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The Higgs Particle

The Higgs Particle. Sarah D. Johnson University of La Verne August 22, 2002. Outline. Mass in the Standard Model Electro-Weak Force Unification and the Higgs Mechanism Searches for the Higgs Particle Future Prospects What We Will Learn When We Find It. Mass in the Standard Model.

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The Higgs Particle

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  1. The Higgs Particle Sarah D. Johnson University of La Verne August 22, 2002 UCI Quarknet

  2. Outline • Mass in the Standard Model • Electro-Weak Force Unification and the Higgs Mechanism • Searches for the Higgs Particle • Future Prospects • What We Will Learn When We Find It UCI Quarknet

  3. Mass in the Standard Model What is the origin of the particle masses? UCI Quarknet

  4. Particle Masses (GeV/c2) UCI Quarknet

  5. Questions: Why is there such a large range of quark masses? Why is there such a large range of lepton masses? Why are the neutrino masses so small? Why do the W and Z have mass, but the photon and the gluon do not? UCI Quarknet

  6. II. Electroweak Force Unification and the Higgs Mechanism 1961 – 1968 Glashow, Weinberg and Salam (GWS) developed a theory that unifies the electromagnetic and weak forces into one electroweak force. Electromagnetic Force – mediator: photon (mass = 0) felt by electrically charged particles Weak Force – mediators: W+,W-, Z0(mass ~ 80-90 GeV/c2)felt by quarks and leptons UCI Quarknet

  7. For two protons in a nucleus the electromagnetic force is 107 times stronger than the weak force, but, at much shorter distances (~10-18 m), the strengths of the weak and the electromagnetic forces become comparable.. UCI Quarknet

  8. GWS ElectroweakTheory The theory begins with four massless mediators for the electroweak force: Wμ1,2,3 and Bμ. Wμ1,2,3, BμW+, W-, Z0, γ This transformation is the result of a phenomenon known as Spontaneous Symmetry Breaking. In the case of the electroweak force, it is known as the Higgs Mechanism. UCI Quarknet

  9. Spontaneous Symmetry Breaking This is a phenomenon that can occur when the symmetries of the equations of motion of a system do not hold for the ground state of the system. UCI Quarknet

  10. Higgs Mechanism Goldstone’s Theorem - The spontaneous breaking of a continuous global symmetry is always accompanied by the appearance of massless scalar particles called Goldstone bosons. In the Higgs Mechanism, as the result of choosing the correct gauge, the massless gauge field “eats” the Goldstone bosons and so acquires mass. In addition, a “mass-giving” Higgs field and its accompanying Higgs boson particle emerge. W’s W+ W- Z0 UCI Quarknet

  11. The Higgs Field and Higgs Boson The neutral Higgs field permeates space and all particles acquire mass via their interactions with this field. • The Higgs Boson • neutral • scalar boson (spin = 0) • mass = ? Ho UCI Quarknet

  12. III. Searches for the Higgs Particle • What properties are important? • The strength of the Higgs coupling is proportional to the mass of the particles involved so its coupling is greatest to the heaviest decay products which have mass < mH/2. For example, if mH > 2Mz then the couplings for decay to the following particle pairs: • Z0Z0 : W+W- : τ+τ- : pp : μ+μ- : e+e- • are in the ratio • 1.00 : 0.88 : 0.02 : 0.01 : 0.001 : 5.5 x 10-6 UCI Quarknet

  13. Mass constraints from self-consistency* of the Standard Model : • 130 GeV/c2 < MH < 190 GeV/c2 • *The discovery of a Higgs boson with a mass less than 130 GeV/c2 would imply “new physics” below a grand unification (GUT) scale energy of 1016 GeV/c2 • Dominant Production Mechanisms : • LEP: e+e- H0 Z0 • Tevatron: gg  H0 • qq  H0W or H0Z UCI Quarknet

  14. Searches at the Large Electron-Positron Collider (LEP) at CERN • Final States with Good Sensitivity to Higgs Boson: • e+e- (H0bb) (Z0qq) BR 60% • e+e- (H0bb) (Z0νν) BR 17% • e+e- (H0bb) (Z0e+e- , μ+μ-) BR 6% • e+e- (H0τ+τ-) (Z0qq) • e+e- (H0qq) (Z0 τ+τ-) BR 10% UCI Quarknet

  15. Aerial view of LEP at CERN UCI Quarknet

  16. LEP Search Results LEP1: 17 million Z0 decays mH > 65 GeV/c2 LEP2: 40,000 e+e- W+W- events e+e- H0Z0 has background from W+W- and Z0Z0 events, but b-tagging and kinematic constraints can reduce these backgrounds. In 2000 at LEP2 with a center of mass energy of > 205 GeV: ALEPH: signal three standard deviations above background with mH  115 GeV/c2 All four experiments: signal reduced to two standard deviations above background mH  115.6 GeV/c2 mH > 114.1 GeV/c2 UCI Quarknet

  17. Searches at the Tevatron Search Methods: qq  (H0 bb)(W  lν) qq  (H0 bb)(Z0  l+l-) (l = e, μ) CDF: also hadronic decays of W,Z Dzero: also Z ν ν Run I: CDF and DZero took 100 pb-1 of data each and no signal seen though cross section limits were set Run II: CDF and DZero expect 10 fb-1 of data each UCI Quarknet

  18. IV. Future Prospects • The Large Hadron Collider (LHC): 2007 • pp collider with a center of mass energy of 14 TeV • ATLAS and CMS detectors optimized for Higgs searches • Higgs mass range between 100 GeV/c2 and 1TeV/c2 • Next Linear Collider: after 2010 • e+e- collisions at 500+ GeV • precision measurements of Higgs couplings to a few percent • measurements of self-interaction via two Higgs final states UCI Quarknet

  19. V. What We Will Learn When We Find It • If H0 found at the expected Standard Model mass, it will validate the GWS Electroweak Theory and complete the model. • Measurements of the Higgs couplings and comparison with particle masses will verify mass-generating mechanism. • A lighter than 130 GeV/c2 mass Higgs boson could support a theory beyond the Standard Model, known as Supersymmetry. • If a Higgs boson with a mass < 1 TeV is not found, it would indicate that the Electroweak symmetry must be broken by a means other than the Higgs mechanism. UCI Quarknet

  20. Supersymmetry • Supersymmetry is a theory beyond the Standard Model that predicts that every particle will have a super-partner. • The Minimal Supersymmetric Standard Model (MSSM) contains five Higgs particles: h0, H0, A0, H+, H- • In the MSSM the lightest Higgs, h0, is expected to have a mass less than 130 GeV/c2 • The current mass limits on MSSM Higgs are: • mH0> 89.8 GeV/c2 mA0 > 90.1 GeV mH > 71.5 GeV UCI Quarknet

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