# LHC – the greatest experiment Prof Nick Evans & the origin of mass University of Southampton on Earth.

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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 contributors Spin 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 = mc 2 1 1-v /c 22 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 ) 222 x ‘ = (1 – v /c ) ( x – v t) 22

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 > h t E > h Or equally

Dirac’s Legacy Electrons 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 = mc 2 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 Mass The 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 Higgs To 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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