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A Top-Down Search for the Strange Charm of Putting Up With Those Quarks at Bottom of the Universe
For part two of our universe-construction project, while the helium models dry, it’s time to delve into the depths of the sub-sub-atomic universe.
Consider those carefully-constructed model atoms. Each contains protons, neutrons, and electrons.
As it turns out, with electrons, there are (so far as physics can determine at present) no smaller particles needed to build an electron. Electrons are part of a group of six elementary particles called leptons. Some of these leptons–the neutrinos–were predicted to not even have any mass, but experiments have shown that while they are incredibly low-mass, neutrinos do have some mass. Interestingly, these experiments leading to even more new developments in fundamental physics and the Standard Model theory. Still, electrons are by far the most numerous leptons (at least in our corner of the multiverse.)
In our candy-based model, we have more than one proton crammed into in a nucleus. Each of those protons has a positive charge, but we all know that objects with the same charge repel each other. Why does the nucleus stay together?
In our model, of course, there is all that sticky candy. But in the real atom, there is also something that, in its own way, makes protons stick together. These other particles are one type of another class of matter, called mesons. These strange, essential, particles are stable only inside the nucleus, where (like our sticky marshmallows) they act as a “glue” to hold protons and neutrons close together.
Given that extremely tiny leptons have been observed, as well as tiny mesons inside the nucleus, protons and neutrons may begin to seem too big to be elementary particles. Sure enough, it turns out that protons and neutrons are also made of smaller particles. And those mesons, too, are made of those same even-smaller particles. And, while it took thirty years to search them all out, a total of six more fundamental particles (on top of the six leptons) have been found. Most of the matter we know about only requires two of those particles–plus the electron–but modern physics predicted six, and sure enough, there are six of them.
Meet the QUARKS. Their six kinds are: up, down, charm, strange, top, and bottom. Each kind comes in a matter form and an antimatter form.
Intriguingly, the terminology for “kinds” of quarks is flavors. Other characteristics of quarks and leptons include color, another clue to the pleasure scientists find in these discoveries. For now, we’ll experiment with the flavors of quarks. Unlike real quarks, we will use macroscopic objects that also happen to taste sweet.
As usual, if you’re working with youngsters, begin by reassuring everyone that there will be plenty of time to eat their quarks later. Each person gets one each of the six flavors of candy…quarks. Because the candies will be handled a lot during the first stage, tell them not to open the wrappers yet. Observe the candies. One side has the brand name on it, and the other side is plain. If we put the candy name-side up, we’ll call it a quark, and if it has the plain side up, we’ll call it an antiquark.
Quark vs Antiquark
A meson is formed by pairs of one quark and one antiquark. Give the group some time to see just how many combinations can be made of such pairs. (A few special mesons combine two or three such pairs, in quark combinations.)
A Small Set of Mesons
This will take some cooperation–participants will want to get together and different groups will organize their tests differently. Meanwhile, if you have access to a whiteboard or poster paper, you can sketch out a list of simple mesons shown below. For smaller (or older) groups, you can also pass out copies of this grid and let everyone check off the combinations as they are discovered.
| quark | antiquark | candy (name) | candy (plain) | |
| bottom eta | b | b | pineapple | pineapple |
| Upsilon | b | b | pineapple | pineapple |
| charmed eta | c | c | purple | purple |
| D+ | c | d | purple | peppermint |
| D0 | c | u | purple | red |
| J/Psi | c | c | purple | purple |
| Strange D | c | s | purple | green |
| Charmed B | b | c | purple | pineapple |
| Kaon0 | d | s | peppermint | green |
| B0 | d | b | peppermint | pineapple |
| Phi | s | s | green | green |
| Strange B | s | b | green | pineapple |
| pion | u | d | red | peppermint |
| kaon+ | u | s | red | green |
| B+ | u | b | red | pineapple |
| Charged rho | u | d | red | peppermint |
| Kaon*+ | u | s | red | green |
What’s important from this exercise is realizing that all of these two-quark combinations can really happen. Some of the mesons are the ones that help stick nuclei together. Others are found in outer space, as cosmic rays. Others are only found when scientists smash other particles together to find out what they are made of. Recently, the last of the mesons described by this model was detected by an international team of physicists, using the Large Hadron Collider at CERN, in Switzerland. This prompted huge celebrations by physicists and the process inspired a documentary film about the search for the Higgs Boson, Particle Fever.
When I ran this project at BayCon in 2017, one of the young participants scanned the list above and said, “What about the top quark?” Trust a science-fiction fan to spot an anomaly. Indeed, none of the known mesons make use of the top quark, which is the most elusive one of all, and in some ways the most peculiar. The top quark is extremely unstable–even more ephemeral than the strange, charm, and bottom quarks–and it requires a large particle accelerator to observe one. (Fermilab managed it first; now CERN‘s Large Hadron Collider holds the record.) Even then, once produced, a top quark vanishes in 1/1,000,000,000,000,000,000,000,000th of a second. The top quark is also amazingly massive, fueling the deep interest in the nature of mass itself, which many think is one of the functions of the Higgs boson, which itself has only recently been (tentatively) observed. Scientists at CERN hope to use the relatively massive top quark as a test laboratory to verify their (provisional) Higgs boson observations.
Three-quark particles are called baryons–the most common of these are protons and neutrons. The next step for our own quark exploration is to find the combination of up and down quarks that yields the proton and the one that forms a neutron. Each person has 2 peppermint and 2 of one other color to play with. Each group can also pool resources (still keeping those candy wrappers on) to mix and match groups of three using only 2 colors of candy.
To sort out which of these combinations works requires one extra piece of information. We know that an electron has a charge of -1, a proton has a charge of +1, and a neutron is neutral, with a charge of zero. Another cool feature of quarks…and one of the hardest things their discoverers had to come to terms with…is that they have fractional charges. Before quarks, everyone used to think of a charge…equal to the electric charge of an electron…as an indivisible thing. Just like an atom. But just as it has turned out that atoms aren’t indivisible, neither is charge.
Up quark’s charge: +2/3
Down quark’s charge: -1/3
So, with just a little arithmetic, we can find out which of our combinations makes a proton and which makes a neutron. Here’s the cheat sheet:
| uuu |
2/3 + 2/3 + 2/3 = 2 |
Positive…but too much for a proton |
| ddd |
(-1/3) + (-1/3) + (-1/3) = -1 |
Negative, so it can’t be a proton or a neutron.
Note: it’s not an electron either–remember, an electron is already an elementary particle. |
| uud
or udu or duu |
2/3 + 2/3 + (-1/3) = 1 |
OK! It’s a proton! |
| (Just a reminder…the order the quarks are listed in doesn’t matter.) | ||
| ddu
or dud or udd |
-1/3 + (-1/3) + 2/3 = 0 |
Yes! We have discovered the neutron! |
Aha, it’s a proton.
Aha, It’s a neutron!
So, the charge calculations show that protons and neutrons are made of two ups plus one down for a proton and two downs plus one up for a neutron.
It’s possible to have participants glue their protons and neutron quark groups together. A dip on the water cup from the atomic marshmallow project will make a candy piece sticky. However, these sticky messes will need to sit aside for a while to dry. If your participants include young children, you might want to skip that possibility, as a glued-up stack of Life-Savers could be a choking hazard.
Speaking of glue, the same BayCon2017 participants also suggested some ideas for incorporating gluons into our model. To cover the topic of quantum chromodynamics would be a fun challenge, but for the present, those lonely orange LifeSavers we’d set aside as those transient top quarks can be added between the red and white candies in our proton and neutron models to represent the color exchanges among the quarks.
So now we have established that everything in matter is made of tiny (and flavorful) points of dancing energy called quarks and leptons. How can we visualize the true relative sizes of these quarks, protons, nuclei, and atoms?
Poke a pin through a piece of paper and hold it up to the light, then pass it around, so everyone can see how tiny that hole is. Think of that bright speck as an electron or a quark. To be at the same scale, our helium nucleus would be about 3 feet across. A handy meter-stick or yardstick will provide a sense of scale, but for drama, bring out a huge balloon (the 36-inch size). It won’t be edible, but it will be fun to play with afterwards. If that big old balloon is the tiny nucleus, then to build a whole helium atom we’d need a marshmallow about seven miles (ten kilometers) across!
So let’s check back on our atom model from the atomic marshmallow project. It’s mostly nothing, just that airy, fluffy marshmallow. Remember how thin the “shell” of the electron cloud is, and how surprisingly hard it is to notice the tiny nucleus once the two little protons and neutrons were placed inside. Even so, in our model, the protons and neutrons are huge compared with the atom. Imagine how fantastic the resulting candy treat would be–and how many people could enjoy it–if we’d tried to make this marshmallow atom model to scale.
