Transcript Document
LHC – the greatest experiment on Earth & the origin of mass Prof Nick Evans University of Southampton CERN - Geneva Probing the structure of matter LHC will begin science in 2008 The ring is 27km round and on average 100m underground 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) 14 TeV centre of mass energy 100 billion protons per bunch 20 collisions per crossing 1 crossing every 25ns 600 million collisions per second 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 - £2.6 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 Spin Offs – CERN invented the www and gave it away… Amazon revenue 2006 - £7bn CERN is 20 European member states plus many international contributors The Story So Far Electrons and their electromagnetic interactions are responsible for chemistry and day to day forces Special Relativity The speed of light is the same for any observer This means nothing can travel with light – nothing can reach v=c! E= 1 mc 2 1-v2/c2 Rest mass = energy Quantum Theory- Wave-Particle Duality The energy in waves comes in lumps or quanta – E = h f The photon is the quantum of the electromagnetic field/ light Sine wave has definite momentum but indefinite position A definite position state is a Fourier sum of all momentum sine waves A compromise wave packet satisfies x t p >h E >h Dirac’s Legacy Electrons can absorb photons But in Relativity, rotating this in space-time gives… The electron travelling back in time is a hole or antiparticle Every particle has a twin of the same mass but precisely opposite charges – particles and anti-particles annihilate into photons. Gauge Theory - QED How do you know which to call particle and which anti-particle? Nature has the same problem – it may make a different choice in causally disconnected bits of space Nature has invented an interaction so that two charged particles can probe the choice each other made – that force is electro-magnetism. 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 (You can’t just create an electron because of charge conservation - but can create electron positron pair) The quantum vacuum is a seething mass of particles appearing and disappearing constantly…. How Can You Tell? The “virtual” particle pairs interfere in electron scattering processes. g-2 is tested to 13 sig figs! The effective charge seen in two electron scattering depends on the separation of the electrons. The Strong Nuclear Force Quarks come in 3 colours! This difference changes the way in which the vacuum is polarized so that… “asymptotic freedom” Gross, Politzer, Wilczek The strong nuclear force is described by a gauge theory … except that the 8 gauge fields, gluons, carry colour charge….. 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 antiquark pairs! The vacuum is really full of quark anti-quark pairs with a density 15 3 like that of an atomic nucleus (10 grams/cm ) !! The Proton Mass The quark pairs are responsible for the proton’s mass Interaction energy provides proton mass QCD & Strings Strongly coupled QCD is a tough maths problem – how do we compute beyond perturbation theory? String theory gets meson properties right because a q anti-q pair look like a string A string is a one dimensional object with tension BUT relativistic strings like to live in 10 dimensions! String theory contains quantum gravity Gauge Gravity Duality In recent years we have realized that strings in 10d are in fact the QCD string… a weird and wonderful alternative description of quarks and glue… Maldacena The extra dimensions are holographic creations. EG a quark is a string with an up label on one end and a colour label on the other Classical General Relativity computations solve strongly coupled quantum problems! Is real gravity a hologram?? If the space-time stretches it the quark becomes massive Quarks in a Dense QCD Plasma Computations of gravity wave propagation tell us about transport properties of a quark gluon plasma Larry Yaffe’s calculations of the shock wave produced by a moving quark The ALICE Connection A lead-lead collider at LHC In heavy ion collisions we squeeze quarks together testing asymptotic freedom. At LHC energies the quark gluon plasma is a strongly coupled liquid Gauge gravity duality is currently our best tool to describe this mayhem! What else have we found? Massive gauge bosons for the weak nuclear force! Why do otherwise identical particles have different masses? The Origin of Mass The strong nuclear force cannot explain the mass of the electron though… Or very heavy quarks top mass = 175 proton mass The Higgs Boson We suspect the vacuum is full of another sort of matter that is responsible – the higgs…. a new sort of matter – a scalar? To explain the W mass the higgs vacuum must be 100 times denser than nuclear matter!! It must be weak charged but not electrically charged The Search for the Higgs EG look for Higgs decay to two photons There are variants…. Is the Higgs some new quark anti-quark pair bound by a new ultra strong force? Should we embrace a new symmetry that requires a scalar for every fermion Supersymmetry… No Loose What if our theories are wrong and there is no higgs? Without the higgs our theory of WW interactions predicts scattering cross sections greater than one… there must be something there… What could it be? – extra space-time dimensions - a bigger gauge symmetry SU(2)xSU(2)x… - something new… Summary Coming soon! The LHC is an amazing technological project As reviewed in the Independent & Times Higher Education Supplement! That will explore the frontier of our understanding of the building blocks of nature It will test our understanding of QCD as the generator of mass And search for the Higgs boson – the missing link of the Standard Model Mostly we hope for the unexpected though – we want new insights into scientific law and natural philosophy A FREE Popular science novel www.hep.phys.soton.ac.uk/~evans/NL