What is the World Made of?

What is the world made of? Depending on whom you ask, you will get very different answers to that question. To a biologist, the world is made of living organisms. To a chemist, the world is made of molecules formed from atoms. If you ask that question of a physicist, the physicist will probably start by talking about atoms, and then proceed to talk about the protons, neutrons and electrons that make up an atom. However, for particle physicists such as myself, there is no ambiguity in the question. We want to know: what are the fundamental, indivisible building blocks from which all the matter in the universe is made? What are the basic interactions that glue these building blocks together to make the matter we see around us? Is there a finite set of building blocks or is matter like an onion with layer upon layer of inner structure? We do not know the answer to these questions, but we have a fairly consistent picture or model of the world at this most fundamental level that explains all of our experimental results to date. Most of us think that this current picture is incomplete and that there is something deeper and more fundamental lurking behind it. The excitement of our work is that we are constantly probing for the chink in the armor or our current understanding that will reveal some clue of the more profound and fundamental structure beneath.

Figure 1

The progression of structure within structure from the atoms that make up the familiar matter we see around us to the quarks that are thought to be the basic building blocks of nature.

What is the world made of? All the matter you see around you is made of atoms. There are over 100 different kinds of atoms and as early as the turn of this century it was felt that there were too many different atoms for them to be the fundamental building blocks of nature. In fact, atoms are not fundamental building blocks. As shown in figure 1, the atom has structure. It was found experimentally that the atom is made of a small nucleus surrounded by a cloud of electrons. As experiments probed the nucleus, structure was again found. The nucleus of the atom is made of individual protons and neutrons, which are themselves made of fractionally charged particles called quarks.

To date, quarks and electrons seem to be indivisible. There is no evidence that they have finite size or structure, and they cannot be broken into smaller more fundamental objects. We think they are the fundamental building blocks of matter. Everything we see around us in everyday life is made from two different types of quarks (called up (u) and down (d)), electrons (e) and neutrinos([[nu]]e), where the neutrino has to be thrown in to explain the radioactive decay of some nuclei.

Unfortunately, the list of building blocks does not stop here. The u and d quark, the electron, and its neutrino form what we call the first generation, and while all the matter we see around us can be formed from these first generation building blocks, two other heavier generations of quarks and electron-like objects (called leptons) are needed to explain the variety of particles that have been observed in cosmic rays and high energy particle accelerators.

Table I

Generation    I                  II                 III

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

Leptons       electron (e)       muon (u)           tau
              electron neutrino  muon neutrino      tau neutrino
The fundamental constituents from which all matter is made. All the matter that has been observed is made from the particles listed in this table and their anti-particles.

Table II

Force               Carrier of Force
Gravity             graviton
Strong              gluon (g)
Electromagnetic     photon
Weak                Z, W+, W-
The fundamental forces of nature and the particles that transmit those forces.
All matter that has ever been observed can be built from the quarks and leptons listed in Table I. The fundamental forces that govern how these particles interact are listed in Table II. These tables summarize a model of how the world is made at its fundamental level that is self consistent, mathematically rigorous, and describes all of the experimental data we have. There are, however, many unanswered questions, and it is not clear that this model can provide answers to them.

Studying Quarks with CESR/CLEO

In order to study quarks, leptons and their interactions, we need to be able to produce them. This is particularly difficult with the particles of the second and third generation that are not abundant in ordinary matter. One very efficient way to produce particles is to collide electrons and positrons (a positron is the anti-particle of an electron) in a circular machine called a storage ring. When an electron meets a positron they can annihilate and make a state of pure energy that can then rematerialize as a quark anti-quark or lepton anti-lepton pair. If the electron and positron have sufficient energy, they can make quark and lepton pairs from the second and third generation.

Figure 2

An overview of the Cornell campus with the location of the CESR tunnel shown. The Wilson Synchrotron lab, which houses the CLEO detector, is in the lower part of the picture.

On the Cornell campus, 15 meters under Alumni field (as indicated in figure 2) sits the Cornell Electron Storage Ring, CESR, which is one of the premier electron storage rings in the world. There is a tunnel that houses a ring of magnets where counter rotating bunches of electrons and positrons circulate at close to the speed of light. About 7 million times per second, a bunch of 10 billion electrons passes through a bunch of 10 billion positrons. Mostly nothing happens, but once every 10 seconds or so, an electron and positron annihilate and a pair of third generation b quarks is formed.

Using an electron-positron storage ring to study quarks is a bit like trying to figure out how a watch works by smashing two watches together and studying the pattern of broken pieces that emerge. We never get to see the b quarks we produce, because they live only a very short time, but we see the fragments from their decay in a large detector called CLEO. A collaboration of 200 physicists from Universities all over the country participate in the collection and analysis of data from CLEO. In terms of budget and size, the CESR/CLEO facility is relatively modest, but in terms of physics productivity, we are rivaled only by the giant European high energy physics laboratory, CERN and the Fermi National Accelerator Lab out side of Chicago. Yet, for all its preeminence as a world class high energy facility, CESR is very much a campus based lab with faculty, graduate students and even undergraduates playing major roles in making it work. The hands on, shirt-sleeves rolled up spirit of everyone pitching in to get things working has proved enormously successful over the years and the 'Cornell style' is famous throughout the field.

How is data from CESR contributing to our understanding of nature at its most fundamental level? With our machine, we can study all of the particles listed in Table I except for the t quark which is too heavy for us to be able to produce it. The focus of our program, however, has been primarily on studies of the b quark, and that is because we hope that the b quark will offer a clue to a very deep mystery--why we are here.

In the very early universe, a few instants after the 'Big Bang,' we think that there were equal amounts of matter and antimatter. Now, more than ten billion years later, the antimatter has gone away. Our solar system is made entirely of matter, and for as far out into the universe as astronomers can probe (about 60 million light years) we can find no evidence for significant amounts of antimatter. Where did it all go? It has been suggested that the matter that we see now is the result of a very tiny excess of matter over antimatter that formed in the very early universe due to a property of the weak interaction called CP violation. The violation of CP means that in a physical process such as the decay of a particle, if one changes particle to antiparticle everywhere and reverses left and right, then the rate for this new process will be different. The result is that matter and antimatter decay slightly differently, which could account for the excess of matter over antimatter that we see around us today. If there had been no CP violation, matter and antimatter would have annihilated in the hot early universe and nothing would have been left to become us and the galaxies that surround us. Clearly this phenomenon of CP violation is very important, and the b quark provides a crucial input to our understanding, because b quarks are predicted to exhibit small but measurable CP violation in their decays. With a data sample of about 30 million b quarks, we might hope to observe it.

At Cornell, we currently have the worlds largest sample of b quark data, but we need 10 times more data than we presently have before we can attack this problem. A great deal of our effort is going into improving the storage ring so that it will produce a pair of b quarks every second or two instead of the current rate of once every 10 seconds. We are also working on improving our detector to make it more efficient at recording b quark events. If we can succeed at finding experimental evidence for CP violation in b quark decays, we will be a big step closer to understanding the origins of the universe. The search will not be finished because we will still be a long way from being able to prove that CP violation in the early universe and CP violation (if observed as predicted) in b quark decays are related. There are many other questions we need to answer as well. Why are there 3 generations of particles and not more or less? Why do the particles listed in Table I have the masses they do? Why is CP violated in the weak interactions but not, as far as we know, in the strong or electromagnetic interactions? Are the particles listed in Table I really the fundamental building blocks of nature, or do they themselves have a structure? My own personal guess is that as we do increasingly precise experiments with our ever expanding data sets, we will find that the simple picture of nature outlined in Tables I and II is inadequate to explain the data and that nature has some big surprises in store for us.

This research was paid for by THE US GOVERNMENT through the National Science Foundation.

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Last modified: March, 1995

Persis Drell, persis@lns62.lns.cornell.edu