LHC Detectors: ATLAS and CMS

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LHC Detectors ATLAS and CMS
Howard Gordon, Brookhaven National Laboratory,
Jirí Dolejší, Charles University Prague
Physicists passed a long way from the table-top
accelerators like the first cyclotron invented
and built for about 25 by Ernest Lawrence in
1930 towards huge accelerators for about 1 G
hidden under the landscape like LHC at CERN ...
Replica of Lawrences cyclotron at CERN Microcosm
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CERN LHC, to be finished in 2007
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Why are physicists building such huge and
expensive machines???
Because there are still many unanswered
questions, like
Do the predicted supersymmetry particles exist?
Where is the awaited Higgs boson?
Are there any extra dimensions predicted by some
theorists?
What gives particles their mass?
A rather simple question might also be Is the
Nature fully described by the today's Standard
Model, nothing beyond? The answer could be hardly
yes! The new machines are huge and therefore
expensive to explore the new energy regions and
to enable studies of extremely rare processes ...
if something was not observable in the past, we
should create the chance to observe it
tomorrow. LHC will accelerate particles, but we
should be able to see them to have
appropriate detectors. Have a look at them
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Here is one of them
A Toroidal LHC ApparatuS
ATLAS
22 m
44 m
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And here the second
Compact Muon Spectrometer
CMS
15 m
22 m
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Why are the detectors at LHC so big???
They should deal with all particles flying from
the collision of accelerated protons.
The protons are not two like on the animation,
but plenty of them grouped into bunches
2808 bunches in each beam, 1,151011 protons in
each bunch, bunch spacing 25 ns what corresponds
to 7.5 m distance (some bunch positions are
empty).
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Each meeting of two bunches results in about 23
proton-proton collisions. The mean number of
particles born in all these collisions is about
1500. The detector should record as many of
them as possible.

• The collision point is watched
• by surrounding detector.
• Some particles just escaped
• from the collision zone,
• the next collision threatens.
• The detector should
• have large coverage
• (catch most particles)
• be precise
• be fast (and cheap and ...)

So boring to paint 1011 protons in each bunch
...
Each proton carries energy 7 TeV. So each bunch
with 1011 protons carries energy 101171012 eV
71023 eV 44 kJ. This is a macroscopic
energy!!! In order to reach such kinetic energy
on a bike, you go with a speed of more than 30
km/h!
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The real detector should have no holes and
expose to particles sufficiently thick layer of
material to detect them (see the chapter
Particle physics experiment for processes which
happen when particles fly into matter).
The collision point is watched by surrounding
detector. Here many particles escape detection.
The Collisison point surrounded by layers of
different detectors
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Let us have a look at interaction of different
particles with the same high energy (here 300
GeV) in a big block of iron
1m
The energetic electron radiates photons which
convert to electron-positron pairs which again
radiate photons which ... This is the
electromagnetic shower.
electron
The energetic muon causes mostly just
the ionization ...
muon
pion (or another hadron)
Electrons and pions with their children are
almost comple- tely absorbed in the sufficiently
large iron block.
The strongly interacting pion collides with an
iron nucleus, creates several new particles
which interact again with iron nuclei, create
some new particles ... This is the hadronic
shower. You can also see some muons from
hadronic decays.
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Expert pages! You dont need to understand them,
but it is a challenge!
Try to answer the following questions What about
interactions of high energy photons? What about
neutral pions which decay very quickly (the mean
lifetime is just 810-17 s, ct 25 nm) to two
photons? To answer these questions think about
the evolution of the electromagnetic cascade
... For a little bit deeper insight to the
electromagnetic and hadronic showers we may
remember the exponential probability of a
projectile to survive without interaction or
without absorption (see the chapter Particle
physics experiment) in the depth t of the
target where we introduced the mean
interaction length t. This quantity determines
the mean distance between collisions of hadrons
with nuclei of the material and therefore it
tells us where the hadronic shower will probably
start and how fast it will evolve. The radiation
length X has almost the same meaning in
evolution of the electromagnetic cascade it
determines the mean path of an electron to
radiate the photon and also the mean path of a
photon to convert to the electron-positron pair.
Look at values of these quantities for several
materials
Material Radiation length X Nuclear interaction length t
water 36,1 cm 83,6 cm
iron 1,76 cm 16,9 cm
lead 0,56 cm 17,1 cm
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Here is the general strategy of a current
detector to catch almost all particles
Magnetic field bends the tracks and helps to
measure the momenta of particles.
electron
Hadronic calorimeter offers a material for
hadronic shower and measures the deposited
energy.
muon
Neutrinos escape without detection
hadrons
Tracker Not much material, finely segmented
detectors measure precise positions of points on
tracks.
Muon detector does not care about muon
absorption and records muon tracks.
Electromagnetic calorimeter offers a material
for electro- magnetic shower and measures the
deposited energy.
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All the detectors are wrapped around the beam
pipe and around the collision point here are a
schematic and less schematic cut through ATLAS
The Electromagnetic calorimeter
The Tracker or Inner detector
The Muon detector
The Hadronic calorimeter
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ATLAS and CMS follow the same principles but
differ in realization
ATLAS CMS
Tracker or Inner Detector Silicon pixels, Silicon strips, Transition Radiation Tracker. 2T magnetic field Silicon pixels, Silicon strips. 4T magnetic field
Electromagnetic calorimeter Lead plates as absorbers with liquid argon as the active medium Lead tungstate (PbWO4) crystals both absorb and respond by scintillation
Hadronic calorimeter Iron absorber with plastic scintillating tiles as detectors in central region, copper and tungsten absorber with liquid argon in forward regions. Stainless steel and copper absorber with plastic scintillating tiles as detectors
Muon detector Large air-core toroid magnets with muon chamber form outer part of the whole ATLAS Muons measured already in the central field, further muon chambers inserted in the magnet return yoke
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So, why are the detectors at LHC so big???
Many tempting questions
Towards higher energy
LHC, 77 TeV
Challenging theoretical predictions
Curiosity to explore the unexplored
Many very ener- getic particles to be recorded
and analysed
ATLAS and CMS in their complexity
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How to get the data from the detector?
The detectors will sense the collisions of proton
bunches every 25 ns, i.e. with the frequency of
40 MHz. With 23 pp collisions in every bunch
crossing it means pp collision rate almost 1 GHz.
Few GHz is the frequency of current computer
processors, so how it could be possible to
collect and elaborate data from such a huge
detector???
One should have in mind, that new beam particles
come to the interaction region with a speed of
light, but signals from the detector move in the
cables always slower. One could therefore expect,
that information from the detector will cumulate
inside and sooner or later explode. Almost every
student knows the feeling of the
potentially exploding head from some lectures or
seminars.
Destiny of ATLAS after first data taking?
The solution is quite human - to concentrate on
the most interesting events and to forget about
all others. This task is performed by the trigger
system. The trigger planned for ATLAS has three
levels and in these three steps reduces the event
rate to about 100 200 events per second which
are written to storage media. The size of data
from one event is about 1 MB.
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What to do with that amount of data?
The data heap will grow fast more than 100 MB
per second, about 10 TB per day, 1 PB (1015 B)
per year. You can translate this amount of data
to usual media ATLAS will need to burn a CD
every 7 seconds, more than ten thousands CDs per
day, more than million CDs per year... The
computing power needed to analyze this huge
amount of data is larger than what is available
now. LHC experiments are actively participating
in the development of a new computing tool to
facilitate the analysis. The solution is a
distributed computing and the corresponding key
word is the grid. The word grid as used here
is analogous to the power grid the distributed
requests for computing resources, data or
computational power will be satisfied by the
tiered structure of computing centers (see figure
on the following page).
You may notice that our estimates are quite
rough. We calculate with a year having 107
seconds instead of having p107 seconds. We
expect that not the whole year could be used for
running the experiment and recording the data.
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How these collaborations work? Where they get
money?
The ATLAS Collaboration includes about 1850
physicists and engineers from 175 institutes in
34 countries. CMS has a similar list of
participants often from the same countries, but
not completely overlapping.
Each institute has specific responsi-bilities as
formalized in a Memorandum of Understanding.
Financial support comes from the funding
agencies of individual participating states.
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Both these experiments have a well defined
democratic structure for steering all
affairs. There has been a heavily docu- mented
process for each subdetector
setting the objectives
Review
developing the detailed technical specifications
Review
procurement and placing contracts
Review
Review
installation
full prototyping of each component
testing
commissioning
Review
Review
fabrication
operation
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These collaborations have orga-nized meetings to
resolve specific design issues and to divide the
work. The meetings can occur all over the
world, often using telephone or video
conferencing, but are mostly held at CERN.
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Decisions and technical specifi-cations are
documented in Technical Design Reports, drawings
and other documents that are available on the
World Wide Web that was invented at CERN by
particle physicists.
The NEXT cube, the first WWW server at CERN
Microcosm and Tim Berners-Lee which together
with Robert Cailliau invented the World Wide Web.
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What is happening now?
Leading industrial companies from all over the
world fabricate components of the detector. Many
of the components are assembled in the various
collabo-rating institutes. Final installation
and commissioning of each component is done at
CERN with the participation of the collaborating
teams.
The cryostat for liquid argon electromagnetic ca
lorimeter.
Hadronic calorimeter being assembled in the
ATLAS experimental cavern.
Toroid magnets of the muon system
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To be continued