AAPTMar01

Report
ATLAS at the Large Hadron Collider
A Particle Physics Detector
at the
Energy Frontier
M. Gilchriese
Lawrence Berkeley National Laboratory
M. Gilchriese - March 2000
Outline
• Physics motivation - why are we doing
this?
• What is the Large Hadron Collider?
• Introduction to the ATLAS detector
• A film interlude
• The experimental environment
• Tour of ATLAS detector technology
• Pointers to additional information
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Physics Motivation
• Experimental and theoretical work over the last three decades
has resulted in the “Standard Model” that, so far, describes with
remarkable success the “fundamental” particles and the forces
that act between them.
• Nevertheless there many mysteries that are not explained by
the “Standard Model”.
• Among these are
– is there a field - the Higgs field or equivalent - that explains the very
different masses of the basic building blocks, the quarks, leptons and the
quanta of the fundamental forces? If so, find this particle(or particles).
– does supersymmetry exist ie. are there partners to each of the known
building blocks with different mass and spin?
– do the quarks and leptons have substructure?
– can we observe direct manifestations of extra dimensions?
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Standard Model
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Colliding Beams
• In parallel with the theoretical developments over the last 30some years, has been the development of colliding beam
accelerators to achieve the highest possible energies.
• During this time, electron-positron, proton-proton and
antiproton-proton colliders have been built with ever increasing
energies to continue to push the “energy frontier” in particle
physics.
• Many(but definitely not all!) of the discoveries of new
quarks(charm, top), leptons(tau) and of the force
quanta(gluons, W, Z) have come from experiments at these
colliders.
• The Large Hadron Collider will be the highest energy collider
in the world, and is likely to remain so for a long time.
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Large Hadron Collider(LHC)
• Located at the CERN, the
European Laboratory for
Particle Physics
• Will be operational by
2006.
• Collides protons-onprotons(and can also do
heavy ions-on-heavy ions).
• Center-of-mass energy 14
TeV
• High intensity(luminosity),
more than 10x current
proton-antiproton colliders.
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Aerial View of the LHC
Circumference of 27 km
Main CERN Site
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LHC Underground Layout
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LHC Magnets
• Superconducting dipoles and
other magnets guide and focus
the proton beams and bring
them into collision at multiple
points around the ring.
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• Magnets are cooled by
superfluid Helium at 1.9oK to
achieve the highest possible
magnetic field with the
“standard” superconductor
M. Gilchriese - March 2000
used.
A Toroidal LHC ApparatuS
Muon Detectors
Superconducting
Toroids
Inner Tracking
Calorimeters
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Tall person
Superconducting
Solenoid
ATLAS
M. Gilchriese - March 2000
International Collaboration
2000 scientists
150 institutions
35 countries
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Roll the Film!
M. Gilchriese - March 2000
LHC Experimental Environment
• Proton collisions are messy!
• Each proton can(crudely) be thought of as a collection of
constituents/virtual particles.
• The “hard”, high energy(actually transverse momentum)
collisions of these constituents are of interest to probe the
smallest distances and to produce the highest mass new particles
• The “soft” collisions are background to the interesting, hard
collisions.
M. Gilchriese - March 2000
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Luminosity and Event Rates
• An interaction between the
counter-rotating protons is called
an “event”.
• The rate of these events depends
of the type of interaction{the cross
section()} and the luminosity(L).
Rate = L x 
• LHC luminosity = 1034 cm-2sec-1
• “Soft” collisions => 109 events
sec-1
• Most interesting events are much,
much rarer, few per second or less
=> on-the-fly filtering of data
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A Toroidal LHC ApparatuS
Muon Detectors
Superconducting
Toroids
Inner Tracking
Calorimeters
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Tall person
Superconducting
Solenoid
ATLAS
M. Gilchriese - March 2000
What Is Detected?
Detect
•
•
•
•
•
•
Electrons
Muons
Taus(not so easy)
Photons
Jets(will explain)
Original quark type(b,c,s)
sometimes
• Neutrinos or other non-interacting
particles
•
•
•
•
•
•
•
How?
Electromagnetic calorimetry/tracking
Absorber/tracking
Tracking/calorimeter
Electromagnetic calorimetry/tracking
Calorimeter/tracking
Secondary vertices/tracking
Calorimeter
Detection
“Onion”
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Basics of Particle Detection
• Liberation and collection(sometimes also amplification) of
charge(ionization) in
– solid(eg. silicon)
– gas(usually argon + other gases)
– liquid(eg. liquid argon)
• or creation of light by ionization(scintillation) that is converted
to charge(eg. by photomultiplier tube)
• Followed by amplification(if needed), storage, processing…in
electronics elements.
• Within ATLAS there are more than 108 individual electronics
elements.
• This is only possible by the extensive use of integrated circuit
technology.
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Tracking Detector
• Silicon pixel and strip
detectors and wire/gas
detectors.
• Inside solenoid(2 Tesla field)
to measure momentum of
charged particles.
7m
Superconducting
Solenoid
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Elements of the Tracking Detector
• Higher granularity and resolution
near collision point.
• Granularity sizes are
– Silicon pixel detector 50µ x 400µ
– Silicon strip detector 80µ x 12cm
– Straw tube/wire gas 4mm x 1m
Straw tubes
Gas-filled tube
Wire at
HV
Strip detector
Wire bonds
Front-end ICs
Hybrid
M. Gilchriese - March 2000
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6cm
Charged Particle Tracking
• Computer reconstruction
• Pattern recognition
TRT
• This particular event shows
the characteristics of “jets”
• Jets are created from the
quarks and gluons formed in
the collisions.
• The quarks and gluons
Silicon
combine to form
pixel
hadrons(pions, kaons,
detectors
protons…) that are
detected(if charged) by the
Silicon strip detectors
tracking detectors or by the
calorimetry(charged or
neutral).
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Electromagnetic Calorimetry
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Liquid Argon Detector
• Ionization created by
electromagnetic(EM)
showers(in lead mostly)
is detected in liquid
argon.
Electromagnetic
Shower
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EM Calorimeter Elements
Location and energy of EM showers measured
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Detecting Electrons
• Electrons are
identified and
their energies
measured by
combining
tracking(is there a
track) and the EM
calorimeter as
shown in this
computer
simulation of a
Higgs particle
decay to four
electrons.
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Hadronic Calorimetry
• Energy from hadrons is
detected in the
combination of the the
EM and Hadronic
calorimetry.
• Goal is to measure well
as much of the energy in
an event as possible =>
smallest possible holes.
• And the direction and
magnitude of the energy.
• Missing (transverse)
energy(eg. from
neutrinos) can also be
inferred.
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Scintillation Tile Calorimeter
Photomultiplier Tube
• Hadrons interact in steel producing
showers of particles.
• These create light in the scintillating tiles.
• The amount of light is proportional to the
energy.
Scintillating Fiber
• Light is converted to charge by
photomultiplier tubes.
Scintillating Tile
Steel
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Detecting Muons
• Combine tracking
information with
fact that muons
penetrate material
- see next slides.
• Computer
reconstruction of
Higgs particle
decay into two
muons and two
electrons
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Big Superconducting Magnets
• Will be world’s biggest
superconducting
toroids to measure
muon momenta using
precision wire/gas
tubes - see next page.
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Muon Position Measurements
Pressurized, gasfilled tube.
Wire at
HV
“Acres” of very precise
(100 Micron) measurements
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Data Processing
• The data obtained from all of the detector elements
ultimately ends up on permanent storage accessible to
computers.
• Only a small fraction, roughly 1 part in 107, of the
events created in the proton-proton collisions ends up
on permanent storage.
• Computing power in all of the collaborating countries
will be used to analyze the data.
• The goal(shared with other areas of science) is to
make analysis of data possible via a world-wide data
network “grid” accessible from as many places as
possible.
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Additional Information
• ATLAS has an educational Web Site (under
continuous construction/improvement) at
http://pdg.lbl.gov/atlas/atlas.html
• This includes copy of the movie.
• And there are additional educational
resources at
http://www-pdg.lbl.gov/
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