Tom Barber

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• Tom Barber

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The Large Hadron Collider

• Located at CERN
• Franco-Swiss border
• 27km diameter ring
• 100m underground
• Accelerates protons to nearly the speed of light
• Two counter rotating beams.

27km
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The Large Hadron Collider

• Using superconducting magnets.
• 300 degrees below zero.
• 800 million collisions per second
• Energy in the beam as much as a 400 tonne train
  at 150 km/h.
• Only 14 TeV of energy in each collision.

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High Energy Physics?

• Units of electron volts (eV)
• LHC produces 7 TeV protons
• But how much is 1 TeV?
• 1 TeV 1012 eV 1000000000000 eV
• 1 eV 10-19 J 0.0000000000000000001 J
• 1 TeV 10-7 J 0.0000001

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Not so High Energy

• Kinetic Energy
• K.E. ½ m v2
• Something small a mosquito
• How heavy?
• m 0.001 kg
• How fast?
• v 0.01 m/s
• Kinetic Energy
• KE 0.001 x 0.01 x 0.01 J
• 0.0000001 J 10-7 J
• 1 TeV !!

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Large Mosquito Collider!
7 TeV
7 TeV

• So why high-energy physics

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Which contains more water?
Clouds?
OR
Water Droplet?
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• Clouds contain more water molecules in total.
• Try standing outside in the rain!
• The drop contains more molecules in a fixed
  volume.
• The water drop has a higher

density
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High Energy Density

• The droplet contains water in a small volume.
• The LHC squeezes the energy into a space
• a million million times smaller
• than a mosquito.
• The same energy density as during the Big Bang!
• Looking back to the start of the universe!

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The ATLAS Experiment

• Protons collide at 4 points on LHC.

• ATLAS at point 1
• Records the aftermath of collisions.

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Who is ATLAS?
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• 2000 physicists
• 166 universities
• 37 countries

• Multi-national collaboration

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The ATLAS Detector

• The detector is made of many parts

• What do they all do?

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Building a Particle Detector

• Time
• Position
• Charge
• Momentum
• Energy
• How do we do it

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Measuring Position

• Particle Motion
• Track particle position using silicon layers.
• Charged particles in silicon cause electric
  current.
• Current measured by electronics.
• Tracks reconstructed by computer programs.

electric current
charged particle
silicon layer
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ATLAS Tracker
420,000 Xenon Straws
6 Million Silicon Strips
135 Million Pixels
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SemiConductor Tracker

• 4 barrel layers
• 9 endcap wheels
• 6 million silicon strips

• Need to configure and read-out each module.
• I work on the Data AcQuisition (DAQ) software.

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Momentum

• What is momentum?
• p m x v
• Charged particles on the move
• Force on charged particle in a magnetic field
• F B x e x v
• Force at right angles
• ? circular arcs
• With a centripetal force
• F m v2 / r
• Put them together

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Making Tracks

• Measure radius of tracks to measure momentum
• p B x e x r
• We need a magnet
• a really big magnet.

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ATLAS Magnets

• Solenoid
• 5.3m long
• 2.4m bore
• 5.7 tons
• 2 T Field

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Toroid Magnets
Endcap
Barrel
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Energy

• Measured by calorimeters

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What about muons?

• Muons are heavy.
• They through most of ATLAS.
• Need their own outer detector.

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Summary

• Now we can measure
• Position
• Momentum
• Energy
• Muons
• Lets put it all together

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An ATLAS Event
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Needle in a Haystack

• Signals converted to data for computers.
• Protons collide
• 40,000,000 times a second
• Enough data to fill 100,000 CDs per second!
• But
• only one in 5,000,000 is interesting.
• How do we decide which to keep?

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Event Selection 1

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

Reject!
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Event Selection 2

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

electron
Accept!
Z boson production
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Event Selection 3

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

Reject!
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Event Selection 4

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

muon
electron
electron
Accept!
Higgs boson production
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Event Selection 5

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

Reject!
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Event Selection 6

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

muon
energy
Accept!
Supersymmetric event
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Event Selection 7

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

Reject!
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Event Selection 7

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

Missing energy
Accept!
More supersymmetry
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Event Selection 8

• Use computers to recognise interesting patterns.
• What do you think of this event?
• Accept
• Or
• Reject?

Accept!
Lots of energy
Black Hole??
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Trigger and Computing

• Now try it 40,000,000 times a second!
• Need a trigger system to filter events.
• 1 Petabyte of data each year
• 4 tonnes of DVDs!
• Need to use the grid
• Data from LHC machines is copied to sites around
  the world.
• Physicists send their analysis code to run on the
  data
• here are some examples of what we do

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Testing the Standard Model
Allowed in SM

• I work on testing one aspect of the Standard
  Model
• Triple Gauge Couplings forbidden!
• Should improve limits by ten times.

Forbidden in SM
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and beyond?

• Our current theories work well, but
• there are lots of problems, e.g.
• Where does mass come from?
• Why are particle masses so different?
• Why do we have different forces?
• How does gravity fit in?
• Are there extra dimensions?
• Is there a Theory of Everything?
• How do Swiss buses run on time?
• Still lots to understand

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The Higgs Boson

• Why do we have mass?
• Peter Higgs proposed a new scalar field.
• Wine bottle potential naturally explains mass.

• Predicts existence of a new particle
• The Higgs Boson
• How does it work?

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Imagine a party of physicists
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a celebrity comes onto the scene
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he wants a drink,
but everyone wants to talk to him.
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Now there is a rumour of free wine
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the rumour spreads through the room.
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The Higgs so Far

• Direct search at LEP
• Mass gt 114 GeV
• Possible candidate at 115 GeV!

• Other experiments indirectly give upper limits
• W mass, t mass
• Current limit
• Mass lt 186 GeV

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Finding the Higgs in ATLAS

• H ? ZZ ? 2e2mu
• Higgs decays into characteristic signature.
• Could ATLAS be the first to see this particle?

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Could ATLAS find it?

• Significance is a statistical measure of signal
  quality.
• SNsignal/vNbackground
• If significance gt 5, we claim discovery!
• ATLAS has good chance over large mass range.

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SUSY

• Why do we have different forces?
• Why are particle masses so different?
• Supersymmetry to the rescue
• Each particle has a supersymmetric partner
• Could explain Dark Matter?

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Black Holes

• Could produce microscopic Black Holes
• Decay via
• Hawking Radiation
• into very complex events.
• Finding them will be a challenge!

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Will the LHC destroy the world?

• Could a black hole/strangelet from the LHC eat
  the world?
• No!
• Particles from space have been bombarding the
  earth for billions of years.
• Cosmic Rays are far more energetic than LHC
  collisions.
• These have not harmed the earth since it formed.
• Dont be scared of the LHC!

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A Golden Age?

• ATLAS is nearly finished!
• The LHC is due to start next year.
• It will run for 15 years.
• Could change our view of the universe.
• You will be able to see the data!
• A very exciting time
• will you be part of it?

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Some Links

• This Talk
• http//www.hep.phy.cam.ac.uk/barber/talks.html
• ATLAS page (videos, webcam, photos)
• http//atlas.ch
• CERN Webpage
• http//public.web.cern.ch/public/