The Quantum Universe John Womersley Director of Particle Physics Rutherford Appleton Laboratory, UK
Download ReportTranscript The Quantum Universe John Womersley Director of Particle Physics Rutherford Appleton Laboratory, UK
The Quantum Universe John Womersley Director of Particle Physics Rutherford Appleton Laboratory, UK 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 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 a revolução está vindo! * 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. But every kind of particle that ever existed is still there, in the quantum fluctuations of empty space. Empty space “still knows” about all the equations, all the symmetries that governed the formation of the universe. John Womersley • 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 John Womersley Fermilab Chicago Inside the Tevatron Tunnel Tevatron Booster Batavia, Illinois p source Main Injector & Recycler John Womersley • Fermilab’s Tevatron collider started operations in 1988 Run I – 1992-95 Run II – 2001-09 50 × more data, increased energy John Womersley Detectors CDF • D-Zero Surround the collision points with arrays of instrumentation to intercept the particles produced – large (thousands of tons) – complex (many subsystems, 106 – 107 channels of electronics) – designed and built by collaborations of university and laboratory physicists John Womersley Tracker Superconducting Magnet protons antiprotons 3 Layer Muon System Electronics John Womersley Muon Electron every 396 ns … Jet (experimental signature of a quark or gluon) John Womersley Missing transverse energy (experimental signature of a non-interacting particle) John Womersley Proton-antiproton collisions • • Proton beams can be accelerated to very high energies (good) But the energy is shared among many constituents – quarks and gluons (bad) Transverse momentum • To select high-energy collisions: look for outgoing particles produced with high momentum perpendicular to the beamline (“transverse momentum”) – Do this 2½ million times a second, as the collisions happen – “triggering” John Womersley Computer programs reconstruct the particle trajectories and energies in each collision (each “event”) John Womersley Displaced vertex tagging • The ability to identify b-type quarks is very important – signatures for the Higgs boson and many other interesting things • b quark forms a B-meson, travels ~ 1mm before decaying B to reconstruct this decay, need to measure tracks with a precision at the 10m level John Womersley Silicon sensor Wire bonds Silicon sensor HDI (flex circuit readout) SVX2e readout chips John Womersley Measuring ladder position after insertion Zeiss coordinate measuring machine at Fermilab’s Silicon Detector Facility John Womersley What do physicists actually do? • • • • • • Design and build hardware – Detectors, electronics Write software Operate the detector Interpret data Present, refine, discuss our results among ourselves Publish papers John Womersley The work of many people… Example: The DØ detector was built and is operated by an international collaboration of ~ 670 physicists from 80 universities and laboratories in 19 nations > 50% non-USA ~ 120 graduate students 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 We found one very high missing-ET event in the first year of data Will we find more? John Womersley Indirect searches for new particles • • • 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 Carry out a “blind analysis” • Current best limits – Observe no events – Probability (Bs ) < 2 10 – Probability (Bd ) < 5 10-8 – Will keep getting better 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 John Womersley John Womersley Higgs searches • Current searches at the Tevatron are 20-100 times less sensitive than will be needed to find a Higgs • We will get a factor 20 more data, but by itself that won’t be enough • Experiments are improving their techniques – should be able to say something interesting, provided the mass of the Higgs is low enough John Womersley The Top Quark • • The top quark offers an indirect window on to the Higgs Because it is the most massive particle known, it interacts most strongly with the Higgs – Precise measurements of the top mass can tell us about the Higgs mass – Measurements of the way the top quark is produced and how it decays may hint at new phenomena associated with the Higgs • The top is in principle “just a very heavy quark” so we can calculate its behavior in detail • Look for any surprises, anomalies John Womersley How to catch a Top quark Neutrino Muon W b t Wt b John Womersley Top production • If the top is “just” a very heavy quark, we can precisely calculate its expected production rate (“cross section”) in proton-antiproton collisions PLB 626, 35 (2005) L=230 pb-1 1 secondary vertex tag Expected Top Signal Looks very much as expected John Womersley Top mass • Because its mass is so large, the top quark should decay very rapidly (yoctoseconds) into a W boson and a b quark; the W decays even more rapidly into either two quarks or a lepton + neutrino • The top mass can be reconstructed from the energy of the b and of the W decay products It can be measured quite precisely – at the 2-3% level • • In the Standard Model, the top mass W mass and Higgs are all related – Hence we can check if it is all consistent – yes, so far – And get an indirect measurement of the Higgs mass • Points to a rather light Higgs John Womersley How does top decay? • Does it really decay always to a W plus a b-quark? – Can test by using the silicon detector to identify b quarks – Distinguish b fromb by charge of particles seen All consistent with SM i.e. 100% top Wb • Does it decay to a W through the normal weak interaction? – Can test by measuring the angular distribution of the W decay All consistent with standard weak decays 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 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 Where do we go from here? John Womersley The Large Hadron Collider 14 TeV proton-proton collider at CERN Magnets being installed Over half the dipole magnets completed First beam in 2007 John Womersley The ATLAS and CMS detectors CMS mid-2005 PbWO4 crystals Final Barrel assembly at CERN September 2005 ATLAS mid 2005 John Womersley The International Linear Collider • • Discoveries at the Tevatron or LHC will leave us more questions than answers: – Have we really discovered the Higgs • Right quantum numbers? • Does it couple to mass? – Have we really discovered supersymmetry? • Superpartners have same properties as their partners? – Have we really discovered dark matter? • Does it have the right properties? An electron-positron linear collider is the way to answer these questions John Womersley • • The ILC is a high priority for the US Department of Energy, provided it is affordable and scientifically justified Seen as a fully international project Northern Illinois (near Fermilab) is a candidate site – Just to show the scale: US study version 47 km long • Fermilab site John Womersley International Linear Collider • • • 500 GeV, upgradeable to 1 TeV Accelerator technology chosen Global design group established Professor Barry Barish Caltech Timeline 12/05 Baseline configuration (done) 12/06 Reference design report with cost estimate 2008 Technical design report 2010 Construction decision? John Womersley Conclusions • We have 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? – What is the structure of space and time? Are there extra dimensions? John Womersley The quantum universe is a wonderful place Perhaps the most wonderful aspect is that it is possible for us to understand it John Womersley Questions, comments… 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