Particle Physics.
"The study of the fundamental subatomic particles, including both matter (and antimatter) and the carrier particles of the fundamental interactions."
1. A Short Timeline.
2. Strange Words for Strange Properties.
3. The Standard Model - Hadrons.
4. The Standard Model - Leptons.
5. The Standard Model - Forces.
6. Antimatter.
7. A (much) Longer Timeline.
1. A Short Timeline.
Unlike the other branches of the tree that is physics, particle physics is unique in the sense that it cannot be traced back to a single point of birth or to a creator whereas things like relativity and astronomy are the brain children of Einstein and Galileo respectively.
Particle physics is the study of the interactions between the smallest building blocks of matter: particles. In some ways then, Democritus the greek philosopher, who lived from 460-370 BC,is the forefather of particle physics as it was he who first proposed the existence of atoms. Indeed the word atom is derived from his greek word 'atomos' meaning indivisible.
Democritus' theory of atoms stood the test of time and was widely accepted until 1897 when J.JThompson discovered electrons and proved that atoms were made of smaller charged particles and so divisible. Thompson, in turn, proposed the plum-pudding model.
Thompson's model was subsequently overturned in 1917 by Ernest Rutherford's famous gold foil experimentthat deduced the existence of both the proton and the nucleus. Niels Bohralso correctly identified that electrons orbited the nucleus in discrete energy shells and later in 1932, James Chadwick conducted beryllium bombardment experiments and discovered the neutron.
All of
these people could be credited with the creation of particle physics but I
believe that particle physics truly began in 1964 when physicists Murray Gell-Mann and George Zweig independently proposed the quark model (more on this later).
This sparked half a century of particle hunting across the globe and placed particle physics as the leading branch of physics at the time.
2. Strange Words for Strange Properties.
Before we dive into particle physics, it is vital we understand the strange language that particle physicists speak. The wacky nature of the particles explored means that even their properties take some getting used to - the subatomic world is so small that everyday descriptions of matter break down.
The first step to mastering the language of particle physics is to understand these new properties of particles. Perhaps particle physicists are overly-imaginative or maybe they just like difficult words but the property names and official definitions are obtuse at best.
All particles have their own unique attributes such as electric charge, mass, colour charge, spin and strangeness. If these words mean nothing to you, don't worry they're just strange words in the particle physics language but the proper definitions (in italics) followed by a more understandable explanation (in normal text) can be found below:
Mass.
"Mass is a measure of how much matter there is in an object."
It's very important to not confuse mass and weight. Mass measures how much stuff there is in the object whereas weight is the force that an object's mass exerts due to a gravitational field. This means that mass is a constant everywhere in the universe but weight is not. An astronaut in the ISS has the same mass as when they stand on earth but a different weight because of the change in gravitational field strength.
In everyday use, mass is measured in grams or kilograms but in the tiny world of particle physics these units are far too large. So a new mass unit was produced: the electron-volt or eV. As Einstein famously proved in his special theory of relativity (see the special relativity chapter), E = m x c^2. This can be rearranged to give m = E / c^2. Energy is measured in electron-volts so it follows that mass is also measured in electron-volts. Therefore to convert between energy and mass in particle physics, just times/divide by c^2.
But wait, if these particles are moving surely they have added mass due to their greater kinetic energy? A light fast-moving particle has the same mass as a heavy slow-moving one. To distinguish between them, physicists measure the rest mass of particles, that is to say the mass when the particle is stationary, and so they can directly compare different particle's mass. Whenever I refer to the mass of a particle from now on, I mean it's rest mass.
Mass is given to objects when they are in a Higgs field by the Higgs Boson (named after Peter Higgs above) but we will discuss this in much greater detail later. In conclusion, mass is a measure of how large the particle is - the word massive is literally mass-ive.
Strangeness.
"Strangeness is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions that occur in a short period of time."
There are lots of overly long words in that sentence. Strangeness can be more simply defined as a measure of the number of strange quarks in a particle. This then changes how it interacts with the strong nuclear force and the electromagnetic force. Strangeness is simply calculated as :
Strangeness = number of strange quarks - number of anti strange quarks.
Strangeness is vital for a concept called the eightfold way that we will meet later and the universe is very strict when it comes to conserving strangeness. In a reaction total strangeness before the reaction MUST equal total strangeness after.
Electric Charge.
"Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field."
As a household name, electricity has muddled the understanding of electric charge. Electric charge is a measure of how the particle behaves in an electric field.
A particle can have positive, negative or neutral electric charge which dictates how it interacts with other electrically charged particles - the old adage of opposites attracts turns out to be true. Much much more on electric fields and subsequently magnetic fields can be found in the chapter titled Electromagnetism.
Colour Charge.
"Colour charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics."
Colour charge is a measure of how the particle behaves with other colour-charged particles. All quarks have a colour charge which is strictly either blue, red or green. It is important to note the quarks themselves are not acctually this colour, a three-way identification scheme was needed and the primary colours were used.
There are three fundamental rules of colour charge: dIfferent coloured charges attract, same charges repel and all particles must be colour neutral. The applications of these rules are explored more fully in the Hadron section below.
Spin.
"Spin is an intrinsic form of angular momentum carried by elementary particles, and thus by composite particles such as hadrons, atomic nuclei, and atoms."
This is perhaps the most unhelpful definition of the 5 because it implies that the particles are tiny spinning spheres which is just not true. Simply put, spin tells you whether or not a particle can make up atoms or act as messenger particles between them.
Spin always comes in values of 1/2. Particles can have any spin as long as it is either an integer or ends in .5. If a particle has non-integer spin, it is called a Fermion (named after Italian Physicist Enrico Fermi who worked on induced radioactivity) and will combine with other non-integer spin particles to make larger blocks of matter.
Alternatively, if it has integer spin it is known as a Boson (named after Indian physicist Satyendra Nath Bose who did work on the photon) and will work as a messenger particle and carry forces between Fermions.
3. Standard Model - Hadrons.
The main goal of particle physics is to explain the everyday interactions of matter. Throughout the 1970s and 80s, particle physics exploded as the most prevalent branch of physics and teams of scientists around the globe worked together to achieve such an explanation. Their answer was the Standard Model of Particle Physics.
The standard model of Particle Physics, or 'particle zoo' is at the heart of particle physics and is essentially a compiled list of the various types of particles and the forces that act between them. It serves to simplify the world around us into 3 sets, two which relate to particles and one which describes the forces acting between the particles.
As far as we know, the standard model is complete: all types of particle are identified and explained.
The next three sections therefore relate to Hadrons, Leptons and the forces that act between them, as described by the Standard model.
Hadrons are in many ways the poster child of particle physics - they make up almost all of the matter on earth. As you read this sentence, the mass of the room you sit in is made up of 99.9999% Hadrons. Their name comes from the Ancient Greek, ἁδρός (hadros) which means thick/large because of their large mass (relative to other particles, relative to anything else they are negligible).
There are two subtly different types of Hadrons - Mesons and Baryons. Both names also come from ancient Greek, Meson comes from the Greek μέσος (mesos) which means intermediate because the predicted mass of Mesons was between that of the electron and proton. Baryon comes from the Greek βαρύς (barys) which means heavy because at the time of their discovery they had the greatest mass of the particles.
However, both belong to the Hadron group. The common overarching definition of a Hadron is:
"Any particle that is made of quarks and so has colour charge and therefore feels the effects of the strong nuclear force."
Quarks.
So far we have casually thrown around the word quark, but what really are they? If you took an outrageously powerful microscope (employing poetic license here) and looked at a piece of wood you would be be able to see hydrocarbon molecules.
If you looked inside the molecules you would be able to see individual atoms of carbon and looking closer still, you would see protons and neutrons inside the nucleus. Inside the protons and neutrons (known collectively as nuclides) exist quarks.
In 1964 physicists Murray Gell-Mann and George Zweig both proposed the quark model. The quark model is both beautifully simple and mathematically faithful. The core concept behind it is as follows:.
"The fundamental building block of matter is the quark. There is no smaller composite of matter (that we have yet discovered). "
As far as we know, quarks are the smallest building block of matter - you cannot look into a quark with a microscope and see something smaller. However, an exciting modern branch of particle physics is string theory which suggests that quarks themselves are made of smaller vibrating strings but this has its own chapter later on so I won't ruin the fun now.
There are 6 different types of quarks, which come in 3 generations: up + down is the first generation then strange + charge and finally top + bottom. Every quark has its own unique associated properties - which are described by the strange particle physics jargon of the last chapter.
As seen in the table, there are lots of trends in the properties of the quarks. All quarks have non-integer spin of 1/2. 3 of them (up, charm and top) have electric charge of +2/3 and are collectively known as up-type quarks. The other 3 (down, strange and bottom) are the down-type quarks and have an electric charge of -1/3
Another noticeable trend in the quark table is in mass. As you move down the generations, mass of the quarks increases. The bottom quark is about 75,000 times as massive as the up quark but still has the same spin.
When describing quarks mathematically, the first letter of the name is used to denote it, eg strange = s
Another property quarks exhibit is colour charge, as discussed earlier. Each quark can have any of the 3 colour types and so there are essentially 3 versions of each quark, for example red/blue/green strange quarks are all valid quarks.
It is important to note that if colour charge is different between two quarks of the same type, nothing else is different between them. A red strange quark has the same mass/spin/electric charge/strangeness as a blue quark. This is vital for Baryon and Meson creation (you're in for some fun when we get to that later).
Antiquarks.
As well as the 6 quarks, there also exists 6 antiquarks. They are simply named anti- followed by the name of the associated quark. The anti-quarks have the exact same properties apart from a reversed charge. So an up-quark with +2/3 charge becomes a antiup-quark with -2/3 charge whilst all the down type antiquarks have a charge of +1/3. Antiquarks can be combined to make antimatter as further described in section 6.
The other difference between quarks and antiquarks is anti-colour which is the colour charge of antiquarks. The three possible anti-colour charges are cyan, magenta and yellow (the 3 secondary colours) and they follow the same basic rules as normals colour charge: opposites attract, like charges repel and antimatter has to remain colour neutral.
Cyan is made of green + blue, magenta is blue + red and yellow is green + red. This means that an antiquark with a cyan colour charge acts like two quarks with red and blue charge respectively.
To denote antiquarks in mathematical models, the same rules that apply to quarks are used but to distinguish between them, antiquarks use the same letter with a horizontal line above, eg u (up) becomes ū (antiup). This is pronounced u-bar. The same bar-notation represents the mean value of a data set in statistics,
This is incredibly unhelpful and infuriating because many particle physics equations require both interpretations of the bar-notation. For example, s-bar could represent an antistrange-quark or it could mean an average of the set of strange quarks. This pattern of brilliant physics and annoying notation is disappointingly common.
Now that we've discussed what quarks and antiquarks are, we can return to our two types of Hadron:
1. Baryons.
Every Baryon (protons and neutrons are examples of Baryons) has its own 'recipe' of 3 quarks that make it up. It is always 3 quarks, never more or less. For example, a proton is made of two up quarks and one down quark.
The overall properties of the particle are determined by the combined properties of the quarks that make it up. In our example, a proton therefore has a total electric charge of +1 (calculated through 2/3 + 2/3 - 1/3). It also has a spin of 3/2 (calculated through 1/2 + 1/2 + 1/2) - and is therefore an electrically charged Fermion.
However, this summation effect does not apply to mass; a proton has a mass of 0. 938 GeV/c^2 which is roughly 90 times larger than the summation of the associated quark's masses. The extra mass of hadrons comes mainly from the energy mass of the gluons which hold the quarks together (see the section on the strong nuclear force).
Baryons also must be colour neutral - this means all baryons are made of exactly 1 red, 1 blue and 1 green quark. It makes no difference to the proton whether it has a green up quark or a blue up quark but it is vital that it remains colour neutral.
A nice analogy to think about this is that baryons have white colour charge - combining the three primary colours gives white light as seen in the diagram above on the right.
Eagle-eyed readers will note the antiproton and antineutron in this diagram. As examples of antimatter they belong in section 6 and so we will steer clear for now.
2. Mesons.
Mesons are also Hadrons but they are different from Baryons in a very simple way. Rather than be composed of 3 quarks, Mesons are made of a quark - antiquark pair.
There are strict colour charge rules on which quarks and antiquarks can be used to construct mesons. Like Baryons, Mesons MUST be colour neutral. As discussed earlier, anticolour acts like two charges. So in order to remain colour neutral, there must be one Quark of each colour in the Meson (the antiquark can be treated as two quarks when it comes to colour charge). A valid Meson pair would be a green quark and a magenta antiquark because magenta is red + blue.
Unlike Hadrons, Mesons are not stable - the heavier Mesons decay into lighter ones whilst the lighter ones decay into electrons and photons (see the later section on the weak nuclear force for more detail). This decay process happens ludicrously fast, even the most stable Mesons only live for around 1/10th of a nanosecond and are created in high energy collisions. In nature the only conditions they were created in was Moment after the Big Bang but in the modern day, Mesons can be produced in particle accelerators.
There are two main types of Meson that live long enough in these collisions to merit their own names and symbols:
Pions (π).
The first and most stable Meson is the pion, represented by the Greek symbol π. Pions are defined by their 'meson recipes': all pions are made up of only up and down quarks/ antiquarks.
There exist three types of pion: the π+ (up-quark + antidown-quark), the π- (antiup-quark + down-quark) and the π0 (up-quark + antiup-quark or down-quark + antidown-quark). The π+ and π- are antiparticles off each other whilst the π0 is it's own antiparticle. Pi
ons - which are still unstable despite being the most stable meson- decay into muons and muon neutrinos (see the section on Leptons).
Kaons (k).
The second type of stable meson is the kaon - although less stable than the pion it still has a long enough lifetime to be detected before decaying into pions which subsequently decay into muons and then electrons.
It is represented by a k and then common definition is any meson that contains a strange quark/anti-quark as well as either a down or up quark/anti-quark. This means that unlike pions, there are 4 types of kaon: k+ (up-quark + antistrange-quark), k- (strange-quark + antiup-quark), k0 (down-quark + antistrange-quark) and k0- (strange quark + antidown-quark).
The k0 and k0- are both electrically neutral but the k- is negatively charged whilst the k+ is positively charged. This also means that the k0 and k0- are antiparticles of each other and the same is true for the k+ and k- mesons.
4. Standard Model - Leptons.
The second class of particles in the standard model are the Leptons. Lepton comes from the Greek λεπτός (leptos) which translates as thin or delicate and is so named because they are much lighter than Hadrons.
Conveniently, the standard definition of the lepton is the exact reverse of that of the Hadron; Leptons are any particle that are not made of quarks and so do not interact with the strong nuclear force. Because they are not made of quarks, Leptons are an example of a fundamental building block of matter - they are not made of anything else.
Like Hadrons, there are 2 types of Lepton:
1. Charged Leptons.
Just like quarks, charged Leptons come in three generations. Electrons are the first generation which are perhaps the only household name in the standard model. The second and third iteration of the charged Lepton are much more obscure: the muon and tau.
The only difference between the three cousins is mass - a muon has roughly 207x the mass whilst a tau is a whopping 3700x larger than the standard electron. All charged Leptons have an electric charge of -1, no colour charge (remember colour charge comes from quarks) and a spin of 1/2. Mass is the only property that differentiates the 3.
In nature, electrons are found orbiting the nucleus of the atom. This fact (first discovered by Niels Bohr in 1913) is responsible for most chemical reactions and so central to chemistry.
The electron's heavier cousins are far rarer however because of their extreme unstableness - muons exist for about 2.2 x 10^-6 seconds and tau for just 3 x 10^-13. The larger the particles are, the faster they decay into lighter ones, this is explored more fully in the chapter on the weak nuclear force.
Because of their short lifetimes. Tau particles can be found anywhere that high-energy collisions are happening. In nature this mostly means cores of stars, blackholes and supernova but impatient humans searching for the elusive heavy leptons have built huge expensive magnetic rings (called particle accelerators) to bring the tau particles down to earth.
Both muons and taus can be produced in particle accelerators but muons are also found naturally in particle showers coming from cosmic radiation. Before this phenomena can be explored though, we must first discuss the other type of Lepton: the neutrino.
2. Neutrinos.
The Austrian physicist Wolfgang Pauli, one of the forefathers of particle physics, predicted the neutrino's existence 26 years before they were detected. He predicted they would be electrically neutral and massless but when detected in 1956, it was revealed they were indeed electrically neutral but did have mass albeit a very small mass.
This lead to Enrico Fermi (another particle physics great) to name them neutrinos which means 'little neutral one' in his native Italian. For each charged Lepton, there is an associated neutrino, each named the electron/muon/tau-neutrino respectively.
Neutrinos are actually incredibly abundant - across the universe they are the most common particle with mass. Around 200 trillion neutrinos haves passed through ur body as you read this sentence, and a further 100 trillion fly right through you for every second you contemplate this. The reason for their huge abundance is partly due to the conditions of the universe just after the Big Bang (see section 7: a longer timeline).
You might think the huge number of them somewhat diminishes Wolfgang Pauli's Nobel-prize-winning prediction of their existence. However despite their abundance, neutrinos are incredibly hard to detect because they hardly interacts with the world around them at all. To put a number on this, for every 10 trillion neutrinos that hit earth (most of which come from the sun fusing), 1 will interact and the rest will pass through as though the earth isn't there.
Because of their minuscule mass, neutrinos don't meaningfully interact with gravity - our best instruments are far too clumsy to detect them this way. They also are electrically neutral and incredibly fast (traveling at almost the speed of light) which makes detecting them doubly hard.
The only thing neutrinos interact with are the Z0 and W Bosons of the weak nuclear force (it's vital you understand these before reading this section so see section 5: Forces).
Cosmic Particle Showers.
5. Standard Model - Forces.
The final leg in the tripod of the Standard Model are the forces. There are 4 fundamental forces of the universe that act between the Hadrons and Leptons (collectively Fermions). Each force has its own Boson or messenger particle and each dictates a certain area of particle interaction:
1. Gravity.
Gravity is the force that pulls masses towards each other, it is an attractive force that acts between objects with mass. It explains everything from Newtons falling apple to the orbits of stars but in the realms of particle physics it is perhaps the least consequential of the 4 fundamental forces as the masses of the particles involved are too small for gravity to take a significant effect. Its effect* is described by Newton's gravity formula:
F= (m1m2 ) / r^2.
This formula tells us that the force (F) acting on two masses (m1 and m2) is equal and opposite to each other. The force between you and the earth is identical in magnitude but only the earth has a much larger mass so is hardly affected by your mass - you and the earth both orbit a point fractionally away from the earth's centre of mass.
It also tells us that the force acting because of gravity (F) follows the inverse square law. As the radius of the circle the two masses orbit (r) gets larger, the force due to gravity gets smaller at an accelerating rate. However, it never reaches 0! You feel a force from gravity of every star in the night sky but they are so distant that the force becomes negligible.
3. The Strong Nuclear Force.
The strong nuclear force is the force that holds the protons and neutrons together in the nucleus. It acts between quarks (more on that later) and is so outrageously strong that it takes more energy to overcome it than it takes to create an entirely new nucleus. As you pump more and more energy into splitting a nucleus, rather than split the nucleus you force the universe to create an entirely new nucleus and go into "energy debt". This is the principle behind atomic bombs.
The strong force is transmitted by gluons, which are massless bosons that interact with quarks in the protons and neutrons of the nucleus. Of the messenger particles, gluons are unique because they interact with themselves - that is to say gluons transmit the strong force between each other. When they do this they form glueballs, which contain nothing but gluons. Glueballs remain hypothetical however as the force between gluons is negligible compared to that between quarks; gluons are yet to be separated from quarks in an experiment setting so glueballs are confined to theortical physics.
2. Electromagnetism.
Electromagnetism is responsible for two phenomena: electric and magnetic attraction and repulsion. The central pillar of the electromagnetism force is that all charged particles interact with one another: opposites attract and like charges repel. The strange part of this, as Maxwell showed, is that in order to do this the charged particles exchange photons. This makes the photon the messenger particle of the force.
The force is only felt by charged particles so electrically neutral neutrinos are completely unaffected. It is the second strongest of the 4 forces, as easily demonstrated it is stronger than gravity. A small bar magnet can raise an object thus ‘beating’ gravitational pull of the earth.
As further explored in the electromagnetism section, electromagnetism is directly involved in the phenomena of light (which is just oscillating electric and magnetic fields).
4. The Weak Nuclear Force.
The fourth fundamental force is the weak nuclear force. It is so named because at the current temperature of our universe (which is around 4K on average) it is not as strong as the strong force or electromagnetism. Although hard to test, it is still theoretically stronger than gravity. Furthermore, as time goes on the universe only cools so the weak force will get gradually weaker in the future.
The weak force is responsible for truly odd interactions that result in the changing of up-type (up/charm/top) quarks to down-type (down/strange/bottom) quarks. Much more on the quark model later. The changing of quarks relates to the changing of nucleons and so the weak force is responsible for radioactive decay. For example in beta decay, a neutron changes into a proton or vice versa in order to stabilise the nucleus.
The weak force is unique in the sense that it has multiple messenger particles. When an up-type quark becomes a down-type one, W+ bosons are exchanged and in the opposite case, W- bosons are exchanged. The third messenger of the weak force is perhaps the weirdest particle in the entire zoo. The Z0 boson is an electrically neutral particle - it is essentially a photon with mass. It exchanges energy but doesn't change anything else about the particles. This has the effect of reshuffling the arrangement of the nucleons in the nucleus and so stabilising the atom.
6. Antimatter.
Just as Baryons are made of quark recipes, Antibaryons are examples of antimatter and so are made of antiquark recipes. These recipes also contain three quarks like Baryon ones, but use the 6 antiquarks rather than the 6 quarks.
An antiproton is two antiup-quarks and one antidown-quark. This makes it the exact counterpart to the proton and similar antineutrons exist as an opposite to the neutron. Antibaryons have the same properties as their normal matter cousins (they have identical spin, mass and overall colour charge). The exception to this is electric charge - antimatter has inversed charge, for examle an antiproton has a -1 charge whilst an anti electron has a +1 charge (more on antineutrinos in the next section!).
Antibaryons do not exist naturally, they are produced in high speed particle collisions in particle colliders. To produce an antiproton, two protons are slammed together with a bit of oomph (I am the undisputed champion of physics understatements).
7. A (much) Longer Timeline.
Now we have understood the standard model and antimatter we have the weaponry available to tackle larger more complex ideas in the world of particle physics. It's time to answer a question that puzzled the scientific community for close on a century: what was the universe like in its infancy?
In order to answer this question, we have to go backwards in time. All the way backwards to the very beginning of time- the big bang. At the moment of the big bang, everything in the universe was one, all matter and energy was condensed into a tiny but incredibly-dense point in space.
We know virtually nothing about this stage of our universes creation but if we move one unit of Planck time (the smallest measurement of time possible in models of quantum mechanics, it is equal to 10^-43 seconds and named after the german physicist Max Planck) forwards in time we arrive at:
1. The Planck Epoch
The Planck epoch is the moment of time at which humans can first deduce anything about the early universe, that is not to say that nothing existed before it, it is just the limit of our knowledge.In the Planck Era (the time between the big bang and the Planck Epoch we know nothing. We can guess at lots of things though:
One guess is that the four fundamental forces of nature (gravity, electromagnetism, strong/weak nuclear forces) were unified -they acted as one 'super' force.This makes the Planck Era an incredibly exciting avenue of study in modern physics because physicist believe it may hold secrets necessary in constructing a theory of everything - a model that explains all of the forces and particles in the standard model (more on this later).
A grand unified theory already exists which couples the strong/weak nuclear forces and electromagnetism together but adding in gravity to the equation is proving rather tricky (of all the understatements in the Grail Diary, this is perhaps the largest). Gravity is not playing nicely with the grand unified theory because we cannot mathematically pair General Relativity (which deals with the very big) and Quantum Mechanics (which deals with the very small).
One way physicists hope to do this is by recreating the conditions the universe was in during the Planck Era in order to observe the 'super' force. These conditions are recreated at high energies in particle colliders but even the largest and most powerful colliders, like the Large Hadron Collider at Cern, cannot achieve the high energy state the universe was in at the big bang so the theory of everything remains elusive.
Another guess we can make about the Planck Era is that at some point during it, gravity detached itself from the super force and became its own separated force. We know this because our models of gravity work up until the Planck Epoch.
However, it could (depressingly) be the case that gravity was independent of the other forces since the beginning and so the search for a theory of everything is pointless and the best we can hope for is a grand unified theory that we've already discovered. Particle physicists don't particularly like this line of thought because it would put them out of a job!