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Life at the LHC without the Higgs: Models

Life at the LHC without the Higgs: Models. Giacomo Cacciapaglia (UC Davis). 2007 E ö tvös-Cornell Workshop Budapest June 28, 2007. Life without a Higgs:. The Standard Model has been tested very accurately, however…

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Life at the LHC without the Higgs: Models

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  1. Life at the LHC without the Higgs:Models Giacomo Cacciapaglia (UC Davis) 2007 Eötvös-Cornell Workshop Budapest June 28, 2007

  2. Life without a Higgs: • The Standard Model has been tested very accurately, however… • we still don’t know the origin of the Electroweak symmetry breaking (and masses…) • The Higgs mechanism is a nice description of such phenomenon, but it suffers from theoretical prejudices (hierarchy problem), and the Higgs boson has not been discovered yet. • What if we don’t see the Higgs at the (early) LHC?

  3. The role of the Higgs: • Weakly coupled doublet H = (h , WL±, ZL) • The same dof’s can arise as composite states of a strongly interacting sector ( ) → no fundamental scalar • Can we decouple h from the theory? What would that imply? Technicolor WL WL scattering violates Unitarity at ~ 1 TeV scale. Strong coupling appears below 1 TeV → broad resonances (techni-ρ, …), theory out of control (precision EW tests, …)

  4. We would like to keep the theory under control up to a safe scale of 5-10 TeV: • Let’s consider the contribution of the -mesons to the WL scattering: • Expanding the scattering amplitude for large Energy: A (WL-WL) ~ A(4) E4 + A(2) E2 + finite terms. • A(4) vanishes due to gauge invariance; • A(2) receives contributions both from the vector resonances and the Higgs: The violation of perturbative unitarity can be delayed up to 10 TeV! We can smoothly go to a Higgsless model (gaugephobic Higgs model) hep-ph/0611385

  5. Many models have been proposed: • Higgsless models in warped space; • models in flat space; • deconstructed models; • minimal 3 site models; • … Models in warped space have less freedom! I’ll use them as a guide to explore no-Higgs phenomenology.

  6. We want to find significant signatures and correlate them • with special features of this class of models, like: • unitarization of WLWL scattering; • the S parameter; • flavour; • top physics – custodial symmetry; • … Such signatures are generic for no-Higgs models! Allow to test various properties of the mechanism!

  7. The model Csaki, Grojean, Pilo, Terning, hep-ph/0310355 bulk SU(2)LxU(1)Y SU(2)DxU(1)X • AdS5 space: • UV brane at z=R, IR brane at z=R’; • SU(2)LxSU(2)RxU(1)X in the bulk; • SU(2)RxU(1)X → U(1)Y on the UV brane; • SU(2)LxSU(2)R → SU(2)D on the IR brane. R R’ SU(2)LxSU(2)RxU(1)X IR UV

  8. Spectrum • The W mass is determined by the two mass scales in the problem: MW2 MIR2/log(R’/R) MIR = 1/R’ • The custodial SU(2)R ensures the correct MW to MZ mass ratio • The KK masses are determined by the master equation: J0 (M(k) R’) ~ 0 → M(1) ~ 2.4 MIR …. • Generically, large corrections to the S parameter, like in Technicolor theories…

  9. Light fermions • Light fermions are doublets of the SU(2)’s: ψL = (ul, dl) = (2, 1, 1/6) bulk mass cL ψR = (ur, dr) = (1, 2, 1/6) bulk mass cR • The bulk masses control the localization of the zero modes: UV for cL > ½, IR for cL < ½. • Masses generated on the IR brane • Splitting between up and down component from kinetic terms on the UV brane.

  10. Delocalizing the l.h. zero modes in the bulk (cL ~ 1/2) it is possible to minimize the EWPTs (S~0). hep-ph/0409126 • The almost flat profile of the zero modes allows to decouple the KK gauge bosons from the light fermions: bounds from LEP and Tevatron relaxed! M(1) < 1TeV allowed!! • Therefore, additional corrections from KK states are small. Both representations choice and localization patterns are preferred by Precision tests and Unitarity! Only way to minimize S… fine tuning?

  11. Flavour: a killer for flat fermions? • Contributions of New Physics to FCNC’s are highly constrained: the SM contribution to such processes is already small! • Typical scale suppressing FCNC’s is >103 TeV! • We need to worry about: • Operators in the bulk (4-fermi operators) suppressed by a ~TeV, • Contribution of KK gauge bosons (masses ~ TeV). • A flavour symmetry in the bulk may not be enough!

  12. A flavour symmetry for the light generations. See A.Weiler’s talk on Tuesday. • Let’s impose a SU(Nf)LxSU(Nf)R in the bulk broken to SU(Nf)D on the IR brane. • Mixing arises from the kinetic terms on the UV brane for the right-handed quarks only! • This ensures the absence of FCNCs at tree level and MVF in the charged sector! • In this scenario, both effects from KK states and higher order operators are under control. • Flavour with flat fermions is possible!

  13. Numerical analysis: M(1) ~ 700 GeV 1% 0.5% 1% 0.5% 0.1% Couplings to gauge KK states: g(1)L ~ 0 – 20% gsm, g(1)R ~ 30% gsm Deviations in the lh and rh couplings.

  14. Third generation: mtop vs Zbb • In Higgsless models it is a challenge to have a heavy top and small corrections to the Zbb coupling (below 1%). • The top must be composite due to its heaviness: the bL is also composite. • The SU(2)R doublet ψR contains a left-handed b’ that mixes with the bl via the top Yukawa. • If we extend the custodial symmetry to O(4) ~ SU(2)LxSU(2)RxPLR, we can use PLR to protect Zbb: need to use new representations that respect O(4). (Agashe, Contino, daRold, Pomarol) Cacciapaglia et al, hep-ph/0607146 • Or, we can exile top and bottom to a different AdS throat (like, we have two independent CFT sectors – Cacciapaglia et al, hep-ph/0505001 and 0604218)

  15. Top and Bottom: a brane on their own • R’t < R’w • Top and bottom decouple from the first KK states (localized on the IRw brane) • The weak-throat is exactly like the model described before • The top-throat is strongly coupled, but it does not affect EWPTs Gauge bosons Light fermions Top- bottom

  16. The new custodian for the bottom. They will contain extra (heavy) degrees of freedom: The top Yukawa (M1) does not involve the bottom! The dangerous mixing is absent. Q(bL, bR) = -1/3; Q(tL,tR,TL,TR) = 2/3; Q(XL,XR) = 5/3.

  17. Comparison between the new (continuous) and old (dashed) representations (neglecting mbottom) • A 4% deviation is still there due to the breaking of PLR on the UV brane. • Such contribution can be canceled tuning the bottom Yukawa.

  18. Higgsless phenomenology: • Higgsless models have some theoretical complications, but also nice phenomenological features: • Effectively, one free parameter: the mass of the KK states M(1). All the other parameters are fixed by matching the SM (MZ, GF, α, …) or constrained by precision tests. • Interesting mass range from perturbative unitarity: 600 GeV < M(1) < 1 TeV. • Perturbative and under control up to ~ 5-10 TeV. • The model can be probed and excluded at the LHC! See next talk by G.Marandella!

  19. Group portrait without the Lady: UV brane IR brane • Gauge bosons and light fermions are flat • Top, bL and KK resonances are localized near the IR brane → highly composite • right handed light fermions and bR are localized near the UV brane → elementary

  20. Unitarity → (600 GeV) < M(1) < 1 TeV Couplings among GB’s • flat fermions • VKK f f couplings • KK fermions deg. to VKK • S parameter → • rh on the UV brane • VKK fr fr couplings • Flavour → • New states • large couplings to VKK • … or topless scenario! • Mtop vs Z b b →

  21. Indirect probes Deviations in the WWZ coupling: close to present bounds, new measurements at LHC (ILC) Couplings of the third generation: • Zblbl = 1.004 gSM; • Zbrbr = 0.993 gSM; • Ztltl = 0.461 gSM; • Ztrtr = 1.908 gSM; • Wbltl = 0.862 gSM. • Wbt measured in single top production at the LHC (and Tevatron) • Ztt at ILC (poor sensitivity at the LHC) Log[R] 3% 2.5% 2% 1.5% cL

  22. Conclusions and outlook • EWSB via a strong sector is an interesting alternative to the (weakly coupled) Higgs mechanism. • Extra dimensions provide efficient and constrained models. • Rich phenomenology: many resonances can be discovered at the early LHC. • Signatures can be traced back to a specific feature of the model: unitarity, S parameter, top. • Limited parameter space: the early LHC can exclude this class of models! • A more careful study of LHC signals is necessary.

  23. Back-up slides

  24. A gaugephobic Higgs Cacciapaglia et al, hep-ph/0611358 • VIR fixes the normalization of v(z): we parametrize it as • In the limit V → VSM ~ 246 GeV, R’→R, and we recover the 4D Standard Model. • Mw = f(V, R’, R) = 80.4 GeV. • The theory has 3 free parameters: V, β, mh (we fix R = 10-8).

  25. Localized on the IR brane RS Higgsless 4D Standard Model V >> TeV β Flat profile 246 GeV V

  26. Unitarity in WL-WL scattering • Expanding the scattering amplitude for large Energy: A (WL-WL) ~ A(4) E4 + A(2) E2 + finite terms. • A(4) vanishes due to (5D) gauge invariance; • A(2) receives contributions both from the gauge KK modes and the Higgs: defines the “higgslessness” of the model .

  27. Numerical results • In RED, isocontours of 1/R’ (MKK ~ 2.4/R’) • In BLUE, isocontours of ξ.

  28. Higgs phenomenology Couplings of the (light) Higgs to the SM fields relative to the SM one. (β = 2) • The heavier the particle, the more suppressed the coupling: mF = ∫dz ψ ∂z ψ + y5∫dz v(z) ψ ψ The larger mF, the larger the bulk contribution (distortion of the wave functions). While, the effective Yukawa coupling is prop to the overlap part.

  29. Benchmark point 1:  ~ 25% h →γγ is not useful anymore (sup. by a factor of ~20) • For a light Higgs, the only channel is tth (suppressed by a factor of ~4). h →γγ not observable. • For heavy Higgs, WW and ZZ channels suppressed by a factor of ~4. • In order to match the discovery potential of the SM, we need ~16 times more integrated luminosity.

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