CMS Silicon Strip Tracker Operation and Performance
Gaëlle Boudoul on behalf of the CMS
collaborationEmail:Gaelle.Boudoul@cern.ch
Abstract:
The Silicon Strip Tracker (SST) of the CMS experiment is, with 9.6 million
readout channels, the largest strip tracker ever built. In order to correctly
interpret and reconstruct the events recorded it needs to be precisely calibrated,
thus ensuring that it fully contributes to the physics research program of the
CMS experiment. In 2009 and 2010, the performance of the SST has been
carefully studied using cosmic muons and tracks from proton-proton collisions
at centre-of-mass energies of 900 GeV, 2.36 TeV and 7 TeV. In this paper, we
present some results of the detector performance.
1. Overview of the CMS experiment at LHC and specifications of the
Silicon Strip Tracker
The CMS experiment at LHC [1] is composed of two tracking detectors (a
silicon pixel and a silicon strip detector) surrounded by the calorimeters and a
large superconducting coil, which provides an axial magnetic field of 3.8 Tesla.
The iron yoke of the flux-return is instrumented with four stations of muon
detectors covering most of the 4π solid angle.
The LHC design luminosity of 1034 cms-2 s-1 and the collision rate of 40MHz
imposed major constraints on the design of the tracker. With an average of 20
minimum bias events per bunch crossing, more than 1000 reconstructible tracks
are expected in the tracker. In order to keep the hit occupancy in the Silicon
Strip Tracker (SST) of the order of a few percents, a fine granularity is
mandatory. The tracking should be precise and robust in order to have a high
efficiency and an excellent parameter resolution for both low and high
transverse momenta. The primary and secondary vertices should be precisely
reconstructed and the mass of the detector should be the lowest possible to
minimize undesired particle interactions. The tracker has to be able to sustain
high radiation doses and present a low risk of failure.
2. Overview of the Silicon Strip Tracker
After more than 15 years of development and construction, the SST was
installed within CMS in 2008. It consists of 4 subdetectors: the Tracker Inner
Barrel (TIB), with 4 cylindrical layers and 2724 modules, the Tracker Inner
Disks (TID), with 3 disks on either side of the interaction point and 816
modules, the Tracker Outer Barrel (TOB), with 6 cylindrical layers and 5208
modules, and the Tracker Endcaps (TEC), with nine disks on each side and 6400
modules. The silicon detectors used in the SST are made of one (thin -320m) in
the inner region or two (thick – 500m) sensors in the outer region. The
Tracker covers the pseudo rapidity region |η|<2.5 [2].
Each detection module is composed of a silicon sensor made of p-type strips on
n-type bulk. The interstrip distance (pitch) varies from 80 to 205 μm depending
on the type of module and its position within the tracker. The readout of the
signal is performed by the APV readout chip, which samples, amplifies, buffers
and processes signal from 128 detector channels at a frequency of 40 MHz. In
an operating mode known as peak, it produces a CR-RC pulse shape with a
relatively slow rise time of 50 ns. In deconvolution mode, which is the standard
mode of operation, it performs additional signal processing to constrain the
signal to a single bunch crossing [3]. Each module is read out by 4 or 6 APV
chips, depending on the number of strips, 512 or 768. On receipt of a Level1
Trigger, pulse height and bunch-crossing from the APV chips are converted to
optical analogue signals through lasers. The data is then sent via optical links to
the Front End Drivers (FED) [4], where the digitization of the data is performed.
The FEDs perform pedestal subtraction, noise suppression, before carrying out
cluster finding.
3. Commissioning and Operation
The SST commissioning is performed in several steps, which consist of tuning
the laser gain for the optical readout links, optimizing the readout chip (pulse
shape, analogue baseline), measuring the noise and the pedestals, and finally
performing a fine delay scan to allow a synchronisation at the module-level
down to 1 ns.
The operational fraction of the SST in 2010 is 98.1%, and the breakdown by
subdetector is shown in Table 1.
Table 1- Operational fraction of the SST in 2010:
TIB/TID
96.25
TOB
98.8
TEC+
98.8
TEC99.1
In operation, the first checks of the SST were done using the Data Quality
Monitoring (DQM) system. First, during the data taking, a fraction of the events
is analyzed (online DQM) in order to give immediate feedback. Later, the full
data-set is analyzed (offline DQM) to certify the data. Every step of the
reconstruction chain is monitored through histograms. Specific tools have been
developed to check automatically the quality and provide a synoptic view of the
detector to the physicists in charge of the data certification.
4. Performance
4.1 Signal To Noise ratio
The signal-to-noise ratio is a benchmark for the tracker performance. It is
measured using clusters associated to tracks and depends on the strip length and
thickness. The signal-to-noise ratio in each subdetector is shown in Table 2. It
varies from 19.2 and 23.7 in deconvolution mode, and is large enough to allow
an efficient identification of particles with negligible background.
Table 2 - Signal to Noise ratio in peak mode (deconvolution) mode during
900GeV (7TeV) collisions:
Signal/noise
TIB
Peak
27.4
Deconvolution 19.4
TID
26.7
18.5
TOB
34.1
22.5
TECthin
28.8
19.2
TECthick
35.7
23.7
4.2 Hit Efficiency
To measure the hit efficiency of a given layer, the track reconstruction is
performed without the layer under study. The efficiency is then computed
considering the interpolation of these tracks onto the layer being studied,
seeking hits on the modules through which the tracks should pass. The measured
efficiencies are shown in Figure 2. Once the modules identified during
commissioning as defective are removed from the measurement, the efficiency
is above 99%. This method is thus also useful to monitor the SST and identify
failures.
Figure 2 _ Hit efficiency as a function of the SST layer
4.3 Hit Resolution
The hit resolution is measured using overlapping modules within a layer,
comparing the measured hit position and the position predicted by the
interpolation. The advantage of this method is that it reduces the amount of
material between two layers and reduces the effects of track extrapolation. The
sensor resolution is known to depend strongly on the angle of the track and the
pitch of the sensor. The results are therefore determined separately for different
sensor pitches and in 10 degree intervals for the track incidence angle. The
minimal value is achieved for an angle that corresponds to the optimal charge
sharing. The results are shown in figure 3.
Figure 3_ Hit resolution as a function of the pitch for 3 different cluster sizes
and for a local track angle between 0 and 10.
4.4 Lorentz Angle.
The 3.8T magnetic field causes the drift direction to tilt by the Lorentz Angle
(LA), and modifies the distribution of the cluster charge on the strips. A
correction must therefore be applied to the reconstructed cluster positions since
any error in the assumed LA would result in an apparent misalignment of the
silicon sensors. The correction is maximal in the barrel where the magnetic field
is perpendicular to the electrical field.
The LA is measured with cosmic tracks by comparing the cluster width with the
track angle on the sensors. The measured value of the tangent of the Lorentz
Angle is 0.07 ±0.01 in the Inner Barrel and 0.09±0.01 in the Outer Barrel.
In deconvolution mode, an additional correction is needed, as a fraction of the
charge from the sensor backplane does not reach the strips in time for readout,
which biases the cluster position. The correction to the cluster position is
measured using collision data and taken into account.
4.5 Particle Identification
The mean energy loss through ionization (dE/dx) of a particle traversing a thin
layer of material is given by the Bethe- Bloch formula [4]. For a fixed
momentum and charge, the energy loss depends on the particle mass and it can
be used for particle identification. A nice agreement is observed between data
and simulation for the proton and kaon peaks (fig 4). The third peak observed in
data is due to deuterons, which are not reproduced correctly in the simulation.
Figure 4 - Left: Reconstructed mass of low momenta hadrons (protons, kaons,
deuterons). Right: Distribution of dE/dx versus momentum from 7 TeV collision
events.
5. Conclusion
The performance of the CMS tracker after calibration with cosmic muon and
collision data is excellent. A signal to noise ratio between 19.2 and 23.7 is
measured, and high hit efficiencies and resolutions are obtained. Furthermore,
the dE/dx measurements allow unambiguous observations of kaon, proton and
deuterons in collision data.
REFERENCES
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[3] M. Raymond et al, “The APV25 0.25 μm CMOS readout chip for the CMS
tracker“, Nuclear Science Symposium Conference Record, 2000 IEEE, 9/1139/118 (vol.2)
[4] F. Drouhin et al., The CERN CMS SiliconStrip Tracker Control System,
CERN-CMS-CR-2004-032
[5]C. Amsler et al. (Particle Data Group), Review of Particle Physics, Phys.
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