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

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 2 Outline Higgs boson as the agent of “electroweak symmetry breaking” 3. unmasking – does accumulated evidence fit theoretical profile enough to convict? secret agent of 3 steps to tracking down the culprit: 1. wiretaps – indirect, virtual evidence 2. at the scene – real Higgs production

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 3 Basics of the Standard Model All forces (except gravity) carried by spin 1 vector bosons All matter composed of spin ½ fermions  electromagnetism (QED) strong (QCD) weak

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 4 Standard Model Basics (cont.) Vector bosons also self-interact electroweak QCD , Z

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 5 Standard Model Basics (cont.) We can (essentially) only compute reaction rates as a perturbative expansion in small parameters (couplings) QED ++ … e3e3  e weak g QCD gsgs

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 6 Asymptotic Freedom Gluon self-interactions make QCD more calculable at high energies Quantum fluctuations of massless virtual particles polarize vacuum Gross, Wilczek, Politzer (1973) – Nobel 2004 QED: electrons screen charge (e larger at short distances) QCD: gluons anti-screen charge (g s smaller at short distances) For N c =3, N f < 17, gluons win

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 7 Asymptotic Freedom (cont.) Running of  s is only logarithmic, slow at short distances (large Q or  ). confining calculable Bethke

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 8 Weak Interactions at High Energy violates unitarity for E > 1 TeV , Z g, like , has 2 helicity states, longitudinal mode needs new dynamics no problems while

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 9 Higgs Mechanism Brout, Englert, Guralnik, Hagen, Higgs, Kibble (1964) physical Higgs boson v modes “eaten” by W,Z A cosmic superconductor: Weak fields screened within fm

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 10 Basic Higgs Properties Higgs boson couples to mass: all masses due to Higgs An elementary spin-0 particle. Novel experimentally, but not theoretically

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 11 Unitarity Revisited , Z no problem now! Even better, theory is renormalizable: quantum corrections calculable in terms of basic electroweak parameters: ‘t Hooft, Veltman (1972) – Nobel 1999

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 12 Whispers of the Higgs Boson QHE or (g-2) e Step 1: Measure 3 electroweak parameters extremely well  lifetime LEP1 Step 2: Compute quantum corrections to other observables – depend on plus only unknown in SM Higgs sector

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 13 Step 2 in more detail For most observables, find: strong, quadratic dependence on m t weak, logarithmic dependence on m H Most important corrections are to W, Z propagators To hear Higgs “whisper” underneath cacophony of top quark, also need a precise value of m t = 178(4) GeV (Tevatron)

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 14 Step 3 Now make very precise measurements of the observables. Many of these come from LEP1, , 18 million Z’s SLC, , 0.5 million polarized Z’s

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 15 A Simple, Powerful Observable: A LR Count numbers of Z’s produced with left- vs. right-handed e - ’s Measure beam polarization P e depends on m t, m H

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 16 Forward-backward asymmetries silicon vertexing to zoom in on b quark decays eLeL eReR

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 17 Combine precision observables for m H Are they all hearing the same whispers??

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 18 Evidence at the scene (direct searches) How to pick out of a crowd? What are the backgrounds? vs. e + e - colliders hadron colliders

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 19 Direct Search, Phase I LEP2, , up to 209 GeV Very clean production mechanism Could see a light enough Higgs boson almost independently of its decay mode Ruled out Higgs mass almost to kinematic limit: E – m Z = 118 GeV Actual limit: m H > 114 GeV

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 20 Direct Search, Phase II Tevatron Run II, 2001—2009? pp collider at E CM = 2 TeV _ Protons = bags of strongly interacting quarks and gluons Large number of possible production mechanisms

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 21 Possible “Disguises” (decay modes) SM decay probabilities, or “branching ratios” (Br) completely determined by m H - still rich set of possibilities Many “disguises” very effective at hadron colliders (bb, cc, gg) __

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 22 Tevatron detectors CDFD0

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 23 Sample direct search Muons “easy” to identify – very penetrating: ATLAS

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 24 muon pair sample dominated by Z production, but can cut on invariant mass m 

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 25 also cut on missing transverse momentum E T

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 26 and muons shouldn’t be back to back; and no energetic jets; and …

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 27 After all cuts imposed, mostly just WW left: Resulting limits still well above Standard Model Higgs expectations

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 28 Another example In some supersymmetric models, hbb coupling enhanced by a large factor of tan  _ Look for 3 jets containing b quarks Limits starting to get interesting

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 29 Third example Best channel for Tevatron for Standard Model Higgs Requires fb -1 of data, doesn’t look likely now

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 30 Direct Search, Phase III: the LHC pp collider, in LEP tunnel: 2007-?? E CM = 14 TeV, Luminosity (collision rate) 10—100 times greater than Tevatron CMS ATLAS

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 31 LHC Detectors ATLAS

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 32 Many production & decay mechanisms Here focus on one of each: largest, but largest QCD uncertainties very small, but also clean bump

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 33 best virtue: smooth background Important to have theoretical control over sizes of: signal backgroundinterference drawback: S/B = 1/20

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 34 Signal Height proportional to Compute  and Br as expansion in  s Series for Br is quite convergent, under good control Series for  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 ( )

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 35 at NNLO To make tractable, use large m t approximation: reduces number of loops by 1 effective vertex Still many amplitude interferences, with different numbers of final state gluons (or quarks). Each diverges; only sum is finite.

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 36 at NNLO Results expressed as K factor: Series stabilized; residual uncertainties estimated at 10—20%

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 37 at NNLO Y = rapidity longitudinal position of Higgs in detector Anastasiou, Melnikov, Petriello (2005) Can also include parton-level event cuts

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 38 background Not as important as signal; experimentalists will measure it Also gg component is not the only one For a long time it was only known at leading-order:   Advances in 2-loop integrals: can compute Bern, LD, DeFreitas (2001) And then apply to NLO corrections Bern, LD, Schmidt (2002)  

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 39 interference? In principle as important as signal, since it contaminates peak main source of a required phase Fortunately, effect is small, ~ 5% in Standard Model LD, M. Siu (2003)

NTU, 5/31/05 Precision Collider Physics & Search for Higgs Boson 40 Higgs properties post-LHC Which in turn should lead to 10%--50% determinations of Higgs couplings to W,Z,g, , heavy fermions Much work by theorists and experimentalists has led to:

NTU, 5/31/05 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 TeV (rough match to LHC parton energies) Recent global selection of acceleration technology Now Global Design begins Collisions in 2015—2020?

NTU, 5/31/05 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

NTU, 5/31/05 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!