## Sunday, November 1, 2009

### PARTICLE PHYSICS - Summary and Essay

When I'm not watching Korean movies, reading speculative fiction, or listening to music from video games, I'm probably reading or thinking about real science, most notably, particle physics. I made a summary of them earlier this year:

### ELEMENTARY PARTICLES

These particles are not comprised of any smaller particles and are generally referred to as point particles, since they are akin to the notion of a point from high school geometry.
There are two main types of elementary particles - fermions and bosons.

FERMIONS
Fermions are the matter particles - we are made of them, along with everything else. Except dark matter. There are at least 24 different fermions, divided up into two main categories.
I have listed the masses of the particles using the accepted unit - the electron volt. For example, MeV is short for million electron volts and GeV is short for billion electron volts.
It's easier to express the masses of particles in units of electron volts because, if we used kilograms, then the numbers would become tiring (even with scientific notation). Just one GeV expressed in kilograms is about .000000000000000000000000001783

QUARKS (The top quark is the heaviest known particle.)
• Up (2 MeV)
• Charm (1.25 GeV)
• Top (171 GeV)
• Down (5 MeV)
• Strange (95 MeV)
• Bottom (4.2 GeV)
LEPTONS (The neutrino masses are only a few electron volts each.)
• Electron (0.511 MeV)
• Muon (106 MeV)
• Tau (1.78 GeV)
• Electron Neutrino
• Muon Neutrino
• Tau Neutrino
As you can see, I only listed 12 fermions, yet I have claimed there are at least 24. This is because they each have an antiparticle, which raises the count to 24. For example, the antiparticle of the electron is called the positron.
Antiparticle names are not always so clever, however. Physicists generally just place anti- before the particle name, such as antimuon. The main difference between matter and antimatter is the charge. Electrons have a negative charge, while positrons have a positive charge.

BOSONS
Bosons are the particles that mediate forces. We are not constructed of bosons, yet we are greatly affected by them. There are four currently accepted forces - strong nuclear force, weak nuclear force, electromagnetism, and gravity.

PHOTON

Photons are the most well known bosons, since light is made of them. This particle mediates the electromagnetic force. It has no mass and has no known range, other than the size of the universe.

W and Z BOSONS

The W and Z bosons are not well known, nor is the force they mediate, the weak nuclear force. This force is responsible for the decay of neutrons and other forms of radioactivity.
W and Z bosons have a very limited range, only .000000000000000001 meters. This force is much weaker than electromagnetism and the strong force, but vastly stronger than gravity.
The mass of the W boson is 80.4 GeV and the mass of the Z boson is 91 GeV.

GLUONS

There are 8 different types of gluons, they have no mass, and they mediate the strong nuclear force. Gluons are cleverly named, since they act like glue and hold quarks together. Quarks make protons and neutrons, which in turn make atoms, which in turn make us.
Since quarks are held tight by gluons, we should be thankful for their adamant adherence to proper behavior. Gluons do not feel the electromagnetic force, nor the weak force. They certainly do not feel gravity as well.

GRAVITON

The graviton is unconfirmed. None have yet been observed, but their existence has been theorized in order to explain what mediates the gravitational force. Gravity is a force and no particle has been discovered to explain it. This may be due to the weakness of gravity.
As an example, it is easy to defy gravity. Just pick up a paper clip. The entire mass of the Earth is holding the paper clip down, yet you are able to pick it up. In numbers, gravity is approximately 1,738,590,000,000,000,000,000,000,000,000,000 times weaker than the weak force.
Since gravity is so weak, any particle that mediates it would need to be quite massive. Massive particles are difficult to find, which explains why we haven't found the graviton (if it exists).

HIGGS BOSON

This is another unconfirmed particle, but many believe the LHC will find it. It is thought to permeate space like a fluid and impede W and Z bosons, which limits their range. We might know much more by 2011. As for now, it's too early to say what the Higgs really is.

### COMPOSITE PARTICLES

Composite particles are constructed of more than one elementary particle. They are all called hadrons, which are themselves divided up into two types.

Hadrons are collections of quarks, in sets of 3 or 2.

BARYONS

These are collections of fermions, specifically, 3 quarks. The proton and the neutron are the most well known baryons.

MESONS

There are not any well known mesons in the general public. These are quark-antiquark pairs. This may have no meaning to you, but the most well known of the mesons are the pion and kaon. Mesons are responsible for binding protons and neutrons through the strong force.
Mesons are typically referred to as composite bosons, even though they are not made of bosons. This is because a meson acts like a boson - it mediates a force.

Bear in mind that my summary above does not even begin to cover how many different particles actually exist. I don't know. No one actually knows that answer. I easily left out more than 100 particles and there's no telling how many more are yet to be discovered. But all particles can be divided up between those that are elementary or composite, or matter or force carriers (I would hope so).

Here is the essay I wrote earlier this year (click to read) while bored at work one evening:

You'll notice I put a part in brackets [They have zero mass, but not zero energy, which seems like a contradiction]. I did that due to my own uncertainty (at the time). I now know that photons can indeed have zero energy, so there is no contradiction. To check this yourself, refer to page 53 of this book:

I strongly recommend that book if you want to check your own knowledge, or simply get a summary of everything known about particle physics up until 2007.