Transcript Particle physics experiment - Institute of Particle and

LHC Detectors:
ATLAS and CMS
Howard Gordon, Brookhaven National Laboratory, Jiří 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 Lawrence’s cyclotron
at CERN Microcosm
1
CERN LHC, to be finished in 2007
2
Why are physicists building such huge and expensive
machines???
Because there are still many unanswered questions, like:
What gives
particles
their mass?
Where is the
awaited
Higgs boson?
Are there any
extra dimensions
predicted by
some theorists?
Do the predicted
supersymmetry
particles exist?
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:
3
Here is one of them:
A Toroidal LHC ApparatuS
ATLAS
22 m
44 m
4
And here the second:
Compact Muon Spectrometer
CMS
15 m
22 m
5
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,
1,15×1011 protons in each bunch,
bunch spacing 25 ns
what corresponds to 7.5 m
distance
(some bunch positions are empty).
6
The collision point is “watched”
by surrounding detector.
Some particles just escaped
from the collision zone,
the next collision threatens.
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 detector should:
• have large coverage
(catch most particles)
• be precise
• be fast (and cheap and ...)
Each proton carries energy 7 TeV.
So each bunch with 1011 protons carries
energy 1011×7×1012 eV = 7×1023 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!
So boring to paint
1011 protons
in each bunch ...
7
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
8
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
electron
The energetic electron radiates photons
which convert to electron-positron pairs
which again radiate photons
which ... This is the electromagnetic shower.
The energetic muon causes mostly just the
ionization ...
muon
pion (or another
hadron)
Electrons and pions
with their “children”
are almost completely 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.
9
Expert pages! You don´t 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 8×10-17 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:
N beam (t )
Nbeam (t )  Nbeam (0).exp( ntargett )  Nbeam (0).exp(t / t )
where we introduced the mean interaction length t. This quantity
determines the mean distance between collisions of hadrons with
t
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
10
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
muon
Hadronic calorimeter:
offers a material for
hadronic shower and
measures the deposited energy.
Neutrinos escape without detection
hadrons
Tracker: Not much material,
finely segmented detectors
measure precise positions
of points on tracks.
Electromagnetic calorimeter:
offers a material for electromagnetic shower and measures
the deposited energy.
Muon detector:
does not care about
muon absorption and
records muon tracks.
11
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 Tracker or Inner detector
The Hadronic calorimeter
The Electromagnetic calorimeter
The Muon detector
12
ATLAS and CMS follow the same principles but differ in realization:
ATLAS
Tracker or
Inner Detector
Silicon pixels, Silicon strips,
Transition Radiation Tracker.
2T magnetic field
CMS
Silicon pixels, Silicon
strips.
4T magnetic field
Electromagnetic Lead plates as absorbers with Lead tungstate (PbWO4)
calorimeter
liquid argon as the active
crystals both absorb and
medium
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
13
So, why are the detectors at LHC so big???
Many
tempting
questions
Challenging
theoretical
predictions
Towards higher energy
LHC, 7+7 TeV
Curiosity
to explore
the
unexplored
ATLAS and CMS
in their complexity
Many very energetic particles
to be recorded
and analysed
14
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???
Destiny of
ATLAS after
first data
taking?
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.
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.
15
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...
You may notice that our estimates are quite rough.
We calculate with a year having 107 seconds instead
of having p×107 seconds. We expect that not the whole
year could be used for running the experiment
and recording the data.
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).
16
17
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 responsibilities as
formalized in a
Memorandum
of Understanding.
Financial support
comes from the
funding agencies
of individual
participating states.
18
19
Both these experiments
have a well defined
democratic structure
for steering all affairs.
There has been
a heavily documented process for
each subdetector:
setting the
objectives
Review
Review
developing the
detailed technical
specifications
full prototyping of
each component
procurement and
placing contracts
Review
Review installation
testing
fabrication
Review
Review
commissioning
operation
20
These collaborations have organized 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.
21
Decisions and technical specifications 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.
22
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 collaborating 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
calorimeter.
Hadronic calorimeter
being assembled
in the ATLAS
experimental cavern.
Toroid magnets of
the muon system
23
To be continued
24