Introduction

The Standard Model is a theoretical framework for organizing the particles and forces that make up our universe, much like a periodic table. "It is the quantum theory that includes the theory of strong interactions (quantum chromodynamics or QCD) and the unified theory of weak and electromagnetic interactions (electroweak)." [1] The standard model is incomplete and has not been proven; a deeper understanding of the Top quark, electroweak symmetry breaking, and identification of the Higgs boson is still needed.

Forces

In their first brush with classical physics, students are introduced to two kinds of forces: forces imparted to objects as a result of contact with other objects, and the poorly understood but very predictable gravitational force. Things become more complicated when the electromagnetic force is introduced; suddenly the world is one in which fields exist everywhere, and an object will experience a force if it is sensitive to that particular kind of field. An object with mass is affected by gravitational fields, whereas an object with charge is affected by electric fields, and an object with both charge and velocity is affected by mangetic fields when aligned perpendicularly.

Quantum field theory, however, views forces as interactions mediated by the exchange of particles specific to the fundamental force acting upon the objects. Another term for mediating particle is field quantum, or "particle of a field." Thus forces are once again interactions with objects, although the rules of subatomic interactions as described by quantum mechanics are quite unlike classical or Newtonian mechanics.

Known Fundamental Forces

The fundamental forces are gravitation, electroweak, and strong. Gravitation is the attraction two objects with mass/energy experience towards one another, mediated by gravitons. Gravitons are theorized to be massless spin-2 bosons, but have not been found experimentally. (In fact, gravity is no longer included in the standard model.) The electroweak force is attractive or repellant, determined by the charges/weak-charges in question, and mediated by photons, W+ and W- bosons, and Z^0 bosons. [2] The strong force is what keeps the positively charged protons in the nucleus, in spite of repulsion due to the electroweak force, mediated by gluons (which "glue" the quarks of a hadron together with so much force that the hadrons are held together as well).[3]

The Grand Unified Theory and the Higgs Boson

Just as the electroweak force is a unification of the electromagnetic and weak forces, the Grand Unified Theory supposes that all the fundamental forces are in fact aspects of one force. In addition to the graviton, physicists have yet to discover the Higgs boson, another hypothetical force carrier predicted by the standard model which interacts with particles to give them mass [a thing may have energy, mass, or a combination of the two, affected by the thing's velocity (relative to something else?)]. "Failure to find [the Higgs boson] would call into question the Standard Model. Experimenters at Fermilab hope to find evidence for the Higgs boson and make further discoveries in the next few years."[4]

Particles

Particles have intrinsic properties: mass, spin, charge. Quarks have additional instrinisc properties which determine how they combine and decay: color-charge, parity (P), isospin (Iz), strangeness, charm, bottomness, and topness. Parity is the point analog of the spread-out wave functions that describe particles -- positive for quarks and negative for antiquarks. Iso-spin is not angular momentum, but does follow the same rules of addition. Color-charge is a property that determines how quarks can be combined to form hadrons; the possible values are red, green and blue for quarks, anti-red, anti-green, and anti-blue for antiquarks, and the resulting hadron must be color-neutral, which is achieved by summing red + green + blue, anti-r + anti-g + anti-b, or a color and its anti-color. Strangeness, charm, bottomness, and topness are binary properties (on or off) that are conserved in strong and electromagnetic interactions, but not in weak ones. Similarly, Parity is conserved in both strong and electromagnetic interactions, while Isospin is conserved only in strong reactions.

Bosons are particles that are exchanged in the conveyance of a force. The more mass a boson has, the shorter the range of the force it conveys. Photons, which convery a very long-range force, are nearly massless, whereas the Z0 which conveys the electroweak force is 91.2 GeV/c^2.

"Quarks and leptons are members of a family of particles called fermions (particles with half integer spins). Both the quarks and leptons come in pairs." [5]

A hadron is a particle composed of quarks. Typical hadrons are protons (composed of two Up quarks and one Down quark) and neutrons (composed of two Down quarks and one Up quark). 3-quark hadrons are known as baryons (Greek for "heavy"), and 2-quark hadrons formed of quark-antiquark pairs are called mesons (Greek for "intermediate").

Particles and Forces of the Standard Model

Forces of the Standard Model
Force Electroweak Strong
Property Charge Weak-charge Color-charge
Strength 10^-2 10^-6 1
Range 1/r^2 10^-3 fm complex
Bosons photon W+, W- Z^0 gluon
Data from Table 11.1 The Standard Model, [2]

Known Bosons
Participants in Electroweak and Strong
Spin = 1/2
Color-charge = r, g, b
Name Mass Charge Mean life or width
photon < 6 x 10^-17 eV < 5 x 10^-30 e Stable
W+, W- 80.425 +- 0.038 GeV +- 1 2.124 +- 0.041 GeV
Z^0 91.1876 +- 0.0021 GeV 0 2.4952 +- 0.0023 GeV
gluon 0 (theoretical) SU(3) color octet  
data from 2004 Review of Particle Physics [6]

Quarks
Participants in Electroweak and Strong
Spin = 1/2
Color-charge = r, g, b
Name Mass Charge (e) Additional Qualities
up 1.5 to 4 MeV + 2/3 Isospin (Iz) = + 1/2
down 4 to 8 MeV - 1/3 Iz = - 1/2
strange 80 to 130 MeV - 1/3 Strangeness = -1
charm 1.15 to 1.35 GeV + 2/3 Charm = +1
bottom 4.1 to 4.4 GeV (MS mass)
4.6 to 4.9 GeV (1S mass)
- 1/3 Bottom = -1
top 174.3 +- 5.1 GeV (direct observation)*
178.1 (+10.4 - 8.3) GeV (Standard Model Electroweak fit)
+ 2/3 Top = +1
data from 2004 Review of Particle Physics [6]
* Most recent observed mass of Top quark is 173.5 [+2.7/-2.6 (stat.) +/- 2.5 (JES) +/- 1.7 (syst.)]GeV/c^2 [7]

Leptons
Participants in Electroweak
Spin = 1/2
Name Mass Charge
electron (e) 0.51099892 +- 0.00000004 MeV -1
e-neutrino < 3 eV 0
muon (mu) 105.658369 +- 0.000009 MeV -1
mu-neutrino < 0.19 MeV, CL = 90% 0
tauon (tau) 1776.99 +0.29 -0.26 MeV -1
tau-neutrino < 18.2 MeV, CL = 95% 0
data from 2004 Review of Particle Physics [6]

References

    2. Nonclassical Physics: Beyond Newton's View, Randy Harris pp. 518-9.
    6. Summary Tables: Bosons, Quarks, and Leptons, 2004 Review of Particle Physics Summary Table, S. Eidelman et al. (Particle Data Group), Phys. Lett. B 592, 1 (2004) (URL: http://pdg.lbl.gov)

Contact: graya( a t )u.washington.edu