This document discusses the Higgs particle and the searches that have been conducted to find it. It begins by explaining how particle masses are generated in the Standard Model through the Higgs mechanism. This involves the Higgs field imparting mass on particles when they interact with it. Experiments like LEP and the Tevatron have searched for the Higgs boson but have so far only found lower limits on its possible mass range. Future experiments like the LHC are expected to directly detect the Higgs if it is light, around 100-1000 GeV/c^2. Discovering the Higgs would validate the Standard Model, while not finding one would require new physics beyond the model.

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

2. 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

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

4. August 22, 2002 UCI Quarknet
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

5. August 22, 2002 UCI 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?

6. August 22, 2002 UCI 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-
, Z0
(mass ~ 80-90 GeV/c2
)
felt by quarks and leptons

7. August 22, 2002 UCI Quarknet
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..

8. August 22, 2002 UCI Quarknet
GWS Electroweak Theory
The theory begins with four massless mediators
for the electroweak force: Wμ
1,2,3
and Bμ.
Wμ
1,2,3
, 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.

9. August 22, 2002 UCI 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.

10. August 22, 2002 UCI 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’s W-
W+
Z0

11. August 22, 2002 UCI 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 = ?
Ho

12. August 22, 2002 UCI 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
< mH/2. For example, if mH > 2Mz then the couplings for
decay to the following particle pairs:
Z0
Z0
: 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

13. August 22, 2002 UCI Quarknet
• 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 → H0
W or H0
Z

14. August 22, 2002 UCI Quarknet
Searches at the Large Electron-Positron
Collider (LEP) at CERN
Final States with Good Sensitivity to Higgs Boson:
1. e+
e-
→ (H0
→bb) (Z0
→qq) BR 60%
2. e+
e-
→ (H0
→bb) (Z0
→νν) BR 17%
3. e+
e-
→ (H0
→bb) (Z0
→e+e-
, μ+
μ-
)BR 6%
4. e+
e-
→ (H0
→τ+
τ-
) (Z0
→qq)
e+
e-
→ (H0
→qq) (Z0
→ τ+
τ-
) BR 10%

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

16. August 22, 2002 UCI Quarknet
LEP Search Results
LEP1: 17 million Z0
decays mH > 65 GeV/c2
LEP2: 40,000 e+
e-
→ W+
W-
events
e+
e-
→ H0
Z0
has background from W+
W-
and Z0
Z0
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

17. August 22, 2002 UCI Quarknet
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

18. August 22, 2002 UCI 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/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

19. August 22, 2002 UCI Quarknet
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.

20. August 22, 2002 UCI 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: 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:
mH
0
> 89.8 GeV/c2
mA
0
> 90.1 GeV mH
±
> 71.5 GeV