Erasmus : What we call the “Standard Model” summarises our knowledge of the basic building blocks of the universe – in terms of the elementary fundamental particles and the forces that interact with them. There are 12 basic fundamental elemental particles. We use the name “fermions” for these.

The Standard Model has antiparticles as well, not appearing in the picture above as well the hypothetical graviton.

Erasmus : This group (fermions) is then subdivided into leptons and quarks.

Six of this family are quarks --- they go by the unusual and distinct names of up, down, charm, strange, bottom and top. (A proton, for instance, is made of two up quarks and one down quark.)

The other six are leptons--- these include the electron and its two heavier relatives (described as generation two and generation three of this type of particle), namely the Muon and the Tau.
Each of these particles: the Electron the Muon and the Tau, have their corresponding neutrino, similarly described as belonging to the 3 progressively heavier generations of matter. (I, II, III).

The Fermion particles (Quarks and Leptons) are all classified into three generations, which are progressively heavier. Each class is divided into pairs of particles that exhibit a similar physical behaviour called a generation (see the table- a different view emphasizing properties).

Kinkajou : There are four fundamental forces in the Universe: 
Gravity, Electromagnetism, and the Weak Nuclear and Strong Nuclear Forces. Each of these is currently thought to be produced by fundamental particles that act as carriers of the force.

Erasmus : The most familiar of these is the photon, a particle of light, which is the mediator of electromagnetic forces. (This means that, for instance, a magnet attracts a nail because both objects exchange photons.) Most of us are familiar with light which essentially travels in straight lines and that we can see as being mediated by photons. Magnetism is mediated by magnetic photons. These are much more wavelike than particle like. They are the reason that the magnetic field of the Earth curves around the planet. We’ll discuss this later.

The strong force is carried by eight particles known as gluons.

The weak force is transmitted by three particles, the W+, the W- , and the Z bosons.

The graviton is the particle or whatever we think may be associated with gravity.

Erasmus : The diagram above shows 4 carriers of force, but not the “graviton”. We have little evidence that a particle such as a graviton should exist, but we know through science and through experience that something of the sort must do so. All the evidence points to the graviton having no mass and no energy. These are things which seem at odds with the description of gravity as a force (due to our belief in equivalence of force and energy). We believe if a particle such as a graviton were to exist, it must have a spin +2. 

Now one more major issue. Each particle has a corresponding antiparticle. For example the most obvious example is the electron and its antiparticle the positron. These particles and antiparticles appear in the table above.

The Standard Model includes 12 elementary particles of spin +1/2, as we have stated are known as fermions. According to the spin–statistics theorem, fermions respect the Pauli Exclusion Principle. This states that two or more identical particles with half-integer spins (i.e. fermions cannot occupy the same quantum state within a quantum system simultaneously.

Each fermion has a corresponding antiparticle.

Erasmus : Fermions are classified according to how they interact (or equivalently, by what charges they carry).

There are six quarks (up, down, charm, strange, top, bottom). Quark’s carry something we call “colour charge”.

There are six leptons (electron, electron neutrino, Muon, Muon neutrino, tau, tau neutrino). Leptons do not carry “colour charge”.

The defining property of quarks is that they carry colour charge, and hence interact via the strong interaction. The phenomenon of colour confinement results in quarks being very strongly bound to one another, forming colour-neutral composite particles called hadrons that contain either a quark and an antiquark (e.g. mesons) or three quarks (baryons).

The lightest baryons are the proton and the neutron. Quarks also carry electric charge and weak isospin. Hence they interact with other fermions via electromagnetism and the weak interaction.

The remaining six fermions do not carry colour charge and are called leptons. The three neutrinos do not carry electric charge either, so their motion is directly influenced only by the weak nuclear force, which makes them extremely difficult to detect. By contrast, by virtue of carrying an electric charge, the electron, Muon, and tau all interact electromagnetically.

Erasmus : Each member of a generation has greater mass than the corresponding particles of lower generations. The first-generation charged particles do not decay; hence all ordinary (baryonic) matter is made of such particles.

Kinkajou : Specifically, all atoms consist of electrons orbiting around atomic nuclei, the nuclei ultimately constituting of up and down quarks. (Forming protons and neutrons)
On the other hand, second- and third-generation charged particles decay with very short half-lives and are observed only in very high-energy environments.
Neutrinos of all generations also do not decay and pervade the universe, but rarely interact with baryonic matter.

Kinkajou : I have heard some physicists describe photons and neutrinos as massless. But isn't energy equivalent to mass, so that can't be true?

Erasmus : To complicate our understanding things, some professionals relate that particles such as photons and neutrinos are massless.  To restate this, they do not have non relativistic mass. That is their energy is tied up in their movement or their velocity: the speed of light for the photon.  We all know that energy has mass equivalence due to Einstein’s equation E= mc2. However, many professionals use the non-relativistic mass -convention.

Neutrinos were originally thought to be massless. However, because neutrinos change flavour as they travel, at least two of the types of neutrinos must have mass. This phenomenon is known as neutrino oscillation,

Erasmus : In particle physics, a massless particle is an elementary particle whose invariant mass is zero.
The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object or system of objects that is independent of the overall motion of the system.

Because of mass–energy equivalence, the rest energy of the system is simply the invariant mass times the speed of light squared. Similarly, the total energy of the system is its total (relativistic) mass times the speed of light squared.

Note that for reasons above, such a rest frame does not exist for single photons, or rays of light moving in one direction. When two or more photons move in different directions, however, a centre of mass frame (or "rest frame" if the system is bound) exists. Thus, the mass of a system of several photons moving in different directions is positive, which means that an invariant mass exists for this system even though it does not exist for each photon. (This just highlights the way physicists think about matter.
We have included these notes so that our readers can cope with statements that seem at odds with what is known).

Erasmus : The invariant mass of a system includes the mass of any kinetic energy of the system’s constituents that remains in the centre of momentum frame, so the invariant mass of a system may be greater than sum of the invariant masses (rest masses) of its separate constituents. For this reason, invariant mass is in general not an additive quantity.

Goo : Complicated. No wonder so many ordinary people reading these about this stuff get confused. What Next?

Kinkajou : What are Gauge Bosons?

Erasmus : In the Standard Model, gauge bosons are defined as force carriers that mediate the strong, weak, and electromagnetic fundamental interactions. The standard model well describes these three interactions, but gravity remains unexplained. Using the standard model, an interaction is described as an exchange of bosons between the objects affected, such as a photon for the electromagnetic force and a gluon for the strong interaction. Those particles are called force carriers or messenger particles.

Interactions in physics are the ways that particles influence other particles. The Standard Model explains such forces as resulting from matter particles exchanging other particles, generally referred to as force mediating particles. When a force-mediating particle is exchanged, the effect at a macroscopic level is equivalent to a force influencing both of them, and the particle is therefore said to have mediated (i.e., been the agent of) that force.

This is an important concept to understand because it creates obvious discrepancies with what we know of the structure of force and matter.

Goo : Do tachyons exist?
Erasmus : Tachyons have never been found in experiments as real particles traveling through the vacuum, but we predict theoretically that tachyon-like objects exist as faster-than-light 'quasiparticles' moving through laser-like media.

A tachyon or tachyonic particle is a hypothetical particle that always travels faster than light. Physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. If such particles did exist they could be used to send signals faster than light. According to the theory of relativity this would violate causality, leading to logical paradoxes such as the grandfather paradox. Tachyons would exhibit the unusual property of increasing in speed as their energy decreases, and would require infinite energy to slow down to the speed of light. No verifiable experimental evidence for the existence of such particles has been found.

Goo : But quantum effects such as with entangled photons can certainly bypass the speed of light. They are in effect instantaneous.

Kinkajou : True. But they still do not travel beyond the speed of light. Effects do travel in excess of light speed though. This is the basis of some of the famous 'Schrodinger Cat' paradox. It can't be there and it can't be happening, but it does and is.

Erasmus : At a macroscopic level, electromagnetism allows particles to interact with one another via electric and magnetic fields. The gauge bosons of the Standard Model all have spin (as do matter particles). The value of the spin is 1, making them bosons. As a result, they do not follow the Pauli Exclusion Principle that constrains fermions: thus bosons (e.g. photons) do not have a theoretical limit on their spatial density (number per volume).

Erasmus : The types of gauge bosons are described below.
* Photons mediate the electromagnetic force between electrically charged particles. The photon is massless (By convention definition) and is well-described by the theory of quantum electrodynamics.

* The W+,  W- , and  Z gauge bosons mediate the weak interactions between particles of different flavours (all quarks and leptons). They are massive, with the Z being more massive than the W±. The weak interactions involving the W± act only on left-handed particles and right-handed antiparticles. The W± carries an electric charge of +1 or -1 and couples to the electromagnetic interaction. The electrically neutral Z boson interacts with both left-handed particles and antiparticles. These three gauge bosons are grouped together, as collectively mediating the electroweak interaction.

* The eight gluons mediate the strong interactions  between colour particles (the quarks).
Gluons are massless. The eightfold multiplicity of gluons is labelled by a combination of colour and antichlor charge (e.g. red–antigreen). Because gluons have an effective colour charge, they can also interact among themselves. Gluons and their interactions are described by the theory of quantum chromodynamics.

Kinkajou : Why Are Some of the Force Carriers Described with Colours? Is This for Real?

Erasmus : The strong interaction is mediated by the exchange of massless particles called gluons that act between quarks, antiquarks, and other gluons. Gluons are thought to interact with quarks and other gluons by way of a type of charge called colour. Colour charge is analogous to electromagnetic charge, but it comes in three types (±red, ±green, ±blue), which results in a different types of force, with different rules of behaviour. These rules are detailed in the theory of quantum chromodynamics (QCD), which is the theory of quark–gluon interactions.

In theoretical physics, quantum chromodynamics (QCD) is a type of quantum field theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. As we’re stated before the proton is composed of two up quarks and one down quark. Similarly a neutron is imposed of two down quarks and one up quark). Mesons are particles composed of two quarks rather than the three quarks typically present in the basic hadrons (protons and neutrons) which make up the universe.

Gluons and the Colour Force

Erasmus :  The QCD analog of electric charge is a property called colour. Gluons are the force carriers of the theory, just as photons are for the electromagnetic force in quantum electrodynamics.  QCD exhibits three salient properties:

* Colour confinement. Due to the force between two colour charges remaining constant as they are separated, the energy grows until a quark–antiquark pair is spontaneously produced, turning the initial hadron into a pair of hadrons instead of isolating a colour charge.

* Asymptotic freedom, a steady reduction in the strength of interactions between quarks and gluons as the energy scale of those interactions increases (and the corresponding length scale decreases).

Asymptotic freedom is a feature of quantum chromodynamics (QCD), the quantum field theory of the strong interaction between quarks and gluons, the fundamental constituents of nuclear matter. Quarks interact weakly at high energies. At low energies, the interaction becomes strong, leading to the confinement of quarks and gluons within composite hadrons.

* Chiral symmetry breaking, the spontaneous symmetry breaking of an important global symmetry of quarks, with the result of generating masses for hadrons far above the masses of the quarks, and making pseudoscalar mesons exceptionally light. 

Goo :  The last force is gravitation which allows particles with mass to attract one another in accordance with Einstein's theory of general relativity.

Kinkajou : What is the Higgs Boson? What Role does it Play?

Erasmus :  A massive spin-zero particle, was proposed as the Higgs boson, and is a key building block in the Standard Model. It has no intrinsic spin, and for that reason is classified as a boson (like the gauge bosons, which have integer spin).
As it turns out, the down quarks interact more strongly with the Higgs [field], so they have a bit more mass. This is why the tiny difference between proton and neutron mass exists. 

The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except the photon and gluon, are massive. In particular, the Higgs boson explains why the photon has no mass, while the W and Z bosons are very heavy.

In electroweak theory, the Higgs boson generates the masses of the leptons (electron, Muon, and tau) and quarks. As the Higgs boson is massive, it must interact with itself.

Because the Higgs boson is a very massive particle and also decays almost immediately when created, only a very high-energy particle accelerator can observe and record it.

Experiments to confirm and determine the nature of the Higgs boson using the Large Hadron Collider (LHC) at CERN began in early 2010

On 4 July 2012, two of the experiments at the LHC (ATLAS and CMS) both reported independently that they found a new particle with a mass of about 125 GeV/c2 (about 133 proton masses, on the order of 10×10E-25 kg), which is "consistent with the Higgs boson". On 13 March 2013, it was confirmed to be the searched-for Higgs boson.

Goo : Many laymen call it the “God” particle due to our belief in its ubiquity in the creation of matter in the early stages of the Big Bang.

Kinkajou : Where does mass come from?
Erasmus :  The Higgs field gives mass to elementary particles, but most of the mass comes from somewhere else.  Where??

The Higgs field gives mass to fundamental particles—the electrons, quarks and other building blocks that cannot be broken into smaller parts. But these still only account for a tiny proportion of the universe’s mass.

The rest comes from protons and neutrons, which get almost all their mass from the strong nuclear force. These particles are each made up of three quarks moving at breakneck speeds that are bound together by gluons, the particles that carry the strong force. The energy of this interaction between quarks and gluons is what gives protons and neutrons their mass.

Keep in mind Einstein’s famous E=mc2, which equates energy and mass. That makes mass a secret storage facility for energy. When you put three quarks together to create a proton, you end up binding up an enormous energy density in a small region in space. 

Kinkajou : But what about neutrinos?
Erasmus :  Neutrinos must get their mass from a Higgs-like field, which is electrically neutral and spans the entire universe. This could be the same Higgs Field that gives mass to the other elementary particles, or it could be a very distant cousin. In some theories, neutrino mass also comes from an additional, brand new source that could hold the answers to other lingering particle physics mysteries.

This new mechanism may also be related to how dark matter, which physicists think is made up of yet undiscovered particles, gets its mass.
It’s possible that there is a new set of particles that explain all of these weird phenomena that we haven't explained yet.

Gravity Lensing of Photons

Kinkajou : Tell Us About Weak Interactions and Decay

Erasmus :  An interaction occurs when two particles (typically but not necessarily half-integer spin fermions) exchange integer-spin, force-carrying bosons. The fermions involved in such exchanges can be either elementary (e.g. electrons or quarks) or composite (e.g. protons or neutrons), although at the deepest levels, all weak interactions ultimately are between elementary particles.

In the weak interaction, fermions can exchange three types of force carriers, namely W+, W+-, and Z bosons. The masses of these bosons are far greater than the mass of a proton or neutron, which is consistent with the short range of the weak force. In fact, the force is termed weak because its field strength over a given distance is typically several orders of magnitude less than that of the strong nuclear force or electromagnetic force.

Erasmus :  Other important examples of phenomena involving the weak interaction include beta decay, and the fission of uranium, plutonium or thorium in typical nuclear power plants. . Most fermions decay by a weak interaction over time. Such decay makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14. It can also create radioluminescence.

 The weak interaction has a coupling constant (an indicator of interaction strength) of between 10E-7 and 10E-6, compared to the strong interaction's coupling constant of 1 and the electromagnetic coupling constant of about 10E-2; consequently the weak interaction is ‘weak’ in terms of strength. The weak interaction has a very short effective range (around 10E-17 to 10E-16 m  At distances around 10E-18 meters, the weak interaction has a strength of a similar magnitude to the electromagnetic force, but this starts to decrease exponentially with increasing distance. Scaled up by just one and a half orders of magnitude, at distances of around 3×10E-17 m, the weak interaction becomes 10,000 times weaker

The weak interaction affects all the fermions of the Standard Model, as well as the Higgs boson;

Erasmus :  Neutrinos interact only through gravity and the weak interaction.
The weak interaction does not produce bound states nor does it involve binding energy – something that gravity does on an astronomical scale, that the electromagnetic force does at the atomic level, and that the strong nuclear force does inside nuclei.

Erasmus :   In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction.

Kinkajou : I Heard that Neutrons that make up most of the matter in the Universe are unstable. That can't be true, can it?

Erasmus :  Along with protons, neutrons make up the nucleus, held together by the strong force. The neutron is a baryon and is considered to be composed of two down quarks and one up quark.
A free neutron will decay with a half-life of about 10.3 - 14.9 minutes but it is stable if combined into a nucleus. The decay of the neutron is associated with a quark transformation in which a down quark is converted to an up by the weak interaction. This decay is an example of beta decay with the emission of an electron and an electron antineutrino. The neutron is about 0.2% more massive than a proton.

Erasmus :  The average lifetime of 14.9 minutes is very long for a particle decay reaction that yields 1.29 MeV of energy. You would expect that this decay is steeply "downhill" in energy and would be expected to proceed rapidly.

It is possible for a proton to be transformed into a neutron, but you have to supply 1.29 MeV of energy to reach the threshold for that transformation. In the very early stages of the big bang when the background thermal energy was much greater than 1.29 MeV, we surmise that the transformation between protons and neutrons was proceeding freely in both directions so that there was an essentially equal population of protons and neutrons.

A more detailed diagram of the neutron's decay identifies it as the transformation of one of the neutron's down quarks into an up quark. It is an example of the kind of quark transformations that are involved in many nuclear processes, including beta decay.
The decay of the neutron is a good example of the observations which led to the discovery of the neutrino.

Momentum and energy for the two-particle decay are constrained to these values.

The fact that the electrons produced from the neutron decay had continuous distributions of energy and momentum was a clear indication that there was another particle emitted along with the electron and proton. It had to be a neutral particle and in certain decays carried almost all the energy and momentum of the decay. This would not have been so extraordinary except for the fact that when the electron had its maximum kinetic energy, it accounted for all the energy Q available for the decay. So there was no energy left over to account for the mass energy of the other emitted particle. The early experimenters were faced with the dilemma of a particle which could carry nearly all the energy and momentum of the decay but which had no charge and apparently no mass!

The mysterious particle was called a neutrino. The present understanding of the decay of the neutron is

This decay illustrates some of the conservation laws which govern particle decays. The proton in the product satisfies the conservation of baryon number, but the emergence of the electron unaccompanied would violate conservation of lepton number. The third particle must be an electron antineutrino to allow the decay to satisfy lepton number conservation. The electron has lepton number 1, and the antineutrino has lepton number -1.

Erasmus :  Protons and Neutrons
Along with neutrons, protons make up the nucleus, held together by the strong force. The proton is a baryon and is considered to be composed of two up quarks and one down quark.
It has long been considered to be a stable particle, but recent developments of grand unification models have suggested that the proton might decay with a half-life of about 10E32 years. 

When we say that a proton is made up of two up quarks and a down, we mean that its net appearance or net set of quantum numbers match that picture. The nature of quark confinement suggests that the quarks are surrounded by a cloud of gluons, and within the tiny volume of the proton other quark-antiquark pairs can be produced and then annihilated without changing the net external appearance of the proton.

Weak isospin is usually given the symbol T or I, with the third component written as T3 or I3. T3 is more important than T. It can be understood as the eigenvalue of a charge operator. Geometrically, an eigenvector, corresponding to a real nonzero eigenvalue, points in a direction in which it is stretched by the transformation and the eigenvalue is the factor by which it is stretched. If the eigenvalue is negative, the direction is reversed. Loosely speaking, in a multidimensional vector space, the eigenvector is not rotated.

In the weak isospin conservation law, weak interactions conserve T3.
It is also conserved by the electromagnetic and strong interactions.
However, interaction with the Higgs field does not conserve T3.

The weak interaction is the only fundamental interaction that breaks parity-symmetry, and similarly, the only one to break charge parity symmetry.

Goo : Awesomely Confusing. I think some of this complex stuff is here just to introduce the concepts. So if someone ,mentions things about matter - you've at least heard a little bit about most of everything.

But is there More?

Kinkajou : What is Chirality?

Erasmus :  Where "left"- and "right"-handed here are left and right chirality, respectively (distinct from helicity). The weak hypercharge for an anti-fermion is the opposite of that of the corresponding fermion because the electric charge and the third component of the weak isospin - reverse sign under charge conjugation.

8 Gluons

I think the picture says much more than the words. So let's move on.

Kinkajou : Tell us About the Strong Interaction

Erasmus :  In nuclear physics and particle physics, the strong interaction is the mechanism responsible for the strong nuclear force, and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation.

 At the range of 10E-15 m (1 femtometer), the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction, and 10E38 times as strong as gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds these neutrons and protons to create atomic nuclei.

Erasmus :  Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton.

The strong interaction is observable at two ranges and mediated by two force carriers. On a larger scale (about 1 to 3 fm), it is the force (carried by mesons) that binds protons and neutrons (nucleons) together to form the nucleus of an atom. On the smaller scale (less than about 0.8 fm, the radius of a nucleon), it is the force (carried by gluons) that holds quarks together to form protons, neutrons, and other hadron particles in the latter context, it is often known as the colour force.

 The strong force inherently has such a high strength that hadrons bound by the strong force can produce new massive particles. Thus, if hadrons are struck by high-energy particles, they give rise to new hadrons instead of emitting freely moving radiation (gluons). This property of the strong force is called colour, and it prevents the free "emission" of the strong force: instead, in practice, jets of massive particles are produced.

Erasmus :   In the context of atomic nuclei, the same strong interaction force (that binds quarks within a nucleon) also binds protons and neutrons together to form a nucleus. In this capacity it is called the nuclear force (or residual strong force). So the residuum from the strong interaction within protons and neutrons also binds nuclei together.

 As such, the residual strong interaction obeys a distance-dependent behaviour between nucleons that is quite different from that when it is acting to bind quarks within nucleons.

Kinkajou : Additionally, distinctions exist in the binding energies of the nuclear force of nuclear fusion vs. nuclear fission. Nuclear fusion accounts for most energy production in the Sun and other stars. Nuclear fission allows for decay of radioactive elements and isotopes. It is largely mediated by the weak interaction.

Erasmus :  The strong nuclear force acts between hadrons, such as mesons and baryons.

This "residual strong force", acting indirectly, transmits gluons that form part of the virtual Pi and Rho mesons, which, in turn, transmit the force between nucleons that holds the nucleus (beyond protium) together.

The residual strong force is thus a minor residuum of the strong force that binds quarks together into protons and neutrons. This same force is much weaker between neutrons and protons, because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (van der Waals forces) are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms.

The rapid decrease with distance of the strong  attractive residual force and the less-rapid decrease of the repulsive electromagnetic force acting between protons within a nucleus, causes the instability of larger atomic nuclei, such as all those with atomic numbers larger than 82 (the element lead).