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

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

August 22, 2002UCI Quarknet I.Mass in the Standard Model What is the origin of the particle masses?

August 22, 2002UCI Quarknet Particle Masses (GeV/c 2 ) Up Charm 1.3 Top 175 Photon 0 Down Strange 0.1 Bottom 4.3 Gluon 0 ν e <1 x ν μ < ν τ <0.02 Z Electron Muon Tau W ± 80.4

August 22, 2002UCI Quarknet Questions: Why is there such a large range of quark masses? Why do the W and Z have mass, but the photon and the gluon do not? Why are the neutrino masses so small? Why is there such a large range of lepton masses?

August 22, 2002UCI Quarknet 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 -, Z 0 (mass ~ GeV/c 2 ) felt by quarks and leptons

August 22, 2002UCI Quarknet For two protons in a nucleus the electromagnetic force is 10 7 times stronger than the weak force, but, at much shorter distances (~ m), the strengths of the weak and the electromagnetic forces become comparable..

August 22, 2002UCI Quarknet GWS Electroweak Theory The theory begins with four massless mediators for the electroweak force: W μ 1,2,3 and B μ. W μ 1,2,3, B μ W +, W -, Z 0, 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.

August 22, 2002UCI Quarknet 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.

August 22, 2002UCI Quarknet 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’sW-W- W+W+ Z0Z0

August 22, 2002UCI Quarknet 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 = ? HoHo

August 22, 2002UCI Quarknet 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 2M z then the couplings for decay to the following particle pairs: Z 0 Z 0 : W + W - : τ + τ - : pp : μ + μ - : e + e - are in the ratio 1.00 : 0.88 : 0.02 : 0.01 : : 5.5 x 10 -6

August 22, 2002UCI Quarknet Mass constraints from self-consistency* of the Standard Model : 130 GeV/c 2 < M H < 190 GeV/c 2 *The discovery of a Higgs boson with a mass less than 130 GeV/c 2 would imply “new physics” below a grand unification (GUT) scale energy of GeV/c 2 Dominant Production Mechanisms : LEP:e + e -  H 0 Z 0 Tevatron:gg  H 0 qq  H 0 W or H 0 Z

August 22, 2002UCI Quarknet Searches at the Large Electron-Positron Collider (LEP) at CERN Final States with Good Sensitivity to Higgs Boson: 1.e + e -  (H 0  bb) (Z 0  qq)BR 60% 2.e + e -  (H 0  bb) (Z 0  νν)BR 17% 3.e + e -  (H 0  bb) (Z 0  e+e -, μ + μ - )BR 6% 4.e + e -  (H 0  τ + τ - ) (Z 0  qq) e + e -  (H 0  qq) (Z 0  τ + τ - ) BR 10%

August 22, 2002UCI Quarknet Aerial view of LEP at CERN

August 22, 2002UCI Quarknet LEP Search Results LEP1: 17 million Z 0 decays m H > 65 GeV/c 2 LEP2: 40,000 e + e -  W + W - events e + e -  H 0 Z 0 has background from W + W - and Z 0 Z 0 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 m H  115 GeV/c 2 All four experiments: signal reduced to two standard deviations above backgroundm H  GeV/c 2 m H > GeV/c 2

August 22, 2002UCI Quarknet Searches at the Tevatron Search Methods: qq  (H 0  bb)(W   l  ν) qq  (H 0  bb)( Z 0  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

August 22, 2002UCI Quarknet 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/c 2 and 1TeV/c 2 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

August 22, 2002UCI Quarknet V. What We Will Learn When We Find It If H 0 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/c 2 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.

August 22, 2002UCI Quarknet 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: h 0, H 0, A 0, H +, H - In the MSSM the lightest Higgs, h 0, is expected to have a mass less than 130 GeV/c 2 The current mass limits on MSSM Higgs are: m H 0 > 89.8 GeV/c 2 m A 0 > 90.1 GeVm H  > 71.5 GeV