is divided into 18 layers whose thickness increases outwards from 2 to 10 cm for a total thickness of 55-60 cm. The gaps between iron plates are filled with active detectors, Resistive Plate Chambers (RPCs) or Limited Sreamer Tubes (LSTs). Both RPCs and LSTs provide two dimensional position information in each plane with a resolution of ~1-2 cm. Muons will produce a track through most if not all of the IFR layers while most pions will interact in the EMC or IFR steel.

Rapid aging and efficiency loss of the original RPCs have 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.

Further details can be found in the IFR Home Page.


To achieve good momentum resolution without increasing the tracking volume and therefore calorimeter cost, it is necessary to have a field of 1.5T. The magnet coil is therefore of superconducting design, with an inner radius of 1.40m for the coil dewar and a cryostat length of 3.85m. The nonstandard features are segmentation of the iron for an Instrumented Flux Return (IFR), and the complications caused by the DIRC readout in the backward direction.

Magnet Home Page

Coordinate Systems, Units and Naming Conventions

BaBar Coordinate System

The BaBar coordinate system is defined in BaBar Note 230. It is defined as a right handed system such that:
  • The +z axis is parallel to the magnetic field of the solenoid and in the direction of the High Energy (nominally the electron) beam.
  • The +y axis points vertically upward
  • The +x axis points horizontally, away from the centre of the PEP-II ring.
  • The origin, (0,0,0), is defined as the nominal interaction point.
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. (TDR)

Labeling and Numbering

Unless there are extremely important reasons to do otherwise, BaBar software should refer to hardware components by the same labels or numbers with which they are physically labeled.

Numbering sequences of items should begin at 0. In general, numbering conventions should be ordered so that increasing numbers correspond to monotonically increasing values of the most relevant coordinate. For example the layers of the drift chamber should be ordered with increasing radius and cell numbers with increasing $\phi$.

The origin of the numbering should start at the smallest used value of the most relevant coordinate quantity. For example, the calorimeter crystal numbering in the polar direction most logically increases with $\cos{\theta}$/ z, so the zero layer is that with the most negative $\cos{\theta}$z. Angular ordering is slightly different; the standard BaBar definition for $\phi$runs from $-\pi$ to $\pi$. The lowest number, however, should be given to the item with the smallest positive $\phi$.For example, cell 0 in the drift chamber should be at $\phi = 0$not $\phi = -\pi$.

Subsystem Acronyms

A Three Letter Acronym has been adopted for each BaBar subsystem:
PEP-II, including beam pipe
Silicon Vertex Tracker
Drift Chamber
Electromagnetic Calorimeter
Instrumented Flux Return

Back to Workbook Front Page

Author: Tracey Marsh
Contributors: Neil Geddes
Joseph Perl

Last modification: 6 June 2005
Last significant update: 10 March 2000