The trajectory of charged particles may be determined by their
ionizing effects on the material they pass through.
The track curvature in a magnetic field can be used to determine
a particle's momentum (and charge),
which in DELPHI's central uniform 1.2 T field is
given in
by
, where
is the radius
of curvature in cm.
Tracking of charged particles in the barrel is provided by
the Inner Detector (ID), TPC, and Outer Detector (OD).
The forward chambers (FCA and FCB) are used in the endcaps.
All these detectors use gases as ionizing media.
Additional extremely precise measurements
are made close to the interaction point by the
silicon Vertex Detector (VD).
See table 2.2 for a comparison of these
detectors' specifications.
A recognition of the importance of precise measurements close to the interaction region to identify and reconstruct short-lived particles, particularly given the emphasis on particle identification prompted by the RICHes, led to the inclusion of a high-precision silicon Vertex Detector in the design of DELPHI.
After initial tests during the pilot run of 1989, two layers of single-sided detectors (measuring only the azimuthal coordinate) at 9 and 11 cm radii (designated inner and outer respectively) were installed in 1990. This was the first use of this technology in a collider [24].
In 1991 an additional (closer) layer was added at 6.3 cm, when experience with LEP operation allowed a reduction of the beampipe radius from 7.9 to 5.6 cm. The new beampipe is composed of beryllium, rather than aluminium to reduce multiple scattering.
For the 1994 run, the closer and outer layers were replaced with double-sided detectors [25], which are capable of measuring both the azimuthal and longitudinal coordinates. The inner layer remained of the old design (and in fact used modules scavenged from the inner and outer layers of the previous detector).
The geometry of the 1994 detector is shown in figure 2.5.
The single-sided detector plaquettes consist of
285
-thick phosphorous-doped n-type silicon
with 7
-wide boron-doped p
strips parallel to
the
-axis spaced at intervals of 25
.
By applying a reverse bias of
V to these diodes,
the n-type silicon is depleted of charge carriers.
The passage of a charged particle through the detector liberates
electron-hole pairs.
The holes drift to the diodes where the deposited charge is detected.
Every other diode (i.e. at 50
intervals) is read out
via capacitive coupling: the aluminium readout strip is
separated from the p
silicon by a 0.23
layer
of silicon dioxide insulator.
Capacitive coupling, rather than direct charge readout,
is used to reduce the leakage current and hence detector noise.
Each unconnected strip is capacitively coupled to the
readout strips on either side of it.
This causes the deposited charge to be distributed over
several readout strips (most often two), allowing the
position measurement to be improved over simple strip
enumeration by using the relative charge deposits on each
strip.
As shown in figure 2.6,
the double-sided detector has n
In order to improve the signal-to-noise () ratio in
, the
n
strip pitch is increased at the ends of the
module, where the larger incidence angle of the tracks
spreads the charge over a larger area.
The two plaquettes on each side of a module are bonded such that the
n-side of one is joined to the p-side of the other,
further improving the (
) of the
-measurement.
The opposite polarities of signals from the two plaquettes
also alleviates the ambiguity produced by multiplexing
(3:1 in the closer and 2:1 in the outer layer)
the connections between the two metal layers on the n-side.
Pairs of plaquettes are wire-bonded in series to form half-modules, which
are read out at either end
by onboard CMOS chips, which each preamplify and multiplex the
charges from 128 channels.
4-10 chips are read out serially at 2.5 MHz on a
single twisted pair cable connected via a line driver and
repeater to the data acquisition crates (in the counting
houses, m away). 36 (1991-3) or 48 (1994-5)
SIROCCO2.5Fastbus modules, each with two readout units
consisting of a flash ADC and digital signal processor
are used to digitize the signal stream and perform
pedestal and noise calculations and zero suppression.
A total of 73,728 (1991-3) or 125,952 (1994-5) strips are
read out.
Temperature and humidity variations (which are
monitored as described in sections 3.2.3
and 3.2.10) as well as
detector interventions can affect the alignment.
Until 1995, the stability of the outer layer position relative to the
ID was
monitored (every 64th beam crossing) to a 1
precision using lasers.
Channels passing a cut
(in 1994, 2.5 on the p-side and 1.4 on the n-side)
are grouped into clusters, which are accepted
if their total
.
The hit position is calculated from
the pulse-height weighted position of the cluster maximum and
its largest neighbour (
),
corrected for nonlinear charge sharing between the two strips.
n-side clusters with a large incidence
angle (
from the normal) where the
charge is spread over several strips use the
strips on the edge of the cluster.
The local or
coordinates cannot be converted into
global DELPHI coordinates without additional information
(usually external tracks).
Before 1994, no
coordinate was available, and
even since then there is no reliable way of directly
associating the p-side hit with its n-side counterpart.
Although the plaquettes are aligned closely with the
-axis, this is not perfect, so even a precise separate
measurement of
or
cannot be made.
For these reasons, it is necessary to
associate initial track measurements from the ID, TPC, and
OD with individual VD hits before conversion to the DELPHI
coordinate system and inclusion into the global track fit.
First
and then (using the resultant
precision improvement)
hits are associated with each track.
Remaining unassociated hits are checked for the possibility
of VD-only tracks (perhaps due to material interaction
outside the VD) using the beamspot position as the external
constraint.
In order to obtain the full precision of the Vertex
Detector, the relative positions of the plaquettes as well
as the global alignment of the VD within DELPHI must be
accurately measured [26].
The relative position of each module within a
half-shell is determined to 25
before insertion into DELPHI
with a combination of microscope (for the strip positions relative
to the module) and mechanical probe (relative module positions)
measurements.
The 2
(1994-5) microscope measurement precision is sufficient for
plaquette-plaquette alignment.
Tracks passing through the regions where two modules
overlap, 3-hit tracks, and dimuon (
)
events are used to improve the module-module alignment,
align the two half-shells relative to each other, and align
the VD with respect to the rest of DELPHI.
An overall alignment precision of the order of half the
intrinsic hit precision is obtained.
All these tasks are performed by a single software library, VDCLAP2.6 [28], which was the first detector physics package in DELPHI to be used at all levels of the reconstruction (DELANA, DSTFIX, as well as analysis programs). VDCLAP operates on VD hit, track, alignment, association, efficiency, dead strip, and beamspot data held in standard COMMON blocks.
Minimum ionizing particles at normal incidence give
(1994-5) between 11 (closer
) and 17 (outer,
and
),
producing a single-hit efficiency of 98-99% and,
as shown in figure 2.7,
The overall precision of a vertex detector may be described
in terms of track impact parameters
(see definition in appendix B)
with respect to the production vertex, usually the
primary vertex (see section 2.11.3).
There are two components to the impact parameter error
(excluding the uncertainty in the production vertex, which
does not concern us here): the intrinsic precision of the VD
hits, extrapolated to the vertex; and the uncertainty due to
multiple scattering in the beam pipe and VD material.
In the
and
planes these may be parameterized with
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The Inner Detector [29] consists of two components: the
jet chamber is a high-resolution drift chamber providing
tracking information, while the trigger layers
provide a rapid readout in both
and
.
The jet chamber consists of 24 azimuthal sectors, each with a plane
of 24 axial wires down its centre.
A variation in the drift field (and hence drift velocity)
with allows for constant drift time for all measurements
of radial tracks, and hence a rapid trigger.
Outside this, the trigger layers consist of 5 layers of
192 multiwire proportional chambers (MWPC; before 1995) or
straw tubes (1995 [30]). As well as measuring the
-coordinate,
the
measurement can be used to help resolve the
left-right ambiguity of the jet chamber.
The TPC [31] is the main tracking detector in DELPHI, providing three-dimensional position information, a momentum measurement (from the track curvature in the magnetic field), and some particle identification based on the specific ionization in the detector.
The layout of the TPC is shown in figure 2.9.
The cylindrical vessel is divided in two by a cathode plane at
The size of the the TPC is constrained by the Barrel RICH surrounding it. The Outer Detector [32] provides an additional track measurement outside this to extend the distance over which the track curvature is measured and hence improve the overall momentum resolution.
The OD consists of 24 overlapping planks, each of 145 drift
tubes, arranged into 5 staggered layers.
The
-coordinate is obtained from the drift time, and
(3 layers only) from the relative timings of the signal
at the two ends of the anode wires.
The Forward Chambers A are mounted on either end of the TPC,
and are thus mechanically part of the barrel.
Each side consists of three modules each of two staggered layers of
drift tubes, operating in limited streamer mode. The wire
orientation of each module is 120
from the other two,
providing two measurements of each of three non-orthogonal
coordinates
.
The Forward Chambers B are positioned outside the Forward RICH.
Each endcap consists of two semicircular modules,
each with 12 planes of wires.
Pairs of staggered wire planes are orientated 120
from their neighbours, providing four measurements of each of
the same
coordinates used in the FCA.
Tim Adye 2002-11-06