The Standard Model of Particle Physics

A Simplified Summary

by Ben Best

Space, Matter and Time — what else can there be? Quantities of these fundamental qualities can be measured in Meters(m), Kilograms(kg) and Seconds(s), respectively — the Système Internationale (SI) standard of measurement (also known as MKS). The universe is believed to consist of matter (4% atoms — 3.6% of which is in intergalactic gas — and 22% dark matter) and energy (74% dark energy). Dark energy uniformly fills all of space with a density of 10−29 grams per cubic centimeter, and is the source of a repulsive force causing the universe to expand. Energy (E) is regarded to be interconvertible with matter (mass, m) by Einstein's famous equation E = mc2, where c is the speed of light.

The so-called Standard Model of particle physics (formulated in the 1970s) describes the universe in terms of Matter (fermions) and Force (bosons). Force is not independent of the fundamental qualities. Force can be expressed in terms of the fundamental qualities as Mass times Acceleration [kg x m/s2] or (equivalently) as change of Momentum per unit Time [(kg x m/s)/s]. The Standard Model describes approximately 200 particles and their interactions using 17 fundamental particles, all of which are fermions or bosons: 6 quarks (fermions), 6 leptons (fermions), 4 force-carrying particles (gauge bosons), and the Higgs boson. Unlike the force-carrying particles, the matter particles have associated antimatter particles, such as the antielectron (also called positron) and antiquarks. [For the purpose of this paragraph, Antimatter (fermions) is being regarded as a kind of Matter (fermions).]

There are four known forces, each mediated by a fundamental boson particle (quantum, known as a carrier particle, "gauge bosons"). Standard model mathematics predicts the existence of the weak, strong, and electromagnetic forces. The weak and strong force are only seen in atomic nuclei or other subatomic particles. Aside from gravity, all the macroscopically observable forces — such as friction & pressure as well as electrical & magnetic interaction — are due to electromagnetic force.

KNOWN FORCES (Gauge bosons)
Strong nuclear gluon 1 0.14 (?) 10-15
Electromagnetic photon 7 X 10-3 none infinite
Weak nuclear W+,W- & Z bosons 10-5 80-90 10-17
Gravitation gravitron (tentative) 6 X 10-39 none infinite

The strong nuclear force holds quarks together to form protons & neutrons, and holds protons & neutrons together in a nucleus (being stronger than the electromagnetic force repelling protons). (The strong force is also repulsive if protons and neutrons become too close.) The weak nuclear force causes transformation of protons to neutrons and vice-versa, along with other radioactive phenomena.

Mass of subatomic particles is described by the mass-energy unit GeV, Giga (billion) electron volts. (The amount of energy an electron gains moving through a potential of one volt in a vacuum is one electron-volt,1eV.) Theoretically gluons have no mass, but small values are observed experimentally. Gluons are mostly energy, and by mass-energy interconversion account for most of the mass of protons & neutrons.

Subatomic matter particles can be described as fundamental or composite. The fundamental matter particles are quarks & leptons. Protons & neutrons are composites of quarks, whereas an electron is a lepton fundamental particle. A proton is 1,800 times more massive than an electron. Quarks are never found alone, whereas leptons never form composites (unless an atom is called a composite). Neutrinos are electrically uncharged leptons that have a mass less than one-millionth of an electron, and can oscillate between three types: electron-neutrino, muon-neutrino, and tau-neutrino. Properties of fundamental particles include spin, electrical charge, color charge, and mass.

Composites made of quarks or antiquarks are called hadrons. Calling matter fermions and forces bosons is an oversimplification because composite particles can be fermions or bosons. A composite particle containing an even number of fermions (such as the carbon−12 nucleus with 6 protons and 6 neutrons) is a boson — and an odd number of fermions (such as the helium−3 atom, with 2 protons, 1 neutron, and 2 electrons) will result in a fermion. A simple table representing the most common forms of matter found on Earth would be:

Dark matter is not made of hadrons, and is speculated to be composed of neutrino-like neutralinos (leptons). The strong nuclear force acts on hadrons, but does not act on leptons (electrons are unaffected by the strong force). The weak nuclear force acts on both hadrons & leptons.

Hadrons are "glued" together by gluons. The larger the number of gluons exchanged among quarks, the stronger the binding force. The most important baryons are the proton and the neutron. Unlike electrostatic forces that weaken with increasing distance, strong force (gluon) weakens with increasing closeness. But if a quark attempts to escape from a proton or neutron by moving outward, the strong force becomes so intense as to make escape practically impossible. Nonetheless, at extreme densities, quark-gluon plasmas have been observed, demonstrating that quarks can exist outside of confinement by protons, neutrons or mesons.

Strong force due to gluon exchange between quarks only occurs within protons & neutrons. The force that holds the nucleus together (nuclear force) is due to "leakage" from gluon exchange resulting in an exchange of pion particles between protons & neutrons. Nuclear force acts nearly the same on protons as it acts on neutrons (independent of charge). Nuclear force is analogous to the van der Waals forces which hold neutral (uncharged) molecules together by electrical polarization effects.

The strong nuclear force (strong interaction) is also called the "color interaction". Analogous to the two-valued electrical charge associated with electromagnetic force is a three-valued "color charge" associated with quarks & the strong force (gluons) that binds quarks together. The colors of the three-valued charge are called red, green and blue — not visual colors, but called a "charge" based on an analogy to visual colors. Just as combining electrical positive & negative charge results in a neutral electrical-charge, combining red, green & blue color-charge gives a neutral color charge (the analogy to visual color being that mixing the red, green & blue primary visual colors gives neutral white). Mesons are color-charge neutral because they combine red, green or blue quarks with anti-red, anti-green or anti-blue quarks, respectively.

All quarks & gluons have color-charge, but leptons do not. All the hadrons (protons, neutrons, mesons) comprised of quarks, antiquarks and gluons have neutral color-charge (analogous to most atoms having a neutral electrical charge). A quark can change color by emitting or absorbing gluons. If a red quark becomes a green quark it must have emitted a gluon carrying the colors red and anti-green. Quarks are constantly changing color-charge by exchanging of gluons with other quarks. The closer quarks come to each other, the weaker the quark color-charge change.

Electromagnetic & gravitational forces vary as the inverse square of distance without limit (to infinity). But the strong & weak nuclear forces are short-range rather than inverse-square forces. Short-range forces only operate at very short range through exchange of particles. It is the non-zero rest mass of the short-range force-mediating particles which causes them to decay quickly and thereby limits their range. For the strong nuclear force the exchange-particle is the gluon (nuclear "glue"). Unlike photons, which uniformly surround electrons forming a spherically symmetric shell, gluons clump together into tubes when linking quarks to quarks or to antiquarks. (Agglomerations of gluons alone are called "glueballs"). Beyond a hadron separation of about 1.7 femtometers (10−15 meters), the strong nuclear force becomes negligibly small.

For the weak nuclear force the exchange-particles are W+, W- or Z bosons. W bosons are named after the Weak nuclear force. W bosons can be positive (W+) or negative (W), each being the antiparticle of the other. A Z particle is electrically neutral, and is its own antiparticle. The Z boson may have been so-named because it has Zero electric charge. Neutrinos are electrically-neutral particles that are only affected by the weak nuclear force. The fact that photons have zero mass allows the range of the electromagnetic force to be infinite, but the fact that the range of W and Z bosons reaches only across the atomic nucleus necessitates that those bosons have mass. The fact that W and Z bosons have mass was evidence for the existence of the Higgs boson, which was proven to exist by July 4th, 2012.

The weak nuclear force is responsible for nuclear decay. The weak nuclear force is mediated by the massive W+ & W bosons (mass of a Bromine atom) & Z-boson (mass of a Zirconium atom). Emission & absorption of the W+ & W bosons is the only way quarks change flavor. A Z boson can decay into either a quark/anti-quark pair or into a lepton/anti-lepton pair (same flavor in both cases). The means by which a neutron decays into a proton (beta-decay) is by emitting a W boson (leaving a proton) which decays further into an electron and an electron antineutrino. Weak W boson nuclear force is responsible for the fact that all the more massive quarks & leptons rapidly decay into the lightest (and most stable) quarks & leptons. Z0 bosons can decay into particles that interact by weak nuclear force, including all the neutrinos: electron neutrino, muon neutrino, and tau neutrino.

A more detailed description of neutron decay shows the role of weak nuclear force. A down quark in the neutron becomes an up quark by the emission of a W boson. The W boson decays into a high-energy electron (beta decay) and an electron neutrino. W emission is seen in the beta decay of cobalt−60 to nickel−60.

Although strong & electromagnetic forces make no distinction between right-handed or left-handed particles (particle invariance), particles subject to weak forces do make this distinction. A right-handed particle is a particle spinning in the direction the right-hand fingers curl when the particle is traveling in the direction pointed-to by the right thumb. Because electrons and neutrinos are the particles most influenced by weak nuclear force, they display the greatest "handedness". Only left-handed (not right-handed) fermions are affected by the weak force. Electrons & neutrinos are left-handed, whereas antielectrons (positrons) & antineutrinos are righthanded (antiparticles have opposite handedness).

The Standard Model is consistent with quantum mechanics and special relativity. Gravity is excluded from the Standard Model — gravitons have never been observed. At very high energies and very small scales the other three forces become almost identical, but the convergence is imperfect.

There are 6 flavors of quarks and 6 flavors of leptons, both grouped in 3 generations as shown in the following two tables. The first two flavors of each table (up & down quarks in the quark table, electron & electron neutrino in the lepton table) are known as first-generation matter — as distinct from the second pair of rows (second-generation) and the third pair of rows (third-generation). First-generation matter is both the least massive and the most stable. The vast majority of matter in the universe is first generation, because second and third generation matter is too unstable to last more than tiny fractions of seconds (decaying into first-generation matter). It is the weak force that mediates flavor changes between quarks and between leptons.

The six flavors of quarks are summarized in the following table:

QUARKS (Fermions)
Up First  0.003  + 2/3
Down First  0.006  - 1/3

Charm Second  1.3  + 2/3
Strange Second  0.1  - 1/3

Top Third  175  + 2/3
Bottom Third  4.3  - 1/3

The most massive quark, the top quark, has the mass of a silver atom — and is so unstable that it was the last quark to be discovered (in 1995, after years of searching). Flavor changes of quarks are only due to the weak nuclear force.

Because the proton & neutron baryons are stable particles, it is not surprising that they are composed of the lightest & most stable quarks: the up-quarks and the down-quarks. A proton is composed of two up-quarks & one down-quark (uud), whereas a neutron is two down-quarks & one up-quark (ddu). A down-quark contains a negative charge that is one-third the negative charge of an electron, whereas an up-quark contains a positive charge that is two-thirds as strong as the negative charge on an electron. In all cases, protons and neutrons are composed of quarks that are one each of red, blue, and green "color charge" to result in a "color-charge neutral" proton or neutron. For a proton, for example, the masses of two up-quarks & one down-quark accounts for only about 2% of the mass and 30% of the spin — showing the considerable contribution of gluons and raw (kinetic & potential) energy (E =mc2) to the total mass and spin of a proton.

A meson is a quark bound by a tube of gluons to an antiquark. Like protons and neutrons, mesons are "color-charge neutral". A red quark combines with an anti-red quark, a blue quark combines with an anti-blue quark, or a green quark combines with an anti-green quark to give a "color-charge neutral" meson. The meson is so-named because its mass is intermediate between the electron and the proton (Greek: mesos=intermediate). Mesons typically have no spin. The lightest and most stable meson is the pion (pi meson), which has a mean life of 2.6 x 10−8 seconds. Neutral pions (π0) are composed of either up & antiup or down & antidown quarks. (Antiup and antidown quarks are antimatter particles that can also be called up and down antiquarks.) A negative pion (π) is composed of a down and antiup quark, whereas a positive pion (π+) is composed of an up and antidown quark. The force holding protons and neutrons together in the nucleus has not only been described as being due to "residual strong force", but due to protons & neutrons exchanging pions.

The heavier kaon (K-meson) has a mean life that is less than half that of the pion. Neutral kaons (K0) are composed of a down quark and a strange anti-quark. A negative kaon (K) is composed of a strange and antiup quark, whereas a positive kaon (K+) is composed of an up and antistrange quark. The decay of a mercury−197 atom to gold−197 is associated with emission of a positive kaon.

The 1964 discovery of the uneven distribution of neutral kaons (K0) between matter and antimatter forms was the first proof of charge-parity violation (CP violation). A charge inversion is a flip between positive and negative (or vice-versa). A parity inversion is like a flip between and object and its mirror image, but it is actually a 3−dimensional flip rather than a reflection inversion (1−dimensional flip). The laws of physics are symmetrical between matter and antimatter for electromagnetism, gravity and strong force, but parity is violated for the weak force. In the Standard Model, weak force is only seen for left-handed components of particles and right-handed components of antiparticles. Charge-parity violation may be behind the fact that there is more matter than antimatter in the universe.

In May 2010 it was discovered that the oscillations of neutral B−mesons (B0) transforming into their own antiparticles and back (flavor oscillation) also exhibits charge-parity violation. The oscillating neutral B−mesons decay into matter (muons) about 1% more often than they decay into antimatter (antimuons).

The six flavors (3 pairs) of leptons can be summarized in the following table:

LEPTONS (Fermions)
Electron (e) First   5.11 X 10-4    - 1
Electron neutrino First  < 10-8      0

Muon (μ) Second   0.106    - 1
Muon neutrino Second  < 3 X 10-4      0

Tauon Third   1.78    - 1
Tauon neutrino Third  < 3.3 X 10-2      0

According to the Standard Model neutrinos have no mass. But whereas the fusion reactions of the Sun only produce electron-neutrinos, the electron-neutrinos coming from the Sun were only one third of those expected. Later it was confirmed that neutrinos oscillate between the three flavors, which is only possible if neutrinos have a very small mass — a mass which should not be due to the Higgs boson according to the Standard Model.

Muons (μ) were once thought to be mesons (and were formerly called "mu mesons"), but are now known to be leptons. The muon was the first fundamental particle discovered that is not found in ordinary atoms, although muons can replace electrons in atoms (making the atoms extraordinary). With a mean lifetime of 2.2 microseconds, muons are not very stable. Muons are created in the atmosphere when cosmic ray protons strike the nuclei of air atoms creating pions (pi mesons) that each decay into a muon and muon-neutrino. The muon further decays into an electron, an electron-antineutrino and a muon-neutrino. The antimatter form of the muon is the antimuon (μ+), which has opposite charge, but equal mass and spin.

All subatomic particles have spin (intrinsic angular momentum). Spin is quoted in units of Plank's constant (h) divided by . Spin is the product of (h/2π) and the spin quantum number, s, where s = n/2 where n is a non-negative integer. For atoms and molecules, it is the sum of the spins of unpaired electrons that gives rise to paramagnetism. For both force-carrier and fundamental particles, spin determines the energy distribution function, which can be either Bose-Einstein (Bosons) or Fermi-Dirac (Fermions). Particles with ½-spin (fermions) are constrained by (obey) the Pauli Exclusion Principle, whereas other particles (bosons) are not. In sum:

Fermions    ½ Fermi-Dirac constrained by (obey)
Bosons 0,1 or 2 Bose-Einstein no constraint (don't obey)

The Pauli Exclusion Principle prevents fermions (protons, neutrons, electrons, quarks, neutrinos, etc.) from being too close if they have the same quantum state (spin), but bosons (photons, gluons, W/Z bosons, gravitrons, etc.) can be close together while sharing the same quantum state — as with masers & lasers for photons. Near absolute zero temperature bosons bunch together, but fermions do not. The bunched bosons are called a Bose-Einstein condensate (predicted by Albert Einstein on the basis of the work of East Indian physicist Satyendra Bose, the namesake of bosons). Bose-Einstein condensates have been created consisting of thousands of atoms (first demonstrated with rubidium atoms in 1995).

Although fundamental particles (quarks, leptons) are fermions, and force carriers are bosons, composites (hadrons, nuclei or atoms) may be bosons or fermions. Baryons are fermions, but mesons are bosons. The helium−4 nucleus is a boson, which allows helium−4 to display superfluidity (zero viscosity and infinite thermal conductivity) at temperatures below 2.17 Kelvin. The helium−3 nucleus is a fermion and does not display superfluidity at 2.17 Kelvin. Weakly interacting fermions can display bosonic behavior, such as superconductivity. (Helium−3 and helium−4 are immiscible liquids below 0.8 Kelvin.)

The Standard Model is based on two quantum field theories. The quantum field theory based on electromagnetic quanta is called quantum electrodynamics (QED), which explains how electrons, positrons & photons interact. The quantum field theory based on strong force quanta is called quantum chromodynamics (QCD), which explains how quarks & gluons interact. Glucons interact not only with quarks, but with other gluons. Color & electromagnetic charge are both conserved. For a QCD description of possible patterns of excitation in continuous quark and gluon fields it is necessary to specify 84 numbers at each point in space: 36 for quark fields plus 48 for gluon fields. For quark fields, 6 flavors, 3 colors and 2 components accounting for spin ("spin-up" and "spin-down") are required (6X3X2 = 36). (Quarks are fermions, which means only the two spin states +½ or −½ times Plank's constant.) For gluon fields there are 8 directions in space, with each direction having 6 fields (3 electric + 3 magnetic) (8X6 = 48).

For every matter particle there corresponds an antimatter particle. Antimatter particles can correspond to matter particles in every respect except that the charge is opposite. Or they may also be opposite in other properties, including spin and color-charge. An antielectron (positron, the only antiparticle with a unique name) has the same mass as an electron, but is electrically positive. Antiquarks have electrical charges −2/3 and +1/3. Associated with the antiquarks are the color-charges: antired, antigreen and antiblue. An antiproton is composed of 2 up antiquarks & 1 down antiquark (opposite the 2 up quarks & 1 down quark of the proton). If a proton has blue & red up quarks and a green down quark, a corresponding antiproton would have antiblue & antired antiup quarks and an antigreen antidown quark (although quark colors change too rapidly for this to be meaningful). An antihydrogen atom is composed of a positron in an s-orbital around an antiproton (composed of two up and one down antiquarks).

When a particle and an antiparticle meet, they annihilate into pure energy and may give rise to energetic neutral force-carrier particles, such as gluons, photons or Z bosons. The collision of a low-energy positron and a low-energy electron gives rise to two gamma ray photons. But at high enough energies electron-positron annihilation can produce a Z boson. In contrast to annihilation, energetic force-carrier particles can give rise to matter particle/antiparticle pairs (pair production). An unsolved mystery of cosmology is why the universe is dominated by matter rather than antimatter.

A free neutron (a neutron not in an atomic nucleus) has a mean lifetime of about 15 minutes, typically decaying into a proton, an electron and an electron antineutrino. A neutron can decay in the same way in an unstable nucleus. Protons in an unstable nucleus may decay either by forming a neutron, a positron and an electron neutrino or by capturing an electron to form a neutron and an electron neutrino. Proton decay outside of a nucleus has never been observed. If proton decay does occur outside of a nucleus, the mean lifetime of the free proton is not less than 1036 years.

The Standard Model
[Standard Model Chart]

In summary, the Standard Model consists of 17 particles. Twelve of the 17 fundamental matter-particles are fermions: 6 quarks and 6 leptons. The remaining five particles are bosons, four of which are physical manifestations of the forces through which particles interact. (At high energies the weak nuclear force merges with electromagnetic force.) The fifth boson is the Higgs boson, associated with the Higgs field which gives mass to electrons, elementary quarks, Z&W bosons, and the Higgs boson itself. [Note that the strong nuclear force associated with the gluon particle gives mass to atomic nuclei, by binding together the three quarks inside protons and neutrons — and all attempts to include gravitons or gravity into the Standard Model have failed.] Gluons interact only with quarks and themselves, but all the other bosons interact with both leptons & quarks. Quarks carry both electrical & color charge, but leptons have no color charge, and only non-neutrino leptons have electrical charge. Neutrinos carry neither electrical nor color charge. About 85% of the mass of the universe is yet unaccounted-for by any of the particles in the Standard Model — missing "dark matter".

According to Big Bang theory, the existing universe emerged from an explosion in a vacuum that occurred 13.7 billion years ago. The four forces were unified until 10−43 seconds after the Big Bang, after which first gravity and then strong nuclear force separated from the other two forces. At 10−12 seconds after the Big Bang electromagnetism separated from the weak nuclear force, and the universe was filled with a hot quark-gluon plasma that included leptons and antiparticles. At 10−6 seconds hadrons began to form. Most hadrons and antihadrons were eliminated by annihilation, leaving a small residue of hadrons by one second post-Big Bang. Between one and three seconds after Big Bang the universe was dominated by leptons/antileptons until annihilation of these particles left only a small residue of leptons. The universe was dominated by photons created by all of the matter/antimatter annihilations, and the predominance of matter over antimatter had been established. Between 3 and 20 minutes after the Big Bang protons and neutrons began to combine to form atomic nuclei. A plasma of electrons & nuclei ("ionized hydrogen & helium") existed for 300,000 years until the temperature dropped to 5,000ºC when hydrogen & helium atoms formed.

If matter and antimatter were perfectly symmetrical, the cooling of the universe would have resulted in particle/antiparticle annihilation that would have left the universe filled only with photons. But for every billion mutual annihilations a particle of matter remained — comprising the existing matter of the universe. The predominance of matter over antimatter is a consequence of charge-parity violation (CP violation). About 99% of the photons in the universe (the cosmic microwave background) are the result of Big Bang annihilations. Photons from stars are a trivial contribution, by comparison.

The standard model used by cosmologists predicts that the universe is composed of 5%  ordinary matter, 27% cold dark matter, and 68%  dark energy. Dark matter reputedly caused hydrogen to coalesce into stars, and is a binding force in galaxies. Dark energy is accelerating the expansion of the universe. The cosmologists' standard model also predicts that within the first 10−32 of a second after the Big Bang, the universe doubled in size 60&nbps;times in a growth spurt known as inflation.

Dark matter does not interact with the electromagnetic force, thus making it transparent and hard to detect, despite the fact that dark matter must permeate the galaxy. Unlike visible matter, dark matter is nonbaryonic — its composition is outside of the (unextended) Standard Model. Neutrinos may be a low-mass example of dark matter. Invisible Weakly Interacting Massive Particles (WIMPs having a few hundred times the mass of a proton) have been hypothesized as being the substance of dark matter. It is believed that the effect of Earth moving through a dark matter "wind" results in a 10% greater dark matter flux when it is summer in the Northern Hemisphere than when it is winter. Some physicists believe that dark matter does not exist, but that theories of gravitation need to be revised (as is proposed by modified Newtonian dynamics).

The most prosaic goal of the Large Hadron Collider (LHC, the enormous particle accelerator that first began operation in September 2008 at CERN, Europe's particle physics laboratory near Geneva, Switzerland) was to find the Higgs boson. The Higgs boson adheres to the W & Z bosons to give them mass, but does not adhere to photons (leaving photons massless). The more particles interact with the Higgs field, the more massive they become. The bosons that mediate electromagnetism (photons) and the strong force (gluons) are massless, but the bosons that mediate the weak force (Z and W bosons) have a mass about a hundred times greater than the mass of a proton. The Higgs field, not the Higgs boson, gives energy to particles. Because of Einstein's E = mc2, giving energy is equivalent to giving mass. Heavier particles interact with the Higgs field more than lighter particles, the heavy top quark more than any other particle. A Higgs field would fill the vacuum of space with Higgs bosons, just as the electromagetic field fills the vacuum of space with photons.

The LHC is designed to create new particles from the energy of proton collisions — consistant with Einstein's E = mc2 equation — with the particle of greatest interest being the Higgs boson. The discovery of a new boson, widely believed to be the Higgs boson, was announced on July 4th, 2012. Whether or not the new boson is the Higgs boson repsonsible for the Higgs field, the new boson is definitely connected to the Higgs field. More than 500 trillion (5 X 1014) high-energy proton collisions were produced before physicists could confidently announce that they had discovered a new boson with a mass of roughly 125 GeV. The probability that the observed boson is due to random background radiation is less than one in three million. The Higgs boson must have a spin = 0. The observed boson does not have spin = 1 because it decayed into two photons, but spin = 2 remains a possibility (and would mean the boson is not the Higgs).

Two detectors were created to search for the Higgs boson: (1) CMS (Compact Muon Solenoid) and (2) ATLAS (A Toroidal LHS ApparatuS). Neither detector could detect a Higgs boson directly, but the Higgs boson rapidly decays into photons, Z or W bosons, or fermions — which CMS and ATLAS can detect. Detection is most accurate for decay into two photons, but that mode of decay only happens 0.2-0.3% of the time. The probability of a Higg boson being produced from a single high-energy proton collision is about one in ten trillion (1 X 1013) because the interaction between quarks and gluons with the strong nuclear force are far more powerful than their interaction with the Higgs field. A Higgs boson could be formed from gluons from the colliding protons fusing together, or by quarks from the protons emitting Z or W bosons that fuse.

The LHC could validate or invalidate models of supersymmetry which double the number of particles in the standard model by pairing every boson with a fermion superparticle — and pairing every fermion with a boson superparticle (somewhat analogous to antimatter). But most particle physicists are hoping to make discoveries with the LHC that gets beyond the Standard Model, including an understanding of nonbaryonic dark matter. The general theory of relativity (the best model for gravity) would be included in a Unified Field Theory that would account for all force fields.

String theory was first formulated in the 1970s to describe strings of energy binding a quark and antiquark to form a meson. A number of superstring theories have been proposed to unify relativistic quantum field theory with general relativity theory. At Planck-length (10−35 meter) dimensions Einstein's equations of general relativity result in such intense fluctuations of energy that "spacetime goes haywire". Instead of boson & fermion particles, the universe is proposed to be made of Planck-length boson & fermion strings — two-dimensional entities vibrating in ten-dimensional space-time. Strings might be closed loops or open — and they must be stretched under tension to vibrate (excite). Unlike particle interactions which occur at a single point in space-time, strings collide over a small but finite distance. Strings vibrate in ten dimensions, six of which are tightly coiled in on an unmeasurably small scale and four of which are in conventional space-time. A variant known as membrane theory (M-theory, "branes" — multi-dimensional membranes) puts gravity in an eleventh dimension and points to an infinite number of solutions — implying (for some) an infinite number of universes.

The Standard Model treats fundamental particles as point-like entities having no dimensions, adjusted for by a kludge called renormalization. String theory removes the need for renormalization and provides mathematically satisfying explanations for many other problems. But string theory has still not fulfilled its promise of unifying gravity and quantum mechanics. Nor has it produced testable hypotheses, because strings could only be measured at energies well beyond the capacities of existing particle accelerators. Some physicists worry that aesthetic elegance is displacing evidence as the basis of physical theory.

(For more details on superstring theory go to For more on infinite numbers of universes, see my essay The Copenhagen Interpretation of Quantum Mechanics.)

For more detailed charts of Standard Model particles & interactions, see

See also the American Physics Society Particle Physics Links and the Image Bank for more tutorials and visual aids.

For more on theories of particle physics see Elementary Particle Physics Today.