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LHC Detectors: ATLAS and

CMSPhysicists passed a long way from the table-top accelerators like the first cyclotron invented and built for about 25$ by Ernest Lawrence in 1930towards huge accelerators for about 1 G$ hidden under the landscape like LHC at CERN ...

Howard Gordon, Brookhaven National Laboratory, Jiří Dolejší, Charles University Prague

Replica of Lawrence’s cyclotron at CERN Microcosm

2CERN LHC, to be finished in 2007

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Are there any extra

dimensions predicted by

some theorists?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:

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?

Do the predicted

supersymmetry particles exist?

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Here is one of them:

ATLAS

22 m

44 m

A Toroidal LHC ApparatuS

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And here the second:

CMS

15 m

22 m

Compact Muon Spectrometer

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They should deal with all particles flying from the collision of accelerated protons.

Why are the detectors at LHC so big???

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).

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Each meeting of two bunches resultsin 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 escapedfrom 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 paint1011 protons in each bunch ... Each proton carries energy 7 TeV.

So each bunch with 1011 protons carriesenergy 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!

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The Collisison point surrounded by layers of different detectors

The collision point is “watched”by surrounding detector. Heremany particles escape “detection“.

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).

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The energetic electron radiates photons which convert to electron-positron pairs which again radiate photonswhich ... This is the electromagnetic shower.

Let us have a look at interaction of different particles with the same high energy (here 300 GeV) in a big block of iron:

electron

muon

pion (or another hadron)

The energetic muon causes mostly just theionization ...

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.

Electrons and pionswith their “children”are almost comple-tely absorbed inthe sufficiently large iron block.

1m

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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, c = 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 hadronicshowers we may remember the exponential probability of a projectileto 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 . This quantitydetermines 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:

Expert pages! You don´t need to understand them, but it is a challenge!

)(beam tN

t

)/exp().0()exp().0()( beamtargetbeambeam tNtnNtN

Material Radiation length X Nuclear interaction length

water 36,1 cm 83,6 cmiron 1,76 cm 16,9 cmlead 0,56 cm 17,1 cm

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Here is the general strategy of a current detector to catch almost all particles:

electron

muon

hadrons

Tracker: Not much material,finely segmented detectorsmeasure precise positions of points on tracks.

Electromagnetic calorimeter: offers a material for electro-magnetic shower and measures the deposited energy.

Hadronic calorimeter: offers a material for hadronic shower andmeasures the deposited energy.

Muon detector: does not care about muon absorption and records muon tracks.

Neutrinos escape without detection

Magnetic field bends the tracks and helps to measure the momenta of particles.

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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 calorimeterThe Tracker or Inner detector

The Hadronic calorimeter The Muon detector

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ATLAS and CMS follow the same principles but differ in realization:

ATLAS CMSTracker 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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Curiosity to explore

the unexplore

d

Challenging

theoretical prediction

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So, why are the detectors at LHC so big???

Many tempting question

s

Towards higher energy

LHC, 7+7 TeV

Many very ener-getic particles to be recorded and analysedATLAS and

CMSin their complexity

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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 cometo the interaction region with a speed of light, but signalsfrom 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 potentiallyexploding head from some lectures or seminars.

How to get the data from the detector?

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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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).

What to do with that amount of data?

You may notice that our estimates are quite rough. We calculate with a year having 107 seconds instead of having ×107 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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Review

Review

Review

setting the objectives Review

Both these experiments have a well defined democratic structure for steering all affairs.

There has beena heavily docu-mented process for each subdetector:

developing the detailed technical

specificationsReviewfull prototyping

of each component

Review

procurement and placing contractsfabricatio

n

testinginstallation

commissioningoperatio

n

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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 serverat CERN Microcosm and Tim Berners-Lee which together with Robert Cailliau invented the World Wide Web.

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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.

What is happening now?

Hadronic calorimeterbeing assembled in the ATLAS experimental cavern.

The cryostat for liquid argon electromagneticcalorimeter.

Toroid magnets of the muon system

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To be continued