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Exploring the Microworld of Forces and Particles

Exploring the Microworld of Forces and Particles. The Particle Mandala. Arthur O. Williams Lecture April 10, 2006 Paul Grannis, Stony Brook. 40 years ago we had four distinct forces, governing different phenomena:.

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Exploring the Microworld of Forces and Particles

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  1. Exploring the Microworld of Forces and Particles The Particle Mandala Arthur O. Williams Lecture April 10, 2006 Paul Grannis, Stony Brook

  2. 40 years ago we had four distinct forces, governing different phenomena: • Electromagnetic – interactions of charged particles and photons (unified by Maxwell in 19th century) responsible for most of the phenemena seen in our laboratories. • Strong – responsible for the force between nucleons (hadrons), and the binding of the nucleus. • Weak – decays of quasi-stable particles, interactions involving neutrinos (e.g. powering the sun). • Gravity – the dominant long range force in the universe, of negligible strength compared to all others on microscale at low energy.

  3. Electromagnetic force Interaction between two charged vector currents (electrons, positrons, protons etc), mediated by the photon (spin 1) which couples (weakly, a=1/137) to charge. The quantum theory QED is renormalizable, and owing to the small coupling, susceptible to accurate calculation by perturbation theory. p e+ Massless photon → long range interaction (~1/r2) g e+ p

  4. p Weak force (beta decay is prototype) n e- n Experiments initially showed ~zero range. This violates unitarity (conservation of probability) at high energy so can’t be correct. Structure seen to be combination of vector (V) and axial vector (A) – parity violation. Fermi introduced current form similar to EM; with charged boson spin 1 force carriers (W±) this postponed the unitarity problem. The observed short range means that W’s are very massive. p • No neutral current analog observed (e.g. K0→ p0 n n ) • 2 charged leptons (e & m), 2 neutrinos (ne & nm ) • Muon and electron number conserved n e- W- n

  5. Weak force The unitarity problem persists – WW scattering violates probability conservation at TeV scale. And a renormalizable, locally gauge invariant, theory requires massless force carriers. Solution: (Higgs, Weinberg, Salam, Veltman, t’Hooft, …) postulate a triplet of massless neutral gauge bosons (w+, w0, w-) and a massless singlet (b0) that mediate the underlying weak interaction. Introduce a set of 4 spin 0 Higgs fields (2 complex doublets) that stimulate spontaneous symmetry breaking. Gives mass to the charged bosons (W+ , W-), and causes a rotation of the neutral states: Z0 = cosqWw0 + sinqW b0 g = -sinqW w0 + cosqW b0 The W+, W- and Z acquire mass in this symmetry breaking. The g remains massless. The longitudinal polarization states needed for massive states ‘swallow’ 3 of the Higgs fields, leaving 1 Higgs boson to be observed.

  6. Weak force This postulate combines the EM and Weak Interactions as unified Electroweak theory. It requires neutral currents of fixed strength relative to charged currents. The Higgs field is responsible not only for giving mass to W and Z, but also all the fermions (e, m, quarks). The weak mixing angle qW was approximately determined by n scattering to yield MZ ~ 90 GeV and MW ~ 80 GeV. Spontaneous symmetry breaking Spontaneous symmetry breaking – mass on thin vertical wire. Ground state minimizes total energy of bent wire and gravitational potential. But direction of the flop has azimithal degeneracy. Ferromagnet breaks the symmetry through the agency of an external magnetic field. The Higgs mechanism is a Rube Goldberg device? You bet – But for 35 years it has agreed with experiments!

  7. d u s s u d The Stong Force – the Quarks By late 1960’s, a host of particles with differing quantum numbers were found – baryons (partners of proton and neutron) and mesons like p, K, r, w etc. ‘Strange’ particles produced in pairs and decay by weak force. etc. etc. ! Gell-Mann, Neeman, Zweig classify these states with just 3 quarks: strangeness And corresponding 3 antiquarks Three spin ½ quarks: Two zero strangeness: u (up) : Qu = +2/3 d (down): Qd = -1/3 One strangeness = -1 s (strange): Qs = -1/3 isospin

  8. (proton = uud; neutron = udd; p+ = u d; K+ = u s; etc. ) The Stong Force The observed particles could be made from combinations of 3 quarks (baryons) or quark + antiquark (mesons) with differing orbital angular momentum. “The Eightfold Way” Particles with quantum numbers not allowed by combining quarks were not seen. Predict new particles (e.g. W- = sss), observed in 1964 (Samios, Shutt et al.) • A statistics problem: baryons made of three identical quarks in relative s-waves and spins aligned (e.g. W- = sss) have wavefunctions symmetric in space and spin. Quarks are spin ½ fermions, so these states violate the Pauli Principle. • Another problem: quarks not observed experimentally • At this stage, quarks were considered to be useful but fictitious memnonics for the particle zoo.

  9. The Stong Force Electron-proton scattering at high energy and momentum transfer Q2 showed evidence for pointlike constituents with charges of uds quarks (Friedman, Kendall, Taylor et al, Bjorken) Electron-proton scattering at high energy and momentum transfer Q2 showed evidence for pointlike constituents with charges of uds quarks (Friedman, Kendall, Taylor et al, Bjorken) The approximate scaling (Q2 independence) would infer that the proton is made only from quarks. But more precise experiments show some Q2 dependence. electron g (carrying momentum Q) proton scattered quark x = fraction of proton momentum carried by struck quark Q2 = mom. transfer squared (“microscope” resolution)

  10. The Stong Force The resolution comes with Quantum Chromodynamics (QCD) – gauge theory of strong interactions involving matter particles (quarks) and massless gauge bosons (gluons) responding to a new type of ‘charge’ called ‘color’ (Yang, Mills, Gross, Politzer, Wilczek … ). Unlike EM interaction where ghas no charge, the gluons have color-charge and thus interact with themselves. • Solves statistics problem if there are three colors for quarks, and wavefunction is antisymmetric in color. • Predicts non-scaling in ep scattering, since a single quark at low Q2 can acquire structure through virtual gluon emission. At high Q2, the quark is resolved into q and virtual g At low Q2, g from electron sees only a single quark QCD is a renormalizable gauge theory, like EW, (but fiendishly difficult to calculate)

  11. Magnetic Electric Gravity Electroweak Strong Electromagnetic Weak Unified Force ?? The dream of force unification What about ‘grand unification’ of Electroweak and Strong to make a single microscopic force? Electromagnetic and Weak forces are unified into Electroweak. Maxwell ! ?? ????? Our current theory does not quite permit this to happen, but if there were new particles, then such a unification could happen at very high energy. And then there is gravity … string theory could perhaps bring it into the fold. The original 4 forces of microscopic matter

  12. So, in the decade from 1965 – 1974 we passed from a view of four distinct and very different forces among the 100’s of hadrons and leptons to the: • STANDARD MODEL • Matter composed of a few quarks (u,d,s) and leptons (e, ne, m, nm) • Unified Electroweak force with both charged and neutral gauge boson force carriers. One surviving Higgs field. • Strong force of QCD, with 8 colored gluons as carriers • (Gravity still outside the picture) Symmetry structure: SU(3) x ( SU(2) x U(1) ) QCD with 3 colors EW with weak isospin and isosinglet pieces QCD and EW just pasted together, no unification

  13. Elementary matter particles and force bosons in 1974 ? Postulated in the SM; not yet observed • Both EW and QCD forces are gauge interactions with fundamental zero mass bosons. • Quarks feel the Strong and EW forces. Anything charged feels the EM interaction. Leptons feel only the EW force.

  14. Outstanding questions in 1974 • The 3 quark model gives far too large weak neutral current effects, and has irredeemable divergences if #quarks ≠ #leptons. Need a fourth quark, partner to the strange quark. • Higgs boson is not seen – no prediction for its mass. • Few direct tests of EW model correctness existed. • W±, Z not seen, but predicted at ~ 80, 90 GeV. • No experimental evidence for the gluon, and only indirect evidence for QCD. The following ~20 years were devoted to resolving these questions, and adding some unexpected surprises.

  15. A whirlwind tour of the news headlines from 1974 to 2005 1974: Simultaneous observation of a narrow resonance at 3.1 GeV, in e+e- collisions (Richter et al. at SLAC) and in pN collisions (Ting et al. at BNL). pN →J e+e-→y The J/Y was quickly inferred to be an s-wave spin 1 bound state of a new charm quark and its anti-charm partner. The c quark (Qc = +2/3) is the isospin partner of the s quark.

  16. 1976:Charmed particles observed directly (Goldhaber, Pierre et al) with expected properties. Observation of the charm quark restores the symmetry between leptons and quarks needed in theory and necessary to suppress neutral currents.

  17. 1976:New charged tau lepton observed (Perl et al.) in e+e-→ e m + missing energy. Its companion nt is inferred from the missing energy. Who ordered this?? This discovery opens up a third generation of leptons, and again gives asymmetry in number of quarks and leptons. Where are the corresponding quarks?

  18. 1977: the charge Q= -1/3 bottom quark is inferred from a new narrow resonance at 9.5 GeV in pp collisions at Fermilab (Lederman et al.) – a bb bound state. It is the first of the third quark generation. Its mass is ~3x that of charm (and 5x proton mass). The bottom hadrons themselves followed at Cornell.

  19. 1979: The gluon is seen experimentally in e+e-→ q q g (experiments at PETRA storage ring at DESY), as events where the radiated gluon is manifest as a third jet (jets = collimated sprays of hadrons emerging from a single quark or gluon progenitor). Quarks and gluons cannot seen directly because the strong color force prevents un-matched color particles from emerging alone. Measurements show the ‘running’ of strong coupling constant 3-jet event – gluon discovery at PETRA

  20. 1983: Discovery of the W± and Z bosons in UA1 and UA2 (Rubbia et al., Darriulat et al. at CERN proton-antiproton collider). Masses and decay patterns are as predicted by EW theory. Then Now 24 events UA1 and UA2 Z to dilepton invariant mass at discovery in 1983. CDF Z → ee invariant mass in 2006.

  21. 1995: Discovery of the top quark by CDF and DØ experiments at the Fermilab proton-antiproton collider. The top mass is about 175 GeV, over 50 times the mass of the next most massive b quark (and ~ mass of Au nucleus). Roster of SM matter particles (quarks and leptons) and force carrying bosons is complete. World avg mass: 172.5 ± 2.3 GeV

  22. ~ 1985 – 2005: 100’s of precision studies of Z, W boson and top quark properties at the CERN and SLAC e+e- colliders and Fermilab proton-antiproton collider pin down the SM Electroweak parameters to great accuracy. Although the Higgs boson remains out of sight, these precision measurements tell us what mass it would have in the SM context. Virtual Higgs loops affect the top and W masses (logarithmically), giving experimental sensitivity to Higgs mass Favored SM Higgs mass is about 120 GeV. LEP-2 rules out Higgs < 114 GeV W and top mass agree with SM, but may hint at something new

  23. 1990 – 2006: The weak quark eigenstates are rotations of the strong quark states. The quark mixing rotation angle, and the CP violation seen in K0 decays in 1964, have been studied in both K and B decays (SLAC and KEK (Japan), BNL, CERN, Fermilab). Unitarity constrains matrix elements. 2005 1995 progress ! Peanut shape is the error on closure of unitarity constraint on quark mixing matrix in SM. Quark mixing and CP violation is seen to be consistent with the 3 generation of quarks in SM.

  24. 1998: Neutrino mass and mixing: nm produced by cosmic ray collisions with upper atmosphere air molecules (and subsequent p decay) disappear after traversing the earth’s diameter (Superkamiokande experiment underground in Japan, Koshiba et al.). This is interpreted as oscillation of nm to (invisible) nt. • Oscillation requires that neutrinos have different masses and the mass eigenstates are mixtures of ne, nm, nt (mixing). m/e (observed / expected) Strictly speaking, n mass and mixing is outside the SM, but is analogous to what we see in the quark sector. with no oscillation cos qzenith → upgoing downgoing

  25. The SM parameters: • 6 quark masses • 6 lepton masses • 4 quark mixing matrix parameters • 4 lepton mixing matrix parameters • 3 force coupling `constants’ • 2 EWSB parameters (e.g. mH , sin2qW ), • 1 phase for strong interaction CP violation 26 arbitrary parameters – to be determined by experiment And if these parameters were different, our universe would be dramatically changed: e.g. if down quark were lighter than the up quark, the proton decays, hydrogen doesn’t exist, stars don’t ignite, universe nearly transparent to light, chemistry vastly changed …

  26. The SM has been validated with 1000’s of measurements showing agreement between theory and data etc. etc.

  27. The SM explains a vast array of experimental data – so why don’t we like it? • Those 26 arbitrary parameters – SM has no explanation for why they are as observed. Masses vary by 10 orders of magnitude ! Why 3 generations? • SM shows CP violation, but not enough to explain why there is the huge asymmetry between number of baryons and antibaryons in the universe. • The Strong and EW interactions are just pasted together in SM. If extrapolate the coupling constants to high energy, they come close to a common value at ~ 1017 GeV – but no cigar g3 g2 g1 No unification

  28. why we don’t like the SM … • Quantum corrections (loop diagrams) would cause the Higgs, W, Z boson masses to diverge to Planck scale unless there is some fantastic accidental tuning of couplings to keep these at TeV scale. (hierarchy problem) • Galaxies show substantial dark matter, also evident in early galaxy formation. DM seems to be massive particles, left from the early universe. SM provides no candidate. • Dark energy, pushing the universe apart in the present epoch, has no explanation in the SM. • The SM would give WL (energy density due to cosmological constant) be O(10120). One might understand some new symmetry causing it to be zero, but WL ~ 1. The biggest fine tuning problem of them all ! • Gravity is not included in SM

  29. So, despite the successes of the SM, we strongly believe it will be supplanted with new physics. First one needs to find what serves the role of the Higgs boson to break EW symmetry. Moreover, to solve the SM defects (fine tuning of Higgs mass, DM particle, desire to unify the forces … ) there needs to be new physics at few 100 – 1000 GeV – the Terascale. The new theory must reproduce the successes of the SM while adding new ingredients – much as Quantum Mechanics gives Classical Mechanics in the correspondence limit. • There are several classes of theoretical models suggested for the new paradigm: • New symmetries of nature • New forces and particles • Extra space dimensions Each model class has many variants

  30. An experimentalists dream – We know there is a new playing field at the Terascale, but have no idea who the players are, or the rules of the game. Go there and find out! And there are two demonstrated new accelerator colliding beam facilities that will give a complementary view of the new terrain: The Large Hadron Collider (LHC), to be commissioned in 2007 at CERN – proton-proton collisions at ECM = 14 TeV The International Linear Collider (ILC) being designed in international collaboration: e+e- collisions at ECM = 0.5 – 1 TeV.

  31. Colliders for the energy frontier • High energy reach • Broad range of parton CM energies at once • Large event rate • Large QCD backgrounds • Pileup – spectator quarks & other pp collisions • Radiation damage issues LHC proton proton • Known initial quantum state • Well-defined ECM and pol’zn • low bkgd → ambitious experimental techniques • Event rates low; need sequential runs at different ECM and polarization • Complex machine detector interface; need exquisite control of beam optics ILC e- e+ LHC & ILC collider characteristics are highly complementary

  32. The LHC Mt. Blanc The 14 TeV (ECM), 27 km circumference Large Hadron proton-proton Collider at CERN on the Swiss-French border – complete in 2007. The LHC will be the highest energy accelerator for many years. Lake Geneva But … The protons are bags of many quarks and gluons (partons) which share the proton beam momentum. Parton collisions have a wide range of energies – up to ~2000 GeV. Initial quantum state is not fixed.

  33. The International Linear Collider Collide beams with energy tuneable up to Ecm =500 GeV (upgrade to Ecm = 1000 GeV). Two identical linear 10 (20) km long accelerators, bringing beams to head-on collision in 6 nm high spot. Fixed parton collison energy; polarized e+ and e- beams in JP=1- initial state allow control of production processes. International planning and design now underway for ILC. Layout of electron arm

  34. e+ Z g,Z e- H The physics program for the LHC and ILC: • Find the agent for Electroweak symmetry breaking – in the SM, the Higgs boson. The LHC should discover the Higgs if it exists up to >1 TeV (10 times higher than SM indicated value). significance Curves denote different Higgs boson spins; ILC data cleanly discriminate. Higgs mass → The ILC will tell us that if what LHC sees is the SM Higgs or some other thing. It can detect the Higgs even if it decays into invisible particles. It can tell us the Higgs quantum numbers, and its couplings to different particles. interaction rate collision energy

  35. 1. Mapping the Higgs boson In the SM, Higgs couplings are directly proportional to mass. Measuring these couplings to few % level at ILC is a sensitive test of whether we have the SM or some new physics. Yukawa coupling Particle mass → The Higgs couplings to other particles is crucial for learning what new physics is operating. New symmetries Extra dimensions bottom tau charm top Higgs W Z glu on SM value

  36. Delineate the new physics:New symmetries Supersymmetry (SUSY) introduces new fermionic space-time coordinates, resulting in a new boson for every existing SM fermion and vice versa. (Partner of the spin ½ electron is a spin 0 selectron). In exact SUSY, selectron mass = electron mass etc. We know this is not true, so SUSY is a broken symmetry. All the other properties of the selectron are like the electron (charge, couplings). There are many model variants. • SUSY boson and fermion higher order contributions to Higgs mass cancel the SM pieces, so the hierarchy problem is solved. • SUSY has a natural DM candidate. • SUSY could provide the CP violation needed. • SUSY modifications to SM predictions are small, so not in conflict with data.

  37. 2. Learning about Supersymmetry The LHC and ILC have complementary strengths in mapping the SUSY spectrum – LHC sees quark and gluon partners; ILC sees lepton and W/Z/Higgs partners. Together they can extrapolate to the scale where SUSY is broken and tell us how this works. Mass unification pattern from ILC & LHC in 2 different models energy → SUSY provides a good candidate for DM (lightest SUSY particle). LHC and particularly ILC can determine its mass. Compare with CMB, underground DM experiments to see if the picture is consistent.

  38. Delineate the new physics: New forces New forces and the particles they introduce provide a new energy scale. This stabilizes the hierarchy problem of the SM. The prototype candidate was a new interaction similar to QCD (‘Technicolor’) with new particles at O(10 TeV). The simplest of these models would produce deviations from the SM that are not seen, but many more complex variants exist. All of these give new quarks and bosons that would be seen at LHC and ILC. dimuon mass An example: a new higher mass Z boson seen at LHC production rate

  39. Delineate the new physics: New dimensions String theory requires at least 6 extra spatial dimensions (beyond the 3 we already know). The extra dimensions are curled up like spirals on a mailing tube. If their radius is ‘large’ (~1 attometer = billionth of an atomic diameter) or larger, they could lower the effective Planck mass, eliminate the hierarchy problem and unify all forces (including gravity?) at the new Planck scale. If a particle created in an energetic collision goes off into the extra dimensions, it becomes invisible in our world and the event shows missing energy and total momentum imbalance.

  40. 4. Untangling New Dimensions Combination of data from LHC and ILC allow the determination of the reduced Planck scale and the number of extra dimensions. Wavefunctions trapped inside a ‘box’ of extra dimensions yields a series of resonance states (like new heavy Z bosons) – indistinguishable at LHC from other sources of such states. dimuon mass production rate ILC measurements of the couplings (vector and axial vector) allow us to distinguish what new physics is operating.

  41. Example of ILC and LHC complementarity Observed final particles 4 ways to produce a ‘signal’ in jet , dilepton and missing energy at LHC a) & b) SUSY with different choices of dark matter particle (lightest SUSY particle) = spin ½ partner of photon partner or neutrino. c) & d) Extra Dimensions models with different character of excited Z. LHC can’t distinguish these interpretations. At ILC, the cross-sections and angular distributions for different initial state polarizations tell us which is happening. This information can in turn be used by LHC to deduce the heavy particle masses.

  42. The important things to note about all the postulated models of new physics: • All known models have observable phenomena within reach at the LHC and ILC. • Each model class has many variants, each with a large degree of freedom of parameters. The LHC and ILC are needed to give complementary, binocular views of new phenomena. Together, they will tell us much more than either alone. “Pardon me, I thought you were much farther away”

  43. The outlook The Standard Model and measurements in hand provide a vista of new unity and interconnectedness of the microscopic world. go here sense whats happening here The experimental tools to take us there are in hand. LHC will start next year. The ILC prospects have improved steadily but the project has yet to be approved by world governments.

  44. The structure of the universe The Particle Mandala A gateway to understanding The eightfold way to unification The connectedness of things

  45. Summary • Over the course of 40 years, our understanding of the fundamental forces and constituents of matter has been revolutionized. • The SM paradigm is about to be broken in ways that we cannot predict. The next generation of experiments will tell us a fascinating new story. • A truly exciting time for particle physics !

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