What Exactly Are Particles?
When we say "particle," it's easy to assume we mean a tiny grain of matter, like a particle of dust. In physics, however, they have a technical definition:
Particles are excitations of quantum fields.
This sounds annoyingly complicated but we will need it later on as we explore. If you look up quantum field theory on Wikipedia, you may be further dismayed at the immediate plunge into advanced theory. What I hope to give you here is simply a feel for it, so I recommend clicking this link and going to the introduction. Even if you don't research quantum field theory on your own, I try to provide enough background so that you can see how it works to help us understand the physics definition of a particle.
The quantum field concept is fleshed out in what we call the Standard Model. This model is quickly becoming the go-to model for those of us trying to understand matter and energy. Everything in the universe is made up of 12 basic building blocks or particles, governed by 4 basic fundamental forces. These building blocks can be further divided into matter particles and force-carrier particles, and there are many of them. Since its development in the early 1970's this model has been able to explain a whole gamut of experimental results and has accurately predicted a wide variety of phenomena.
To understand particle physics, we will begin by figuring out what a quantum field is.
Excitation in a Quantum Field
A quantum field is a measurable physical property that is a part of every point in space-time. Space-time, you might remember, is a mathematical model that combines the three dimensions of space together with time. You can think of it as the ultimate backdrop of the universe. Everything else happens in space-time. It can move and it is stretchy - it can be distorted and even twisted. It is this kind of movability that requires us to use Einstein's theory of relativity. Phenomena can occur in different frames of reference and we must take this into account when we measure things like velocity and even time. Within this framework, one or more quantum fields may operate. Quantum field theory describes these operations, which we can measure as fundamental forces, such as the weak force, the strong force, electromagnetism and, possibly eventually, gravity.
Forces between particles are carried out by other particles. For example, the electromagnetic force between two electrons, perhaps during a chemical reaction or magnetic attraction, is carried out, or mediated by, an exchange of photons. The weak force, associated with radioactivity, is mediated by particles called W and Z bosons, and the strong force is mediated by particles called gluons. Each of these force-carrying particles, the photon, W and Z bosons and the gluon, all referred to as gauge bosons, is a virtual particle.
A virtual, or force-carrying, particle can be thought of as a quantized excitation in a field. It does not exist by itself; it cannot be measured except by measuring the force it is carrying. Real particles, on the other hand, can indeed by measured and they do exist by themselves.
Real (Elementary) Particles
All matter is made up of two kinds of elementary particles, leptons and quarks. Ordinary everyday matter consists of specific leptons called electrons as well as quarks, which make up the neutrons and protons within the atomic nucleus.
Ordinary Matter - The Atom
All matter, from helium to aluminum to uranium, is composed of atoms. The universe consists of at least 118 different kinds of atoms, which are differentiated from each other by the number of protons they have in their nuclei. All electrically neutral atoms have the same number of electrons as protons. If electrons are lost or added, these atoms become ions and they have a charge. For example an oxygen ion has a charge of -2, because it has two extra electrons associated with it, 10 electrons rather than 8. The number of neutrons can also vary among the same kind of atom. Most oxygen atoms have 8 protons and 8 neutrons, but some oxygen atoms may have as many as 9 or 10 or as few as 13 neutrons within their nuclei. These represent less stable nuclear arrangements, so these oxygen isotopes, as they are called, are unstable and radioactive as a result.
This is a pre-quantum simplified diagram of what an atom looks like:
Negatively charged electrons orbit around a nucleus composed of positively charged protons and neutrally charged neutrons. The modern quantum atomic model is far more complex but for our purposes right now, the simplistic Bohr-like model above works. Each neutron and proton consists of smaller elementary particles called quarks, which are bound to each other by gluons, virtual particles that carry out the strong force, shown by yellow lines below.
Categories of Particles - Fermions and Bosons
Protons and neutrons are called baryons. Ordinary matter, made up of protons and neutrons, is called baryonic matter. Quarks (baryonic) and electrons (leptons as we will see later) are all examples of fermions.
Particles can be placed in one of two broad categories based on their quantum state - either as fermions or bosons.
Subatomic particles, both elementary such as electrons and composite such as protons, are examples of fermions. Fermions have a half-integer spin. What is this spin? The spin of an elementary particle, as far as we know, is an intrinsic physical property of it - the particle has no inner structure. Its spin is a specific angular momentum that can't be altered; it defines one kind of particle from another kind. The spin of composite particles, on the other hand, is usually understood as the total angular momentum of its constituent particles. Because fermions have half-integer spins (again, for reasons that are still not fully understood) they obey a law called the Pauli Exclusion Principle, which means that no two fermions in the same quantum state can exist in the same place at the same time. More than one boson, on the other can occupy the same quantum state at the same time. This difference in quantum state separates fermions (almost always associated with matter) from bosons (usually force carrier particles).
Every fermion has an antimatter twin particle. An electron's antimatter twin, for example, is a positron. These anti-particles can be fundamental or composite. There are antiquarks, anti-protons, etc., and they make up antimatter. An antimatter twin of you is theoretically possible but the jury is still out as to how much antimatter actually exists in the universe. At one point, shortly after the Big Bang, most researchers believe the universe was equally seeded with both kinds of matter. When a fermion and its antiparticle meet, they instantly annihilate into a burst of pure energy, which appears in the form of a fundamental boson, such as a photon. Bosons, force carrier particles, cannot be classified as antiparticles. They are simply their own antimatter twin, so there is no such thing as an anti-strong force or anti-electromagnetic force, for example.
Besides quarks and electrons, the building blocks of ordinary matter, there are many other elementary particles as well. We'll explore some of these next.
Different Kinds of Quarks; Different Kinds of Leptons
The quarks inside atomic nuclei are either up quarks or down quarks. There are four other kinds of quarks beside these, called bottom, top, strange and charm. These four all have higher masses and are unstable - they quickly decay into the stable up and down quarks of matter. They are created only in very high-energy collisions such as in cosmic rays or inside particle colliders. All quarks interact through the strong force.
Electrons are by far the most common kind of lepton but there are several other kinds of leptons as well. Electrons have the least mass and are the most stable of all the charged leptons. The charged leptons interact through the electromagnetic force as well as the weak force, but no leptons interact through the strong force. Only quarks do. The heavier charged leptons (the muon and the tau particles), like the exotic heavy quarks we just learned about, are made only in cosmic collisions and particle colliders. They quickly decay into ordinary electrons. In addition to charged leptons there are neutral leptons called neutrinos (which also come in different classes - electron neutrino, muon neutrino and tau neutrino). These particles are rarely observed because they rarely interact with anything. They have a very tiny mass and pass right through ordinary matter almost unaffected. They are produced during radioactive decay and fusion reactions (inside the Sun for example) and interact exclusively* through the weak force. Neutrinos have a very interesting story to tell, which we will read more about later on.
The diagram below lays out the 12 matter and 3 force-carrier particles we have explored so far. Fermions are either purple or green (notice that they all have a 1/2 integer spin). You might wonder what eV means. Instead of grams, the masses of subatomic particles are expressed in electron volts (eV), the energy equivalent of mass. To explain this, consider the annihilation of an electron and a positron. Both have an equivalent mass of 0.511 MeV. When they annihilate, 1.022 MeV of energy is produced, in this case in the form of a photon of that energy. This unit is a mass-energy equivalent unit. The bosons are orange, and we will explore these in more detail next. (Did you notice that the W and Z boson each have a mass value. Intriguing isn't it?)
copyright: MissMJ (Wikipedia)
All fermionic and bosonic elementary particles cannot be broken down into smaller substructures. Mathematically these particles are treated as single points, although some particle theories such as string theory give them a physical dimension. All elementary particles are thought to obey the laws of quantum mechanics. Remember that they are, strictly speaking, excitations of quantum fields. Within the quantum equations that describe these fields, particles are not points at all, but instead are wave functions. They express both particle and wave behaviours, depending on how they are being measured. This has been experimentally proven with both electrons and photons. A particle is also a probability amplitude of position and momentum. That means that its position and momentum (or energy or velocity) cannot both be measured at the same time. This follows the rules of the Uncertainty Principle. You will never be able to observe an electron or proton or gluon as some kind of moving particle under a powerful futuristic microscope. We cannot directly observe a wave function. However, this hasn't stopped physicists from being able to image individual atoms, or at least get a good approximation of them, using a new high-resolution electron microscope with a maximum resolution of less than 50 picometres. The diameter of a typical atom is about 100 picometres. Nanoparticles gave the scientists a great test material to use because they have a very small number of atoms spread out over a large surface area. This is a computerized image of a nanoparticle with each individual atom depicted as a yellow sphere:
Copyright: Swiss Federal Laboratories for Materials Science and Technology
It is important to keep in mind that this is an approximation. All atoms jostle around and vibrate, even as they approach absolute zero. They don't sit perfectly still in networks. However other weird things do start to happen to atoms when they get very cold. To find out, check out my article called "Very Hot, Very Cold."
These are, for the most part, force-carrier particles. They all have an integer spin. There are composite boson particles such as mesons, superfluids and Bose-Einstein condensates, as well as fundamental boson particles. The fundamental bosons are all considered to be force-carriers. These are called gauge bosons. Their interactions are described by gauge theory.
The meson is made up of a quark and an antiquark. It is highly unstable and is formed only in cosmic ray collisions and colliders. These interesting particles interact with both the strong and the weak force, and there are many of them. Mesons, although they consist of quarks, are not fermions, nor are they constituents of baryonic matter. The quarks have their spins anti-aligned so together they make up composite particles of zero spin, and that makes them bosons. There are charged and uncharged mesons, and there are different mesons based on the types of quarks that make them up. Charged mesons are unique in that they interact with all four fundamental forces, weak, strong and electromagnetic, as well as, presumably, gravity, as they do have mass as well. Like the gauge bosons, the lighter mesons are indeed force carrier particles (the pion, for example, helps mediate the strong force) while the heavier ones may not be. Superfluids and Bose-Einstein condensates, on the other hand, are examples of atoms of ordinary matter acting like bosons under extreme conditions, and they are not force-carrier particles.
Here I will concentrate on elementary bosons - the gauge bosons (photons, W and Z bosons and gluons) as well as two theoretical gauge bosons - the Higgs boson thought to impart mass on other particles, and the graviton, the sought-after particle of gravity.
These bosons carry out the fundamental forces, as virtual particles. W and Z bosons carry out the weak force. Photons carry out the electromagnetic force. And gluons carry out the strong force. What is interesting about these particles is that the W (2 kinds of these) and the Z bosons have mass while the photon and gluon do not. The reason for this is believed to be the result of symmetry breaking very early on as the universe evolved. This is a process through which scientists think our fundamental forces "settled out" of the universe as it expanded and cooled after the Big Bang (an explosion of tremendous pure energy from which the universe began). You can think of this process as analogous to ice "settling out" of water as it cools and freezes. If we run the universe's evolution backward through time, we see evidence that the separate forces and matter we have today recombine step by step as we move backward through an ever-increasing energy environment. We ultimately reach a point where all energy and matter are recombined into one universal "mother" energy, the grand unified force.
This simple diagram shows how the fundamental forces broke from a theoretical grand unified force soon after the Big Bang:
The image below displays the evolution of the fundamental forces as well as particles and structures such as stars:
(copyright: John von Neumann Institut fur Computing)
Each colour band is a phase transition, where symmetry is broken. Notice the energy decreasing over time.
The weak force and the electromagnetic force were coupled together into one force called the electroweak force when the universe was very young and filled with intense energy (in any environment where the energy is high enough, these forces would recombine, as in high energy colliders for example). This is the point where the Higgs boson comes into the story. The Higgs boson is a massive theoretical force-carrying particle that is thought to impart mass on other particles, through an interaction with its force field, called the Higgs field. According to the Higgs theory, at this point in the universe's evolution, the three weak force bosons and the Higgs boson were all coupled together to a Higgs field, gaining mass due to the Higgs mechanism. When the electroweak force broke into the weak force and the electromagnetic force (through symmetry breaking), the W and Z bosons remained coupled to the Higgs field (and a Higgs boson was released) while the photon did not, and so that is why the photon is massless. The W and Z bosons, though massive, do exist as virtual particles mediating the weak force. Their mass, in fact, explains why they have such a short range of influence. Like the photon, W and Z bosons also exist as real particles as well. Gluons are thought to exist de-confined, or free from quarks (which themselves are de-confined from nucleons), as real particles in quark-gluon plasma inside the incredibly hot dense cores of neutron stars.
Higgs theory predicts a massive Higgs boson, which has not yet been observed. A great deal of energy would be required to create it (equivalent to the energy of the very early universe) and physicists at the Large Hadron Collider at CERN are currently working on producing one. In fact, just today as I write this article (December 2010), scientists at CERN released an announcement that they may have already glimpsed the Higgs boson! They saw excess energy spikes corresponding to around 124 - 125 GeV during very powerful particle collisions, and these spikes correspond to a predicted Higgs energy signal. Now they need to rule out background noise. A more definitive announcement could come within the next year.
The strong force broke from the electroweak force even earlier on, mediated by gluons. One of the figures above shows gravity breaking the earliest of all the fundamental forces. It is a guess; no one knows if gravity, like the other three fundamental forces, came into existence with a symmetry-breaking event and an associated lowered energy state, releasing a field and an associated gauge boson particle, the graviton. There are many theorists currently working on this problem. Somehow the acquirement of mass is coupled to gravity, as if mass is equivalent to the gravitational charge of a field. Is there an interaction between the Higgs field and the gravitational field? We must also consider that gravity, according to general relativity, more accurately acts on the energy of a particle (it acts on the particle's stress-energy tensor). This means that gravity only looks like it's interacting just with the mass of an object when it is non-relativistic (when it is at rest). When that particle approaches light speed, gravity is interacting more accurately with its momentum, its total energy in other words. As well, some theorists think of gravity not as a fundamental force at all but an emergent property of the universe itself.
Fermions attain their mass through interaction with the Higgs field, but not in the same way as W and Z bosons do. We know that force particles arise from fields spread out over space and time. Parameters in the equations for the field associated with the Higgs boson can be chosen so that the lowest possible energy state of that field (the empty space of our universe today) is a non-zero value. The equations predict that all particles that can interact with the Higgs boson gain mass from the interaction (which comes from the non-zero energy). The mass of the particle in this sense comes from its inertia or resistance to motion when we try to move it while it is being "grabbed at" by the Higgs particle with which it is interacting. In terms of thinking of Higgs as a field, when a particle moves through the Higgs field, it distorts it - the field clusters around the particle, giving it mass. If the Higgs boson is discovered, it will fill in a major piece predicted by the Standard Model, giving us even more confidence in the validity of this model.
We now have an idea of what some of the various fermions and bosons are and how they interact with each other. The diagram below illustrates those interactions:
You might have some questions about this diagram. Quarks, for example, interact with the strong force through gluons, as we now know. But you will see here that they also interact with the weak force through the W and Z bosons. This occurs during a process called quark transmutation. During nuclear fusion and radioactive decay, up quarks decay into down quarks (fusion) and down quarks decay into up quarks (decay). You might also wonder how quarks interact with photons. It occurs through pair production, a process in which two very high energy gamma photons smash into each other, annihilating themselves and producing a proton/antiproton pair (other particle/antiparticle pairs can be produced, depending on the energy of the two photons). Remember that protons, like neutrons, are composed of quarks.
Next, we will move beyond the Standard Model to explore some particles that are more difficult to define.
The Standard model does an impressive job of describing three of the four fundamental forces in terms of how they evolved, how they are related to each other, and how they are mediated by particles. Gravity, on the other hand, remains a mystery. Gravitons, theoretical spin-2 bosons, are unique in that they describe the behavior of space-time itself while the other forces play out "on the space-time stage." Any attempts to describe gravitons in the same way as other bosons are met with severe difficulties. And yet, it seems reasonable to expect a force-carrier particle that mediates the fourth fundamental force of gravity. Please take a look at my article called "Gravity" for a full exploration of gravity and the hypothetical graviton.
Dark Matter Particles?
In astronomy, dark matter is matter that neither emits nor scatters light (or any other electromagnetic radiation) so we cannot observe it. However, it is believed to make up about 83% of the matter in the universe. It is the "missing mass" that accounts for higher than expected orbital velocities of galaxies inside galaxy clusters and observations of gravitational lensing caused by galaxy clusters.
Some of this matter may just be ordinary matter that doesn't emit any electromagnetic radiation. These could be unassociated planets, brown dwarfs, dead carcasses of burnt out stars and possibly even lone black holes, but researchers think these together could only make up a small fraction of the total dark matter, and the rest, they believe, is nonbaryonic matter. Recall that a baryon is simply a composite particle made up of three quarks, protons and neutrons. Non-baryonic matter is matter that isn't made up of atoms, and in this case it also does not interact with ordinary matter through the electromagnetic spectrum. That means it has no electric charge, for example. The only way we can detect it is through its interaction with gravity. One of the leading theories suggests that this dark matter may be made up of neutrinos. These particles have no charge, and a very small (and still disputed) mass. They do however interact very occasionally with ordinary matter.
The Enigmatic Neutrino
Neutrinos are created during some kinds of radioactive decay, nuclear reactions and when cosmic rays hit atoms. The young universe may have been flooded with these particles. Today, we get most of our neutrinos from the Sun. About 65 billion of them pass through each square centimeter of Earth every second! The 1995 Nobel Prize in Physics went to a research group who managed to directly detect the neutrino, or at least its antimatter twin: Antineutrinos created in a nuclear reactor reacted with protons producing neutrons and positrons.
Neutrinos come in different flavours - electron neutrinos, tau neutrinos and muon neutrinos. Strangely enough, they can oscillate between these flavours as they propagate through matter (you need to detect all three when you are measuring them). We already know that neutrinos have no charge. However if they have even a tiny mass, they may also have a small magnetic moment and because of this moment they could theoretically interact weakly* with the electromagnetic spectrum, adding to the neutrino puzzle, which now has an even larger mysterious piece to contemplate: New research reveals that neutrinos may travel faster than light speed! If neutrinos are massless, as was first thought, there is no reason why they shouldn't be able to travel at light speed just as photons do. But if they do have even a tiny mass, Einstein's law of special relativity states that their mass would approach infinity at light speed (or put another way, it would take an infinite amount of energy to get a particle with mass to light speed). And yet, the OPERA collaboration experiment measured neutrinos exceeding light speed in September and again in November this year (2011). Obviously, researchers are wary about this new data and are busy trying to further confirm it. If it turns out to be true, we have a big problem on our hands: Einstein's theory of special relativity, upon which the cornerstone of all relativistic theory is based, states that nothing can travel faster than light speed. And yet, general relativity does not necessarily rule out all faster than light speed: If a distortion in space-time allows space-time itself to move faster than light speed (and there is no rule forbidding this), it can carry a particle (or anything else) along with it, at faster-than-light speed. In this case the particle, by way of the same theory, would travel backward through time, much like a theoretical particle called a tachyon does. Neutrino research is a very hot area right now, for good reason.
Besides neutrinos, dark matter could also be made up of exotic particles such as axions or supersymmetric particles.
Axions - Another Candidate for Dark Matter
Axions are theoretical particles that have no electric charge, a very small mass, on scale with neutrinos, and very low interaction with the strong and weak forces. As a result, they interact only very weakly with ordinary matter. They are predicted to change into and from photons in the presence of strong magnetic fields and this may be used to try to detect them. Axions may have been made en mass during the Big Bang and they should be very stable (like neutrinos) and so they may contribute significantly to the hidden mass of galaxy superclusters.
Finally, supersymmetric particles may contribute to the mass of dark matter. According to supersymmetry theory, every fermion should have a partner boson and every boson should have a partner fermion. These shadow particles are heavier, with masses up to a thousand times greater than the their corresponding real particle, so it will not be easy to create one in a collider. However, the Large Hadron Collider at CERN can now create the extreme high-energy environment needed to begin to look for these theoretical particles.
(From: The Particle Adventure (http://particleadventure.org/supersymmetry.html))
There is no direct evidence for the existence of supersymmetry. It is a possible solution to several theoretical problems in which the universe now exists as a broken symmetry in which supersymmetric particles are far heavier than their real twins and therefore only exist when an extreme high-energy environment is recreated. I find it intriguing to think of supersymmetry as a theoretical mirror in which you can see the force particle twin of a particle of matter and vice versa, and that the line we easily draw between matter and energy can be completely erased. In fact, the line didn't even exist right after the Big Bang, not until some expansion and cooling, some settling of energy, could allow for the first of many symmetries to be broken. There is snag here: Supersymmetric particles are all high-energy particles thought to exist only under the universe's extreme energy conditions. How could any particle exist today in our relatively quiescent universe and impart hidden mass? This is where a new particle, the neutralino, comes in. There are four theoretical neutralinos that are fermions with no charge, the lightest of which should be stable and could exist in the universe today. They are all supersymmetric partners of bosons. It won't be easy to find, however. In most models, all supersymmetric cascade decays (which can hopefully be produced soon at CERN) end up decaying into this particle, which then will leave the detector unnoticed, because these light neutralinos should only interact with W and Z bosons. The only way to infer their existence is to look for unbalanced momentum in the detector. Even so, if the neutralino could be detected, it would boost both supersymmetry theory and help us figure out what dark matter is. Other attempts at direct neutralino detection are now being considered as well.
We are all fairly familiar with the particle theory of matter. Molecules break down into atoms and atoms break down further into electrons, proton and neutrons, and these nucleons break down further still into quarks. It is quite easy to visualize this concept of matter. For us to move beyond this to understand both matter and energy as particles, however, we must get comfortable with a new way of approaching the problem. We must embrace the idea of a particle, either matter and energy (or force), as a quantum entity, a wave function. It isn't easy or even comfortable to revisit the atom as a cloud of probabilities. In our everyday experience, atoms seam so much more steadfast and reliable than that. How can the atoms in my cup of coffee be wave functions?
This new approach pays off for us because now we can conceptualize (almost) all the forces that operate in the universe as well as matter. We can begin to appreciate the idea that particles of force and particles of matter are actually very closely related to each other, perhaps they are even mirrors of each other. And with this quantum approach we can begin to peel away at the ultimate mystery of how forces and matter came to be in our universe.
For an extensive list of currently known particles, click on this link.
This 5-minute video from CERN explores the Standard Model of particle physics in an easy-to-follow format, nicely summing up this article:
I also recommend an excellent on-line resource called The Particle Adventure. I suggest starting with the Standard Model and going from there. It will help you understand much of what I've tried to cover and it's great for kids from age 12 to 112!