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The Hunt for the Higgs

The Hunt for the Higgs. Nigel Glover Institute for Particle Physics Phenomenology Durham University on the occasion of Professor Higgs’ 80th Birthday. The Higgs Boson. Why do we need it? What is it? Why haven’t we found it yet? How are we going to find it?. Why do we need it?. 2004.

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The Hunt for the Higgs

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  1. The Hunt for the Higgs Nigel Glover Institute for Particle Physics Phenomenology Durham University on the occasion of Professor Higgs’ 80th Birthday

  2. The Higgs Boson • Why do we need it? • What is it? • Why haven’t we found it yet? • How are we going to find it?

  3. Why do we need it?

  4. 2004 1979 1999 1991 2008 1997 2008 The Standard Model of Particles • Gauge Sector • Strong Interactions • Electroweak Interactions • Flavour Sector • Quark Mixing • Electroweak Symmetry Breaking Sector 2007

  5. Force Carrying Quanta • Photon (electromagnetic) • verified 1922 • mass of photon = 0 • W,Z bosons (weak force) • verified 1983 • MW, MZ: 80 GeV/c2, 91 GeV/c2 • Gauge symmetry is fundamental to electrodynamics • when extended to electroweak theory, requires massless W,Z • how can we accommodate their large masses?

  6. Why do we need the Higgs? Fundamental symmetries of nature require that all the elementary particles and force carriers are massless • in an “ideal” world all elementary particles would be massless but in the real world the elementary particles have widely differing masses • so the symmetry must be broken This is what the Higgs mechanism and electroweak symmetry breaking is all about

  7. What is it?

  8. What is symmetry breaking? Consider a smooth ball at the top of a very smooth symmetric hill The ball can roll in either direction … there is a left-right symmetry But the ball can only fall in one direction … the symmetry is broken

  9. More symmetry breaking above Tc below Tc Came to particle physics from condensed matter physics Heisenberg ferromagnet Theory has rotational invariance; ground state is not invariant  Symmetry has been broken

  10. Global symmetry breaking <> Consider a model with a complex scalar field φ L = μφ* μφ – V(φ*φ) with V(φ*φ) = -μ2φ*φ+λ (φ*φ)2 • The global U(1)symmetry is broken by a vacuumexpectation value <φ>ofthe φ-field given, at the classical level, by the minimum of V. • degeneracy of vacuum leads to masslessNambu-Goldstone oscillations Yoichiro Nambu Jeffrey Goldstone

  11. <φ> <φ> Aμ Aμ Gauge symmetry breaking Consider the same model with gauge interactions L = Dμφ* Dμφ – V(φ*φ) -1/4 FμνFμν with Dμ= μ+ieAμ, φ=<φ>+h Expanding φ around the true vacuum <φ> generates a mass for the “photon” Aμ M2 = e2<φ>2

  12. Where did the Goldstone mode go? propagation of Goldstone mode corresponds to rotation of vacuum orientation • equivalent to local gauge transformation and therefore unobservable • violation of Goldstone Theorem • produces extra “longitudinal” mode of massive gauge field

  13. Francois Englert Peter Higgs Robert Brout The men behind gauge symmetry breaking 1964 Physics Letters (15 September), Physical Review Letters (19 October) 1964 Physical Review Letters (31 August) 1997 European Physical Society Prize

  14. Higgs Mechanism in Particle Physics SU(2)xU(1) Electroweak “Standard Model” relies on spontaneous symmetry breaking • Complex SU(2) doublet • Higgs Field (four real scalars) • Spontaneous symmetry breaking • vacuum expectation value v • three Goldstone bosons Goldstone bosons give mass to W±,Z MW2 = ½ g22 v2 MZ2 = ½ (g12+g22) v2

  15. So what is the Higgs boson? • The Higgs boson is the quantum fluctuation of the Higgs field • produced by self interactions Mh2= λ <φ>2 • In the Standard Model, Mh, is a free parameter h <φ> <φ> h

  16. Hmmm. The Higgs boson has no spin at all! Government policy! Mr Blair explains the Higgs boson to Professor Stirling

  17. Properties of the Higgs boson h In the Standard Model, the Higgs boson couples to the fermions – quarks and leptons • Higgs couplings are proportional to the fermion masses • So it couples most strongly to the most massive particle  <φ> h   φ= <φ>+h 

  18. Dawn of the Electroweak Standard Model Papers with Higgs in the title ICHEP Fermilab ‘t Hooft Veltman Higgs Brout/Englert Weinberg Salam citations

  19. Theoretical constraints on Mh Radiative corrections change the shape of the Higgs potential at large and small Higgs boson mass • Triviality Λ < v exp(4π2v2/3Mh2) • Vacuum Stability Λ < v exp(4π2Mh2/3yt4v2)

  20. Unitarity Higgs exchange needed to prevent unitarity violation in WW scattering at high energies • Mh < 780 GeV • New phenomena required at the TeV scale

  21. Why haven’t we found it?

  22. LHC construction LEP construction …in more than 20 years of experiments costing nearly £10B? Papers with Higgs in the title Tevatron II running LEP running citations

  23. Peter Reid

  24. Precision measurements • LEP operated at CERN throughout the 1990’s • 3 light neutrinos • precision weak interaction measurements • established gauge theory of strong interaction • Measuring the Z mass to this accuracy is like measuring your body weight with an error of 1 gram • the weight of a lungful of air MZ = 91.1875 +/- 0.0021 GeV

  25. Indirect limits • Making precise measurements means sensitivity to quantum fluctuations • The Higgs has a small but measurable effect

  26. 95%confidence Indirect limits • The net effect of the precision measurements is to place a limit on the Higgs boson mass • At 95% confidence mH > 32 GeV mH < 185 GeV

  27. Direct searches at LEP • With enough energy in a collision, one could just produce a Higgs boson • But there is also background

  28. Signal or Background? Identified by detector Fixed by accelerator

  29. A Higgs event?

  30. Where is the Higgs? September …… December 2000

  31. 95% Ruled Out Results from LEP It should be around here!

  32. The TEVATRON at Fermilab The current highest energy accelerator on earth

  33. The Higgs signal at the TEVATRON • Enough energy to produce a Higgs boson … and trigger on the b quarks • But there is also background… again

  34. Signal or Background? Fixed by accelerator in this case proton and antiproton Identified by detector

  35. Search update • CDF and D0 have spent the last six years looking for the Higgs Best sensitivity in H -> WW* channel

  36. Higgs search: Status March 2009 Tevatron starting to rule out some of the possible Higgs boson mass range

  37. How are we going to find it?

  38. Unification of couplings? Smallness of neutrino mass Unitarity of WW scattering Hierarchy problem? αs αw Grand unification? Mgut αEM hierarchy E SUSY? susy The right energy scale! E MPl Quantum Gravity LHC collisions TeV Mweak EWSB Physics by scale

  39. Electroweak Symmetry Breaking • Standard Model (SM), SUSY, . . . : Higgs mechanism, elementary scalar particle(s) • Strong electroweak symmetry breaking (technicolour, .): new strong interaction, non-perturbative effects, resonances, • Higgsless models in extra dimensions: boundary conditions for SM gauge bosons and fermions on Planck and TeV branes in higher-dimensional space • New phenomena required at the TeV scale

  40. The Large Hadron Collider at CERN World’s most powerful particle accelerator Superconducting magnets – 8.3T at 1.9K 2 beams of protons will collide 40 million times a second In construction since 1998 Due to start later this year CMS ALICE ATLAS LHCb

  41. Finding the Higgs • 800,000,000 proton-proton interactions per second • ~100,000,000 electronic channels • 0.0002 Higgs per second Starting from this event… We look for this “signature” Selectivity: 1 in 1013 Like looking for 1 person in a thousand world populations Or for a needle in 20 million haystacks!

  42. The Higgs signal at the LHC

  43. Observability of the SM Higgs in CMS with 105 pb-1. The ATLAS and CMS detectors can probe the entire mass range up to MH ~ 1 TeV with a signal significance well above 5σ Depends on the number of proton-proton collisions the LHC can deliver. Maybe can do this by 2012 Higgs discovery

  44. Summary – Higgs Boson • Why do we need it? to give masses to the fundamental particles • What is the Higgs boson? a quantum fluctuation of the Higgs field • Why haven’t we seen it? hints at LEP, but too few events looking now at the TEVATRON • How are we going to find it? If its there, will definitely find at the LHC in 2011+ If it isn’t there, then theoretical framework of Standard Model is in big trouble, and expect to see other even more exciting new phenomena

  45. Discovering the Higgs will be a massive step forward BUT just a discovery will not be sufficient • Is it a Higgs boson? • What are its mass, spin and CP properties? • What are its couplings to fermions and gauge bosons? • Are they really proportional to the masses of the particles? • What are its self-couplings? • Are its properties compatible with the SM. . . ? • How many Higgs bosons are there?

  46. a lot of questions remain! • What is the origin of the fermion mass? • Why is the gauge structure SU(3)xSU(2)xU(1)? • Why are there three families? • Why is the electroweak symmetry broken? • Why are there 3+1 space-time dimensions? • How is gravity involved? GUT? STRING THEORY? Exciting times ahead!!

  47. Peter Higgs by Ken Currie

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