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Introduction To Particle Physics

The history of particle physics is long and complex, with contributions from many different characters in many different places. This article presents an overview of the main steps that led to our current understanding of nature at the most fundamental level. This information is by no means complete, but aims to at least give an idea of where we are today and how we got here.

"You have to learn the rules of the game. And then you have to play better than anyone else."

--- A. Einstein


1) Elemental

High Energy Physics (also known as "particle physics") is the study of matter and interactions on the smallest scales. We seek to find that which is truly fundamental, which cannot be deconstructed into smaller parts or described in simpler terms. The ultimate goal is therefore none other than to find and understand the purest nature of reality.

People have long sought to classify the universe in terms of components. From ancient times there were the "classical elements", sometimes referred to simply as the "elements", namely the loose concepts of "air", "water", "earth" and "fire", which were much later generalized in scientific terms as "gas", "liquid", "solid" and "energy". Perhaps the most significant idea put forward by early thinkers was that of the "atom", which is a Greek word that literally means "indivisible". Before this idea, it was thought that the pieces of the world around us could be continuously cut and subdivided into ever smaller pieces. The proposal of the atom was that there was some ultimate building block level that could not be divided further, with pieces that could somehow be combined differently to give the different substances we see on our own scale.

As the scientific method developed and careful experiments were done on the materials found in nature, a number of substances were found that could not be further refined into component substances, each therefore being seemingly pure and unique. In terms of the atomic idea, it was considered that each of these pure substances had its own unique atom on some smallest scale. Such substances came to be known as the "chemical elements", or simply the "elements", so replacing the ancient use of the word.

Studies eventually revealed many tens of elements, and while they were all apparently unrelated, they could nonetheless be grouped in terms of similar properties. A formal categorization led to the creation of the "periodic table" in the mid-19th century, which remains a true masterpiece of the scientific method. However, in an ironic twist to the story, the fact that the elements could be periodically arranged in terms of property suggested that they were not truly fundamental, but that they actually had ingredients, and that any similarities were due to similarities in their ingredients. In other words, the atoms of the elements seemed not to be "atomic" in terms of the original meaning of the word. There was a deeper level to be found.


2) Atomic

Experiments into electricity began to lead the way to the next stage of understanding. It has been known for many hundreds of years that objects can be given "electric charge", which can be made to move as "electricity". It was discovered that metals are particularly good at conducting electricity, and, importantly, that the metal itself does not appear to move when this happens i.e. the atoms of the metal stay put. So whatever the charge is must be smaller than the atoms themselves. In 1897 this idea was proven experimentally, and the particle of electricity came to be called the "electron". This was the birth of subatomic particle physics.

The last century has seen this topic grow rapidly, with theoretical and experimental discoveries being incorporated into new technology that enables further discoveries, and so on. It was realized that atoms could not be comprised entirely of electrons, as all electrons have the same type of electric charge which causes them to be forced apart. An atom must therefore also have enough particles with opposite electric charge to the electron in order to balance the electric forces and keep the atom intact. Experiments in 1917-1919 led to the discovery of a particle that satisfied this requirement, which came to be called the "proton". However, even though the electrons and protons together balance the charge of an atom, they do not usually add to the total mass of an atom. A massive but electrically neutral particle was therefore predicted soon after the discovery of the proton, which was subsequently discovered about a decade later in 1932 and dubbed the "neutron".

The properties of all atoms, and so the patterns of the periodic table, can be explained in terms of electrons, protons and neutrons. An atom of one element is distinguished from an atom of another element by the number of protons it has, which in turn defines the number of electrons it has in order to balance the electric charge. The protons exist in a central nucleus along with the neutrons, which together provide an atom with most of its mass as the electrons are much lighter. The electrons themselves exist in specific orbits around the nucleus, and the number of electrons and their orbital structure determine how the atom behaves chemically.

Despite answering questions about the periodic table of elements, the nuclear model of the atom generated its own set of questions. How come the electron and proton have electric charge, but not the neutron? How come the protons and neutrons exist together in the nucleus, despite all of the protons repelling each other electrically? How come neutrons are not seen on their own outside of the nucleus? And on and on.


3) Down To Earth

Around the same time as the discovery of the neutron, a number of completely new particles were also discovered that did not fit into the atomic picture at all. These particles were unstable, existing only fleetingly before disintegrating, and were only seen in sensitive detector experiments that were not possible when the chemical elements were being catalogued. Such particles were therefore completely unknown and of no concern at all to earlier scientists, but added to the list of questions present in the 1930's.

On the theoretical side, the best mathematical description of the electron suggested that it should be possible to have something just like an electron but with opposite electric charge, in other words just like a proton but much less massive and free of any nucleus. Lo and behold, a particle with these exact properties was then discovered, making it the first to be predicted from a mathematical theory. The electric charge of the electron had been labelled as "negative", with that of the proton as "positive", and so the new positive partner to the electron was called the "positron", which was thought of as an anti-electron. This began the notion of "antiparticles" as a general concept.

One by one new particles were found. At first they were discovered coming naturally from space, in so-called "cosmic rays", but it was later realized that they could be artificially created. One of Einstein's great achievements showed that mass is another type of energy, and so, as energy can be converted from one type to another, it was determined that mass can actually be generated. By taking known stable particles and colliding them at high speed, the kinetic energy they carry can sometimes be forced to turn into mass, or in other words forced to turn into new particles. Higher energy collisions allow for the creation of either more particles of lesser mass, or fewer particles of greater mass. As long as the total energy adds up before and after the collision, anything goes. In this way, generation after generation of particle accelerator has been constructed, operating at ever higher energy, and particle after particle has been discovered.


4) The Zoo

While discovering things is great fun, having no understanding of what you're finding is no fun at all. There was no clear pattern to the particle "zoo" that was being populated, and so no idea why nature would allow such things that were unstable and disappeared. That's not to say they disappeared without a trace, but rather the heavier particles always disintegrated into some set of lighter particles, all the way down to the very lightest stable particles, such as the electron.

After many years of experimental discovery, theory finally seemed to catch up with some potential answers in the 1960's. More particles meant more data, and more data meant that sooner or later a pattern was likely to be found, and that is exactly what happened. Just as the chemical elements were discovered and categorized by property, leading to the periodic table and the notion of atomic substructure, the new particles could also be categorized too - but only when certain new properties were theorized to exist, which were based on speculative concepts. At the heart of the new theory was, as might be expected at this point, further substructure.

In the atomic case, it was the patterns of the periodic table, and the phenomena of charge and electricity, which suggested some kind of atomic substructure, and the observation of the electron then confirmed it. In the case of the new entities created in accelerators, despite patterns that suggested further substructure, there was no sign of a telltale particle that did the same confirmation job as the electron had done decades earlier. Many of the new particles certainly seemed to be made of smaller pieces based on the patterns, but nothing that could be a single one of those pieces was ever seen on its own.


5) Strength To Strength

Physics is determined by forces and things that are affected by forces, which gives rise to particular behaviour. Gravity, for example, causes particles with mass to accelerate towards each other. In other words, mass is the property a particle can have, the "gravitational charge", which allows it to be affected by the force of gravity, which causes the accelerating behaviour. Different kinds of particle behaviour indicate different particle properties related to different forces.

It was the patterns in the particle data, from the time the "zoo" was being populated, that revealed the nature of the underlying forces at work. It was clear that not all particles obeyed the same forces, meaning that some had properties that others did not. This was understood even from the time when just electrons and protons were known, as a group of protons could somehow overcome their mutual electric repulsion and coexist as a group, which was not a feature of electrons. Protons, therefore, had some property that allowed them to obey some force that was even stronger than the electric force. This force came to be known, imaginatively, as the "strong force".

The electric force only has two types of electric charge, which is the property a particle must have in order for it to obey the electric force. As mentioned, these charges are labelled "negative" (as carried by electrons) and "positive" (as carried by protons and positrons). Like charges repel, and unlike charges attract. Simple. It was determined that, whereas the electric charges cancel in pairs, the strong charges must cancel either in pairs or in triplets. This calls for six types of charge rather than two, and this complexity is one of the reasons that the particle zoo was so difficult to interpret.

The electric charges were labelled as "negative" and "positive" for the very reason that these are mathematical words that provide an image of two-way cancellation to a "neutral zero". A set of mathematical words that provide an image of three-way cancellation is not possible, but this is possible with the colour words "red", "green" and "blue", as together these combine to give a "neutral white" for the strong force triplet combination. That's not to say that a particle with strong charge actually looks "red", "green" or "blue", these are just labels that work well to describe the force. The strong force pair combination is described by the further notion of each colour having an opposite with which it cancels directly, and so we have "anti-red", "anti-green" and "anti-blue", which completes the set of six strong charges mentioned above.

So why was all this needed to explain the observations? Well, interestingly, the new particles that seemed to have substructure could only be explained in terms of constituents that obeyed the strong force. These constituents, which were never seen on their own, were called "quarks", and it was reasoned that each single quark must carry a single strong charge. A number of particles could be explained as being made of two oppositely coloured quarks bound together, for example red plus anti-red, whereas others could be explained as being made of three differently coloured quarks bound together, either red plus green plus blue, or the equivalent addition of all the anti-colours.

There is one further twist to the strong force that differentiates it further from the electric force, and which is a key characteristic. The electric force allows electric charges to be free of their cancelling opposite, so we may take an electron on its own and play with it separately from a proton. But the strong force appears to demand absolute neutrality at all times, and as such it does not allow for single quarks to be pulled out from some composite particle and inspected individually. If you try to do so, the job will always require at least enough energy to create yet more quarks, which will have strong charges that cancel those of the pieces you pulled apart. It is said that "bare" colour can never be seen.

The complexity and subtlety of the strong force was the very reason that many years' worth of particle data took so long to interpret. Researchers had expected to be able to smash particles down to their very building blocks and inspect them individually, but in reality many of these building blocks were quarks and so could not be seen in isolation due to the strong force neutrality condition. We now know of six unique quarks, three of which carry positive electric charge and three of which carry negative electric charge. All can carry any of the red, green or blue strong charges, and all have a counterpart antiparticle with the opposite electric and strong charges. This all allows for a multitude of various combinations.


6) Bias

Table of quarks and leptons
Quarks and leptons are all types of "fundamental fermion"

Despite there being many particles made of quarks, there are others which are not. Such particles do not carry strong charge and so do not obey the strong force, and they are referred to as "leptons". Both the electron and positron are leptons, for example, yet there are others which don't even carry electric charge called "neutrinos". As far as we can presently tell, all leptons are truly fundamental, so have no internal constituents whatsoever, simply being points in space with no spatial extent but which carry certain properties that make them more than nothing at all.

Quarks and leptons together explain the entire zoo of material particles, and are together referred to as the "fundamental fermions". There are three so-called "generations" of fundamental fermion, each having two quarks and two leptons, and each being the same as the next generation except for a difference in mass. There is no current evidence for a fourth generation, but the reason why nature is capped at three is currently unknown.

Each fundamental fermion has an antiparticle with opposite properties. The positron has already been introduced as the antiparticle form of the electron, carrying positive electric charge rather than negative electric charge, and the same goes for the antiparticle forms of the other charged leptons. Despite neutrinos being electrically neutral, they too still have anti-partners that are distinguished in other ways. As mentioned in the previous section, an anti-quark carries not only the opposite electric charge to its regular counterpart, but also the opposite strong charge. A quark with positive electric charge and green strong charge therefore has an anti-partner with negative electric charge and anti-green strong charge.

There is nothing particularly unusual about antiparticles, and the lightest generation of antiparticles seem to be just as stable as the lightest generation of regular particles. We can therefore envision making an anti-universe entirely out of the stable antiparticles, with anti-nuclei formed of anti-quarks, and anti-electrons (i.e. positrons) in orbit about these anti-nuclei to form anti-atoms, and these anti-atoms making up anti-stars and anti-planets and anti-everything-else. Physically there is nothing wrong with that at all.

The interesting thing with antiparticles is not that they can exist, but rather what this tells us about the existence of regular particles. When a regular particle meets its antiparticle, their opposite properties cancel each other out and they completely annihilate, with their mass energy converting to light energy and radiating away. In short, particles and antiparticles cannot coexist. However, if particles and antiparticles are true opposites, there should have been no preference for the creation of either class at the time the universe came into being. In that case, as soon as the universe was created, it would have completely destroyed itself in the ensuing annihilation of equal amounts of opposite particle pairs. As we have a universe to talk about at all, that clearly did not happen. This means that there is some kind of preference for the creation of regular particles rather than antiparticles. There is some idea of why this might be, but it remains a big open question.


7) (Don’t) Blame The Messenger

In the course of the theoretical and experimental evolution of particle physics, it was realized that each force not only needs some kind of charge, but also some kind of messenger particle to communicate between the particles that carry the charge. These messenger particles are referred to as the "fundamental bosons" or the "force bosons". If two electrons are placed a short distance apart then they repel due to the nature of like charges in the electric force. But how does each electron know that the other is present to begin with? That's the job of the electric force bosons. It is envisioned that carrying a particular type of charge causes a particle to continually emit the associated bosons of the force in question. This does not cost any net energy, so the particle can do this continually. If a boson of a certain force from a certain particle meets another particle that obeys the same force, then the boson will interact with it and give rise to the force's effect. That second particle would have been emitting its own bosons the whole time too, some of which will interact with the first particle. In this way there is a force signal between the particles which tells them what they have to do to satisfy the force, such as move apart if they are two electrons.

The required force bosons have all been experimentally verified in particle decay data, which supports the theory of all particles and their interactions that has come to be known as the "Standard Model". The electric force is mediated by a boson called the "photon", and the strong force is mediated by a set of bosons called the "gluons". There is also another force which allows certain interactions that are not provided by either the electric or strong forces, which is referred to as the "weak force" and which is mediated by two types of boson labelled as "W" and "Z". An outstanding puzzle is that the most familiar force of all, namely gravity, does not fit in the existing Standard Model picture, but if it can be incorporated then it too will have a mediating boson, which has been preemptively called the "graviton".

Photons and gluons are both massless, but the W and Z bosons are not. In order to explain the mass of these particles it was necessary to propose a further boson in the theory, called the "Higgs boson", which is not associated with any force at all. This boson comes from space itself, and interacts to varying degrees with different particles, not only the W and Z, and gives rise to their attribute of mass. This may not be the mass mechanism for all particles, but it is thought to be so for most of them.

The Higgs boson was proposed in 1964 as an answer to a theoretical problem, allowing for the mathematics of the Standard Model to be cooked up in just the right way. The predicted observations that such a particle would provide were so subtle that the theory creators never believed it would be discovered, given the state and trends of technology at the time. However, about 50 years later it turned out that technology had actually evolved enough for the Higgs boson to be indirectly observed. The announcement was made in 2012 that two detectors, called ATLAS and CMS, which operate at the Large Hadron Collider at the CERN laboratory in Geneva, had strong enough evidence that the Higgs boson was indeed found. This was a resounding success for both theory and experiment, and provided the Standard Model with its crowning keystone.


8) Onwards And Downwards

The achievements leading to the state of particle physics today have been significant. Theory and experiment have gone hand in hand to reveal the secrets behind many of nature's puzzles, and the Standard Model provides startling accuracy when it comes to predicting experimental outcome. But the story is not over yet, as many puzzles remain. The trend of question leading to answer leading to question continues, and this cycle drives technology and techniques to ever greater heights of sophistication.

Aside from the ultimate goal of understanding nature at its most fundamental level, the search has provided numerous practical applications that further justify the work. From various medical applications, in terms of both diagnosis and treatment, through to aspects of everyday computing, the progress made at the forefront of particle physics research has given us plenty of benefits. And so the work continues, for new generations to be part of, finding new questions and developing new answers and everything else that goes with it. Where the end lies is anyone's guess, but the very path of discovery is full of surprises along the way, and the world can only benefit.


This article can also be seen on the Simon Fraser University HEP group website.


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