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Home , Bevatron, Hadron collider, Large Hadron Collider, Proton Synchrotron, Super Proton Synchrotron

The Large (LHC)

Benjamin Radburn-Smith RAL • Aim to understand the laws of physics at a fundamental level

• Use subatomic to probe this domain – , , , , , • Use high machines for our experiments… Size! • Size matters – Higher energy  Shorter wavelength  probe smaller scales • discovered

– J.J. Thomson (Cambridge, 1897) Size! • (1955 )and (1956) discovered

– 6.5 GeV accelerator (Berkley, USA) Accelerator: Large Accelerator:

• Accelerates bunches of proton to 99.9999991% the , 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 works on the smallest scales • This theory of is called the (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 or … i.e. 96% of the Universe – It doesn’t explain (SM explains the world of the small scale where gravity is negligible) – Does it account for ? – How does the Universe exist?  Matter/ – There are inconsistencies at higher (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 is a quantum of this field! Finding the Higgs • If the Higgs field exists, then we expect to find the • 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 ? • New particles at higher energies? • Black holes? • (SUSY: doubles the number of particles and solves many problems)? • Extra ?

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)? • ? • 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 for the source of protons, by injecting them into a duoplasmatron and ripping off the the – 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 Booster to 1.4 GeV Accelerator Complex

• The accelerates the protons to 25 GeV

• The 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 ! 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 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 ( & 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, 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 ( 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 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 systems (but with gas systems, for example)

• Neutrinos – Can’t be detected – These & any others we don’t know about and can’t detect have to be inferred from missing energy – We expect there to be zero net momentum perpendicular to the beams – Look for imbalances from all other particles Hermetic detectors! Recap

Experiment: Compact Muon Soleniod • One of two general purpose detectors around the LHC • Over 14 ktonnes; 21 m long and 15 m tall • Like a big camera: records the results of protons (&HI) smashing into each other • Filters interesting collisions from 100’s millions/s -> 100 thousand/s -> 100/s saved (triggers to save the data)

RAL built this! Experiment: ATLAS • ATLAS: the other general purpose experiment works in a very similar way – Although uses different technologies for some of the detectors

RAL built this! Triggering and Data taking • >100 million readout channels (from all subdector modules) • Signals are digitised for processing

Triggering and Data taking • Fast special electronics identify interesting signals as part of the first stage of triggering • Selection done in different stages of increasing complexity • Later stages of triggering uses clusters of computers • Computers then reconstruct the particles from the data recorded off the detectors  reconstruct the event • ~ 100 events are saved per second Data analysis • Data is distributed across the globe (e.g. via a node here at RAL to the rest of the UK) • These are then analysed by the • Data is turned into information/knowledge! How has the LHC done? • Went off to a bit of a shaky start (2008) – Superconducting connection broke – Liquid coolant evaporated … quickly – Ripped 35 ton magnets out of the ground, blasted doors off 100 m above • Restarted in 2010 – Starting to ramp up in number of collisions – 3.5 TeV per beam – Re-established known physics

60’s 70’s

80’s/ 90’s How has the LHC done? • 2011 – Still running at 3.5 TeV per beam but dramatically increase the number of collisions (recording the same amount of data as all of 2010 each day) • 2012 – Move to 4 TeV per beam – Conditions becoming a real challenge!

Found the Higgs! • On the 4th of July 2012 two of the experiments at CERN, CMS and ATLAS showed evidence of the discovery of the Higgs Boson

CMS ATLAS Found the Higgs! • On the 4th of July 2012 two of the experiments at CERN, CMS and ATLAS showed evidence of the discovery of the Higgs Boson

CMS ATLAS Found the Higgs! • On the 4th of July 2012 two of the experiments at CERN, CMS and ATLAS showed evidence of the discovery of the Higgs Boson

CMS ATLAS What happens next? • Now we have stopped the machines  Long Shutdown 1 – Upgrade all the connections between the magnets in the LHC – Upgrade the experiments – Aim to return with close to designed energies of ~ 7 TeV per beam! • Then run for a few more years • Long Shutdown 2 (~2018) – Upgrade injection chain – Upgrade experiments • Then run for a few more years • Long Shutdown 3 (~2022) Measure its properties – Upgrade LHC – Upgrade experiments • Return with High Luminosity LHC! • Plenty to investigate with over the next few decades!

Fin

• Thanks to Kristian Harder and Monica Wielers for a selection of slides More Information • Higgs announcement: – http://cms.web.cern.ch/news/observation-new-particle-mass-125-gev – http://www.atlas.ch/news/2012/latest-results-from-higgs-search.html

• LHC status: – http://op-webtools.web.cern.ch/op-webtools/vistar/vistars.php

• LHC: – http://public.web.cern.ch/public/en/LHC/LHC-en.html

• CERN: – http://public.web.cern.ch/public/en/About/About-en.html

• Youtube: – http://www.youtube.com/cern – http://www.youtube.com/user/CMSExperimentTV – http://www.youtube.com/user/fermilab

Credits • Images of CERN facilities and the experiments are from cds: – http://cds.cern.ch/?ln=en • Pictures of the particles (plushies): – http://www.particlezoo.net/ • Angels and Demons picture from: – http://www.sputnik7.com/2009/05/mpc-brings-more-rome-to-angels-demons/ • Einstein Art picture from flickr (wokka) via boredpanda – http://www.boredpanda.com/80-beautiful-street-crimes-done-by-banksy/ • Good idea: – http://ghazalehdesign.com/thestorefront/