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DETECTORS AT THE LHC: ATLAS AND CMS
UNIVERSIDAD COMPLUTENSE
MADRID- SPAIN
INTRODUCTION
Beatriz González Merino and Clara González-Santander de la Cruz
The aim of this work is to summarize the main characteristics of the detectors that will be used at the LHC
accelerator. We will study their principal components and some experiments that will be carried out.
Física Nuclear y de Partículas (347). Facultad de Ciencias Físicas
WHAT IS THE LHC?
ATLAS AND CMS
LHC stands for Large Hadron Collider. Large due to its size, Hadron because it accelerates protons
or ions (Pb), which are hadrons, and Collider because these particles form two beams, travelling in
opposite directions, which collide at four points around the machine’s circumference. The energy of
the collision is the sum of the energies of the two beams, bigger than the one produced at accelerators
where a beam collides with a stationary target. The collision at such high energy could produce new
particles to study.
• Located at CERN, Geneva (Switzerland).
• Currently under construction, it is scheduled to begin operation in 2008.
• Funded and built in collaboration with over two thousand physicists from thirty-four
countries, universities and laboratories.
• Experiments at the LHC: ATLAS, CMS, ALICE, LHCb, TOTEM, LHCf.
They are general purpose detectors designed to cover the widest possible range of physics at the LHC,
from the search for the Higgs boson, supersymmetry (SUSY) and extra dimensions. The difference between
them is the technical solution.
This modern general-purpose high-energy physics detector
needs to be hermetic, so most of the particles are detected.
For engineering convenience, ATLAS and CMS adopt the
barrel plus end-caps design where a cylindrical detector
covers the central region and two flat circular end-caps cover
the angles close to the beam (the forward region).
Function
(from the inner to the outer region)
Carries the beam of particles- bunches of
protons (100 billion approximately)
TRACKING DEVICE
Reveal the paths of electrically charge particle
when they interact with detector's atom's
substance → measures the momentum under a
magnetic field due to the curvature of the
trajectory
Measures the energy lost by the particle while it
travels through it, and it is designed to fully stop
or absorb most of the particles coming from a
collision event. Provides the main way to
identify neutral particles
CALORIMETERS
MUON DETECTOR
Semiconductor detectors: the passing particle creates electrons and
holes in a reversely-biased semiconductor, usually silicon. The
devices are divided intro strips or pixels (resolution 10 microns)
Tile calorimeter
Difference between charged
and not charged particles
Number of quadrupoles
858
Nominal energy, protons
7 TeV
Weight
7000 tonnes
12500 tonnes
Nominal energy, ions
2.76 TeV/u
Location
Meyrin, Switzerland
Cessy, France
Design luminosity
1034 cm-2s-1
Numbers of turns per second
11245
Number of collisions per second
600 million
Figure 4
Hadronic calorimeters (HCAL): measure the energy of the particles
made of quarks (hadrons) when they interact with atomic nuclei by
the production of charged particles.
Pions, protons, or neutrons
Drift Tubes (DT)
Cathode Strip Chambers
(CSC)
Thin Gap Chambers (TGC)
Resistive Plate Chambers
(RPC)
Figure 6
Tracking Electron. Hadronic
Muon
device Calorimeter calorimeter detector
Photon
Photon, electrons or
positrons
Consists of a steel plate structure spread with 500000
plastic tiles of 3 mm thickness, it senses the hadronic
shower particles. It is formed by modules which are
controlled and read by three optical fibres. It also has a
liquid argon end cap formed by a liquid argon device
made of copper plates.
Muons are the only charged particle
that can travel through all of the
calorimeter material. The Muon
System determines the signs and
momenta of muons with better
precision than the inner tracking
system does. It has some different
parts: MDT (tube resolution 80µm),
CSC (resolution 60µm), TGC and
RPC (electric field of 5000V/mm).
1232
21 m log, 15 m high,
15 m wide
Figure 5
Electromagnetic calorimeters (ECAL): measure the energy of light
particles when they interact with the electrically charged particles
inside matter (from the electromagnetic shower)
Tile calorimeter (hadronic)
Number of dipoles
46 m long, 25 m high,
25 m wide
Figure 3
Electrons or
positrons
Muons
Figure 8. DATA ANALYSIS: Trigger and data
acquisition system: an event has to pass two independent
sets of tests or trigger levels, for selecting 100 interesting
events per second out of 1000. The data acquisition system
channels the data from the detector to the storage. Physicists
from around the world use this data for their studies.
Pions or
protons
Neutrons
Muons
Figure 7. Onion-like structure of experiments. The particles trough
the detector and their interaction in each of the regions from the
innermost to the outermost layer as explained in the left.
ATLAS: A Toroidal LHC ApparatuS
Muon detector
9300
Particles detected
Gaseous chambers: the medium ionized is a gas and the ions or
electrons are collected on electrodes under strong electric fields
(spatial resolution 50-100 microns)
Determines the signs and momenta of muons
Number of magnets
CMS
Size
Figure 5 and 6. Tracks of a simulated event
detected at ATLAS and CMS, respectively
Types
BEAM PIPE-COLLISION
REGION
26659 m
Figure 3 and 4. Simulated event in the
ATLAS and CMS detector, respectively
The particle detectors are made to record and visualize the particles which come from the collisions at accelerators
(LHC: collision between protons at 14 TeV). They give information about their charge, mass or speed. The shape,
the size, the length, the shape, the direction and the depth are only some parameters we can study.
Detector zone
Circumference (previous LEP)
ATLAS
Figure 2. General detector shape
DETECTOR GENERALITIES
Figure 1. The LHC system
Important quantities for the LHC:
CMS: Compact Muon Solenoid
Tracker
Liquid Argon calorimeter
(electromagnetic)
Consists of thin lead plates (about 1,5 mm
thick), immersed in a bath of liquid argon
(-183ºC), separated by sensing devices.
Particle showers in the liquid argon
produce ions which are read out as electric
pulses by Kapton electrodes.
Cylinder of a length of 6m and a diameter of 2.6m. Fine pitch
silicon strip detectors provide precise hits (25000 silicon strip
sensors over 210m2). In the central region the momentum
resolution is given by ∆p/pi=0.005+0.15pi (pi in TeV)
Electromagnetic calorimeter (ECAL)
Nearly 80000 crystals of scintillating lead tungstate (PbWO4) are used to
measure accurate energies of electrons and photons. A preshower detector,
which contains two thin lead converters followed by silicon strip detector
planes placed in front of the ECAL, helps in the γ-π0 separation.
The inner tracker and its parts.
Silicon tracker
Toroid Magnets
There are an additional set of
magnets located in the regions
outside of the calorimeters, they
produces a magnetic field which
lines are circles centred on, and
perpendicular to the beam line,
and encircling it. The toroidal
field deflects the muons in the
plane defined by the beam axis
and the muon position
Electromagnetic calorimeter
Transition Radiation Tracker
(TRT)
Solenoid Magnet
It is located outside the inner detector. A
magnetic field (2T) is produced by a large
electrical current (8000A) inside a hollow
cylindrical coil. It deflects each charged
particle coming from the collision:
Pixel detector
Solenoid magnet
•Particle perpendicular to the beam: travels
in a circle.
•Not perpendicular to the beam: the field
changes the trajectory to a circular helix.
Semiconductor Tracker (SCT) - Strip Detectors
It is used to give additional position measurements further from the
collision point. It has layers of silicon subdivided into narrow strips, each
layer has two sets of strips. On the cylinders, the strips run parallel to the
the beam axis, on the disks, the sets go radially.
When a charged particle goes through the strip detector, signals identify
which strip in each set has been touched. The dissipated power in the
detector, more than 30kW, is removed by and evaporating cooling system,
keeping the silicon temperature at –7ºC. The intersection of those two
strips gives a 3-dimensional position measurement. This is very expensive,
that is why this is not used for larger detector areas.
Is the closest sensor to the collision point. These
devices are made of thin layers of silicon
subdivided into pixels of dimensions
50×400µm2. The ATLAS detector will have
approximately 140 million of these tiny pixel
sensors. Closer to the collision, pixels are placed
cylindrically, whereas they are located on disks
further away. Each time a charged particle
traverses a layer a signal is produced, that gives a
precise measure of particle position to know if
the particle has been originated at the collision
point, or a few millimetres from it as a decay
product of another.
They are formed by gas-wire drift
detectors made of tubes with thin
wires running through the
centres. They are filled with an
appropriate gas, and high voltage
is applied between the wire and
the metallized tube wall. When a
particle traverses a tube, its wire
produces a discharge and that
determines how far from the wire
has the particle passed.
In the centre section the tubes
run parallel to the beam pipe, and
in the two end sections the tubes
are positioned radially.
TRT
Magnet system: superconducting coil+ magnet yoke
(barrel and encap) + vacuum tank + cryogenics.
Hadronic calorimeter (HCAL)
Hadronic barrel (HB) and hadronic endcap (HE) calorimeters are sampling
calorimeters with 50mm thick copper (selected because its density) absorber
plates which are interleaved with 4mm scintillator sheets. HB is made of two
4.4m length half-barrels, and HE has two large structures, situated at each end
of the barrel detector and within the region of high magnetic field.
It is 13m long and 6m in diameter, and its refrigerated superconducting
niobium-titanium coils, cooled at -270ºC, produces a magnetic field of 4 Teslas.
The magnetic field is obtained by a solenoid because with the field parallel to
the beam, the bending of the muon tracks is in the transverse plane (with an
accuracy of 20µm) and the momentum measurement starts at zero radial
distance from the beam pipe.
Inner tracker
HIGGS PHYSICS
The Standard Model (SM) of Particle Physics has unified the electromagnetic interaction (carrier: γ –
massless-) and the weak interaction (carriers: Z0, W+, W – quite massive: 80-90GeV-). According to the
SM, particles acquire mass trough their interaction with the Higgs field, which implies the existence of a
new particle: the Higgs boson H0.
MEDICAL
The theory does not predict the H0 mass, but it predicts its production rates and decay modes
for each possible mass. Measuring the decays at ATLAS and CMS detectors the MH could be
determined.
Figure 11
TECHNICAL
Figure 11 and 12. Different simulations
of the Higgs decays depending on its
mass. The Higgs signal that will be
measured is indicated in each case.
H0→ZZ*→2l+2l-
If the mass is in the range 130-700 GeV the most promising channels are
or H0→ZZ→2l+2l-. The detection relies on the excellent performance from the muon
chambers, the tracker and the electromagnetic calorimeter
CULTURE
SUPERSYMMETRY (SUSY)
In SUSY each fermion (matter particle, spin -1/2) has a “superpartner” of spin 0 while each boson (force
carrier, integer spin) has a spin -1/2 superpatner.
Nowadays, no superpartners have been observed, it may be caused by the different mass of
the superpartners relatively to their partner particles. If the scale is in the TeV it will be
measured at the LHC.
Figure 13. Production of sparticles may reveal itself
REFERENCES
Eyes on the LHC, F. Gianotti and C. Quigg, Physics Today, September 2007
http://public.web.cern..ch http://atlas.ch http://cms.cern.ch
trough some kinematical spectra with a pronounced edge
in the l+l- (leptons) mass spectrum, as in the inclusive dilepton mass spectrum image.
Muon detectors
They are placed behind the calorimeters and the coil, and consist of four muon
stations interleaved the iron return yoke plates. In the barrel region they are
arranged in concentric cylinders around the beam line, and in perpendicular disks
to the beam line in the endcaps.
CMS uses three types of detector: drift tubes (DT), cathode strip chambers (CSC)
and resistive plate chambers (RPC).
Drif tube cross section
Parts of the muon chamber
OTHER APLICATIONS FOR THE TECHNICAL ADVANCES USED IN ATLAS AND CMS
Figure 10. Decay rates depending
on the Higgs mass
If the mass is in the range 80-140 GeV H0→γγ is the most promising channel.
Hadronic forward (HF) calorimeter: two situated at each extreme of the CMS
detector. It is made of steel absorbing plates and quartz fibers are inserted into
them. The pass of charged particles trough the quartz fibers produces Cerenkov
light signals, which are used to measure the energy of the jets.
The inductance of the magnet is 14 henries and the nominal current is 19500
amperes, giving a total stored energy of 2.66GJ. There are dump circuits to
safely dissipate this energy. The circuit resistance (0.1 milliohms) leads to a
circuit time constant of nearly 39 hours, which is the longest time constant of
any circuit at CERN.
SCT
Figure 9. Production of
Higgs boson at the LHC
Detail of the HB
There are different types of HCAL:
Superconducting magnet
Pixel detector
SOME PHYSICAL EXPERIMENTS AT THE LHC THAT
WILL BE DETECTED AT ATLAS AND CMS
Simulated event in
the CMS
Advances used in detectors
Other applications
Multi picture
elementary counters
Miniature electronic silicon chips
Biomedical imaging (mammography)
Computer tomography
XPAD, photon counting detector
Can be combined with Positron Emission
Tomography (PET)
Retina project
Silicon microstrip detector technology
Used to understand how living neural systems
process
X-Ray detector
XPIX is an X-ray detector using the XPAD
chip developed for ATLAS
Studying the structure of proteins and solid
state materials
Ultrasound gas
analysis
Measure the fluorocarbon vapours in the
cooling system of the inner detector
Analyse the gas mixtures in semiconductor
production
Emergency personnel
location
System for finding and rescuing persons in
case of an accident in the CERN area
Is suitable in large areas such as mines where
people are difficult to find
Sound reproduction
Measure and align the individual silicon
detectors of the ATLAS Semiconductor
Measure the shape of the groove on
mechanical sound carriers.
Grid computing
Data taken are distributed and accessible to
scientists of LHC experiments.
Applications running on this infrastructure:
Education
Students are involved in the development of
parts of the new detectors
This work provides experience in modern
laboratory work
Figure 14. Ultrasound gas
analysis
Figure 15. Digital
radiograph of a
hornet.
CONCLUSIONS
Figure 12
The knowledge about Particle Physics is fundamental to understand the real nature of matter
It requires the newest technology → challenge for physicists and for technological advances
Detectors play a fundamental role because they are our eyes in the particles world, they are the instrument used to
study and be near of them. They will be doing the same in the next years because with the new detectors we have broaden
our possibilities of making new important discoveries.