The BaBar Detector
Provides background on HEP detectors in general and the
BaBar detector in particular.
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
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
Proceedings of the ICFA School on.
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.
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.
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 (e.g.as 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 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.
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
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
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
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).
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 events will result from electron positron collisions at the
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
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
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,
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
BaBar Coordinate System
The 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.
- 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
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
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
z, so the zero layer is that with the
z. Angular ordering is slightly different; the
standard BaBar definition for runs from to . The
lowest number, however, should be given to the item with the smallest
positive .For example, cell 0 in the drift chamber should be at
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
Contributors: Neil Geddes
Last modification: 6 June 2005
Last significant update: 10 March 2000