Physics for the 21st Century
The Basic Building Blocks of Matter Interview with Featured Scientist Bonnie Fleming
An exploration of particle physics
Interviewer: What is Fermilab?
BONNIE: Fermilab is the high-energy physics laboratory in the United States. It is up until very recently home to the most powerful particle accelerator in the world, the Tevatron, which collides protons and anti-protons at near the speed of light to produce all sorts of things that we’ve learned about over the past decades. Now that the LHC has turned on in Europe, the Large Hadron Collider, the so-called energy frontier has moved to the Large Hadron Collider at CERN. And, here at Fermilab we continue to run the Tevatron and will for some time to come. And, the future Fermilab will move into the intensity frontier, meaning high intensity beams to probe the fundamental particles of nature.
Interviewer: What is the motivation for your research?
BONNIE: One of the most fundamental questions in particle physics today is why do we see a preponderance of matter over anti-matter in the universe? The field that I work in, Neutrino Physics, is a sector of particle physics that hopes to address that very fundamental question. So, specifically, neutrinos are one of the twelve so-called building blocks of matter, elementary particles of matter.
Interviewer: What are the basic building blocks of matter?
BONNIE: The basic building blocks of matter are the bits of particles that we cannot seem to break apart into any smaller particles. And, they include the twelve elementary particles of the standard model. So, as a Particle Physicist, I smash things open at higher and higher and higher energies to try to see particles that I can no longer smash open, that no longer are comprised of some other kinds of particles. And, that process of going from a macroscopic object down to something that I can’t break open anymore, what I’m left with are the fundamental particles of nature, the six quarks and the six leptons.
Interviewer: What is the Standard Model?
BONNIE: So, particle physicists take the world and try to break the world down to the smallest components they can break it down to. And, we come up in the end with twelve particles. The standard model is to particle physics like the periodic table is to chemistry. It’s a description of the building blocks of matter and how they interact. And, it’s a beautiful description. It fits together extremely well.
A hundred years ago, just to give you a sense of what we knew a hundred years ago versus now, people thought that the most fundamental particles were the proton, the neutron, and the electron. In fact, there are famous quotes of scientists saying particle physics is done. We know the fundamental particles. The rest is in the details. A hundred years later, we know that protons and neutrons are not fundamental particles. So, of the twelve fundamental particles, meaning particles, which we can’t seem to break down into any smaller particles, there are six quarks and six leptons. The lepton family is comprised of the very familiar electron, a heavier version of the electron called a muon, and a still heavier version called the tau. And each of those is paired with a particle called the neutrino. And, the neutrino is even within the standard model, elementary particles rather unique, very small. That’s the “ino” part of neutrino and neutral, electrically neutral. And, there are three different flavors associated with those three different flavors of leptons.
Interviewer: What are neutrinos?
BONNIE: Neutrinos are three of those twelve elementary particles in the standard model. They interact individually in nature in the sense that they’re flying around all the time individually. They interact weakly only. They’re the only particle of the standard model that interacts via only one fundamental force of nature, the weak force. They’re neutral. They don’t have any electrical charge. And hence, do not interact electromagnetically. And, there are three different flavors that are paired with the three charged leptons.
Interviewer: How do we know neutrinos have mass?
BONNIE: That’s an excellent question, because it took us a long time to convince ourselves that neutrinos have mass. One of the things that set neutrinos apart from the other building blocks of matter is that their mass is at least extremely small, orders of magnitude smaller than any of the other fundamental particles. So, you can’t measure their mass in the way that we measure the mass of the other fundamental particles. Other particles you can, from a particle physics perspective, put on a scale and measure the mass. Neutrinos, we don’t have a scale that’s essentially precise enough to do that. We can’t use the remnants of an interaction and use what we know from the conservation of energy and momentum to figure out what the neutrino mass is. So, we had to look for another phenomena that would be a suggestion that neutrinos have mass. And it’s only been within the last decade or so that neutrino physicists have convinced themselves that there is a phenomena called neutrino oscillations that occurs. And, that because neutrinos oscillate, we know they have mass.
Interviewer: What are neutrino oscillations?
BONNIE: A neutrino oscillation is when one of flavor of neutrinos spontaneously turns into another flavor of neutrino. So, you start with say an electron neutrino. And, it flies through space and you measure it at some point after it’s traveled some distance. And, you see in fact it’s become a muon neutrino. And, that is an oscillation. It oscillated from one flavor of neutrino to another flavor of neutrino. And, that phenomena actually also occurs in the quark sector. So, it sounds kind of crazy. But, we see it a fair amount in particle physics. For that phenomena to occur, neutrinos must have mass.
Interviewer: Why is it not possible for particles to oscillate into each other and be mass-less?
BONNIE: So, a particle that is mass-less travels at the speed of light, like the photon. The light particle travels at the speed of light. For a particle that’s traveling at the speed of light, the concept of time doesn’t exist, because you’re traveling at the upper speed limit in the Universe. So, nothing changes as a function of time. By contrast, a particle that’s massive, because it has mass, cannot ever get to travel fully at the speed of light. Therefore, it has a concept of time. And, it can evolve as a function of time. Used to be that we thought neutrinos were mass-less and therefore, traveled at the speed of light. That’s kind of a special feature for a particle. The photon that travels at the speed of light is a pretty special particle. When we observe that neutrinos oscillate or change flavor as a function of time, that’s a behavior that you associate with something that can see a concept of time, and therefore, a particle that cannot be traveling at the speed of light, because it is evolving as a function of time, and therefore, the neutrino has to be a massive particle.
Interviewer: What did the discovery that neutrinos have mass mean to the standard model?
BONNIE: People have not figured out how neutrino mass falls out of the standard model. You can put it in by hand. But, there’s a number of different theories for how the standard model generates neutrino mass. And, it will take future experiments and more knowledge about how the standard model works overall to be able to figure out what causes neutrinos to have mass in the standard model. That’s still an open question.
Interviewer: What is MiniBooNE (Mini-Booster Neutrino Experiment)?
BONNIE: The MiniBooNE experiment was designed to address an excess of events that was observed by the LSND experiment at Los Alamos in the 1990’s. It’s not completely understood what that excess of events of is still today.
Interviewer: Describe the MiniBooNE detector.
BONNIE: The MiniBooNE detector is looking for Cherenkov radiation signatures of neutrino interactions in a sphere of mineral oil, which is twelve meters in diameter. And, it’s a technique that allows you to have a quite large detector and still be able to differentiate electron neutrinos well from muon neutrinos. And, MiniBooNe’s goal was to confirm or rule out this LSND signal in the one GeV neutrino energy range.
Interviewer: What is Cherenkov radiation?
BONNIE: Cherenkov radiation is a great phenomena that is the mechanism that we use for particle identification on MiniBooNE. Now, you know what Mock One is, when you go faster than the speed of sound, you hear a big boom. You jump out of your sound cone. Very fast airplanes can do this. In fact, you can jump out of your own light cone; the same phenomena can happen with light. Now, that’s confusing to begin with, because Einstein tells us that nothing can travel faster than the speed of light. But, you have to be careful what Einstein tells us is that nothing can travel faster than the speed of light in vacuum. Light can slow down in media. In the oil that is the MiniBooNE detector, light actually goes slower than the electrons and the muons that are produced from neutrino interactions. Those particles can jump out of their light cone. And, just like you can get a sonic boom when you jump out of your sound cone, you can get a sonic light boom when you jump out of your light cone. And, you get interference effects when you do this that causes light to come off of the path of the particle at angles that travel to the edge of the detector and produce rings inside the edge of the detector. And, the electron and the muon jump out of their light cone in different ways, because the masses of the particles are very different. And therefore, an electron ring from Cherenkov radiation looks very different than a muon ring from Cherenkov radiation. So, we use this ring produced by this phenomena of Cherenkov radiation to identify the type of particle in the detector.
Interviewer: What are the results of the MiniBooNE detector?
BONNIE: MiniBooNE in the end did not observe the same excess at the net energy range as we would have expected for muon type neutrinos oscillating into electron type neutrinos as suggested by what LSND saw. Instead, MiniBooNE surprised us. So, one should always be open to surprises in the field. MiniBooNE saw an excess of events at lower energies than we expected, events that we don’t understand. These events could be electron neutrinos or they could be photons. That’s two classes of particles which we cannot differentiate using the MiniBooNE detection technique. So, the new set of experiments that I’ve now begun working on use a different kind of detection technique, a liquid argon time projection chamber, which can differentiate between electrons and photons, which is both crucial for addressing this excess of events that we see on MiniBooNE and trying to explain what they are, and crucial for the next generation experiments which will look for muon neutrino to electron neutrino appearance, but now using beams that travel over hundreds of kilometers, travel over a thousand kilometers, to understand our matter dominated Universe.
Interviewer: What are the possible interpretations of the LSND and MiniBooNE results?
BONNIE: So, possible explanations for the LSND results and the excess of events that MiniBooNE sees at low energy include a gamut of beyond the standard model physics, possible so-called sterile neutrinos. Sterile neutrinos that travel in extra dimensions, new forces, violation of a CPT invariance, a whole host of different possible theories, which could revolutionize physics if we could find suggestions that they are in fact the cause of those anomalies. But, until we further investigate the MiniBooNE low energy excess, understand in particular if it’s electrons or photons, we can’t move forward in addressing the question. If those events are photons, then it’s interesting but background that we didn’t depreciate. If it’s electrons, then they’re coming from electron neutrinos. And then, it’s probably something quite unexpected.
Interviewer: What is a sterile neutrino?
BONNIE: A sterile neutrino is a kind of neutrino that can oscillate with the standard model neutrinos, but cannot interact via the weak interaction. We know from results from the LEP experiments at CERN that there can only be three standard model neutrinos, meaning three neutrinos that interact via the weak interaction, the electron neutrino, the muon neutrino and the tau neutrino. There cannot be a so-called fourth generation. So, if there are more neutrinos, they have to be neutrinos that can oscillate with the standard model neutrinos, but aren’t quite the same.
Interviewer: What do the results of MiniBooNE mean for the Standard Model?
BONNIE: We don’t expect to see electron neutrino appearance at the energies that we saw in excess on MiniBooNE within the standard model. So, if they are really electrons from electron neutrinos, we’ll have to rethink neutrino oscillations within the standard model. In fact, neutrino oscillation is already outside of the standard model. The standard model does not predict that neutrinos have mass. Neutrino oscillations and therefore the implication that neutrinos have mass is really the first beyond the standard model physics in a long time.
Interviewer: What is ArgoNeuT?
BONNIE: ArgoNeuT is a test experiment here at Fermilab. ArgoNeuT stands for argon neutrino teststand. And, it uses a newer technology for neutrino physics called a liquid argon time projection chamber. It’s a precision technique unlike for example the MiniBooNE detection technique. It’s on the order of an order of magnitude more precise in terms of spatial resolution. And, it’s a technology that appears scalable to very large sizes.
Interviewer: How does a liquid argon time projection chamber work?
BONNIE: You have a volume of argon, liquid argon, which is liquid at eighty-seven to ninety degrees Kelvin. A charged particle goes through the argon and knocks off the electrons on the argon atoms, ionizes the argon. You set up an electric field to drift those ionization electrons to the edge of the detector where you read them out by passing them through planes of electrodes. And, you read the electrical signals induced and collected on the electrodes at the edge of the detector. And therefore, reconstruct the image of those tracks of ionization electrons in the detector. If you know the time that the particle went through the detector, and you know how long it took to drift those particles to the edge of the detector, you can project back in time and figure out where in the middle of the detector the interaction took place.
Interviewer: What is a challenge of this new neutrino detection technique?
BONNIE: To do this kind of detection technique, you need extremely pure argon, so the electrons don’t get gobbled up by oxygen, by water essentially, along the way. You need very low noise electronics, because there’s no amplification of the signal in the liquid. And, you need a cryogenic system that can handle large volumes of liquid argon. And so, to be able to scale these up to the biggest sizes, we’re starting at a small scale and observing neutrino interactions.
Interviewer: What experiments are you thinking about beyond ArgoNeut?
BONNIE: We’d like to develop these liquid argon time projection chambers to very large scales to address the very fundamental question of CP violation in the neutrino sector. We look around and we see a preponderance of matter in our Universe. When we produce matter in the laboratory, we can only do so when we produce an equal amount of anti-matter. And when that matter and anti-matter meet, they annihilate into a puff of energy. So, how can it be in the big bang, the beginning of our Universe that we produced an excess of matter when you have to do that only by producing the same amount of anti-matter, which then annihilates. Everything would annihilate away. And, we would just have an empty Universe. And, instead, we have this little excess of matter that gave rise to what we see in the Universe.
Interviewer: What evidence do you look for that would explain a matter dominated universe?
BONNIE: So, what we look for in particle physics is a difference between particles and anti-particles to somehow explain this a symmetry that occurred in the early Universe to create us. We’ve looked for that in the quark sector. And, we can’t find what we need to explain the Universe. So, the last hope is the neutrino sector. Can we somehow look for differences between neutrinos and anti-neutrinos that could explain our matter dominated Universe? And, to do so, we need to look for differences in neutrino versus anti neutrino oscillations at base lines greater than a thousand kilometers and energies in the one to ten GeV range. So, the experiments that I work on are developing these precision detection techniques to address this next very fundamental question. Use liquid argon TPC’s to look for muon neutrinos oscillating to electron neutrinos and compare that to anti-muon neutrinos oscillating into anti-electron neutrinos. And, if those rates differ, that could be a real clue to a difference between neutrinos and anti-neutrinos that could be some mechanism that give rise to our matter dominated Universe. And that is something very unexpected and does not fit into our theory of the standard model.
Interviewer: What does doing your research look like?
BONNIE: I do something different every day. And, I find that quite interesting, quite fascinating and all contributing to understanding the fundamental particles of the Universe, which is fantastic fun. I get to collect data, teach students, build detectors, all towards addressing some of the most fundamental questions in the Universe. That’s fantastic.
1.2 The Basic Basic Building Blocks of Matter — Video
In the past century, physicists have discovered new constituents of matter—quarks, gluons, neutrinos, and many others. These basic building blocks have been linked together into a theoretical framework, the Standard Model of particle physics, that has been very successful in making predictions that were later confirmed by experiment. Even so, there are hints that the Standard Model is incomplete, and that a deeper theory lies behind it, waiting to be teased into the open. See how scientists are seeking answers to these questions with the latest neutrino experiments and the search for the Higgs boson.
Supplementary: Unit 1: The Basic Building Blocks of Matter — Printable Online Textbook
Supplemental resource for educators and students