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A Brief Introduction to Particle Physics

Particle physics
Origin of Muons
Why Study Muons
Detecting Muons

A Detailed Introduction to Particle Physics

Particle Adventure

Other Particle Physics Links

General Information
Muon and Cosmic Ray Research
Accelerator Resources
Detector Resources
Lists of Links

What is particle physics?

Particle physics is the study of the fundamental particles of the universe, or, in other words, particle physics is the study of really, really small stuff. As of right now, we know of 12 fundamental particles: six quarks and six leptons. (See the particle periodic table to the right.) This is known as the Standard Model in the physics world. There are currently hundreds of identified particles made from combinations of these twelve fundamental particles and scientists are still finding more.

So now you ask, "What exactly are quarks and leptons?"

What are quarks?

Quarks are the fundamental building blocks of nature. They combine to form larger particles, such as protons and neutrons. There are six different types of quarks: up, down, charm, strange, top, and bottom. If you look at the table above, the mass of the particle increases as you go to the right, meaning the top quark is much heavier than the up quark. Quarks in the top row have a charge of +2/3 of an electron's charge (where e = 1.9 x 10-19 C) and quarks in the second row have a charge of -1/3 of an electron's charge. You may think this is strange because you were taught that you can't have a fraction of the charge on an electron. Your instinct is correct: because of this fractional charge, quarks can not exist independently; they must combine to form larger particles.

Quarks combine to form most of the matter in the universe. In fact, most of the matter in the universe is made from just two quarks: the up and the down. For example, a neutron is made of two down quarks and one up quark (add the charges, it makes sense! -1/3e + -1/3e + 2/3e = 0) and a proton is made of two up quarks and a down quarks (+2/3e + 2/3e -1/3e = 1e). In general, you need three quarks to make a particle, so that the charge always adds up to a whole number.

What are leptons?

Leptons are the six particles at the bottom of the periodic table. The bottom row shows the electron, the muon, and the tau particles. You are probably familiar with the electron; the muon and the tau are the heavier, less well known cousins to the electron. The muon and tau are rarer than the electron, have the same negative charge that the electron has. We'll talk more about muons below. The top row shows the neutrinos. There is one neutrino for each of the electron, the muon, and the tau. Leptons are much lighter than the quarks, in fact the neutrinos are so light that there is debate whether they have mass at all!

What is anti-matter?

It's not just science fiction! There actually is something known as anti-matter is real physics. Every particle we talked about above, both quarks and leptons, has an anti-particle. The anti-particle is exactly the same as the particle (i.e. same mass) but all the properties are opposite. For example, an up quark has a charge of +2/3, so an anti-up quark would have a charge of -2/3.

The anti-matter quarks can interact and form new particles in the same way the quarks do. The difference is that now you need a quark and an anti-quark to make a particle, instead of the three quarks like we stated above.

When a particle meets its anti-particle, they annihilate and release a large amount of energy

What is a muon?

A muon is a negatively charged particle, similar to an electron, but about 200 times heavier. Muons fall into the class of particles known as leptons.

Where do muons come from?

Most muons come from what are known as cosmic rays. What are cosmic rays? Cosmic rays are caused by high energy protons from stars in outer space that interact with the Earth's atmosphere. Scientists are unsure of the origin of the highest energy cosmic rays. As they fall toward the earth, they ionize the atmosphere forming a shower of matter and anti-matter particles. These particles are known as pions (p) and are made up of up and down quarks and anti-quarks. You may have never heard of a pion before and that is because they don't last very long. They quickly decay into lighter things, such as leptons and electromagnetic radiation. This is where our muons come from: they are the results of an interaction between a proton and the atmosphere that produces a particle that decays into a muon, among other things. Other leptons, such as electrons and neutrinos are also emitted, but the muons have a higher energy so are more likely to make it down to the Earth's surface. These showers are happening all the time. About 600 particles pass through your body each minute!

The picture to the left illustrates a cosmic ray shower. The yellow streak is the proton, the red and purple streaks are pions and the others are leptons. The white squiggly lines are electromagnetic radiation, such as photons. In the picture, you may notice that the muon (µ+) and some of the electrons (e-, e+) have positive charges. We said about that the muons and electrons were negatively charged, so the positive charge indicated that it is the anti-particle. As far as decays are concerned, the anti-particle behaves the same way as the particle would.

Why study muons?

The simple answer is that we study muons because we can. Muons from cosmic rays are relatively easy to detect. The telescopes are portable and can be used in the classroom for a variety of experiments. By studying the cosmic rays, we can find how the rate that muons are detected depends on the weather, direction, or extra-terrestrial events, such as solar flares. The question to answer in your research project is: why do these factors affect the muon rate?

How can we detect cosmic rays?

Cosmic rays, and muons in particular, are hard to detect because they are traveling very fast and pass through most materials without interacting. The trick to detecting them is to take advantage of the fact that they are charged particles. When a charged particle passes through a particular substance it can ionize the surrounding particles and leave a trail. For example, in a cloud chamber, the air is cooled to the point that when an atmospheric particle is ionized, it will cause the air to condense and thus leaves a visible trail. With the cloud chamber, you can see both muons and electrons, but to the untrained eye, it is hard to tell the difference.

General Information

  • The Particle Adventure is a great introduction to the world of particle physics. It is easy to navigate and provides information on particle detection methods, as well as the standard model.
  • Reflections on Matter teaches you particle physics in 10 easy lessons.
  • FermiILab is a national physics lab run under the Department of Energy devoted to the study of fundamental particles. At Fermilab is the Tevatron, a proton accelerator four miles in circumference and the world’s largest particle accelerator. Fermilab was named for the Italian physicist Enrico Fermi for his work in groundbreaking particle physics.
  • Inquiring Minds is a list of links to particle physics related web pages from Fermilab.
  • Brookhaven National Lab is also a part of the Department of Energy. Many things from physics to chemistry and biology are studied here, but most interesting to particle physics is the Relativistic Heavy Ion Collider (RHIC). By colliding gold nuclei, they can get a look at what the first moments after creation of the universe may have been like. It was in these moments after creation that the fundamental particles were formed.
  • The Laboratory For Elementary Particle Physics (LEPP) at Cornell University is a research center for both experimental and theoretical particle physics and accelerator physics. Their facilities include an electron-positron accelerator (CESR). The CLEO detector is primarily used in studying bottom and charm quarks.
  • CERN is the world’s largest particle physics research facility located in Switzerland. CERN also has a program in which high school physics teachers can do experimental research in particle physics. The website also has many good resources for teachers.
  • The PARTICLE Program at the University of Rochester is a program designed to educated teachers in the field of particle physics and provide them the opportunity to do experimental research with their students.
  • Acronyms of High-Energy Physics is a guide to keep you from getting confused with all these letters swarming around.
  • A Bedtime Primer on Physics for Children and Adults Alike. This is an entertaining tale of particle physics. The links inside the story are also informative and amusing.
  • European Particle Physics Outreach Group gives a good introduction to particle physics and the wider research community. It has a fantastic collection of links to other resources.
  • Smalest Particles, Biggest Machine is a power point presentation of the basics of particle physics.
  • High Energy Physics Made Painless

Muon and Cosmic Ray Research

  • PARTICLE Teachers’ research pages. Muon research being done at high schools through the PARTICLE Program at the University of Rochester.
  • Adelaide Muon Monitor data is available from the University of Adelaide going back to July 2003. Also, some background information on muons and cosmic rays.
  • Stanford Linear Accelerator (SLAC) has a cosmic ray detector with pictures of the equipment and data posted online.
  • Astronomical Links provided by Rob Penna at the University of Rochester. This is the page used in the workshop with Rob and Alice Quillen during the Summer Institute. The links include places to get astronomical and weather data, as well as data from other muon telescopes.
  • Quarknet Online Cosmic Ray Detector has archived data and tutorials to help with analysis.
  • Cosmic Ray Observation Program (CROP) involves high school student in Nebraska studying cosmic rays.
  • Pierre Auger Observatory in Argentina studies high energy cosmic rays.
  • WALTA is a Washington state based high energy cosmic ray investigation program for middle and high school students. WALTA is a part of Quarknet.
  • New York Schools Cosmic Particle Telescope is a project to expose students to real scientific research

Accelerator Resources

  • Event Simulator shows what happens when a Z boson decays. This is a nice interactive java applet.
  • The FermiLab Accelerator Chain is a collection of accelerators that make up Fermilab.
  • Particle Accelerator is a game that shows how the magnetic field reverses to accelerate a particle.
  • The World of Beams explains in simple language what a beam is and why it is useful. This page is sponsored by the Center for Beam Physics at the Lawrence Berkeley National Lab (LBNL).
  • The SLAC Virtual Visitor Center takes you on a tour of the linear accelerator and explains how it works.

Detector Resources

Lists of Links

Department of Physics and Astronomy, University of Rochester, PO Box 270171, Rochester, NY 14627-0170, (585)-275-5306