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ATLAS and the vision for particle physics John Womersley Director of Science Strategy STFC John Womersley What is the universe made of? • A very old question, and one that has been approached in many ways • The only reliable way to answer this question is by observation and direct enquiry of nature, through experiments – The ‘scientific method’ is one of the greatest human inventions John Womersley John Womersley The structure of matter • Centuries of experimentation and subsequent theoretical synthesis have led to an understanding of – Molecules, made of atoms – electrons orbiting nuclei – Chemistry – the interactions of these electrons – Nuclear physics – nuclei made of protons and neutrons – Quarks – the components of protons and neutrons • Culminates in what we call the “Standard Model” – A theory of matter and forces – A quantum field theory describing point-like matter particles quarks and leptons which interact by exchanging force carrying particles photons, W± and Z, gluons Lightest particles stable make up everyday matter John Womersley So we understand what matter is made of, then? Yes – but there are two big problems. First: a problem with what’s in the Standard Model John Womersley The revolution is coming John Womersley The revolution is coming • • The standard model makes precise and accurate predictions It provides an understanding of what protons, neutrons, atoms, stars, you and me are made of But (like capitalism!) it contains the seeds of its own destruction • Its spectacular success in describing phenomena at energy scales below 1 TeV is based on – At least one unobserved ingredient • the Higgs Boson – Whose mass is unstable in quantum mechanics • requires additional new forces or particles to fix – And in any case has an energy density 1060 times too great to exist in the universe we live in • The way forward is through experiments at particle accelerators John Womersley Why accelerators? • Today’s universe is cold and empty: only the stable relics and leftovers of the big bang remain. The unstable particles have decayed away with time, and the symmetries that shaped the early universe have been broken as it has cooled. We use particle accelerators to pump sufficient energy into a point in space to re-create the short-lived particles and uncover the forces and symmetries that existed in the earliest universe. • Accelerators, which were invented to study the structure of matter, are also tools to study the structure of space-time, the fabric of the universe itself • With current accelerators we are exploring the forces that governed the universe when it was about one trillionth of a second (one picosecond) old We know the energy scale that is required to reach the region where the “revolution” happens: ~ 1 TeV in the parton-parton centre of mass we can design accelerators to do this John Womersley John Womersley The Large Hadron Collider The Large Hadron Collider is the first accelerator that will clearly reach the required energies It will – we believe – transform particle physics John Womersley Experimental challenges • Doing physics at a supercollider is not trivial: – The luminosity required is set by small cross sections for new physics • multiple pp collisions per bunch crossing – Each hadron is a broadband projectile of quarks and gluons – Triggering, pattern recognition… • Fortunately nature has given us some help – W and Z – calibrate leptons – And at LHC – the top quark – calibrate jets, b-tags and MET John Womersley How to catch a Top quark Neutrino Muon W b t Wt b John Womersley Second big problem: what’s not in the Standard Model John Womersley Meanwhile, back in the universe … • • • What shapes the cosmos? – Old answer: the mass it contains, through gravity But we now know – There is much more mass than we’d expect from the stars we see, or from the amount of helium formed in the early universe • Dark matter – The velocity of distant galaxies shows there is some kind of energy driving the expansion of the universe, as well as mass slowing it down • Dark Energy We do not know what 96% of the universe is made of! Quarks and leptons 4% John Womersley Don’t let the bright lights fool you The stars are only a few percent of what’s out there The galaxies and the entire universe itself have been shaped by invisible dark matter… … and dark matter is not any of the standard model particles we are familiar with John Womersley Particle Physics Experiments Accelerators Underground Astronomy Experiments Telescopes Satellites Quantum Field Theory (Standard Model) Standard Cosmology Model 10–18 m 1026 m A Quantum Universe John Womersley Particle Physics Experiments Accelerators Underground Astronomy Experiments Telescopes Satellites Quantum Field Theory (Standard Model) Standard Cosmology Model Consistent understanding? 10–18 m 1026 m A Quantum Universe John Womersley Particle Physics Experiments Accelerators Underground WIMPs Astronomy Experiments Telescopes Satellites Quantum Field Theory (Standard Model) Standard Cosmology Model Dark Matter Consistent understanding? ? Goals for particle physics - can we detect cosmic dark matter on earth? - is it consistent with cosmological observations? John Womersley • Dark Matter low rate, small energy deposits – Very sensitive detectors – Well shielded – Underground to avoid cosmic rays 1100 m The Boulby Underground facility is opened, 2003 John Womersley Boulby Underground facility ZEPLIN II liquid xenon detector in shield and associated gas system Interactions in the xenon UK Dark Matter program – Designed and constructed a series of experiments – Currently commissioning the ZEPLIN II detector over half a mile underground • Uses Liquid Xenon to measure scintillation light and ionisation from dark matter John Womersley Intriguingly, dark matter points to the same place where the standard model starts to break down … Standard Model Higgs Boson etc. Dark matter particles mass and interactions John Womersley Particle Physics Experiments Accelerators Underground Quantum Field Theory (Standard Model) WIMPs Astronomy Experiments Telescopes Satellites Standard Cosmology Model Supersymmetry Dark Matter Consistent understanding? ? Goals for particle physics - can we discover supersymmetry at colliders? Something else? - is it consistent with cosmic dark matter? John Womersley What is this “Supersymmetry?” • • • A proposed enlargement of the standard model – We know all the particles have corresponding antiparticles – If supersymmetry is correct, they would also have new, but much more massive relatives called superpartners Theoretically this is very nice – eliminates mathematical problems in standard model – allows unification of forces at much higher energies – provides a path to the incorporation of gravity and string theory These nice properties come at a cost: lots of new particles! – multiple Higgs bosons – squarks and gluinos, sleptons, charginos and neutralinos – their masses depend on unknown parameters – None of these particles has yet been seen – but they are expected to be within reach of current accelerators Lightest supersymmetric particle has all the right properties for cosmic dark matter John Womersley How would we make a discovery? • • • • Standard model predicts how Highest missing ET event many events expected as a function of missing ET Supersymmetry models modify this prediction: more events expected Challenge: how do you calibrate the SM + detector expectation for MET? Are there ways to select two event samples, one where a signal is expected and one where it is not expected, as a benchmark? John Womersley Time to revisit the Higgs Boson • • • Photons of light and W and Z particles interact with the same strength – “Electroweak unification” Yet while the universe (and this room) is filled with photons, the W and Z are massive and mediate a weak force inside atomic nuclei Where does their mass come from? Massive W, Z Massless fields mix Higgs field Massless Higgs boson The “Higgs Mechanism” • This Higgs field has never been seen. Is this picture correct? – A question to be answered experimentally – One clear prediction: there is a neutral particle which is a quantum excitation of the Higgs field • The “Higgs boson” John Womersley Particle Physics Experiments Accelerators Underground Astronomy Experiments Telescopes Satellites Quantum Field Theory (Standard Model) Standard Cosmology Model Higgs Field Dark Energy Consistent understanding? NO! > 1060 Goals for particle physics - what can we learn about the Higgs field? - is it as simple as we think? John Womersley Does there have to be a Higgs? • No one has seen this particle – so why do we think it exists? – The W and Z have mass – Precision measurements of Top quark and W properties – Ultimate test: “WW scattering” q W X W q • • probability becomes > 1 as energy increases – unless there is a Higgs This is a real experiment – can’t have a nonsense answer The Higgs doesn’t have to be a single elementary particle. But something has to play its role John Womersley Higgs searches • If the standard model is correct, we can predict the particles into which the Higgs boson will decay – e.g. if its mass is relatively low, it will decay to two photons – Then see the Higgs as an excess of events with two photons that correspond to a particular mass … except in real life the Higgs signal will not be coloured red! PbWO4 crystals High resolution photon detector for CMS (UK responsibility) VPT light detector John Womersley Particle Physics Experiments Accelerators Underground Astronomy Experiments Telescopes Satellites Quantum Field Theory (Standard Model) Standard Cosmology Model Small CP violation in quark decays Matter dominates Consistent understanding? Not really Goals for particle physics - search for new sources of CP violation using quark mixing - search for CP violation in the neutrino sector John Womersley Indirect searches for new physics • • • Measure the rate of the rare decay of Bs and Bd In the Standard Model, cancellations lead to a very small decay probability – 3 10-9 and 10-10 New particles (e.g. SUSY) contribute additional ways for this to happen, increase probability – up to 10-6 Mass of muon pairs Current best limits (Tevatron) – Probability (Bs ) < 5.8 10-8 – Probability (Bd ) < 1.8 10-8 John Womersley Particle Physics Experiments Accelerators Underground Astronomy Experiments Telescopes Satellites Quantum Field Theory (Standard Model) Standard Cosmology Model Small CP violation in quark decays Matter dominates Consistent understanding? Not really Goals for particle physics - search for new sources of CP violation using quark mixing - search for CP violation in the neutrino sector John Womersley The Neutrino Programme MINOS • Operations and analysis Technology demonstration T2K • Secure a strong role in detector and accelerator development and in physics analysis • Build up UK neutrino community Learn more about neutrino mixing angles (govern CP violation) MICE • Demonstrate muon cooling Explore CP violation Neutrino Factory • Build community, international scoping study design study • RAL is one credible site John Womersley T2K • • • Primary aim – Measure final mixing angle θ13 (µ → e) – CP violation if large Tokai to Super-Kamiokande – 50 GeV PS, 0.75MW (Phase I) – Existing SK Detector, off-axis • ‘narrow band’ ~ 1 GeV beam Accelerator construction underway – Beam in April 2009(!) J-PARC neutrino beamline • • UK participation – RAL, QMUL, IC, Sheffield, Liverpool, Lancaster, Warwick Focus on – Beamline and beam dump • Accelerator expertise – Near (280m) detector • ECAL funding TBD John Womersley Make no little plans… Harwell Science and Innovation Campus • • • • • ISIS ISIS 1MW upgrade ESS-class 5MW spallation source Neutrino factory Future multi-TeV muon collider John Womersley Scientific Conclusions • We have a theory – the standard model – which makes precise and accurate predictions but which we know is incomplete – theoretically – points to the Higgs boson (or something else) – experimentally – dark matter and dark energy • By connecting experiments at particle accelerators and in underground labs with astronomical observations we can understand far more about the universe than from either approach alone – What is the cosmic dark matter? Is it leftovers of Supersymmetry? – Is the universe filled with energy? How does this relate to the Higgs field? – Why is the universe made of matter? Is this related to neutrinos? John Womersley In the past year, particle physics has stumbled into crisis (at least in the UK and US) The field’s internal planning and priority-setting processes have worked – but they have somehow become disconnected from an ability to convince external stakeholders to support these priorities (notably the ILC) The potential for excitement and transformative physics at the LHC is the best hope to re-frame the debate and start to remedy this situation Such opportunities come rarely - let’s not waste this one! John Womersley Questions, comments… John Womersley Particle Physics Experiments Accelerators Underground Astronomy Experiments Telescopes Satellites Quantum Field Theory (Standard Model) Standard Cosmology Model Supersymmetry Extra Dimensions Quantum Gravity Inflation Consistent understanding? Superstrings! Goals for particle physics – can we see evidence of extra dimensions? John Womersley Calorimeter (energy measurement) Superconducting Magnets Tracker protons protons Muon detectors (penetrating particles) physicists John Womersley Muon Electron every 25 ns … Jet (experimental signature of a quark or gluon) John Womersley Missing transverse energy (experimental signature of a non-interacting particle) John Womersley What? Extra dimensions? • • String theories predict that there are actually 10 or 11 dimensions of space-time The “extra” dimensions may be too small to be detectable at energies less than ~ 1019 GeV – To a tightrope walker, the tightrope is one-dimensional: he can only move forward or backward – But to an ant, the rope has an extra dimension: the ant can travel around the rope as well John Womersley Detecting extra dimensions • If there are particles than can travel around the extra dimension(s), we’d interpret this motion as being additional mass – If the dimension is small, the motion would be quantized – would look like a series of new, more massive relatives of a known particle • “Kaluza-Klein modes” • But what if none of the known particles can enter the extra dimension except for gravity? – We (the things we are made of) may be trapped on a (3+1)-dimensional “brane” – the surface of a 10 or 11 dimensional universe – This could explain why gravity seems so weak – Extra dimensions could be large – even infinite – The energies required to “see” them could be much lower • within reach of current accelerators? John Womersley We are searching • Look for a “Kaluza-Klein” excitation of the graviton – Assumed to decay to two electrons or photons Putative signal data High-mass electron pair event mass = 475 GeV, cos * = 0.01 • Look for enhancement to the production of pairs of high energy photons or electrons • See no deviation from 3+1 dimensions – We can set limits on the size and properties of extra dimensions John Womersley