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LHC – the greatest experiment
Prof Nick Evans
& the origin of mass
University of Southampton
on Earth
The ring is 27km round and on average 100m
underground
CERN - Geneva Probing the structure of matter
LHC will begin science in 2008
The Large Hadron Collider will collide the nuclei of atoms with 10 times higher energy than has previously been achieved (14 TeV)
1232, 35 ton, superconducting dipole magnets accelerate ions and focus them into bunches for collision
36,000 tons of coolant below 2K!
Proton-Proton collisions (hydrogen atom nuclei)
100 billion protons per bunch
20 collisions per crossing
1 crossing every 25ns
600 million collisions per second
14 TeV centre of mass energy
To store all collision data would involve storing 10 Petabytes of data a year ie a 20km high stack of CDs… more than can be made
Detectors at collision sites:
Atlas
CMS
LHC-b
Alice
Track particles
Measure energy
Measure momentum
LHC Budget - £5 billion over 10 years
Football on Sky for 3 years - £1bn
Film Titanic has grossed - £1bn
Tesco 2006 revenue - £19bn
UK NHS yearly expenditure - £20bn
UK Army yearly budget - £35bn
CERN is 20 European member states plus many international contributorsSpin Offs – CERN invented the www
and gave it away…
Amazon revenue 2006 - £7bn
Where we are now….
Relativity
The speed of light is the same for any observer
This means nothing can travel with light – nothing can reach v=c!
E = mc211-v /c2 2
Rest mass = energy
Space - Time
A flash of light causes a spherical wave front even if you move relative to source
*
This only makes sense if space and time mix!
t ‘ = (1 – v /c ) ( t – v x / c )2 22
x ‘ = (1 – v /c ) ( x – v t)2 2
Quantum Rules of Motion
Energy comes in lumps
E = h f
Fields can look like particles
The photon is the quantum of the electromagnetic field/ light
Quantum Dynamics
The quantum in some sense travels by both paths….
There is an uncertainty in the position and momentum of the quantum
Heisenberg’s Uncertainty Principle x p > ht E > hOr equally
Dirac’s LegacyElectrons can absorb photons
But in relativity observers do not agree on time ordering of events… so can we have
What does it mean for an electron to travel backwards in time? We only measure charge…
It looks like a +ve charge electron moving forward in time
We have discovered anti-particles!
Accelerator Physics
Electron positron annihilation to a photon allows us to convert their energy to look for all the particles that make up nature
What have we found?
Why do otherwise identical particles have different masses?
Understanding Mass - The Quantum Vacuum
E t > h
The vacuum can borrow energy for short periods
E = mc2
The borrowed energy can be used to create particles
The quantum vacuum is a seething mass of particles appearing and disappearing constantly….
(You can’t just create an electron because of charge conservation - but can create electron positron pair)
How Can You Tell?
The effective charge seen in two electron scattering depends on the separation of the electrons.
The “virtual” particle pairs interfere in electron scattering processes.
The Strong Nuclear Force
The strong nuclear force is described by a theory that is similar to electromagnetism… except that the fields carry (colour) charge…..
This difference changes the way in which the vacuum is polarized so that…
Confinement
You can never pull hard enough to liberate a quark from a proton…
The Quantum Vacuum
Every so often quantum effects create a quark anti-quark pair.
The attractive force is so strong that
binding energy >> mass energy
The vacuum has lower energy if it fills itself with quark anti-quark pairs!
The vacuum is really full of quark anti-quark pairs with a density
like that of an atomic nucleus (10 grams/cm ) !!15
The Proton Mass
The quark pairs are responsible for the proton’s mass
Interaction energy provides proton mass
3
The Origin of MassThe strong nuclear force cannot explain the mass of the electron though…
The Higgs Boson
We suspect the vacuum is full of another sort of matter that is responsible – the higgs….
Or very heavy quarks such as the top quark
top mass = 175 proton mass
To explain the top mass the higgs vacuum must be 100 times denser than nuclear matter!!
The Search for the HiggsTo find the higgs we must “excite” the vacuum – produce a higgs particle… we collide electrons, protons etc so there is 100 times nuclear energy density in some region….
The Large Hadron Collider in Switzerland will switch on in 2008…
We haven’t found it so far but…
There are many versions of the “higgs theory” – when we find it we can study its properties in detail….
No Lose
What if our theories are wrong and there is no higgs?
Without the higgs our theory of WW interactions predicts scattering probabilities greater than one… there must be something there…
What could it be? – extra space-time dimensions
- new forces…
- something entirely new…
Overview• Particle physics has a concise description of matter and forces
• The particles obey very strange laws at high energy and small scales
• The proton mass is a result of the vacuum being full of quarks
• The missing element is the higgs that generates other masses (plus explanation of why the building blocks are what they are)
• Still much to do - our theories of particles don’t fit with theories of gravity
• The LHC will switch on soon and begin to provide answers…
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