At work as an astronomer

The purpose of the science of astronomy is to explain the existence and evolution of objects in the Universe. In other words, its goal is to answer questions such as: Where/When/Why/How did the Universe begin? Will the Universe end? How/When will it end?

Do you have any idea what astronomers currently think the answers to these questions are? Are there other questions about the Universe you have wondered about? List some.

Almost every civilization on Earth has had a history of practising astronomy; consequently, it is one of the oldest sciences known. Early civilizations would observe objects in the sky with the naked eye and record their motion. In this way they could develop useful calendars for agriculture or rituals. Figure 3.1 shows the calendar developed by the Aztec civilization of South America.

Figure 3.11

In the next few pages you will be given a short introduction to what astronomers are currently working on, what telescopes they are using and a (very short) list of some of the important questions they are currently trying to answer about the Universe. For more information on the history of astronomy, see Appendix A.

1. “Aztec sun stone,” photographic reproduction of an original two-dimensional work of art, Wikimedia Commons (http://commons.wikimedia.org : accessed July 10, 2007). This image is in the public domain.

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Current astronomy

In the early 1900s, it was discovered that light could be divided into many different components, some with smaller wavelengths than others. For example, radio waves, infrared and X-rays are all types of light with different properties. When scientists think of all these different types of light as a group, they refer to them as the electromagnetic spectrum. Figure 3.2 illustrates how all these types of light relate to one another. For example, radio waves are very spread out and have a very small amount of energy in them. On the other hand, X-rays and gamma rays can be thought of as very compact waves that carry a lot of energy and thus can be dangerous to us.

Figure 3.22

2. “The Electromagnetic Spectrum,” digital image, Hershel Space Laboratory, Langley Research Center (http://www.nasa.gov/centers/langley : accessed July 11, 2007). This image is in the public domain.

Modern observatories

Nowadays, the work being done in astronomy involves observing objects in the Universe at all wavelengths of the electromagnetic spectrum. For example, astronomers use telescopes on various parts of the Earth, such as the Gemini telescopes located in Chile and Hawaii. These telescopes observe the Universe at visible wavelengths. On the other hand, some telescopes have to be put out into space because the atmosphere does not allow certain types of light from outer space to reach the Earth. This is extremely lucky for all of us! However, astronomers want to study this type of light produced by objects in space. For example, the Chandra X-ray Observatory is in orbit around the Earth and observes the Universe at X-ray wavelengths (see Figure 3.3).

Figure 3.33

Say you are an astronomer or engineer who wants to build a ground-based astronomical observatory. To research where to do this, visit a few existing observatories virtually. For example, try typing “Arecibo Observatory, Puerto Rico” into Google maps (http://maps.google.com). Change the viewing option to “Satellite,” and you will be able to see an image of the observatory and its surroundings. The Arecibo Observatory in Puerto Rico is the largest telescope on Earth; it consists of an antennae (basically a large dish) constructed to lie inside a natural hole in the ground that is about 300 metres in diameter. It observes the Universe at radio wavelengths.

3. The Chandra X-ray Observatory in orbit around the Earth. “Spacecraft Illustration with Earth Background,” digital image, CXC/NGST, Chandra X-ray Observatory (http://chandra.harvard.edu : accessed July 11, 2007). This image may be used for non-commercial educational and public information purposes.

You can also visit the website of the University of Hawaii Institute for Astronomy (www.ifa.hawaii.edu) and click on “Mauna Kea” to see the many different telescopes located on the mountain of that name. Now answer the following questions:

1) How many telescopes can you see on the main image of Mauna Kea?

2) Slide your mouse over the image to read a description of each telescope. How big is the largest telescope for submillimetre astronomy?

3) What similarities can you see between the locations of these telescopes?

4) Would you build your observatory close to populated areas (i.e., close to towns or cities)? Why or why not?

A (very short) list of current problems in astronomy

As you can see, astronomers have a very large number of telescopes available for them to conduct their studies. So what kinds of things are they studying at the moment? The following is a very short list of some of the problems astronomers are working on now:

Black holesBlack holes are objects predicted to exist in the Universe when a lot of matter is concentrated into a very small space. Because of this concentration, when objects (including light waves) come close to a black hole, it will bend space around itself such that the objects are able to come close enough that the pull of gravity is extremely strong. As such, we have never been able to see black holes, but we have been able to detect their presence by the effect they have on the objects around them. The following are some of the questions astronomers are currently trying to answer about black holes: How many of them are there in our galaxy, the Milky Way? How many of them are there in the Universe? Were they one of the earliest objects to form in the Universe? If so, did they contribute to the formation of other objects (such as galaxies)?

Dark matterDark matter is hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic light to be observed directly. Dark matter may account for 25% of the mass in the Universe, while bright matter (or visible matter) only accounts for 5%. Astronomers infer that a lot of dark matter exists because we see the gravitational effect that it has on other objects, but we do not see it emit light. These are some of the questions related to dark matter: What objects make up this matter? How is it distributed in the Universe? What role does it play in the formation of other objects in the Universe?

Figure 3.44

Dark energyCurrent data leads astronomers to believe that the Universe is expanding, that is, that everything in it is moving away from every other thing at a rate that increases with time. To explain these observations, a hypothetical form of energy has been proposed: dark energy. Dark energy is thought to permeate all of space and make up 70% of the Universe (see Figure 3.4). Some of the current questions related to dark energy are the following: What exactly is dark energy? Can we find physical laws to describe it? Is the Universe really expanding at a faster rate with time?

Sky-watching activity

As the Earth moves around the Sun in space, the objects in the night sky change from month to month (and even from day to day). For this activity, you will be an astronomer wondering what kind of objects you can look at in the sky during the current month. First, visit the Amazing Space website at http://amazing-space.stsci.edu and click on “Tonight’s Sky” to watch a video with some highlights of the current objects in the night sky. While watching the video, fill in the table on the next page with information about what objects are visible, where in the sky you can find them and at what time. Sometimes there are special celestial events happening (for example, a lunar eclipse or a meteor shower); make sure you take note of these, too.

OBJECTS IN THE NIGHT SKY

Date Time Type Object Direction

4. Illustration of the composition of the Universe. “Cosmological composition,” digital image, NASA, Wikimedia Commons (http://commons.wikimedia.org : accessed July 11, 2007). This image is in the public domain.

13/10/2007 10 p.m. Constellation Gemini South (Overhead)

You can then try to observe some of these objects yourself with the naked eye, binoculars or even a telescope. If you want a printed version of the objects in the sky, download the one available on the Sky Maps site by visiting: www.skymaps.com. Click on “Downloads” at the top of the page and then select the appropriate file to download.

As you observe these objects, write down descriptions of what you see on a separate sheet of paper, make drawings of what the objects look like, take digital pictures of them or record your observations in some other way. A very simple example of what you could do is given in Figure 3.5. In addition, a blog has been set up where you can share your observations with other students across the province at http://ppo.wiki.zoho.com. Click on “Sky-watching activity” to see the comments already posted. You can type your notes into the blog by clicking on “Post a comment.”

As an astronomer, you will want to keep informed about current research and events in the field of astronomy. To do so, visit the Science Daily website at www.sciencedaily.com. For astronomy news, click on “Space/Time” in the “Latest News” drop-down menu.

Figure 3.55

Counting galaxies

Galaxies are massive groups of stars, gas and dust spread throughout the Universe and held together by gravity. We live in a galaxy called the Milky Way. The Solar System, all the stars in the sky and all the nebulae you can see are all inside the Milky Way, just as we are. All the other galaxies that we see in the sky are millions and billions of light years away, and they each have millions to trillions of other stars in them. The light from these galaxies takes such a long time to reach us that it can tell us what the Universe was like billions of years ago. Galaxies come in different sizes and shapes. They are generally classified into three categories:

Spiral galaxies are large collections of gas and dust where new stars are continuously being born. The gas and dust settle into a kind of flat disk that rotates and causes arm-like structures to form. The bright, round centre is mostly composed of older stars. Figure 3.6 is an image of the spiral galaxy NGC 1300.

Figure 3.66

5. Example of an observation entry. Image created by Marjorie Gonzalez. Used with permission from the McGill Let’s Talk Science Partnership Program.6. “Barred spiral galaxy NGC 1300 photographed by Hubble telescope,” digital image, NASA/ESA, Wikimedia Commons (http://commons.wikimedia.org : accessed July 11, 2007). This image is in the public domain.

Elliptical galaxies are very old objects containing mostly old stars and very little dust or gas. Their shape is mostly round or elliptical, hence their name. Their size ranges from the smallest to the largest galaxies known. Figure 3.7 is an image of the elliptical galaxy M59.

Figure 3.77

Irregular (or peculiar) galaxies are galaxies unusual in their size and shape. They are thought to be the result of enormous collisions and interactions between one or more original galaxies, which have become distorted and formed the objects we observe. Figure 3.8 is an image of the irregular galaxy NGC 1427A.

Figure 3.88

For this activity, you will be an astronomer looking at an image taken with the Hubble Space Telescope. The image is filled with galaxies so distant that they look dim and small. Astronomers observe many properties of each galaxy including type, size, shape, brightness and colour. This information helps astronomers to determine how galaxies may have formed and evolved in the Universe.

Figures 3.9 and 3.10 are of the Hubble Deep Fields. The first one was taken in the northern part of the sky; the second one, in the south. In astronomical terms, a deep field is an image taken over a long period of time to view very faint objects. There are only a few stars in these images; most of the objects you see are distant galaxies. In total, more than 2000 galaxies can be seen in the two images.

7. Messier 59, an elliptical galaxy. “M59,” digital image, NOAO/AURA/NSF, Wikipedia (http://en.wikipedia.org : accessed July 11, 2007). This image is in the public domain.8. Irregular galaxy NGC 1427A, digital image, NASA, Wikimedia Commons (http://commons.wikimedia.com : accessed July 11, 2007). This image is in the public domain.

Figure 3.99

9. Hubble Deep Field North. “HDF-N,” digital image, ID STScI-PRC1996-01a (January 15, 1996), Robert Williams, the Hubble Deep Field Team (STScI) and NASA, HubbleSite Newscenter (http://hubblesite.org/newscenter : accessed August 17, 2007). This image is in the public domain.

Figure 3.1010

10. “Hubble Deep Field South Unveils Myriad Galaxies,” digital image, ID OPO9841b (November 23, 1998), R.  Williams (STScI), the HDF-S Team and NASA/ESA, The European Homepage for the NASA/ESA Hubble Space Telescope (www.spacetelescope.org : accessed August 17, 2007). This image is in the public domain.

One of the things that astronomers have done is look at these images and count how many galaxies of each type are present in them. As an astronomer, you can now do the same and compare your results with what other astronomers have found. Count as many galaxies as you want in both of the images (you don’t have to count thousands of them!) and classify them in the following table.

Number of galaxies % of total

Spiral

Elliptical

Irregular

TOTAL

The number of galaxies of each type that you find can tell you something about how the Universe works. For example, if you happen to find more irregular galaxies in these images than any other type, this would tell you that collisions and interactions between galaxies in the Universe were more common in the past than they are now.

You can find astronomers’ conclusions in the Answer Key. Compare your answers to theirs.

1) Were your answers similar to theirs?

2) If not, what are some of the reasons you can think of for the difference?

For some very interesting computer simulations of collisions between galaxies, go to the following website: www.liensppo.qc.ca. Click on “POP Links” and then scroll down to the “Videos” section under “Physics.” Click on the links to the videos to watch the clips.

These simulations take into account the real physical laws that will determine what will happen to galaxies when they collide.

At work as a particle physicist

Particle physics tries to answer the following questions and many more: What is the Universe made of? What are the smallest components of matter? What particles existed just after the Big Bang? If matter and antimatter were created in equal quantities during the Big Bang, why is the Universe currently made of matter alone? There are many mysteries in particle physics that have yet to be answered.

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This section of the tool kit will describe what particle physicists work on, what experiments they conduct and what facilities they use to conduct them. You will then try a few exercises related to this field to give you a better idea of what particle physicists try to accomplish in their work.

What do particle physicists do?

Particle physicists try to answer some of the above questions using large laboratories where they study the smallest, most fundamental components of matter in the Universe. Currently, physicists believe that matter is made up of different atoms. In turn, atoms are made up of other particles as illustrated in Figures 4.1 and 4.2.

Figure 4.111 Figure 4.212

Particle physicists try to study the properties of these fundamental particles of matter, how they interact with each other, what holds them together and other related questions.

Modern experiments

In everyday life, you explore new objects by using your senses to look at, taste, hear, smell or touch them. However, the objects that particle physicists want to study are so small that something different is required to “look” at them. For example, imagine that you are alone in a dark cave.13 Suddenly, you hear a noise in front of you and wonder what is making it. Fortunately, you happen to have glow-in-the-dark balls with you! You decide to throw them at the object in front of you, and if they bounce back, you will be able tell that they have hit something. First, you decide to use basketballs as your probes, but they are too big, and you can only figure out that the object in front of you is large and wide (see Figure 4.3, left). Then you decide to use tennis balls because they are smaller, but you still cannot figure out what the object

11. The fundamental particles that make up atoms. Image created by Marjorie Gonzalez. Used with permission from the McGill Let’s Talk Science Partnership Program.12. “The Scale of the Atom,” image found in the “What Is Fundamental?” section of the Particle Adventure, Particle Data Group of Lawrence Berkeley National Laboratory, The Particle Adventure (http://particleadventure.org : accessed July 11, 2007). Copyrighted image reproduced with permission.

13. This example is taken from the “How Do We Detect What’s Happening?” section of the Particle Adventure (http://particleadventure.org) and reproduced with permission from the Particle Data Group of Lawrence Berkeley National Laboratory.

is (see Figure 4.3, centre). Finally, you decide to use marbles, and they are small enough to let you see the details of the object in front of you (see Figure 4.3, right).

Figure 4.314

When physicists try to study the smallest components of matter, they also need to use small probes. One way to make a small probe is by taking an already large particle and accelerating it to very high speeds. Why does this help, you ask? Well, there exists a well-known physics phenomenon whereby objects that move fast actually get smaller!15

So, what physicists do is the following:

1) Put a probing particle into an accelerator.2) Give the particle lots of momentum by speeding it up almost to the speed of light, resulting in

a particle that has a great deal of momentum and whose size is very small.3) Slam this probing particle into the target and record what happens.

Figure 4.416

Physicists build accelerators with electrodes (that use currents and voltages to make the particles move fast) and magnets (that change the direction the particles move in). After an accelerator has

14. How you can use “probes” of different sizes to learn about the world around you. “Wavelength, the Cave,” images found in the Particle Adventure, Particle Data Group of Lawrence Berkeley National Laboratory, The Particle Adventure (http://particleadventure.org : accessed July 11, 2007). Copyrighted images reproduced with permission.15. This comes from quantum mechanics: particles can be thought of as “waves” with specific wavelengths. If you make them move faster their wavelength gets shorter and shorter.

16. Particle collisions, images found in the “How Do We Experiment with Tiny Particles?” section of the Particle Adventure, Particle Data Group of Lawrence Berkeley National Laboratory, The Particle Adventure (http://particleadventure.org : accessed July 11, 2007). Copyrighted images reproduced with permission.

pumped enough energy into its particles, they collide either with a target or each other (see Figure 4.4). Each of these collisions is called an event. The physicists’ goal is to isolate each event, collect data about it and check whether what they see in each event agrees with the current physics theories about what should happen.

Modern laboratories

There are many particle accelerators around the world, and international collaborations take place in these laboratories. Furthermore, professors, students and other researchers work at different universities around the world and analyze the data over the Internet. Examples of such laboratories are the Fermi National Accelerator Laboratory, also known as Fermilab, near Chicago in the U.S.; the Stanford Linear Accelerator Center (SLAC), near San Francisco; the European Organization for Nuclear Research (CERN) near Geneva, Switzerland; the Joint Institute of Nuclear Research in Dubna, Russia; and TRIUMF, Canada’s national laboratory for particle and nuclear physics, in Vancouver. Other countries also have smaller laboratories.

Canada plays a leading role in particle physics research, both in Canadian experiments at TRIUMF in Vancouver and at SNOLAB in Sudbury, Ontario.

Additional applications

Other applications of particle physics can be found outside the field of physics, as can job opportunities related to them. Medical imaging such as positron emission tomography and CT scans are based on particle physics experiments, as are X-ray, proton and neutron cancer treatments. Machines used for airport security checks are also based on particle physics. There are jobs related to developing instruments in these areas, testing them and studying their effectiveness.

Current work in particle physics

Particle physicists have created a Standard Model that describes the fundamental particles of matter, the forces acting upon them and the ways they interact with each other to create other particles (see the table below).

Fundamental particles (each also has an antiparticle)

Leptons electron(e-), muon (), tau (),e- neutrino (e-), neutrino (), neutrino ()

Quarks up (u), down (d), strange (s)top (t), bottom (b), charm (c)

Fundamental forces

Weak Force carriers: W bosons (W-, W+), Z boson (Z0)Electromagnetic Force carrier: photon ()Strong Force carrier: gluon (g)

Hadrons (higher-level particles held together by the strong force)

Baryons 3 quarks or 3 antiquarksExample: proton (p) = u+u+d

Mesons 1 quark + 1 antiquark. Example: pion (+) = u+

You may have noticed that the fundamental force of gravity is missing from the above table. Although it is important for very massive objects the size of human beings, planets and stars, the gravitational force is very weak compared to the other forces and is thought to be unimportant for particles as small as those discussed here.

In the Standard Model, particles interact through the weak and electromagnetic forces 17 to decay into difference particles (i.e., naturally change from one particle to another), or they create other particles by combining with others. See Figure 4.5 on the following page for examples.

17. According to current theory, these two forces are different only at low energies; if the interacting particles have a lot of energy (as is thought to have occurred in the Big Bang, for example), these two merge into a single electroweak force.

d

Figure 4.518

However, the Standard Model leaves many questions unanswered: Why are there three types of quarks and leptons of each charge? Is there some pattern to their masses? Are there more types of particles and forces yet to be discovered in even higher-energy accelerators? Are the quarks and leptons really fundamental, or are they, too, made up of smaller particles? What particles form the dark matter in the Universe?

Questions such as these drive particle physicists to build and operate new accelerators, where higher-energy collisions may show, for example, new particles interacting in new ways and provide clues to answering some of the above questions.

18. “Particle Processes,” enlargement of the Particle Chart, Contemporary Physics Education Project (www.cpepweb.org : accessed July 11, 2007). This image is copyrighted; however, permission is granted for teachers and students to print these copyrighted images for personal or classroom use.

What particle is it?

For this activity you will be a particle physicist trying to figure out the velocity of a particle to help you determine what the particle is. Imagine that you are in a laboratory looking at pictures from a bubble chamber, an apparatus that lets physicists see the paths taken by different particles when they are accelerated. Figure 4.6 is an example of what you can see from a bubble chamber:

Figure 4.619

The image on the left is the original image from the bubble chamber; the one on the right explains what you are seeing. The bubble chamber creates tracks that describe how particles move in it. The particles move from the bottom to the top of the image as they travel in the chamber. In this case, a kaon (K-) hit a proton and caused many other particles such as pions (-) and other kaons to form. You can see that the particle 0 has no track because it has no electric charge, and the chamber can only see it after it decays into a proton (p) and a pion (-), which have positive and negative electric charges.

You are now in your laboratory looking at Figure 4.7, the image you got from your bubble chamber. One of the tracks you notice is the one marked by the arrows. At one point, a particle interacted with another one (green arrow) and produced another particle that you cannot see but

19. Bubble chamber. “Example of a knock-on electron (or delta ray),” digital image, CERN, High School Teachers at CERN (http://teachers.web.cern.ch/teachers : accessed July 11, 2007). Copyrighted image reproduced with permission.

that further along produced two other particles (starting at the red arrow) that moved in spiral paths in opposite directions.

Figure 4.720

20. Bubble chamber, enlargement of a digital image, CERN, High School Teachers at CERN (http://teachers.web.cern.ch/teachers : accessed July 11, 2007). Copyrighted image reproduced with permission.

Velocity DistanceTime

0.115 m0.000000000385 s

m /s

A very important clue in determining what these particles are is to calculate their velocity. For example, you can measure on your original copy of the bubble chamber image that the unseen particle travelled a distance of 11.5 cm (0.115 m) in the very short time of 3.85 x 10-10 sec (0.000000000385 sec)! Now calculate its velocity:

_______________

Does this number remind you of anything—say, the speed of light? It should, since that is what this particle is! You are actually seeing a little packet of light, called a photon, being created, and then you see it suddenly change into two particles again (see Figure 4.8).

Figure 4.821

More specifically, what you see is the following chain of events:

1) The incoming electron hits a stationary positron, and they create a photon:e- + e+

2) The photon then travels the distance you see above, but since it does not have an electric charge, we cannot see its path in the bubble chamber image.

3) Finally, the photon breaks up into another pair composed of an electron and a positron: e- + e+

21. Diagram of a photon being created. Image created by Marjorie Gonzalez. Used with permission from the McGill Let’s Talk Science Partnership Program.

New particles22

When scientists study any system, they must ask themselves two basic questions:

1) What are the basic objects or building blocks from which this system is made?

2) What are the interactions or basic forces between these objects?

For this activity, you will be a particle physicist trying to figure out the basic building blocks and forces of new particles you have just discovered. Imagine that the image on the right presents information that was obtained about your new particles from an accelerator.

The different shapes represent higher-level particles made up of smaller fundamental particles. The figure on the left shows particles that you observed, while the figure on the right shows particles that were not observed. Answer the following questions:

1) What are the fundamental particles that make up the new particles? (For example, can the above shapes be made up of circles and squares?)

The fundamental particles you found above were combined in different ways to make up the shapes shown on the left. The ways in which they were combined can be thought of as the “rules” for the interactions between these fundamental particles. You can now try to figure out what these rules are. Don’t forget that the shapes that are not observed can also give you important clues.

2) What are the rules for connecting these fundamental particles? (For example, if you find that one kind of fundamental particle is a square, a rule could be that a square can touch another square only on one of its sides. Could that be a rule in this case?)

22. This activity and accompanying image are adapted from Activity 2 of the Educational Materials found on the Particle Adventure site (http://particleadventure.org/edumat.html) and reproduced with permission from the Contemporary Physics Education Project and the Particle Data Group of Lawrence Berkeley National Laboratory.

Guess the shape23

This activity will give you an idea of what physicists experience when they try to find out the properties of something new without being able to see it. The goal is to guess the shape of an unseen object (which could represent new, recently discovered particles) by rolling marbles against this hidden object and observing the deflected paths that the marbles take. Remember that you must not know the shape of the object before conducting this experiment! Now, follow these steps:

Step 1Have another student in your class take the piece of Styrofoam provided in your kit. Using the utility knife, he or she must cut the Styrofoam into a flat shape about 10 cm in size. (Suggest that your fellow student choose a triangle, square, circle, half-moon or other simple shape.)

Step 2The other student then hides the shape from you under the paperboard provided.

Step 3After the shape has been hidden, place a sheet of paper on top of the piece of paperboard. Roll marbles against the unseen shape from different angles, and sketch the paths of the marbles on the sheet of paper. Then answer the following questions:

1) What is your best guess for what the unseen shape looks like? Draw it.

2) How could you find out whether the shape has features that are smaller than those of your best guess?

3) Without looking at the shape, how can you be sure of your conclusions?

23. This activity is adapted from Activity 3 of the Educational Materials found on the Particle Adventure site (http://particleadventure.org/edumat.html) and reproduced with permission from the Contemporary Physics Education Project and the Particle Data Group of Lawrence Berkeley National Laboratory.

Optional activity: Quiz time24

What is the world made of? What holds it together? People have asked these questions for thousands of years. However, only recently has a clear picture of the building blocks of our Universe been developed. The scientists who have developed this picture work in the exciting and challenging field of particle physics.

How much do you know about the latest theories and research on these ancient questions? Read each of the statements below and place a checkmark in the appropriate box to indicate whether you agree or disagree with each statement. You can then compare your answers to those of scientists (see the Answer Key). The goal of this exercise is not for you to answer all the questions correctly; rather, it is for you to think about the questions and learn from the answers given.

Statement Agree Disagree

1. Some subatomic particles have no mass and no electric charge.

2. Some particles can travel through billions of kilometres of matter without being stopped (or interacting).

3. Antimatter is science fiction and not science fact.

4. Particle accelerators are used for cancer treatment.

5. The smallest components of the nucleus of an atom are protons and electrons.

6. Particle physicists need larger accelerators in order to investigate larger objects.

7. Gravity is the strongest of the fundamental forces of nature.

8. There are at least 100 different subatomic particles.

9. All known matter is made of leptons and quarks.

10. Friction is one of the fundamental forces of nature.

24. This quiz is adapted from Activity 1 of the Educational Materials found on the Particle Adventure site (http://particleadventure.org/edumat.html) and reproduced with permission from the Contemporary Physics Education Project and the Particle Data Group of Lawrence Berkeley National Laboratory.