The BaBar Detector
Provides background on HEP detectors in general and the
BaBar detector in particular.
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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.
PEP-II
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:
- Silicon Vertex Tracker (SVT) - provides precise position information on charged tracks, and is also the sole tracking device for very low-energy particles.
- Drift Chamber (DCH) - provides the main momentum measurements for charged particles and helps in particle identification through dE/dx measurements.
- Detector of Internally Refected Cerenkov radiation (DIRC or DRC) - provides charged hadron identification.
- Electromagnetic Calorimeter (EMC) - provides particle identification for electrons, neuatrl electromagnetic particles, and hadrons.
- Solenoid (not a subdetector) - provides the 1.5 T magnetic field for needed for charge and momentum measurements.
- Instrumented Flux Return (IFR) - provides muon and neutral hadron identification.
(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.
SVT home page
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.
Drift Chamber Home Page
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.
DIRC Home Page
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.
Calorimeter Home Page
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.
IFR Home Page.
Solenoid
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.
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:
- 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.
Page maintained by Adam Edwards
Last modified: January 2008
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