Particle Physics

One of the most frequently asked questions & primary goals in physics today is "What is the Universe made of?"  In asking this question you are also asking the question "what are we made of?"  This question has been asked for many thousands of years, but  luckily today we are starting to understand exactly what it is that we are made from.

Everybody is aware that the stuff we see in everyday life is composed of smaller and smaller particles, eventually ending with the atom.  In every day life this is really about as far as your knowledge need take you.  For the scientific minded person, this breakdown of matter goes even further.  It is here we enter the world of particle physics.

•  It was the Greek philosopher Democritus who first proposed around 400 B.C. that matter was made up of indivisible particles called atomos (a = not, tomos = cut, uncuttable).

•  Some 2,000 years later in 1803, John Dalton an English chemist and physicist, proposed that the atom was actually a small solid sphere.  Part of "Daltons theory" was that "Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process".  He was of course wrong, but other points made by Dalton were revolutionary and indeed correct.

•  In 1897, an English physicist by the name of J. J. Thomson redefined the view of what an atom was, with his "plum pudding model".  He suggested that the atom was in-fact a "a spherical sea of positive charge" in which negative electrons existed.  The sea of positive charge would equal the total negative charge, to explain the overall neutral charge of an atom.  He was awarded the Nobel prize in physics in 1906 "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases".  It was here that the electron was discovered.

•  It was Earnest Rutherford, a physicist from New Zealand who in 1911, while deciphering results from a previous experiment, now called "Rutherford scattering", discovered the nucleus.  He did this by firing alpha particles at gold nuclei, and noted that some of the alpha particles bounced directly back from where they had originated.  Rutherford has been quoted as saying "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."  He later came to the conclusion, that atoms were indeed made up of a nucleus orbited by electrons, and that the positive charge of the nucleus was observed only at the extreme centre of the total volume of the atom.  It wasn't until the 1920's that the word Proton was being used to describe the hydrogen nucleus.

•  Until the 1930's, the model of the atom was still believed to be considered to be composed of protons and electrons.  The total mass of the nuclei "A" was also known to be slightly more than twice the mass of the proton number "Z" for the majority of elements.  In 1932 an experiment carried out, in which helium, nitrogen, and paraffin were bombarded with radiation emitted from a polonium source.  The emitting radiation was energetic enough to eject protons and consisted of alpha particles (now known to be helium nuclei).  Given the emitting radiation was electrically neutral, and high in energy, the culprit was originally thought to be gamma radiation.  The gamma photon however is not energetic enough to eject a heavy proton, so by comparing the energies of recoiling charged particles, Sir James Chadwick could determine that the neutral particle had a mass almost equal to that of the proton.  It was called the Neutron, and James Chadwick received the Nobel Prize in physics in 1935 for its discovery.

Although electrons are elementary particles (they are what they are), protons and neutrons can be broken down further into smaller particles called quarks.

Particle families

Elementary particles

As mentioned above, an elementary particle is one which cannot be further divided. They can be grouped into two categories based on their spin.  All particles have this spin property which is basically their intrinsic angular momentum.  The spin of an elementary particle can be divided into two groups: Integer spin 1...2...3...etc, or half-integer spin i.e. 1/2...3/2...etc.  Those with integer spin are called bosons, while those with half-integer spin are called fermions.

Bosons are particles which transmit the four fundamental forces of nature (see bottom of page).

Fermions are the basic building blocks of matter i.e. electrons and quarks, and can be divided yet further into two groups.  Those which are mediated by Gluons (quarks) and those which are not (leptons).

Both quarks and leptons come in 6 types, and both come with their own individual antiparticles.

Quarks

Leptons (further divided into two groups, those with mass, and those without)

Composite particles

Once we have left behind the elementary particles, we can talk about those particles which are composed of other particles i.e. protons and neutrons.  Called composite particles (for obvious reasons), they interact through the strong force mediated by gluons.  They are collectively known as Hadrons and can be further divided into the two groups mentioned above, fermions (half-integer spin) or bosons (integer spin). 

Believe it or not, it gets even more complicated, because hadrons which are bosons are known as mesons, and hadrons which are fermions are known as baryons.

They are easy to tell apart, because baryons are composed of 3 quarks, while mesons are composed of just 2.

Both the proton and neutron are baryons due to their composition of a combination of a total of 3 up and down quarks.  The pion however is a meson, as it is composed of a quark and an anti-quark.

Summary

Elementary particles = Stand alone particles composed of just themselves.

Composite particles = Particles composed of other smaller particles.

Fermions = Particles with half integer spin.

Bosons = Particles with integer spin.

Quarks = Fermions which interact with the strong force (see below).

Leptons = Fermions which do not interact with the strong force.

Hadrons = Composite particles which interact with the strong force.

Meson = Hadron with integer spin

Baryon = Hadron with half-integer spin

Four fundamental forces

The above few paragraphs are rather complicated to understand without knowledge of the forces with which particles interact.  These are called the four fundamental forces of nature, and shape the way the laws of physics work.  Everything you see or feel can be described by the way particles interact with one another.  These interactions vary depending on the particles in question, but all interact by mediating one of just four particles.  These are the elementary bosons mentioned in the table above.

A force of any kind is a physical interaction of some sort between one or more objects.  It is the exchange of these particles which give rise to the forces we experience.

The electromagnetic force: Force carrier = Photon

The electromagnetic force only acts on particles that are charged like those found in atoms (protons, electrons & quarks).  If the photon had a large mass, then the exchange of the force would only act over short distances.  As we know the electromagnetic force (light) is carried over infinite distances, and that it travels at the speed of light, we can convincingly say that the photon is massless.  

Photons are responsible for holding atoms together to form molecules and to hold molecules together as well.  The Photon is also what carries light from the Sun to our eyes, the signal to your car radio, the particles that heat your food in a microwave, and the x-rays that are used in hospitals.  Theoretically there is no limit to the energy a photon can have, as it is massless.  The wavelength of photons can vary from trillionths of a meter to hundreds of miles.  The visible part of the spectrum (the entire range to which our eyes are sensitive) covers only a very tiny part of the total electromagnetic spectrum.

The electromagnetic force is extremely strong (1 trillion trillion trillion times that of gravity).

The strong nuclear force: Force carrier = Gluon

This force is the strongest of all, in-fact it is 100 times stronger than the electromagnetic force.  It is this force that holds together the quarks inside the protons and neutrons, and the protons and neutrons inside the atomic nucleus.  The Gluon only interacts with itself and quarks.  It combines "blind" quarks into their respective colours to form the protons and neutrons.  Gluons like quarks are themselves split into red + green + blue to = white.  This combination equals a glueball.

The strong force looses some of its strength at high energies, and in a high speed proton anti-proton collision for example, the energies are reduced enough to almost free the quarks and become isolated.  This has the opposite effect on the weak nuclear force & the electromagnetic force which become stronger at high energies.  Although the strong force is immensely powerful, it only acts over very small distances, becoming almost useless over radii larger than that of a proton.

Weak nuclear force: Force carrier = W ± & Z⁰ Boson

This force acts on all particles with half-integer spin like the electron.  Despite its name, the weak force is far from weak.  It is in-fact 10 trillion trillion times stronger than gravity, but 10 trillion times weaker than the strong force, hence its name.  It was unified with the electromagnetic force in 1967 by Abdus Salam and Steven Weinberg who theorised the Weinberg-Salam theory, and later won the Nobel prize in physics in 1979.  This unification of the electromagnetic force and the weak nuclear force is called the electroweak force. 

The exchange of intermediate vector bosons (W ± & Z⁰) are capable of changing the "flavours" of quarks i.e. top --> bottom or strange --> charm.  The weak force is most noticeable for it is the cause of Beta decay.

β decay: The weak force converts a down quark into an up quark through the exchange of a W^- boson, which in turn changes a neutron into a proton, emitting an electron and an electron antineutrino.  This is called beta plus decay.

n → p + e^- + ū

On the other hand, a proton can be converted into a neutron by changing an up quark into a down quark through the exchange of a W⁺ boson, emitting a positron and an electron neutrino.  This is called beta minus decay.

p → n + e⁺ + ٧

The weak force acts over a VERY short distance.  Due to its very large mass it acts over just 10^-18m (0.1% proton diameter).

Gravity: Force carrier = Graviton

Gravity is by far the weakest of all the four forces.  It is 100 trillion trillion trillion times weaker than the strong force.  The act of walking, standing and being able to lift your arms above your head demonstrate how weak gravity is. 

The particle responsible for gravity has been called the graviton, even though it has never been detected.  The graviton has a spin of 2, making it a fermion, and is also believed to be massless based on the fact that gravity acts over infinite distances. 

The graviton must be exchanged from one body to another and back again, each time increasing the force between the bodies and in turn bringing them closer together.  As gravity follows the inverse square law, the force between the bodies increases by the square of the distance between them i.e. two bodies that double in separation, only experience 1/4 the gravitational force.