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Precision Collider Physics and the Search for the Higgs Boson. Lance Dixon, SLAC Physics Colloquium, National Taiwan University May 31, 2005. secret agent of . Outline. Higgs boson as the agent of “electroweak symmetry breaking”. 3 steps to tracking down the culprit:.
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Precision Collider Physics and the Search for the Higgs Boson Lance Dixon, SLAC Physics Colloquium, National Taiwan University May 31, 2005
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
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
Standard Model Basics (cont.) • Vector bosons also self-interact electroweak g*, Z QCD Precision Collider Physics & Search for Higgs Boson
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
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
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
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
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
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
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
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
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
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
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
eL eR Forward-backward asymmetries silicon vertexing to zoom in on b quark decays Precision Collider Physics & Search for Higgs Boson
Combine precision observables for mH Are they all hearing the same whispers?? Precision Collider Physics & Search for Higgs Boson
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
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
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
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
Tevatron detectors CDF D0 Precision Collider Physics & Search for Higgs Boson
Sample direct search Muons “easy” to identify – very penetrating: ATLAS Precision Collider Physics & Search for Higgs Boson
muon pair sample dominated by Z production, but can cut on invariant mass mmm Precision Collider Physics & Search for Higgs Boson
also cut on missing transverse momentum ET Precision Collider Physics & Search for Higgs Boson
and muons shouldn’t be back to back; and no energetic jets; and … Precision Collider Physics & Search for Higgs Boson
Resulting limits still well above Standard Model Higgs expectations After all cuts imposed, mostly just WW left: Precision Collider Physics & Search for Higgs Boson
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
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
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
LHC Detectors ATLAS Precision Collider Physics & Search for Higgs Boson
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
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
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
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
at NNLO Results expressed as K factor: Series stabilized; residual uncertainties estimated at 10—20% Precision Collider Physics & Search for Higgs Boson
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
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
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
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
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
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
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