# Particle Physics 2

## Fundamental Forces

The first two you are familiar with, gravity is the attractive force between all matter, electromagnetic force describes the interaction of charged particles and magnetics. Light (photons) is explained by the interaction of electric and magnetic fields.
The strong force binds quarks into protons, neutrons and mesons, and holds the nucleus of the atom together despite the repulsive electromagnetic force between protons. The weak force controls the radioactive decay of atomic nuclei and the reactions between leptons (electrons and neutrinos).

Current physics (called quantum field theory) explains the exchange of energy in interactions by the use of force carriers, called bosons. The long range forces have zero mass force carriers, the graviton and the photon. These operate on scales larger than the solar system. Short range forces have very massive force carriers, the W+, W- and Z for the weak force, the gluon for the strong force. These operate on scales the size of atomic nuclei.

So, although the strong force has the greatest strength, it also has the shortest range.

## Bosons (Force Carriers)

Bosons are the particles which transmits the different forces between the matter particles, they normally have a whole number spin, 0, 1 or 2. And Fermions which are matter particles they often have spin 1/2. Real particles are the ones you are familiar with, all Fermions are real particles. The Bosons can sometimes be virtual and sometimes real. Virtual particles are the particles which transmits the force between the particles, e.g. virtual photon carries the electromagnetic force between e.g. electrons. They are called virtual particles because they can't be directly detected, you can't 'see' them so to speak. But their effect can be noticed, by e.g. the actual forces between particles.

## Higgs Field and Boson

In the Standard Model of particle physics, the fundamental forces of nature known to science arise from laws of nature called symmetries, and are transmitted by particles known as gauge bosons. The weak force's symmetry should cause its gauge bosons to have zero mass, but experiments show that the weak force's gauge bosons are actually very massive and short ranging (now called W and Z bosons). Their very short range – a result of their mass – makes structures like atoms and stars possible, but it proved exceedingly difficult to find any way to explain their unexpected mass.

By the early 1960s, physicists had realized that a given symmetry law might not always be followed (or 'obeyed') under certain conditions. The Higgs mechanism is a mathematical model devised by three groups of researchers in 1964 that explains why and how gauge bosons could still be massive despite their governing symmetry. It showed that the conditions for the symmetry would be 'broken' if an unusual type of field happened to exist throughout space, and then the particles would be able to have mass.

According to the Standard Model, a field of the necessary kind (the "Higgs field") exists throughout space, and breaks certain symmetry laws of the electroweak interaction. The existence of this field triggers the Higgs mechanism, causing the gauge bosons responsible for the weak force to be massive, and explaining their very short range.

Some years after the original theory was articulated scientists realized that the same field would also explain, in a different way, why other fundamental constituents of matter (including electrons and quarks) have mass.

For many years’ scientists had no way to tell whether or not a field of this kind actually existed in reality. If it existed, it would be unlike any other fundamental field known in science. But it was also possible that these key ideas, or even the entire Standard Model itself, were somehow incorrect.Only discovering what was breaking this symmetry would solve the problem.

The existence of the Higgs field – the crucial question – could be proven by searching for a matching particle associated with it, which would also have to exist—the "Higgs boson". Detecting Higgs bosons would automatically prove that the Higgs field exists, which would show that the Standard Model is essentially correct. But for decades scientists had no way to discover whether Higgs bosons actually existed in nature either, because they would be very difficult to produce, and would break apart in about a ten-sextillionth (10−22) of a second. Although the theory gave "remarkably" correct answers, particle colliders, detectors, and computers capable of looking for Higgs bosons took over 30 years (c. 1980 – 2010) to develop.

As of 2013, scientists are virtually certain that they have proven the Higgs boson exists, and therefore that the concept of some type of Higgs field throughout space is proven.

In Summary

The universe as we know it is truly filled with fields and what we call ‘particles’ is localized disturbances within these fields. Each particle of matter that we observe has a specific field attributed to it.

In order for nature to be consistent over time and distance (i.e. the laws of nature remain unchanged and behave the same throughout all of space and all of time) Physicists invoke the ‘Law of Symmetry’, Symmetry is what allows nature to behave consistently from place to place and from time to time. Physicists soon realized when developing the ‘Standard Model’ of Quantum Mechanics that in order to preserve the symmetries within nature, they had to add in additional types of fields (and the corresponding ‘particles’) as force carriers. These force carriers (and the corresponding particles) have become known as the Photon, Gluon and the Z&W Bosons. Unfortunately, there was a big problem with this model; these force carriers had zero mass according to the math. Why was this a problem? In order to preserve symmetry, these force carriers were required. However, these force carriers must be massless while ‘real world’ observation clearly indicated some of these carriers in fact have a quantifiable mass. Enter the Higgs field; a field, which permeates all of space and it’s the interaction with this field that would give these particles their observed mass.

## Baryons and Mesons

Quarks combine to form the basic building blocks of matter, baryons and mesons. Baryons are made of three quarks to form the protons and neutrons of atomic nuclei (and also anti-protons and anti-neutrons). Mesons, made of quark pairs, are usually found in cosmic rays. Notice that the quarks all combine to make charges of -1, 0, or +1.

Thus, our current understanding of the structure of the atom is shown below, the atom contains a nucleus surrounded by a cloud of negatively charged electrons. The nucleus is composed of neutral neutrons and positively charged protons. The opposite charge of the electron and proton binds the atom together with electromagnetic forces.

The protons and neutrons are composed of up and down quarks whose fractional charges (2/3 and -1/3) combine to produce the 0 or +1 charge of the proton and neutron. The nucleus is bound together by the nuclear strong force (that overcomes the electromagnetic repulsion of like-charged protons)

Quarks in baryons and mesons are bound together by the strong force in the form of the exchange of gluons. Much like how the electromagnetic force strength is determined by the amount of electric charge, the strong force strength is determined by a new quantity called color charge.

Quarks come in three colors, red, blue and green (they are not actually colored, we just describe their color charge in these terms). So, unlike electromagnetic charges which come in two flavors (positive and negative or north and south poles), color charge in quarks comes in three types. And, just to be more confusing, color charge also has its anti-particle nature. So there is anti-red, anti-blue and anti-green.

Gluons serve the function of carrying color when they interact with quarks. Baryons and mesons must have a mix of colors such that the result is white. For example, red, blue and green make white. Also red and anti-red make white.

There can exist no free quarks, i.e. quarks by themselves. All quarks must be bound to another quark or antiquark by the exchange of gluons. This is called quark confinement. The exchange of gluons produces a color force field, referring to the assignment of color charge to quarks, similar to electric charge.

The color force field is unusual in that separating the quarks makes the force field stronger (unlike electromagnetic or gravity forces which weaken with distance). Energy is needed to overcome the color force field. That energy increases until a new quark or antiquark is formed (energy equals mass, E=mc2).

Two new quarks form and bind to the old quarks to make two new mesons. Thus, none of the quarks were at anytime in isolation. Quarks always travel in pairs or triplets.

## Quantum Electrodynamics

The subfield of physics that explains the interaction of charged particles and light is called quantum electrodynamics. Quantum electrodynamics (QED) extends quantum theory to fields of force, starting with electromagnetic fields.
Quantum electrodynamics, or QED, is a quantum theory of the interactions of charged particles with the electromagnetic field. It describes mathematically not only all interactions of light with matter but also those of charged particles with one another. QED is a relativistic theory in that Albert Einstein's theory of special relativity is built into each of its equations. Because the behavior of atoms and molecules is primarily electromagnetic in nature, all of atomic physics can be considered a test laboratory for the theory. Agreement of such high accuracy makes QED one of the most successful physical theories so far devised.

In 1926 the British physicist P.A.M. Dirac laid the foundations for QED with his discovery of an equation describing the motion and spin of electrons that incorporated both the quantum theory and the theory of special relativity. The QED theory was refined and fully developed in the late 1940s by Richard P. Feynman, Julian S. Schwinger, and Shin'ichiro Tomonaga, independently of one another. QED rests on the idea that charged particles (e.g., electrons and positrons) interact by emitting and absorbing photons, the particles of light that transmit electromagnetic forces. These photons are virtual; that is, they cannot be seen or detected in any way because their existence violates the conservation of energy and momentum. The particle exchange is merely the "force" of the interaction, because the interacting particles change their speed and direction of travel as they release or absorb the energy of a photon. Photons also can be emitted in a free state, in which case they may be observed. The interaction of two charged particles occurs in a series of processes of increasing complexity. In the simplest, only one virtual photon is involved; in a second-order process, there are two; and so forth. The processes correspond to all the possible ways in which the particles can interact by the exchange of virtual photons, and each of them can be represented graphically by means of the diagrams developed by Feynman.

Besides furnishing an intuitive picture of the process being considered, this type of diagram prescribes precisely how to calculate the variable involved.

Under QED, charged particles interact by the exchange of virtual photons, photons that do not exist outside of the interaction and only serve as carriers of momentum/force.

Notice the elimination of action at a distance, the interaction is due to direct contact of the photons.
In the 1960's, a formulation of QED led to the unification of the theories of weak and electromagnetic interactions. This new force, called electroweak, occurs at extremely high temperatures such as those found in the early Universe and reproduced in particle accelerators. Unification means that the weak and electromagnetic forces become symmetric at this point, they behave as if they were one force.

Electroweak unification gave rise to the belief that the weak, electromagnetic and strong forces can be unified into what is called the Standard Model of matter.

## Quantum Chromodynamics

Quantum chromodynamics is the subfield of physics that describes the strong or "color'' force that binds quarks together to form baryons and mesons, and results in the complicated force that binds atomic nuclei together.
Quantum chromodynamics, or QCD, is the theory that describes the action of the strong nuclear force. QCD was constructed on analogy to quantum electrodynamics (QED), the quantum theory of the electromagnetic force. In QED, the electromagnetic interactions of charged particles are described through the emission and subsequent absorption of massless photons, best known as the "particles" of light; such interactions are not possible between uncharged, electrically neutral particles. The strong force is observed to behave in a similar way, acting only upon certain particles, principally quarks that are bound together in the protons and neutrons of the atomic nucleus, as well as in less stable, more exotic forms of matter. So by analogy with QED, quantum chromodynamics has been built upon the concept that quarks interact via the strong force because they carry a form of "strong charge," which has been given the name of color; other particles, such as the electron, which do not carry the color charge, do not interact in this way.

In QED there are only two values for electric charge, positive and negative, or charge and anticharge. To explain the behavior of quarks in QCD, by contrast, there need to be three different types of color charge, each of which can occur as color or anticolor. The three types of charge are called red, green, and blue in analogy to the primary colors of light, although there is no connection whatsoever with color in the usual sense.

Color-neutral particles occur in one of two ways. In baryons (i.e., particles built from three quarks, as, for example, protons and neutrons), the three quarks are each of a different color, and a mixture of the three colors produces a particle that is neutral. Mesons, on the other hand, are built from pairs of quarks and antiquarks, and in these the anticolor of the antiquark neutralizes the color of the quark, much as positive and negative electric charges cancel each other to produce an electrically neutral object.

Quarks interact via the strong force by exchanging particles called gluons. In contrast to QED, where the photons exchanged are electrically neutral, the gluons of QCD also carry color charges. To allow all the possible interactions between the three colors of quarks, there must be eight gluons, each of which generally carries a mixture of a color and an anticolor of a different kind.
Because gluons carry color, they can interact among themselves, and this makes the behavior of the strong force subtly different from the electromagnetic force. QED describes a force that becomes weaker as the distance between two charges increases (obeying an inverse square law), but in QCD the interactions between gluons emitted by color charges prevent those charges from being pulled apart. Instead, if sufficient energy is invested in the attempt to knock a quark out of a proton, for example, the result is the creation of a quark-antiquark pair--in other words a meson.

## Theory of Everything

Is that it? Are quarks and leptons the fundamental building blocks? Answer = maybe. We are still looking to fill some holes in what is know as the Standard Model.
The Standard Model is a way of making sense of the multiplicity of elementary particles and forces within a single scheme. The Standard Model is the combination of two schemes; the electroweak force (unification of electromagnetism and weak force) plus quantum chromodynamics. Although the Standard Model has brought a considerable amount of order to elementary particles and has led to important predictions, the model is not without some serious difficulties.

For example, the problem of quantum gravity is unresolved in the Standard Model. Also undefined are the values of the various constants of Nature (the speed of light, charge on the electron, etc.). In fact, why the Universe is built the way it is (3D+1D macroscopic world) is unclear, although various other dimensional shapes can be ruled out as shown by the diagram below.

Another problem is that the Standard Model contains a large number of arbitrary constants. Good choice of the constants leads to exact matches with experimental results. However, a good fundamental theory should be one where the constants are self-evident. For example, the choice of 4 spacetime dimensions is understood under the Standard model since other choices are ruled out by mathematics.

The Standard Model does not include the unification of all forces and, therefore, is incomplete. There is a strong expectation that there exists a Grand Unified Field Theory (GUTS) that will provide a deeper meaning to the Standard Model and explain the missing elements.