Mechanical stability of the CMS strip tracker measured with a laser alignment system

TL;DR: In this article, the mechanical stability of tracker components during physics operation was monitored with a few micron resolution using a dedicated laser alignment system as well as particle tracks from cosmic rays and hadron-hadron collisions.

Abstract: The CMS tracker consists of 206 square meters of silicon strip sensors assembled on carbon fibre composite structures and is designed for operation in the temperature range from -25 to +25 degrees C. The mechanical stability of tracker components during physics operation was monitored with a few micron resolution using a dedicated laser alignment system as well as particle tracks from cosmic rays and hadron-hadron collisions. During the LHC operational period of 2011-2013 at stable temperatures, the components of the tracker were observed to experience relative movements of less than 30 microns. In addition, temperature variations were found to cause displacements of tracker structures of about 2 microns/degree C, which largely revert to their initial positions when the temperature is restored to its original value.

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Mechanical stability of the CMS strip tracker measured with a laser alignment system
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2017 JINST 12 P04023

2017 JINST 12 P04023
Published by IOP Publishing for Sissa Medialab
Received: January 8, 2017
Accepted: April 8, 2017
Published: April 21, 2017
Mechanical stability of the CMS strip tracker measured
with a laser alignment system
The CMS collaboration
E-mail:
cms-publication-committee-chair@cern.ch
Abstract: The CMS tracker consists of 206 m
2
of silicon strip sensors assembled on carbon fibre
composite structures and is designed for operation in the temperature range from −25 to +25
◦
C.
The mechanical stability of tracker components during physics operation was monitored with a
few µm resolution using a dedicated laser alignment system as well as particle tracks from cosmic
rays and hadron-hadron collisions. During the LHC operational period of 2011–2013 at stable
temperatures, the components of the tracker were observed to experience relative movements of
less than 30 µm. In addition, temperature variations were found to cause displacements of tracker
structures of about 2 µm/
◦
C, which largely revert to their initial positions when the temperature is
restored to its original value.
Keywords: Detector alignment and calibration methods (lasers, sources, par ticle-beams); Large
detector systems for particle and astroparticle physics; Particle tracking detectors; Particle tracking
detectors (Solid-state detectors)
ArXiv ePrint:
1701.02022
© CERN 2017 for the benefit of the CMS collaboration, published under the terms of the
Creative Commons Attribution 3.0 License by IOP Publishing Ltd and Sissa Medialab
srl. Any further distribution of this work must maintain attribution to the author(s) and the published
article’s title, journal citation and DOI.
doi:
10.1088/1748-0221/12/04/P04023

2017 JINST 12 P04023
Contents
1 Introduction
1
2 Mechanical design of the CMS tracker 2
3 The laser alignment system 6
4 Tracker alignment 9
4.1 Alignment with the laser system 9
4.2 Alignment with particle tracks 12
5 Tracker mechanical stability 12
5.1 Long-term stability 13
5.2 Stability during temperature variations 15
6 Summary 18
The CMS collaboration 21
1 Introduction
The silicon strip tracker of the CMS experiment at the CERN LHC is designed to provide precise and
efficient measurements of charged particle trajectories in a solenoidal magnetic field of 3.8 T with
a transverse momentum accuracy of 1–10% in the range of 1–1000 GeV/c in the central region [
1].
It consists of five main subdetectors: the tracker inner barrel with inner disks (TIB and TID),
the tracker outer barrel (TOB), and the tracker endcaps on positive and negative sides (TECP and
TECM) [
2, 3]. The silicon strip sensors have pitches varying from 80 µm at the innermost radial
position of 20 cm, to 205 µm at the outermost radius of 116 cm, delivering a single-hit resolution
between 10 and 50 µm [1]. As a general criterion, the position of the silicon modules has to be
known to much better accuracy than this intrinsic resolution.
Silicon sensors exposed to a large radiation fluence require cooling, and the CMS tracker is
designed to operate in a wide temperature range from −25 to +25
◦
C. The mechanical stability of
the tracker components is ensured by the choice of materials and by an engineering design that
tolerates the expected thermal expansion and detector displacements. These displacements have to
be measured and accounted for in the form of alignment constants used in the track reconstruction.
The absolute alignment of individual silicon modules is performed with cosmic ray muons and
tracks from hadron-hadron collisions collected during periods of commissioning or collision data
taking [
4–6]. A significant advance in the track-based alignment came with the introduction of a
global χ
2
algorithm that combines reconstruction of the track and alignment parameters [
7]. This
algorithm, implemented in the millepede package [
8], was successfully used in various experiments
at the LHC, HERA, and Tevatron. The actual accuracy of the track-based alignment depends on
the number of objects requiring alignment and the size of the track sample.
The movement of the tracker components over much shorter time scales is monitored in the
CMS experiment with an optical laser alignment system (LAS) [
9]. Lasers were already used in the
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2017 JINST 12 P04023
alignment of several silicon-based tracking detectors, for example, in the ALEPH [10], ZEUS [11],
and AMS02 [
12] experiments. Moreover, the CMS experiment also uses lasers for linking the
tracker and muon subdetectors together in a common reference frame [
13]. There is an alternative
method of optical alignment based on the RasNiK system that was implemented, for example, in
the CDF [
14] and ATLAS [15] experiments. The RasNik system uses a conventional light source
with coded mask, a lens, and a dedicated optical sensor. Both methods have similar performance,
but lasers have some advantages for operation in the CMS tracker. First, the infrared laser light
penetrates the silicon sensors, hence simplifying the alignment system. Second, the laser light
produces a signal similar to ionizing particles that permits the use of the same radiation-hard silicon
detectors employed for tracking, instead of dedicated sensors. Yet another method, implemented in
the ATLAS experiment, is based on the laser frequency scanning interferometry [
16].
The LAS of the CMS tracker is one of the largest laser-based alignment systems ever built in
high-energy physics. Forty infrared laser beams illuminate a subset of 449 silicon modules, and
monitor relative displacements of the TIB, TOB, and TEC subdetectors over a time interval of a
few minutes with a stability of a few µm [
9]. Alignment with particle tracks and laser beams are
complementary techniques and together they ensure the high quality of track reconstr uction. While
the track-based alignment is used to reconstruct the alignment constants of individual modules, the
LAS identifies short-term displacements of large structures in order to exclude such periods from
the offline analysis of the experimental data.
In this paper we describe the mechanical str ucture of the tracker and the LAS in detail. We
review the alignment procedure of using laser beams and particle tracks. The measurements of the
mechanical stability of the tracker components during the LHC data taking period in 2011–2013, as
well as during the LHC long shutdown period spanning 2013–2014, are presented and discussed.
2 Mechanical design of the CMS tracker
The silicon strip tracker of the CMS detector is composed of 15 148 silicon strip detector modules
with a total area of about 206 m
2
and is described in refs. [
2, 3]. Below we discuss in more detail
the components of the tracker that are relevant to the mechanical stability of the detector. The
mechanical concept of the tracker is sketched in figure 1. The CMS coordinate system has its origin
at the centre of the detector with the z-coordinate along the LHC beam pipe, in the direction of the
counterclockwise proton beam, and the horizontal x- and the vertical y-coordinates per pendicular
to the beam (in the cylindrical system r is the radial distance and ϕ is the azimuth). The inner radii
from 4.4 up to 15 cm are occupied by the silicon pixel detector, which is operated independently
of the strip tracker. The silicon strip modules are mounted around the beam pipe at radii from
20 cm to 116 cm inside a cylinder of 2.4 m in diameter and 5.6 m in length. The TIB extends in z to
±70 cm and in radius to 55 cm. It is composed of two half-length barrels with four detector layers,
supplemented by three TID disks at each end. The TID disks are equipped with wedge-shaped
silicon detectors with radial strips. The TOB surrounds the TIB+TID. It has an outer radius of
116 cm, ranges in |z| up to 118 cm, and consists of six barrel layers. In the barrel part of the tracker,
the detector strips are oriented along the z-direction, except for the double-sided stereo modules in
the first two layers of the TIB and TOB, where they are rotated at an angle of 100 mrad, providing
reconstruction of the z-coordinate. The TECP and TECM cover the region 124 < |z| < 282 cm
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2017 JINST 12 P04023
9/21/161
y
x
z
Δx
Rz
Ry
Rx
Δy
+x
+z
Tracker support bracket
Tracker support tube
Tracker
bulkhead
T
E
C
M
T
E
C
P
T
OB
p
i
x
e
l
T
I
B
T
I
B
T
I
D
T
I
D
E
C
A
L
Δz
Figure 1. Mechanical layout and mounting of the tracker subdetectors (bottom half is shown). The TIB+TID
are mounted inside the TOB, while the TOB, TECP, and the TECM are mounted inside the TST. The red
arrows indicate the connection points and their kinematic constraints.
and 22.5 < r < 113.5 cm. Each TEC is composed of nine disks, carrying up to seven rings of
wedge-shaped silicon detectors with radial strips, similar to the TID. Rings 1, 2, and 5 are also
equipped with stereo modules for reconstruction of the r-coordinate.
Each module of the silicon strip detector has one or two silicon sensors that are glued on carbon
fibre (CF) frames together with a ceramic readout hybrid, with a mounting precision of 10 µm.
Overall, there are 27 different module designs optimized for different positions in the tracker. The
detector modules are mounted on substructures that are, in turn, mounted on the tracker subdetectors.
The TIB is split into two halves for the negative and positive z-coordinates allowing easy
insertion into the TOB. The TIB substructures consist of 16 CF half-cylinders, or shells. The
mounting accuracy of detector modules on the shells is about 20 µm in the shell plane. The
modules are assembled in rows that overlap like roof tiles for better coverage and compensation
for the Lorentz angle [
3]. An aluminium cooling tube, with 0.3 mm wall thickness and 4×1.5 mm
2
rectangular profile is glued to the mounts of the detector modules. Each row has three modules
on one cooling loop and each cooling pipe is connected at the edges of the shells to the circular
collector pipe that gives extra rigidity to the whole TIB mechanical structure. The overall positional
accuracy of the assembly of all shells is about 500 µm.
The TOB main str ucture consists of six cylindrical layers supported by four disks, two at the
ends and two in the middle of the TOB structure. The disks are made of 2 mm thick CF laminate
and are connected by cylinders at the inner and outer diameters. The cylinders are produced from
0.4 mm CF skins glued onto two sides of a 20 mm thick aramid-fibre honeycomb core. The detector
modules are mounted on 688 substructures called rods. The rods are inserted into openings on the
– 3 –