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Indiana University Bloomington

Nuclear Physics

Theory

neutronNeutron rich matter is expected to have a rich structure at sub nuclear densities. All conventional matter is “frustrated”. Nucleons cluster at short distances because of attractive strong interactions and are anti-correlated at large distances because of Coulomb repulsion. Often these length scales are well separated so nucleons bind into nuclei, which are then segregated on a crystal lattice. However at densities of 10^{13} to 10^{14} g/cm^3 (just below nuclear density of 3 x 10^{14} g/cm^3) these attractive and repulsive length scales are comparable. There interference can lead to complex plate, rod and bubble structures for neutron rich matter which are known as pasta phases. Pasta phases are expected in neutron star crusts and in core collapse supernovae. What is shown is a sample configuration of neutron rich pasta from a semiclassical simulation at subnuclear densities and a temperature of 1 MeV. The light (red) spheres are neutrons and dark (blue) sphere are protons.

Nuclear physics investigates the fundamental interactions governing the world of subatomic particles. The building blocks of atomic nuclei: protons and neutrons constitute over 90% of the visible mass in the Universe. They are composites of more fundamental particles known as quarks and gluons. Formulation of the underlying mathematical description of quark-gluon interactions commonly referred to as Quantum Chromo-dynamics (QCD), by F.Wiczek, D.Gross and D.Politzer lead to the 2004 Nobel Prize in Physics. The theory of QCD is complicated and so far only a limited number of predictions have been made theoretically and experimentally validated. The main focus of the research in theoretical nuclear is in development of tools and techniques to study the subatomic structure of matter.

With development of new theoretical and experimental tools the boundaries that traditionally separated nuclear physics, high-energy physics, condensed matter and many-body physics have been dissolving. Today nuclear physics experiments may be using energies that are higher than those of some high-energy laboratories while high-energy physicists may be conducting experiments at nuclear physics facilities. Several phenomena that govern strongly interacting quark-gluon systems have analogies in atomic or condensed matter physics. Physics of the stellar evolution involves nuclear physics when addressing the question of the origin of elements and fate of stars.

Nuclear Theory Center

The mission of the Nuclear Theory Center (NTC) is to carry out fundamental research in subatomic and nuclear physics. It provides theoretical support for the experimental nuclear and high-energy programs at the Indiana University Cyclotron Facility (IUCF) and the Indiana University (IU) Physics Department. The Center provides research and study opportunities for students and postdoctoral associates, it brings collaborators to the center, and organizes seminars and workshops at local, national and international levels.

Current research directions include:

  • Fundamental studies of QCD in the continuum and on the lattice
  • Phenomenology of nuclear structure and interactions
  • Supernova Physics and Neutron Star Structure
  • Effective Models of Hadron and Nuclear Structure

Experiment

Nuclear physics investigates the fundamental interactions governing the world of subatomic particles. The building blocks of atomic nuclei: protons and neutrons constitute over 90% of the visible mass in the Universe. They are composites of more fundamental particles known as quarks and gluons. Formulation of the underlying mathematical description of quark-gluon interactions commonly referred to as Quantum Chromo-dynamics (QCD), by F.Wiczek, D.Gross and D.Politzer lead to the 2004 Nobel Prize in Physics. The theory of QCD is complicated and so far only a limited number of predictions have been made theoretically and experimentally validated. The main focus of the research in theoretical nuclear is in development of tools and techniques to study the subatomic structure of matter.

STAR, THE ORIGIN OF PROTON SPIN

We know that the proton has spin, that it is made of quarks, and that the quarks also have spin. So what’s the problem? All experiments for the last two decades have shown that the quarks are so randomly aligned that together they barely account for a quarter of the spin of the proton. So where does the rest come from? One possibility is that the gluons, the fleeting carriers of the strong force between the quarks, may themselves have their spins aligned. So how do we find out?

When the Relativistic Heavy Ion Collider started running, an opportunity became open. The machine could also run proton beams, and it could align their spins.

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Steve Vigdor and Jim Sowinski work on calorimeter installation.

If we could see when a quark from one proton strikes a gluon in the other proton, we could use the fact that the quark has some spin alignment to check in which way the gluon spin points. RHIC has a large detector system, STAR, with nearly ideal properties. But the gluons most likely to tell us about spin would appear as gamma rays coming out the end of STAR. So a new detector had to be added. Called an electro- magnetic calorimeter, its 23 layers of lead and plastic flash whenever a gamma ray appears. In 2004, the detector and its electronics was in place and ready to go. Then, in spring 2005 the first running with polarized proton beams began. Longer runs aimed at the completion of a full data set will start in early 2006.

STAR was built originally to search for evidence of a quark-gluon plasma whenever two heavy nuclei such as gold collide at relativistic speeds. This plasma is thought to be the state of matter that existed early in the Big Bang before the universe had cooled to the point where neutrons and protons could form. The RHIC machine provides an opportunity to have enough energy in a heavy ion collision to demonstrate that the plasma forms and to study its properties. Since we joined the STAR Collaboration, we're also helping with this search as well. Both experiments are expected to yield a wealth of new data over the next few years.

COLD NEUTRONS, PROBING FUNDAMENTAL PHYSICS

Sometimes the most interesting effects in physics are those that are so tiny they are right at the edge of being observable at all. This is typical of experiments that try to measure the properties of the weak interaction. This interaction causes particles to decay. It also violates parity. This means that, unlike the other forces of nature, it would behave completely differently in a universe that is the mirror image of our own. At this time, we still do not have a good measure of even the strength of some of the pieces of this force. The place to look is at the tiny changes that the weak force causes in processes whose main properties we think we understand.

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The final target and detector for np→dy parity violation

So we start with the simple. Combine a single neutron and proton to make the nucleus of deuterium. A gamma ray is given off when the nucleus is formed. Do it with neutrons whose spin axis points along the neutron's motion. Repeat with the spin pointing the other way. If these two cases are not completely identical, then parity has been violated. The size of the difference is related to the weak force strength. The apparatus for this very sensitive measurement has been under construction for some time. The figure shows 48 crystals of CsI for detecting gamma rays arrayed around a reaction chamber that will contain liquid hydrogen. Neutrons will enter from the left and get captured by the protons that make up the hydrogen nuclei.

The first runs on this experiment started over a year ago. But they're just tests to make sure that everything works. Serious data taking is yet to come.

Spins are often the key that unlocks effects due to the weak interaction. Another way to try this, again with neutrons, is to align the neutron spins and then to send the neutrons through a material in which there is no preferred direction. Without a weak force, nothing happens to the spin orientation. With a weak force, the spin direction rotates slightly, an effect that also violates parity. One good uniform material would be liquid helium since the helium nucleus has no net spin; the two electron spins pair off and cancel, and helium does not form molecules. This experiment is being assembled for running at a reactor near Washington, D.C.

In both cases, these experiments have to be sensitive to very small effects, of the order of one part in 1-10 million. At this level, lots of tiny effects have to be cancelled or removed. Notice the cage with brown magnet windings in the figure. These cancel the small magnetic field of the earth. Unquestionably, such experiments are challenging, not only to the skill of the experimenter but to the imagination to find and eliminate the ways that some subtle problem can arise and spoil the results of the experiment.

Neutrinos, PARTICLES THAT ALMOST AREN’T THERE

Neutrinos were invented to patch up a problem. When the neutron decays into a proton and an electron, it seemed as if some energy and momentum were lost. So it was postulated that another unseen particle, called the neutrino, was produced. Postulating that the neutrino is massless seemed to work well. Since they are generated in a weak interaction process, they would only show themselves if they initiated another weak process. This happens so rarely that most neutrinos can pass through the earth with no effect. So detectors for neutrinos have to be massive and big.

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Schematic view of the neutrino detector. The yellow dots represent photomultiplier tubes.

The solar system has a huge source of neutrinos, the nuclear reactions that fuel the sun. Experiments set up to detect these neutrinos found some, but not as many as expected. So what was wrong? Were our solar models bad? Did our detectors not work as we thought? Or did something happen to the neutrinos between the sun and the earth? Only recently has it become clear that perhaps the least likely idea, that neutrinos mutate in flight, is really the right idea. And if the mutate, they also have mass. So a whole new physics field has opened up. How often do neutrinos change from one type to another? What really is their mass? This is leading to a new generation of large detectors for neutrinos and special experiments to use them. Indiana University is part of a group working at Fermilab on one piece of the puzzle. They are looking for muon neutrinos, made when pions from the Booster accelerator decay, to turn into electron neutrinos. Seeing the electron neutrinos requires a large tank of mineral oil and scintillator, watched very closely by 1280 photomultiplier tubes (figure). The signatures are subtle, as the neutrinos can bounce off the nuclei in the mineral oil or create reactions making pions which subsequently decay. So some time and a lot of data will be needed to unravel these processes and to decide just how many electron neutrinos are being made.

This detector is only the beginning. Called “MiniBooNE”, it is intended to be the first step. If neutrino oscillations are seen, there will be more to study and learn. Then a larger version of this detector will be built. There are other things that neutrinos can tell us if we use them to initiate nuclear reactions. They can measure the presence of strange quarks in the proton, and perhaps could tell us whether these quarks carry any of the missing spin of the proton.

NUCLEAR CHEMISTRY, STUDYING NUCLEAR MATTER

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Cabling around the Indiana Silicon Sphere (ISiS).

The atomic nucleus is the densest material we can study directly. Thinking about it as a drop of incompressible fluid has proven to be an excellent model. So what happens when you heat it up? The addition of calibrated amounts of energy through bombardment by energetic particles, including anti-particles, has shown that the nucleus also has a gaseous phase. Seeing the evidence for this requires catching simultaneously most of the protons, neutrons, and light nuclei that are boiled away as the nucleus cools down again. To do this requires a detector with many segments (figure) that surround a small target chamber. This detector can identify any charged particle or nuclear fragment, giving information on its mass, isotopic character, and energy. With this information, powerful computer simulations are used to reconstruct what happened and to test new ideas about the physics of the processes involved.

Looking over the edge

What we now know about matter has been gathered up into what is called the Standard Model. This model starts with the fundamental particles, such as quarks, electrons, and neutrinos, and the forces that act between them. From there, it is possible to build other particles, such as the proton and neutron. Is this all? Do we have all we need to understand nature? Perhaps not. Why, after the Big Bang, did the universe wind up with an excess of matter rather than nothing? We might have a clue if we could detect the spontaneous separation of positive and negative charge along the spin axis of some particle. Searches have so far have not found such an electric dipole moment.

We intend to keep looking. One standard method people have used is to place a particle in a strong electric field and see if its spin precesses. Such experiments are now pressing their limits of sensitivity, so we need to find ways to search more particles more sensitively. In some crystals, a strong electric field would cause an electric dipole to align along the field. If it Is tied to spin, it is also tied to the magnetic field inside the crystal, and alignment would generate a small magnetic field that could possibly be seen by the most sensitive instruments.

We are also considering ways that accelerators might be used to push the sensitivity of such searches. As the particles accelerate to close to the speed of light, relativistic effects take the magnetic fields that confine the beam inside the machine and generate electric fields at the particle that are much stronger than that fields that can be made in the lab. Some theorists think that we are close to discovering electric dipoles.