The Higgs BosonThe search for the God

Particle

The Standard Model

• Most successful description of fundamental particles and forces

• Provides a theoretical framework incorporating all known particles and forces (except gravity)

Forces

• Electromagnetic

• Strong

• Weak

• Gravity (not actually included in the standard model)

Quantum Field Theory (QFT)

• A theory of fields that is consistent with quantum mechanics

• Utilizes gauge particles which travel back and forth between charged particles (in the case of the electromagnetic field), and are the origin of force

• Electroweak theory

– Incorporates Quantum ElectroDynamics (QED) and theory of weak interactions

• Quantum ChromoDynamics (QCD) is the QFT for color force

Fundamental Particles

• 6 Quarks:

– top, bottom, charmed, strange, up, down

• 6 Leptons:

– electron, muon, tau, and their neutrinos

• 6 bosons

– Electromagnetic force: photon

– Strong force: gluon

– Weak force: W+, W-, Z

– Gravitational: graviton

Shortcomings of the Standard Model

• Doesn’t incorporate gravity

• Doesn’t explain the masses of the particles

– Why should something as simple as a quark even have a mass

– Why do they have different masses

– For instance, photons are massless, but the W and Z particles are huge

– Why there is such a range of masses

• Doesn’t explain the origin of mass

Criteria for a New Theory

• Explain the symmetry breaking between electromagnetism and the weak force

• Describe a mechanism for imparting mass to particles

• Cannot introduce any new physics

Enter the Higgs Field

• There is a Higgs field that fills the universe (a scalar field)

• Any particles that pass through this field distort it

• This distortion slows the particles down, preventing them from travelling at the speed of light

• This is, in fact, the particle’s mass

• Analog to physics in solids – an electron moving through a positively charged crystal lattice acts as if it is 40 times more massive

The Higgs Boson

• Since the Higgs field is a quantum field, it has it’s own carrier particle, the Higgs boson

• The Higgs has spin 0, because the Higgs field is a scalar field

• If it had a spin, there would be a preferred direction, which there is not

• In fact, the Higgs boson is the analog of the phonon in crystals; it is a perturbation in the field itself, without a particle

• Might be a top – antitop combination

The Higgs Boson (cont.)

• Causes W and Z bosons to be massive, which limits the range of the weak force

• Photons are not affected, so they are massless, travel at the speed of light, and have an infinite range

• In other words, the symmetry of the electroweak force is broken

The Mass of the Higgs Boson

• The mass of the Higgs particle is important because different theories predict different masses

• Experiments at the LEP showed that the mass of the Higgs must be > 113 GeV

• Supersymmetric: < 130

• Standard model: < 170

• Technicolor: > 160

Finding the Higgs• Basically, you accelerate protons and

antiprotons in opposite directions

• When they come together, they annihilate in a burst of energy

• This energy forms a Higgs particle, which then decays

• The decay particles are picked up by a detector, and their velocities are measured

• Using conservation of momentum and energy, physicists work backwards to find the mass of the Higgs

Decay Process for the Higgs

• Higgs is heavy, so it can decay into almost anything

• Branching ratios are strongly dependent on the Higgs mass, thus making a variety of tagging and detection algorithms necessary

Summary of Decays Available

Indistinguishable jets produced, but important for Higgs production

Decay into gluon pairGluon Decay

Weak branching ratio but clean signature

Decay into photon through higher order processes

Two Photon Decay

Coupling strongest to top quark (unless mH<2MT)

Decay into any fermion

Fermionic Decay

Decay into Z or W boson

Vector Boson Decay

DiagramRemarksDescriptionHiggs Decay Process

History of the Higgs

• Peter Higgs proposed the idea in 1964

• The SSC was built, in large part, to look for the Higgs. However, the project was terminated in 1993

• In 2000, the LEP collider was scheduled for shutdown

• However, the physicists decided to max the collider in a final bid to find the Higgs

• They found 5 possible appearances, but the collider was shut down without the Higgs having been found

History of the Higgs (Cont.)

• It was hoped that Fermilab would be able to detect the Higgs, but that turned out to not be the case

• LHC at CERN should be finished in 2007 and should be able to detect it

Conclusion

• The Higgs boson gives particles mass• To be consistent, the standard model requires the

existence of the Higgs boson• The mass of the Higgs boson will pick out the most

valid theory• Finding the Higgs boson is not trivial. Further

searches will increase the precision of current measured values, and provide a better estimate for the Higgs mass

Conclusion (Cont.)

• If and when the Higgs is discovered, it will be one of the most important discoveries in particle physics

• Finding the Higgs boson will further the longer-term goal of unifying all forces

• There are other theories to explain how particles get their mass - proving that the Higgs boson does not exist would be just as scientifically valuable as proving that it does

Higgs Himself

Dismantling the LEP

Computer reconstruction of a possible Higgs decay

Part of the LHC (under construction)

Bibliography

• The God Particle: If the Universe is the Answer, What Is the Question?, by Leon Lederman, Dick Teresi, Houghton Mifflin Co; (January 1993)

• http://www.wired.com/wired/archive/12.04/grid_pr.html• htttp://physicsweb.org/article/world/12/12/12/1• htttp://physicsweb.org/article/news/4/9/2• htttp://physicsweb.org/article/world/12/12/12/1• http://en.wikipedia.org/wiki/Higgs_boson• http://phy.uct.ac.za/courses/phy400w/particle/higgs1.htm• http://phy.uct.ac.za/courses/phy400w/particle/higgs2.htm• http://phy.uct.ac.za/courses/phy400w/particle/higgs3.htm• http://phy.uct.ac.za/courses/phy400w/particle/higgs4.htm• http://phy.uct.ac.za/courses/phy400w/particle/higgs5.htm