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The Dawn of the LHC ERA

The Dawn of the LHC ERA. A Confrontation with Fundamental Questions. Michael Dine UCSC 2007. Aerial view of LHC. Size of LHC. In a magnetic field B, a particle of charge q and momentum momentum p travels in a circle of radius R given by. At the LHC, the desired beam energy is 7 TeV and the

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The Dawn of the LHC ERA

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  1. The Dawn of the LHC ERA A Confrontation with Fundamental Questions Michael Dine UCSC 2007

  2. Aerial view of LHC

  3. Size of LHC In a magnetic field B, a particle of charge q and momentum momentum p travels in a circle of radius R given by At the LHC, the desired beam energy is 7 TeV and the state of the art dipole magnets have a field of 8 Tesla. Plugging in and converting units gives a radius of 3 km and a circumference of 18 km. Addition of quadrupoles, RF cavities, etc., increases the circumference of LHC to 27 km.

  4. Magnet Pictures 2 in 1 superconducting dipole magnet being installed in the CERN tunnel LHC dipoles waiting to be installed.

  5. Detecting Particle Collisions When high energy particles collide, they produce many particles. Simulation of an event in ATLAS detector. White lines are the four muons. The other tracks are due to particles from quarks in the protons.

  6. ATLAS Detector

  7. Tracker Pictures Tracker Inserting silicon detector into tracker Inserting solenoid into calorimeter

  8. Calorimeter Installation

  9. Muon Toroids Muon superconducting toroids.

  10. Endcap muon sector Endcap Muon Sectors

  11. SCALE OF THE PROJECT • The stored energy in the beams is equivalent roughly to the kinetic energy of an aircraft carrier at 10 knots (stored in magnets about 16 times larger) • There will be about a billion collisions per second in each detector. • The detectors will record and stores “only” around 100 collisions per second. • The total amount of data to be stored will be 15 petabytes (15 million gigabytes) a year. It would take a stack of CDs 20Km tall per year.

  12. Fundamental Questions of Another Era If one asked a physicist in 1935, what are the great questions of “fundamental" physics, might have answered: • How does one quantize electrodynamics? • What accounts for the strong interactions? • Does the Fermi theory provide a description of the weak interactions. The focus was to provide an understanding of how protons, neutrons, electrons and neutrinos interact to describe the world around us.

  13. Evolving horizons By 1960, QED understood, well tested; four Fermi theory well tested, basic structure understood. But a proliferation of strongly interacting particles, as well as the discovery of the muon and its neutrino. Now might have asked: • What is the nature of the strong interactions? Is it described by a quantum field theory, like QED? • The four fermi theory of the weak interactions provides a successful phenomenology, but can't describe physics at very high energies. What takes its place? E.g. does it arise from exchange of massive vector fields (short range?). • Given the success of field theory for some interactions, tempting to look at the quantum mechanics of general relativity. But hard to make sense of it (e.g. Feynman)

  14. The Standard Model Quarks and leptons, interacting through exchange of gauge particles (photon, W§, Zo, gluons).

  15. The Standard Model (I) 1995 2000 quantum field theory, describing interactions between pointlike spin-1/2 particles (quarks and leptons) via exchange of spin-1 vector bosons (photon, W and Z, gluon) fundamental particles (fermions) 2 (particle pair) * 3 (generations)* 2 (anti-particles)

  16. By 1995, the strong and weak interactions were understood at the sort of precision level of QED in 1960. the Standard Model was triumphant; no interesting discrepancies. All questions in our list answered (except general relativity)!

  17. If past fundamental questions were truly fundamental, we would quit. But intoxicated by our success, we are now far more ambitious. We are also deeply dissatisfied; the very success of the Standard Model is a source of frustration.

  18. Candidate Fundamental Questions • The Standard Model possesses many parameters. Some are extremely peculiar; e.g. me/mt = 3 x 10-6. • The electric charges of the quarks and leptons are exact rational multiples of one another (e.g. Qe=Qp). Why? • General relativity cannot be combined sensibly with the Standard Model, without some significant modification. • The Standard Model cannot account for most of the energy density of the universe. About 25% dark matter; about 70% dark energy; only 5% baryons. • The Standard Model cannot explain why there are baryons at all (baryogenesis).

  19. Parameters of the Standard Model • Lepton masses • Quark masses • Quark mixing angles . me = 0.511 MeV mm = 113 MeV mt = 1.777~ GeV mu¼ 1.5-4~ MeV md¼ 4-8 MeV ms¼ 80-130 MeV mc¼ 1.15-1.35 GeV mb¼ 4.1-4.4~ GeV mt¼ 174.3 § 5 GeV

  20. MZ= 91.1876(21) GeV a(MZ) ¼ 1/128 as(MZ) = 0.1187(20) sin2(qw)= 0.23120(15) MH > 115 (more later)

  21. Cosmological Parameters (WMAP Satellite)

  22. A theory parable: Grand Unification (a little math, but bear with me -- spectacular results) Standard Model: Interactions with three gauge groups, SU(3) x SU(2) x U(1). SU(2): very familiar. E.g. isospin is a good symmetry of strong interactions. In the Standard Model, a similar symmetry:

  23. Why not enlarge the symmetry (Georgi and Glashow)? Group quarks and leptons into larger objects: SU(3) acts on the d's; SU(2) on the leptons. G is an SU(5) matrix. The SU(5) must be a broken symmetry, but this is already true of SU(2). And right away we explain one of our big fundamental questions, and make some progress on another:

  24. Charge quantization Electric charge must be one of the generators of the group, like t3= s3 for SU(2). Like the Pauli matrices, it must be traceless. In this case: So electric charge is quantized, automatically!

  25. Coupling unification: This theory has one gauge coupling, rather than three. So we should be able to compute two of the couplings in terms of the third. This isn't quite right; it turns out we need one more parameter, the energy scale at which the symmetry breaks, Mgut. It is easy to compute these.

  26. Coupling constants EM Weak Strong

  27. A more exciting prediction – and spectacular failure Quarks carry B= 1/3; in this theory they can turn into leptons. E.g. dc X; MX=1014 GeV u e+ d Leads to p ! e+po, t¼ 1026 years

  28. Super Kamiokande t(p -> e+po)>1033 years

  29. And one even more stupendous failure: Dirac: existence of magnetic monopoles ) charge quantization. The arrow seems to go the other way; any time theorists invent a framework for charge quantization, they find monopoles. The SU(5) theory has monopoles as solitons (static solutions of the non-linear field equations). Beautiful idea. But very heavy (almost a microgram), produce lots of them in early universe. (about 10-5/cc) Certainly not there!

  30. One success: Baryogenesis. Because baryon number and CP not conserved, these theories produce some net amount of matter (baryon number =. The same heavy particles which mediate proton decay are produced in the hot big bang; their decays need not produce same number of protons and antiprotons. Can barely account for the observed matter/antimatter asymmetry

  31. _ q _ X q X _ e e Rates for process and charge conjugate process not exactly the same (need loop corrections), due to CP violation, departure from equilibrium when T ¼ MX.

  32. So we have a prototype for understanding quantization of electric charge and for explaining the value of a coupling. Also for connections between theoretical ideas and cosmology. But overall a failure.

  33. The Standard Model Higgs Boson Higgs Search at LEP: mass limits: obs. exp. mh > 114.4 GeV mh > 115.3 GeV time [year] Last missing particle in SM (EW symmetry breaking – mass) Light SM Higgs preferred MH = 126 +73 -48 GeV < 280 GeV (95% CL)

  34. Hierarchy Problem and the LHC But, apart from our failure to discover it up to now, the Higgs field presents a deeper puzzle. It may be too heavy to see without an LHC but the real puzzle is that it is so light. g H e pi e pi

  35. If you are not an expert at relativistic Feynman diagrams, you can see the problem from some simple quantum mechanics. The first two factors represent matrix elements and density of states; last is the energy denominator. This expression is badly divergent for large p. Still the theory, mathematically, makes sense. The magic of renormalization hides the real difficulty. If you cut off the integral at p = L, you have a correction to the mass: m2 = mo2 + L2. So unless mo is very finely tuned, one would expect m2¼L2.

  36. The need to cut off the integral reflects our ignorance of phenomena at scales L and higher.  So this argument strongly suggests that there must be some new phenomena at an energy scale less than the TeV scale, which cuts off the divergence.Many proposals:  technicolor, large extra dimensions, supersymmetry.  Each makes distinctive predicdtions for phenomena at the LHC.

  37. An attractive Extension: Supersymmetry Symmetry between Fermions↔Bosons (matter) (force carrier) ... doubled particle spectrum ... ☹

  38. Supersymmetry Symmetry between Fermions↔Bosons (matter) (force carrier) ... doubled particle spectrum ... ☹

  39. Interaction Strength in Supersymmetry 1 TeV without SUSY • ... BUT some Standard Model Problems solved ... • ... extension in string theory is candidate for Grand Unified Theory ... • ... lightest SUSY particle stable ⇨candidate for dark matter ... • ... unification of forces ... with SUSY Interaction energy in GeV

  40. l c l c l c q q l l c g q l q Production and decay of superparticles at the LHC. Here, jets, Leptons, missing energy.

  41. In a broad class of supersymmetric models, the lightest new particle is stable (R-parity); typically the partner of the Higgs or Z boson or photon. Produced in early universe. N W+ N W-

  42. Range of supersymmetry parameters consistent with dark matter density; here partner of photon is essentially the dark matter. Range of supersymmetry parameters consistent with dark matter constraints

  43. I am a fan of the supersymmetry hypothesis; I'm not alone. About 12,000 papers in the SPIRES data base. If true, quite exciting: a new symmetry of physics, closely tied to the very nature of space and time. Dramatic experimental signatures. A whole new phenomenology, new questions. But neither the limited evidence nor these sorts of arguments make it true; there is good experimental as well as theoretical reason for skepticism. • This is not the only explanation offered for the hierarchy, and all predict dramatic phenomena in this energy range. • Large extra dimensions • Warped extra dimensions • Technicolor • It’s just that way (anthropic?)

  44. Hypothetical answers to our fundamental questions: • Too many parameters • Charge quantization • Quantum general relativity • Dark Matter • Dark energy • Baryogenesis Other proposals have some success with each of the starred items; perhaps fair to see that supersymmetry does best.

  45. STRING THEORY String theory has pretensions to attack the remaining problems on this list: • A consistent theory of quantum gravity • Incorporates gauge interactions, quarks and leptons, and other features of the Standard Model. • Parameters of the model can be calculated, in principle. • Low energy supersymmetry emerges naturally – all of this proliferation, which seemed artificial, almost automatic.

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