14.1early discoveries 14.2the fundamental interactions 14.8accelerators 14.3classification of...

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14.1 Early Discoveries 14.2 The Fundamental Interactions 14.8 Accelerators 14.3 Classification of Elementary Particles 14.4 Conservation Laws and Symmetries 14.5 Quarks 14.6 The Families of Matter 14.7 Beyond the Standard Model CHAPTER 14 Elementary Particles “If I could remember the names of all these particles, I’d be a botanist.” Enrico Fermi Steven Weinberg (1933 - ) “I have done a terrible thing: I have postulated a particle that cannot be detected.” Wolfgang Pauli (after postulating the existence of the neutrino)

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Page 1: 14.1Early Discoveries 14.2The Fundamental Interactions 14.8Accelerators 14.3Classification of Elementary Particles 14.4Conservation Laws and Symmetries

14.1 Early Discoveries14.2 The Fundamental Interactions14.8 Accelerators14.3 Classification of Elementary Particles14.4 Conservation Laws and Symmetries14.5 Quarks14.6 The Families of Matter14.7 Beyond the Standard Model

CHAPTER 14Elementary Particles

“If I could remember the names of all these particles, I’d be a botanist.”

Enrico Fermi

Steven Weinberg (1933 - )

“I have done a terrible thing: I have postulated a particle that cannot be detected.”

Wolfgang Pauli (after postulating the existence of the neutrino)

Page 2: 14.1Early Discoveries 14.2The Fundamental Interactions 14.8Accelerators 14.3Classification of Elementary Particles 14.4Conservation Laws and Symmetries

Elementary Particles

Elementary Particles

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14.1: Early Discoveries

In 1930 the known elementary particles were the proton, the electron, and the photon.

Thomson identified the electron in 1897, and Einstein defined the photon in 1905. The proton is the nucleus of the hydrogen atom.

Despite the rapid progress of physics in the first couple of decades of the twentieth century, no more elementary particles were discovered until 1932, when Chadwick proved the existence of the neutron.

That would have seemed sufficient…

James Chadwick (1891-1974)

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But particle physics measurements were happening… Energetic particles collide with stationary

particles in a “bubble chamber,” vaporizing the nearby matter and leaving a visible track.

A magnetic field (pointing into the screen) causes charged particles to take curved paths.

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The Positron

In 1928 Dirac introduced the relativistic theory of the electron when he combined quantum mechanics with special relativity.

He found that his wave equation had negative, as well as positive, energy solutions.

His theory can be interpreted as a vacuum being filled with an infinite sea of electrons with negative energies.

If enough energy is transferred to the “sea,” an electron can be ejected with positive energy leaving behind a hole that is the positron, denoted by e+.

Paul Dirac (1902-1984)

E 0

Vacuum Electron & positron

Positron!

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Anti-particles

Dirac’s theory yields anti-particles, which:

Have the same mass and lifetime as their associated particles.

Have the same magnitude but are opposite in sign for such physical quantities as electric charge and various quantum numbers.

All particles, even neutral ones (with some exceptions like the neutral pion), have antiparticles.

Magnetic field into screen

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The Positron

Carl Anderson identified the positron in cosmic rays. It was

easy: it had positive charge and was light.

Carl Anderson (1905-1991)Anderson’s cloud chamber photo of

positron track

Cosmic rays are highly energetic particles, mostly protons, that cross interstellar space and enter the Earth’s atmosphere, where their interaction with particles creates cosmic “showers” of many distinct particles.

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Positron-Electron Interaction

The ultimate fate of positrons (anti-electrons) is annihilation with electrons.

After a positron slows down by passing through matter, it is attracted by the Coulomb force to an electron, where it annihilates through the reaction:

All anti-matter eventually meets the same fate. A lot of energy is released in this process: all of the matter is converted to energy.

Star Trek’s “dilithium crystals” supposedly

contain anti-matter, which powers the Enterprise.

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Feynman DiagramsFeynman presented a particularly simple graphical technique to describe interactions.

It predicts that, when two electrons approach each other, according to the quantum theory of fields, they exchange a series of photons called virtual photons, because they cannot be directly observed.

The action of the electromagnetic field (for example, the Coulomb force) can be interpreted as the exchange of photons. In this case we say that the photons are the carriers or mediators of the electromagnetic force.

Example of a Feynman space-time diagram. Electrons interact through mediation of a photon. The axes are normally omitted.

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Yukawa’s Meson

The Japanese physicist Hideki Yukawa had the idea of developing a

quantum field theory that would describe the force between nucleons—analogously to the electromagnetic

force.

To do this, he had to determine the carrier or mediator of the nuclear

strong force analogous to the photon in the electromagnetic force which he

called a meson (derived from the Greek word meso meaning “middle”

due to its mass being between the electron and proton masses).

Hideki Yukawa (1907-1981)

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Yukawa’s meson, called a pion (or pi-meson or -meson), was identified in 1947 by C. F. Powell (1903–1969) and G. P. Occhialini (1907–1993)

Charged pions have masses of 140 MeV/c2, and a neutral pion 0 was later discovered that has a mass of 135 MeV/c2, a neutron and a proton.

Yukawa’s Meson

Feynman diagram indicating the exchange of a pion (Yukawa’s

meson) between a neutron and a proton.

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Other Mesons, Quarks, and Gluons

Yukawa’s pion is responsible for the nuclear force.

Later we’ll see that the nucleons and mesons are part of a general group of particles formed from even more fundamental particles: quarks. The particle that mediates the strong interaction between quarks is called a gluon (for the “glue” that holds the quarks together); it’s massless and has spin 1, just like the photon.

Computed image of quarks and gluons in a nucleon

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The Weak Interaction

In the 1960s Sheldon Glashow, Steven Weinberg, and Abdus Salam predicted that particles that they called W (for weak) and Z should exist that are responsible for the weak interaction.

They have been observed. Sheldon

Glashow (1932- )

Abdus Salam (1926-1996)

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The Graviton

It has been suggested that the particle responsible for the gravitational interaction be called a graviton.

The graviton is the mediator of gravity in quantum field theory and has been postulated because of the success of the photon in quantum electrodynamics theory.

It must be massless, travel at the speed of light, have spin 2, and interact with all particles that have mass-energy.

The graviton has never been observed because of its extremely weak interaction with objects.

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The Fundamental Interactions

One of the main goals of particle physics is to unify these forces (to show that they’re all just different aspects of the same force), just as Maxwell did for the electric and magnetic forces many years earlier.

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The Fundamental Interactions

A finite range effectively confines the particle, which, by the uncertainty principle, gives it a minimal momentum and hence a minimum kinetic energy and mass. Photons and gravitons are massless. W and Z bosons are heavy.

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14.8 AcceleratorsParticle accelerators generate high enough energies to create particles 1 GeV/c2 or greater.

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Accelerators

There are three main types of accelerators used presently in particle physics experiments: synchrotrons, linear accelerators, and colliders.

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Synchrotron Radiation

One difficulty with cyclic accelerators is that when charged particles are accelerated, they radiate electromagnetic energy called synchrotron radiation. This problem is particularly severe when electrons, moving very close to the speed of light, move in curved paths. If the radius of curvature is small, electrons can radiate as much energy as they gain.

Physicists have learned to take advantage of these synchrotron radiation losses and now build special electron accelerators (called light sources) that produce copious amounts of photon radiation used for both basic and applied research.

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Linear Accelerators

Linear accelerators or linacs typically have straight

electric-field-free regions between gaps of RF voltage

boosts. The particles gain speed with each boost, and the voltage boost is on for a

fixed period of time, and thus the distance between gaps

becomes increasingly larger as the particles accelerate.

Linacs are sometimes used as pre-acceleration device

for large circular accelerators.

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Colliders

Because of the limited energy available for reactions like that found for the Tevatron, physicists decided they had to resort to colliding beam experiments, in which the particles meet head-on.

If the colliding particles have equal masses and kinetic energies, the total momentum is zero and all the energy is available for the reaction and the creation of new particles.

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Large Hadron Collider

Counter-propagating protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV.

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14.3: Classification of Elementary Particles

Particles with half-integral spin are called fermions and those with integral spin are called bosons.

This is a particularly useful way to classify elementary particles because all stable matter in the universe appears to be composed, at some level, of constituent fermions.

Fermions obey the Pauli Exclusion Principle. Bosons don’t.

Photons, gluons, W±, and the Z are called gauge bosons and are responsible for the strong and electroweak interactions.

Gravitons are also bosons, having spin 2.

Fermions exert attractive or repulsive forces on each other by exchanging gauge bosons, which are the force carriers.

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One other boson that has been predicted, but not yet detected, is necessary in quantum field theory to explain why the W± and Z have such large masses, yet the photon has no mass.

This missing boson is called the Higgs particle (or Higgs boson) after Peter Higgs, who first proposed it.

The Standard Model proposes that there is a field called the Higgs field that permeates all of space.

By interacting with this field, particles acquire mass. Particles that interact strongly with the Higgs field have heavy mass; particles that interact weakly have small mass.

The Higgs boson is very heavy, and it hasn’t been observed yet.

The search for the Higgs boson is of the highest priority in elementary particle physics.

Simulated event featuring the appearance of the Higgs boson.

The Higgs Boson

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Boson Properties

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Leptons: electrons, muons, taus & neutrinos

The leptons are perhaps the simplest of the elementary particles.

They appear to be point-like, that is, with no apparent internal structure, and seem to be truly elementary.

Thus far there has been no plausible suggestion they are formed from some more fundamental particles.

Each of the leptons has an associated neutrino, named after its charged partner (for example, muon neutrino).

There are only six leptons plus their six antiparticles.

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Muon and tau decay

The muon decays into an electron, and the tau can decay into an electron, a muon, or even hadrons.

The muon decay (by the weak interaction) is:

e

e

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Neutrinos

Neutrinos have zero charge.

The electron neutrino occurs in the beta decay of the neutron.

Their masses are known to be very small. The precise mass of neutrinos may have a bearing on current cosmological theories of the universe because of the gravitational attraction of mass.

Like all other leptons, they have spin 1/2, and all three neutrinos have been identified experimentally.

Neutrinos are particularly difficult to detect because they have no charge and little mass, and they interact very weakly (they easily pass through the earth!).

Picture of the sun, taken not with light, but with neutrinos, made at the Japanese neutrino observatory Super-Kamiokande.

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Neutrino Oscillations

One of the most perplexing problems over the last three decades has been the solar

neutrino problem: the number of neutrinos reaching Earth from the sun is a factor of 2 to 3 too small if our understanding of the energy-

producing (nuclear fusion) is correct.

Neutrinos come in three varieties or flavors: electron, muon, and tau. The solution was found when researchers saw neutrinos generated in

the Earth’s atmosphere (from cosmic rays) changing or “oscillating” into another flavor (the sun only emits electron neutrinos).

Also, this could only happen if neutrinos have mass.

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Hadrons

Hadrons are particles that act through the strong force.

Two classes of hadrons: mesons and baryons.

Mesons are particles with integral spin having masses greater than that of the muon (106 MeV/c2). (Mesons are made up of pairs of quarks—a quark and an anti-quark.) They’re unstable and rare.

Baryons have masses at least as large as the proton and have half-integral spins. Baryons include the proton and neutron, which make up the atomic nucleus, but many other unstable baryons exist as well. The term "baryon" is derived from the Greek βαρύς (barys), meaning "heavy," because at the time of their naming it was believed that baryons were characterized by having greater mass than other particles. (They’re made up of three quarks.) All baryons decay into protons.

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The Hadrons

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Fundamental and Composite Particles

We call certain particles fundamental; this means that they are not composed of other, smaller particles. We believe leptons, quarks, and gauge bosons are fundamental particles.

Although the Z and W bosons have very short lifetimes, they are regarded as particles, so a definition of particles dependent only on lifetimes is too restrictive.

Other particles are composites, made from the fundamental particles.

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14.4: Conservation Laws

Physicists like to have clear rules or laws that determine whether a certain process can occur or not.

It seems that everything occurs in nature that is not forbidden.

Certain conservation laws are already familiar from our study of classical physics. These include mass-energy, charge, linear momentum, and angular momentum.

These are absolute conservation laws: they are always obeyed.

Additional conservation laws will be helpful in understanding the many possibilities of elementary particle interactions.

Some of these laws are absolute, but others may be valid for only one or two of the fundamental interactions.

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Baryon Conservation

In low-energy nuclear reactions, the number of nucleons is always conserved.

Empirically this is part of a more general conservation law for what is assigned a new quantum number called baryon number that has the value B = +1 for baryons and −1 for anti-baryons, and 0 for all other particles.

The conservation of baryon number requires the same total baryon number before and after the reaction.

Although there are no known violations of baryon conservation, there are theoretical indications that it was violated sometime in the beginning of the universe when temperatures were quite high. This is thought to account for the preponderance of matter over anti-matter in the universe today.

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Lepton Conservation

The leptons are all fundamental particles, and there is a conservation of leptons for each of the three kinds (families) of leptons.

The number of leptons from each family is the same both before and after a reaction.

We let Le = +1 for the electron and the electron neutrino; Le = −1 for their antiparticles; and Le = 0 for all other particles.

We assign the quantum numbers Lμ for the muon and its neutrino and Lτ for the tau and its neutrino similarly.

Thus three additional conservation laws.

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Strangeness

The behavior of the K mesons seemed very odd.

There is no conservation law for the production of mesons, but it appeared that K mesons, as well as the Λ and Σ baryons, were always produced in pairs in the p + p reaction. One would expect the K0 meson to also decay into two photons very quickly, but it does not.

A new quantum number was defined: Strangeness, S, which is conserved in the strong and electromagnetic interactions, but not in the weak interaction.

The kaons have S = +1, lambda and sigmas have S = −1, the xi has S = −2, and the omega has S = −3.

When the strange particles are produced by the p + p strong interaction, they must be produced in pairs to conserve strangeness.

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Unifying all these interactions proved difficult.

In the 1950s, it was rumored that Heisenberg had done it, and just the details remained to be sketched in. But nothing ever emerged from Heisenberg. So Wolfgang Pauli responded with the following:

“Below is the proof that I am as great an artist as Rembrandt; the details remain to be sketched in.”

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The Weak Interaction: The Electroweak Theory

In the 1960s Sheldon Glashow, Steven Weinberg, and Abdus Salam unified the electro-magnetic and weak interactions into what they called the electroweak theory, much as Maxwell had unified electricity and magnetism into the electromagnetic theory a hundred years earlier.

Sheldon Glashow (1932- )

Abdus Salam (1926-1996)

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Unification of the Strong and Electroweak Interactions: The Standard Model

Over the latter half of the 20th century, numerous physicists combined efforts to generate The Standard Model.

It is a widely accepted theory of elementary particle physics at present.

It is a relatively simple, comprehensive theory that explains hundreds of particles and complex interactions with six quarks, six leptons, and three force-mediating particles.

It is a combination of the electroweak theory and quantum chromodynamics (QCD), but does not include gravity.

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Quarks

Three quarks were proposed, named the up (u), down (d), and strange (s), with the charges +2e/3, −e/3, and −e/3, respectively. The strange quark has the strangeness value of −1, whereas the other two quarks have S = 0.

Quarks are believed to be essentially point-like, just like leptons.

With these three quarks, all the known hadrons (at the time) could be specified by some combination of quarks and anti-quarks.

Murray Gell-Mann (1929- )

In 1963 Murray Gell-Mann and, independently, George Zweig proposed that hadrons were formed from fractionally charged particles called quarks. The quark theory successfully described the properties of the particles and reactions and decay.

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Charm, Truth, and Beauty

A fourth quark called the charmed quark (c) was proposed to explain some additional discrepancies in the lifetimes of some of the known particles.

A new quantum number called charm C was introduced so that the new quark would have C = +1 while its anti-quark would have C = −1 and particles without the charmed quark have C = 0.

Charm is similar to strangeness in that it is conserved in the strong and electromagnetic interactions, but not in the weak interactions. This behavior was sufficient to explain the particle lifetime difficulties.

Two additional quarks, top and bottom (or truth and beauty), were also required to construct some exotic particles (the Upsilon-meson).

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Quark Properties

The spin of all quarks (and anti-quarks) is 1/2.

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Quark Description of Particles

Baryons normally consist of three quarks or anti-quarks.

A meson consists of a quark-anti-quark pair, yielding the required baryon number of 0.

1/3e

-2/3e2/3e

-1/3e2/3e

1/3e

2/3e-2/3e

-1/3e

-1/3e

2/3e

-1/3e-1/3e

1/3e

2/3e

2/3e

-1/3e

-2/3e2/3e

-2/3e

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Other Particles

What about the quark composition of the Ω−, which has a strangeness of S = −3? Its quark composition is sss. And its charge is 3(−e/3) = −e, and its spin is due to three quark spins aligned, 3(1/2) = 3/2. There is no other possibility for a stable omega (lifetime ~10−10 s) in agreement with the table.

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Quantum Chromodynamics (QCD)

Because quarks have spin 1/2, they are all fermions. According to the Pauli exclusion principle, no two fermions can exist in the same state. Yet we have three identical strange quarks in the Ω−!

This is not possible unless some other quantum number distinguishes each of these quarks in one particle.

A new quantum number called color circumvents this problem and its properties establish quantum chromodynamics (QCD).

Discovery of the -

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There are three colors for quarks we call red (R), green (G), and blue (B) with anti-quark color antired ( ); antigreen ( ) and antiblue ( ). (A “bar” above the symbol is usually used to describe the “anti-color”).

Color is the “charge” of the strong nuclear force, analogous to electric charge for electromagnetism.

Color

R G B

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Quark-anti-quark creation

Physicists now believe that free quarks cannot be observed; they can only exist within hadrons. This is called confinement.

This occurs because the force between the quarks increases rapidly with distance, and the energy supplied to separate them creates new quarks.

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The Families of Matter

The three generations (or families) of matter. Note that both quarks and

leptons exist in three distinct sets. One of each charge type of quark and lepton make up a generation. All visible matter in the universe is

made from the first generation; second- and third-generation

particles are unstable and decay into first-generation particles.

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Grand Unifying Theories (GUTs)

There have been several attempts toward a grand unified theory (GUT) to combine the weak, electromagnetic, and strong interactions and explain why:

Current experimental measurements have shown the proton lifetime to be greater than 1032 years. Current theory has it at 10-29 to 10-31 years.

Neutrinos may have a small, but finite, mass. This has been confirmed.

Massive magnetic monopoles may exist. If one exists anywhere in the universe, it explains why charge is quantized. There is presently no confirmed experimental evidence for magnetic monopoles.

The proton and electron electric charges should have the same magnitude.

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Another challenge:Matter-Antimatter

According to the Big Bang theory, matter and antimatter should have been created in exactly equal quantities. But it appears that matter dominates over antimatter now in our universe, and the reason for this has puzzled physicists and cosmologists for years.

Events in the early universe may be responsible for this asymmetry. But explanations go far beyond the standard model.

Could this galaxy be made entirely of

anti-matter?

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Including Gravity: String Theory

For the last two decades there has been a tremendous amount of effort by theorists in string theory, which has had several variations. The addition of super-symmetry resulted in the name theory of super-strings.

In super-string theory, elementary particles do not exist as points, but rather as tiny, wiggling loops that are only 10−35 m in length.

Further work has revealed that they describe not just strings, but other objects including membranes and higher-dimensional objects. The addition of membranes has resulted in “brane” theories.

Presently super-string theory is a promising approach to unify the four fundamental forces, including gravity.

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Super-symmetry

Super-symmetry is a necessary ingredient in many of the theories trying to unify all four forces of nature.

The symmetry relates fermions and bosons. All fermions will have a super-partner that is a boson of equal mass, and vice versa.

The super-partner spins differ by ħ / 2.

Presently, none of the known leptons, quarks, or gauge bosons can be identified with a super-partner of any other particle type.

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M-theory

Recently theorists have proposed a successor to super-string theory called M-theory.

M-theory has 11 dimensions (ten spatial and one for time) and predicts that strings coexist with membranes, called “branes” for short.

The number of particles that have been predicted from a variety of different theories include the fancifully named sleptons, squarks, axions, winos, photinos, zinos, gluinos, and preons.

Only through experiments (which no one currently knows how to do) will scientists be able to wade through the vast number of unifying theories.