Lecture johnson

August 22, 2002 UCI Quarknet

The Higgs Particle

Sarah D. Johnson

University of La Verne

August 22, 2002


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


I. Mass in the Standard ModelWhat is the origin of the particle masses?


Particle Masses (GeV/c2)

Up

0.003

Charm

1.3

Top

175

Photon

0

Down

0.006

Strange

0.1

Bottom

4.3

Gluon

0

νe

<1 x 10-8

νμ

<0.0002

ντ

<0.02

Z

91.187

Electron

0.000511

Muon

0.106

Tau

1.7771

W±

80.4


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?


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


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..


GWS Electroweak Theory

The theory begins with four massless mediators for the electroweak force: Wμ

1,2,3 and Bμ.

Wμ1,2,3, Bμ W+, 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.


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.


Higgs MechanismGoldstone’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


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


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


• 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


Searches at the Large Electron-Positron Collider (LEP) at CERN

Final States with Good Sensitivity to Higgs Boson:

1. e+e- (H0bb) (Z0qq) BR 60%

2. e+e- (H0bb) (Z0νν) BR 17%

3. e+e- (H0bb) (Z0e+e- , μ+μ-) BR 6%

4. e+e- (H0τ+τ-) (Z0qq)

e+e- (H0qq) (Z0 τ+τ-) BR 10%


Aerial view of LEP at CERN


LEP Search ResultsLEP1: 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


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


IV. Future ProspectsThe 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


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.


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

Technology

Lecture johnson