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The Large Hadron Collider (LHC)

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The Large Hadron Collider (LHC) The Large Hadron Collider (LHC) Benjamin Radburn-Smith RAL Particle Physics • Aim to understand the laws of physics at a fundamental level • Use subatomic particles to probe this domain – Protons, antiprotons, lead ions, neutrinos, neutrons, muons • Use high energy machines for our experiments… Size! • Size matters – Higher energy Shorter wavelength probe smaller scales • Electron discovered – J.J. Thomson (Cambridge, 1897) Size! • Antiproton (1955 )and Antineutron (1956) discovered – Bevatron 6.5 GeV proton accelerator (Berkley, USA) Accelerator: Large Hadron Collider Accelerator: Large Hadron Collider • Accelerates bunches of proton to 99.9999991% the speed of light, so they circulate 11 245 times/s Accelerator: Large Hadron Collider • 700 million proton-proton collisions per second at CMS/ATLAS The SM • We have constructed a theory of how the Universe works on the smallest scales • This theory of particle physics is called the Standard Model (SM) • Explains particles and the forces that act between them – Has produced predictions which agrees with experimental data to unprecedented accuracy! – It has been correct for decades – … but ? The SM • There are a few problems with the SM: – It doesn’t explain Dark Matter or Dark Energy … i.e. 96% of the Universe – It doesn’t explain gravity (SM explains the world of the small scale where gravity is negligible) – Does it account for mass? – How does the Universe exist? Matter/Antimatter – There are inconsistencies at higher energies (we are still dealing with energies which are relatively low) – A number of free parameters – Different mass scales Designed and constructed the LHC to investigate these problems The BEH Mechanism • In the SM, forces are mediated by particles ( , , , ) – The mathematics only works if some of these are massless: ( , ) but they are heavy! • Brout, Englert and Higgs (and others) introduced a theory: The BEH mechanism – Massive particles appear massive due to some background interaction? – The force carrier is the Higgs particle ( ) "It's an energy field created by all living things. It surrounds us and penetrates us; it binds the galaxy together.” The BEH Mechanism • In the SM, forces are mediated by particles ( , , , ) – The mathematics only works if some of these are massless: ( , ) but they are heavy! • Brout, Englert and Higgs (and others) introduced a theory: The BEH mechanism – Massive particles appear massive due to some background interaction? – The force carrier is the Higgs particle ( ) "It's an energy field created by all living things. It surrounds us and penetrates us; it binds the galaxy together.” - Obi Wan-Kenobi The BEH Mechanism: Analogy • Cocktail Party: Room full of people represents Famous person is then surrounded space filled with the Higgs field by people creating a resistance, which represents a heavy particle interacting with the Higgs field • The Higgs Boson is a quantum of this field! Finding the Higgs • If the Higgs field exists, then we expect to find the Higgs Boson • The Higgs is predicted to decay into certain combination of particles, e.g. – Higgs to photons – Higgs to leptons Why the LHC? • Search for new things! • Find the Higgs Bosons? • New particles at higher energies? • Black holes? • Supersymmetry (SUSY: doubles the number of particles and solves many problems)? • Extra dimensions? Why the LHC? • Search for new things! • Find the Higgs Bosons? • New particles at higher energies? • Black holes? • Supersymmetry (SUSY: doubles the number of particles and solves many problems)? • Extra dimensions? • Something we didn’t think about? • How does the LHC work? • How do the experiments work? • How do we get data? • How do we turn the data into information? How does the LHC work? • Objective: Smash protons into each other with enormous energy and study the debris that results • LHC is part of the CERN accelerator complex Accelerator Complex • Use a bottle of hydrogen for the source of protons, by injecting them into a duoplasmatron and ripping off the the electrons – Need lots: 2 beams of around 3000 bunches each containing around 100 billion protons • These protons are then accelerated using a Linear accelerator to 50 MeV • Then accelerated by the Proton Synchrotron Booster to 1.4 GeV Accelerator Complex • The Proton Synchrotron accelerates the protons to 25 GeV • The Super Proton Synchrotron accelerates the protons to 450 GeV • This then feeds the LHC in both directions Movie: http://cds.cern.ch/record/1228924 What is the LHC? • Holds two beams of protons in a vacuum pipes – To stop them from interacting with any dust particles – LHC: 1/10 000 000 000 000th of atmospheric pressure! – (better vacuum than space around the ISS) • These protons are then accelerated using RF cavities – To kinetic energies of a proton beam equal to the Eurostar at around 100 mph! • Need get the beams to repeatedly pass through the accelerating cavities (taking ~20mins to get up to energy) • For this we need strong magnets! What is the LHC? • Contains a total of ~9300 magnets – With 1232 large 15m long cryodipole magnets to steer the beams Strong magnets require huge currents need superconducting magnets! LHC is the largest fridge on the planet! 6000 tons kept at -271°C Corresponds to ~150 000 household fridges at a temperature colder than the coldest regions of space What is the LHC? • The LHC takes protons from the SPS with 2808 bunches per beam and 1011 protons per bunch • Accelerates them up to (4) 7 TeV per beam • Collides the beams at 4 places around the ring • Then dumps the beams into large carbon/steel cylinders • ~30 collisions every 50 ns • Need huge numbers of collisions to analyse the rare ones Movie: https://cds.cern.ch/record/1406040 But what about the Moon?! • We need accurately control the position of the protons in the pipe – to collide the micrometer size beams head on inside a kilometer scaled machine – Extremely precise beam position monitoring and feedback system • LHC is sensitive to the location of the Moon! – Through ground (25 cm) and water tides – Varies the circumference by 1 mm • LEP used to be sensitive to the TGV! 4 Main Experiments • ATLAS (general purpose) – 7000 tons, 25 m diameter, 46 m length • CMS (general purpose) – 14500 tons, 15 m diameter, 22 m length 4 Main Experiments • LHCb (b physics) – 5600 tons, 13 m width, 21 m length • ALICE (heavy ion physics) – 10000 tons, 16 m diameter, 26 m length (No RAL involvement) Who runs it? • We are an international bunch! Officially (roughly) – ATLAS: 35 countries – CMS: 40 countries – LHCb: 15 countries – ALICE: 30 countires How do we run the machines? • With computers in control centres How do we run the machines? • With computers in control centres Angels and Demons How do we run the machines? • With computers in control centres CMS ATLAS FNAL LHC Role of the Experiments • The LHC smashes the protons (quarks & gluons) together • This produces interesting massive particles • These decay almost immediately • Decay products fly off in all directions • Intercept and analyse these with detectors • Attempt to reconstruct the interesting part of the event using computers and our brains! What we detect • Most interesting particles decay almost immediately What we detect • Most interesting particles decay almost immediately – Can’t see these What we detect • Most interesting particles decay almost immediately – Difficult to see these Never appear isolated: produce whole bundles of protons, neutrons, mesons etc (jet) What we detect • Most interesting particles decay almost immediately – Can’t see these Never appear isolated: produce whole bundles of protons, neutrons, mesons etc (jet) Undetectable here How to detect particles • Tracking • Charged particles ionise the material they pass through – Use a small amount of material to detect the ionisation charges left find the position • Then use many layers to follow the path of the charged particles • Typically we use silicon for this job – Similar to your camera sensors – However these are quite thin, high resolution and radiation hard How to detect particles • Tracking trick! • Remember that charged particles bend in a magnetic field – Immerse the silicon detectors in a magnetic field – Path of the particle is bent – We can then calculate the momentum of the particles • However: we are dealing with particles with high energies – Small curvature – Need large tracking detectors (approx meters) – High spatial resolution (to a few micrometers) How to detect particles • Tracking works for charged particles, but not for neutrals • Use calorimeters: – Large amounts of material which absorb the energies of the particles (as the particles interact with the material) – This creates showers of secondary particles • Absorbers: stop the particles (kinetic energy into showers) • Detectors: measure the shower energies • Derive the energy and position of the initial particle from the properties of the showers How to detect particles • 2 main classes of calorimeters: • Electromagnetic – Absorption via electromagnetic cascade of lightweight particles (electrons, photons) • Hadronic – Nuclear interaction with absorber material (for protons, neutrons, mesons etc) – As we are dealing with high energy: need lots of material e.g. few meters thick using dense material (lead, iron, uranium) How to detect particles • Muons – Are a special case: they are almost unstoppable – Need specific muon detectors to find them – E.g. CMS uses iron absorbers to stop any other particles from reaching the muon detectors, which work in a similar way to tracker
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