Precision Collider Physics and the Search for the Higgs Boson

1. Precision Collider Physics and the Search for the Higgs Boson Lance Dixon, SLAC Physics Colloquium, National Taiwan University May 31, 2005

2. secret agent of Outline • Higgs boson as the agent of “electroweak symmetry breaking” • 3 steps to tracking down the culprit: 1. wiretaps – indirect, virtual evidence 2. at the scene – real Higgs production 3. unmasking – does accumulated evidence fit theoretical profile enough to convict? Precision Collider Physics & Search for Higgs Boson

3. n g weak electromagnetism (QED) Basics of the Standard Model • All forces (except gravity) carried by spin 1 vector bosons • All matter composed of spin ½ fermions strong (QCD) Precision Collider Physics & Search for Higgs Boson

4. Standard Model Basics (cont.) • Vector bosons also self-interact electroweak g*, Z QCD Precision Collider Physics & Search for Higgs Boson

5. g + + … QED e e3 weak g QCD gs Standard Model Basics (cont.) • We can (essentially) only compute reaction rates as a perturbative expansion in small parameters (couplings) Precision Collider Physics & Search for Higgs Boson

6. For Nc=3, Nf< 17, gluons win Asymptotic Freedom Gross, Wilczek, Politzer (1973) – Nobel 2004 Gluon self-interactions make QCD more calculable at high energies Quantum fluctuations of massless virtual particles polarize vacuum QED: electrons screen charge (e larger at short distances) QCD: gluons anti-screen charge (gs smaller at short distances) Precision Collider Physics & Search for Higgs Boson

7. Asymptotic Freedom (cont.) Running of as is only logarithmic, slowat short distances (large Q or m). Bethke confining calculable Precision Collider Physics & Search for Higgs Boson

8. g, Z violates unitarity for E > 1 TeV while no problems g, like g, has 2 helicity states, longitudinal mode needs new dynamics Weak Interactions at High Energy Precision Collider Physics & Search for Higgs Boson

9. physical Higgs boson modes “eaten” by W,Z A cosmic superconductor: Weak fields screened within 0.003 fm Higgs Mechanism Brout, Englert, Guralnik, Hagen, Higgs, Kibble (1964) v Precision Collider Physics & Search for Higgs Boson

10. Basic Higgs Properties An elementary spin-0 particle. Novel experimentally, but not theoretically Higgs boson couples to mass: all masses due to Higgs Precision Collider Physics & Search for Higgs Boson

11. no problem now! Even better, theory is renormalizable: quantum corrections calculable in terms of basic electroweak parameters: ‘t Hooft, Veltman (1972) – Nobel 1999 Unitarity Revisited g, Z Precision Collider Physics & Search for Higgs Boson

12. Step 2: Compute quantum corrections to other observables – depend on plus only unknown in SM Higgs sector Whispers of the Higgs Boson Step 1: Measure 3 electroweak parameters extremely well QHE or (g-2)e m lifetime LEP1 Precision Collider Physics & Search for Higgs Boson

13. Most important corrections are to W, Z propagators For most observables, find: strong, quadratic dependence on mt weak, logarithmic dependence on mH To hear Higgs “whisper” underneath cacophony of top quark, also need a precise value of mt = 178(4) GeV (Tevatron) Step 2 in more detail Precision Collider Physics & Search for Higgs Boson

14. Now make very precise measurements of the observables. Many of these come from Step 3 SLC, 1989-1998, 0.5 million polarized Z’s LEP1, 1989-95, 18 million Z’s Precision Collider Physics & Search for Higgs Boson

15. depends onmt, mH A Simple, Powerful Observable: ALR • Count numbers of Z’s produced with left- vs. right-handed e-’s • Measure beam polarization Pe Precision Collider Physics & Search for Higgs Boson

16. eL eR Forward-backward asymmetries silicon vertexing to zoom in on b quark decays Precision Collider Physics & Search for Higgs Boson

17. Combine precision observables for mH Are they all hearing the same whispers?? Precision Collider Physics & Search for Higgs Boson

18. vs. Evidence at the scene (direct searches) How to pick out of a crowd? What are the backgrounds? e+e- colliders hadron colliders Precision Collider Physics & Search for Higgs Boson

19. Direct Search, Phase I • Very clean production mechanism LEP2, 1996-2000, up to 209 GeV • Could see a light enough Higgs boson almost independently of its decay mode • Ruled out Higgs mass almost to kinematic limit: E – mZ = 118 GeV • Actual limit: mH> 114 GeV Precision Collider Physics & Search for Higgs Boson

20. Direct Search, Phase II Tevatron Run II, 2001—2009? pp collider at ECM = 2 TeV _ Protons = bags of strongly interacting quarks and gluons Large number of possible production mechanisms Precision Collider Physics & Search for Higgs Boson

21. Many “disguises” very effective at hadron colliders (bb, cc, gg) _ _ Possible “Disguises” (decay modes) SM decay probabilities, or “branching ratios” (Br) completely determined by mH - still rich set of possibilities Precision Collider Physics & Search for Higgs Boson

22. Tevatron detectors CDF D0 Precision Collider Physics & Search for Higgs Boson

23. Sample direct search Muons “easy” to identify – very penetrating: ATLAS Precision Collider Physics & Search for Higgs Boson

24. muon pair sample dominated by Z production, but can cut on invariant mass mmm Precision Collider Physics & Search for Higgs Boson

25. also cut on missing transverse momentum ET Precision Collider Physics & Search for Higgs Boson

26. and muons shouldn’t be back to back; and no energetic jets; and … Precision Collider Physics & Search for Higgs Boson

27. Resulting limits still well above Standard Model Higgs expectations After all cuts imposed, mostly just WW left: Precision Collider Physics & Search for Higgs Boson

28. In some supersymmetric models, hbb coupling enhanced by a large factor of tanb _ Look for 3 jets containing b quarks Limits starting to get interesting Another example Precision Collider Physics & Search for Higgs Boson

29. Third example Best channel for Tevatron for Standard Model Higgs Requires 10-15 fb-1 of data, doesn’t look likely now Precision Collider Physics & Search for Higgs Boson

30. CMS ATLAS Direct Search, Phase III: the LHC pp collider, in LEP tunnel: 2007-?? ECM = 14 TeV, Luminosity (collision rate) 10—100 times greater than Tevatron Precision Collider Physics & Search for Higgs Boson

31. LHC Detectors ATLAS Precision Collider Physics & Search for Higgs Boson

32. Here focus on one of each: largest, but largest QCD uncertainties very small, but also clean bump Many production & decay mechanisms Precision Collider Physics & Search for Higgs Boson

33. Important to have theoretical control over sizes of: interference signal background best virtue: smooth background drawback: S/B = 1/20 Precision Collider Physics & Search for Higgs Boson

34. Series for s is poorly behaved: first correction (NLO) is 80% ! Dawson; Djouadi, Graudenz, Spira, Zerwas (1991) • Drove big theoretical effort to compute NNLO term Catani, DeFlorian, Grazzini; Harlander, Kilgore; Anastasiou, Melnikov; Ravindran, Smith, van Neerven (2001--03) Signal • Height proportional to • Compute sand Br as expansion in as • Series for Br is quite convergent, under good control Precision Collider Physics & Search for Higgs Boson

35. effective vertex Still many amplitude interferences, withdifferent numbersof final state gluons (or quarks).Each diverges; only sum is finite. at NNLO To make tractable, use large mtapproximation: reduces number of loops by 1 Precision Collider Physics & Search for Higgs Boson

36. at NNLO Results expressed as K factor: Series stabilized; residual uncertainties estimated at 10—20% Precision Collider Physics & Search for Higgs Boson

37. at NNLO Anastasiou, Melnikov, Petriello (2005) Y = rapidity longitudinal position of Higgs in detector Can also include parton-level event cuts Precision Collider Physics & Search for Higgs Boson

38. g • Advances in 2-loop integrals: can compute Bern, LD, DeFreitas (2001) g • And then apply to NLO corrections Bern, LD, Schmidt (2002) background • Not as important as signal; experimentalists will measure it • Also ggcomponent is not the only one g • For a long time it was only known at leading-order: g Precision Collider Physics & Search for Higgs Boson

39. interference? LD, M. Siu (2003) • In principle as important as signal, since it contaminates peak main source of a required phase • Fortunately, effect is small, ~ 5% in Standard Model Precision Collider Physics & Search for Higgs Boson

40. Higgs properties post-LHC Much work by theorists and experimentalists has led to: Which in turn should lead to 10%--50% determinations of Higgs couplings to W,Z,g,g, heavy fermions Precision Collider Physics & Search for Higgs Boson

41. The International Linear Collider • Designed as the next step after the LHC • Much improved precision at high energies • e+e- annihilations at 0.5 -- 1 TeV (rough match to LHC parton energies) • Recent global selection of acceleration technology • Now Global Design begins • Collisions in 2015—2020? Precision Collider Physics & Search for Higgs Boson

42. ILC Higgs measurements • Few-% precision for big channels; 10-40% for rare ones • The last word in precise unmasking of the Higgs boson Precision Collider Physics & Search for Higgs Boson

43. Conclusions • Quest for Higgs boson is 40 years young • Indirect measurements and the first direct searches have told us where to look • Most likely, discovery and initial characterization awaits the LHC • Full unmasking may take the ILC • Life may well be much more interesting than the simplest Standard Model Higgs! Precision Collider Physics & Search for Higgs Boson