Arts & Sciences

Newsletter
Spring 1995 Vol.16 No.2

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, molecules formed from atoms. A 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 of matter-or are there any? 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 something more fundamental lurks behind it. The excitement of our work is that we are constantly probing for the chink in the armor of our current understanding that will reveal some clue to the more profound and fundamental structure beneath.

What is the world made of? All the matter you see around you is made of atoms-over 100 different kinds. As early as the turn of this century, however, scientists 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 (ve), 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 quarks, and the electron and its neutrino (which are called leptons), form what we call the first generation of elementary particles. While all the matter we see around us can be formed from these first-generation building blocks, the pattern of two quarks and two leptons is repeated, making two other, heavier generations of building blocks. The second and third generations of quarks and leptons are needed to explain the variety of particles that have been observed in cosmic rays and high-energy particle accelerators.

All matter that has ever been observed in cosmic rays or in ordinary worldly matter 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, a model 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 c an provide answers to them.

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 antiparticle 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/antiquark or lepton/antilepton pair. If the electron and positron have sufficient energy, they can make quark and lepton pairs from the second and third generation.

On the Cornell campus, fifteen meters under Alumni Field (as indicated in Figure 2 (not shown) ) sits the Cornell Electron Storage Ring, CESR, one of the premier electron storage rings in the world. A tunnel 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 ten 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. Two hundred physicists from universities all over the country collaborate 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 outside of Chicago. Yet, for all its preeminence as a world-class high-energy facility, CESR is very much a campus-based lab with faculty members, 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 early universe, a few instants after the Big Bang, we think that there were equal amounts of matter and antimatter. Now, more than 10 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 and does the mirror-image of the experiment where left and right are reversed, 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 have the world's largest sample of b quark data, but we need ten times more data than we currently 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 ten 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 three 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.

Persis S. Drell (physics) came to Cornell in 1988 and was a recipient of a Presidential Young Investigator Award from the National Science Foundation in 1988. She is working on the CLEO experiment and thinks that Cornell is the best place in the world to do high-energy physics because the physics is great and the style is fun.