IUCF was originally formed as a laboratory to study the nuclei of atoms using the new cyclotrons constructed under funding from the National Science Foundation. Nuclei are the massive tiny core of atoms that give them their identity as specific isotopes of a given element. They are made up of protons, the number of which determine the element, and neutrons, the number of which determine the isotope. The goal of understanding the structure of nuclei has led to the exploration of the fundamental forces, the strong force and the weak force, which are important at the nuclear level and their symmetries; the underlying quark and gluonic structure of the protons and neutrons; and nuclei under extreme conditions. IUCF continues as a base for these exciting research endeavors and the equipment development necessary to carry them out.
Neutrinos
The neutrino study group at IUCF is currently involved with the MiniBooNE and SciBooNE experiments, both located at Fermilab. MiniBooNE is a neutrino oscillation experiment which allows for the study of neutrino mass. Neutrino mass is important because it may lead us to physics beyond the Standard Model. MiniBooNE's primary goal is to investigate the neutrino oscillation signal reported by the Los Alamos LSND experiment. The LSND collaboration presented strong evidence for the oscillation of muon anti-neutrinos to electron anti-neutrinos that lead to mass-squared differences (between neutrinos) around 1 eV2. If M iniBooNE confirms this signal, then significant changes are demanded of our current model for understanding the building blocks of nature. In particular, a positive result for oscillations will tell us that nature contains at least four different types of neutrinos, at least one of which would be almost totally non-interacting (or sterile). Masses in the range accessible to MiniBooNE will expand our understanding of how the universe has evolved.
The SciBooNE experiment has recently commenced running and will study the interactions of neutrinos with matter. This will allow for better measurements of oscillations by MiniBooNE and other experiments. It will also lead to greater insight into the physics of neutrino interactions with matter.
Weak Interactions
The thrust of this research activity is the investigation of the weak interactions of low energy neutrons. These experiments use intense beams of low energy neutrons produced either at a nuclear reactor or at an acclaerator-based neutron source, such as the IUCF LENS platform. Examples of weak interactions research performed by the IU group, led by Mike Snow, include several fundamental results. Measurements of the weak interactions of low energy neutrons with heavy nuclei at LANSCE have been analyzed within statistical models of the nucleus to learn about the weak interactions between nucleons. At NIST we measured the decay rate of the neutron in an experiment using a Penning trap to hold and later count the protons one-by-one. The neutron lifetime is useful in determining o ne of the parameters of the Standard Model and testing the "universality" of the weak interaction. In addition, it essentially determines the amount of 4He in the universe after the early part of the Big Bang is over (but before stars formed and burned hydrogen into helium). We participated in precision measurements of the coherent neutron scattering amplitudes from hydrogen, deuterium, and 3He using the Neutron Interferometer and Optics Facility at NIST. These results disagreed with almost all of the predictions using existing NN interaction potentials and have been used in conjunction with other measurements in nuclear few body systems to constrain models of nuclear 3-body forces. The NPDGamma collaboration is examining the weak interaction of a neutron with a proton. This interaction, which has never been measured, is the only remaining interaction of the known forces among the particles of "normal" matter which has yet to be determined. This will be the first experiment on the Fundamental Neutron Physics beamline at the Spallation Neutron Source (SNS). We are also working on an experiment at NIST to observe parity violation in n+4He using the phenomenon of neutron spin rotation.
Solenoidal Tracker At RHIC (STAR)
The hadronic interactions group works at the STAR detector located in the Relativistic Heavy Ion Collider (RHIC) accelerator at Brookhaven National Laboratory (BNL). Although most of the scientists at RHIC are working to study the Quark Gluon Plasma discovered there recently, the main focus of our group is to study the spin structure of the proton. This is done by using RHIC as the first polarized proton-proton collider which makes use of Siberian snakes, first studied extensively here in the IUCF Cooler, to preserve the polarization while accelerating the protons to 100 GeV. Only a small amount of the proton's spin is found in Deep Inelastic Scattering to come from the spin of quarks. To investigate whether the gluons might possibly carry the rest of the spin we look at violent collisions of the quarks and gluons which produce jets of particles and high energy gamma rays. These are detected with the STAR detector. To enhance the capabilities of the STAR detector at forward angles our group built and installed an Endcap Electromagnetic Calorimeter (EEMC).
aCORN ("Little 'a'")
The project measuring the electron-antineutrino CORrelation in Neutron beta-decay (aCORN), or "little 'a'" probed one of the experimentally determined parameters in the formula predicting neutron decay probability. The formula was derived by Jackson, Treiman and Wyld in a 1957 paper. Among the four parameters "little 'a'" is the least precisely measured. It quantifies the correlation between the electron and anti-neutrino following the decay.
The weak interaction is one of the four basic forces which physicists have described. It is most commonly seen in beta decays. The simplest of beta decays is the splitting of a neutron into a proton, electron, and a neutrino. Despite being abundant in nature, neutrons are unstable outside of nuclear matter. In its rest frame a neutron has a half life of roughly 10 minutes. In a mathematical sense, "little 'a'" is the parameter that measures the dependence of the neutron decay on the angular relationship between the electron and neutrino momentum. For instance, if a was negative the decay would favor the electron being emitted in the opposite direction of the neutrino. Likewise, if a was identically 0 the electron's initial momentum would be independent of the neutrinos.
Many Body Nuclear Dynamics
The focus of this research is heavy-ion reaction dynamics at intermediate energy (between 20A and 200A MeV). Many body nuclear dynamics examines the nuclear equation of state (EOS), in particular how the density dependence of the symmetry energy affects the properties of nuclear matter. This group, headed by Romualdo de Souza and Sylvie Hudan, is also interested in the interplay between the statistical and dynamical break-up of nuclei under extreme conditions of density, temperature, shape, and isospin (neutron-proton asymmetry). This research requires development of sophisticated detectors and associated electronics such as segmented arrays using silicon strip technology which provide isotopically resolved identification of reaction products formed in nuclear reactions (IMFs and LCPs) and an advanced multi-channel signal processing system (MASE) using highly segmented detector arrays.
Neutron EDM Collaboration
The search for a permanent electric dipole moment of the neutron (nEDM) has been the subject of experimental investigations for 50 years, and has recently emerged as one of the most sensitive tests of physics beyond the Standard Model. A non-zero nEDM would be a signal for time-reversal symmetry violation - a key ingredient for understanding the matter-antimatter asymmetry in the universe. The most sensitive nEDM upper limit to date is derived from experiments performed on ultra-cold neutrons (UCN). UCN have kinetic energies of a few hundred nanoelectron volts - less than the Fermi potential of many substances - and are easily trapped. The neutron velocities in these experiments are small and randomized, which is very advantageous for the suppression of systematic effects. A new nEDM search, with a projected sensitivity two orders of magnitude better than the current experimental limit, is underway using UCN produced via the superthermal process (the downscattering of cold neutrons by phonons) in a superfluid helium bath and subsequently held in the bath. Cold neutrons will be provided on a dedicated beam line for the experiment at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory. The SNS nEDM experimental collaboration consists of about 60 researchers from 15 institutes, including IUCF.
The SNS nEDM experiment employs a technique common to previous experiments, in which a sample of polarized neutrons is held in a static magnetic field. The neutron spins are tipped perpendicular to the field, whereby they precess at the Larmor frequency. If, in addition to the magnetic moment, the neutron possesses a small electric moment, additional precession will be observed when a strong electric field is applied parallel to the magnetic field. Sensitivity to an EDM signal scales as the size of the electric field; to attain sensitivity well below the current experimental limit, the target electric field in the experiment is several tens of kilovolts per centimeter. IUCF researcher Josh Long has developed a prototype system for generating the required fields across the large volumes of superfluid necessary for the experiment.
The nEDM experiment uses a combination of SQUID magnetometry and scintillation light detection to measure the polarized neutron precession. The density of UCN in the superfliud is too low for the precession to be detected by SQUIDs alone. Therefore, a precise concentration of polarized 3He atoms is injected into the superfluid. This concentration is high enough to be detected by the SQUIDs. The polarized neutrons and 3He atoms react with each other, resulting in charged particles that produce scintillation light. Since the neutrons and 3He atoms precess at slightly different rates, and reaction from a singlet state is about 200 times more likely than from a triplet state, the scintillation rate is an indicator of the angle between the polarization vectors of the neutron and 3He ensembles. The scintillation light signal is expected to be extremely small, however, and complicated by losses as it is extracted from the superfluid to detectors at room temperature. With the goal of reducing the extraction path, IUCF researcher Hans-Otto Meyer is investigating the operation of photomultiplier tube detectors at cryogenic temperatures. Furthermore, it is not known how the magnetometry will be affected by the nearby electric fields. To investigate this, IUCF researcher Chen-Yu Liu is studying SQUID operation in a small prototype cryostat fitted with high-voltage electrodes.
Electron EDM Collaboration
A search for the permanent electric dipole moment (EDM) of the electron using a paramagnetic solid sample is expected to yield immediate results in the search for physics beyond the Standard Model. As it is free from hadronic uncertainties, the electron EDM is particularly sensitive to phenomena predicted by new physics, and the sensitivity of current EDM measurements is approaching the level of the predictions of several new models. One such high sensitivity electron EDM search is currently being carried out at Indiana with plans to convert this experiment to a more sensitive version at sub-Kelvin temperatures. As opposed to the atomic beam systems used in other EDM searches, these experiments are conducted on a solid state system, and promise several orders of magnitude improvement over the beam experiments solely from the available electron density.
An electrically-insulating paramagnetic material will become spin polarized in the presence of an electric field if there exists an electron EDM. A sample polarized in that way develops a net magnetization that might be detectable using state-of-the-art SQUID magnetometers. This electric-field-induced spin order increases as the sample temperature decreases. We have identified a candidate material, gadolinium gallium garnet, that has a low conductivity and a high concentration of heavy ions, Gd3+. The net electric dipole moment of unpaired electrons in the valence shell of Gd3+ is predicted to be high.
With a practical sample size of 100 cubic centimeters and a typical applied electric field of 10 kilovolts per centimeter, we expect, after 10 days of data averaging, to place an experimental limit on the electron EDM of approximately 1e-29 e-cm, which is about 100 times better than the current limit set by the thallium beam experiment at Berkeley.
Over the past few years we have built a fully functional preliminary version of this experiment at Los Alamos. It consists of a 10 cubic centimeter sample held in a liquid helium cryostat at 2 degrees Kelvin and exposed to an electric field of 5 kilovolts per centimeter. To date it has achieved a sensitivity of 1e-24 e-cm, already twenty times more sensitive than the pioneering experiments in the late 1970s. Works on the study of systematic effects in this experiment continues, and parts for a larger-scale version of this experiment are on order at Indiana.
Deuteron EDM Collaboration
The Deuteron electric dipole moment (dEDM) Collaboration is developing a search for electric dipole moments on charged particles by placing a polarized beam of them into a storage ring and observing the precession due to the electric field induced in the particle frame of reference by the ring dipole magnets. These electric fields are at least an order of magnitude larger than what can be routinely achieved with a static electric field in the laboratory. Without further intervention, the precession driven by the particle magnetic moment in the ring dipole fields will severely limit the effect of an EDM by canceling its contribution on each revolution of the particle's spin. To break this cancellation, the storage ring will be equipped with RF cavities that produce forced synchrotron oscillations that are in phase with the magnetic moment precession. If the polarization is initially horizontal, this broken cancellation allows the EDM to drive the accumulation of a vertical component to the beam polarization. Initial efforts will be aimed at searching for this vertical EDM polarization using the deuteron since it has a relatively small anomalous magnetic moment and a polarization that is straightforward to produce and measure. In addition, EDMs that arise from the brief appearance of super-symmetric particles (if they exist) in the quark-quark interaction are favored by the structure of the deuteron, making this nucleus particularly sensitive to their presence. The spin polarization of the deuteron will be measured by slowly extracting the beam from the storage ring and observing its spin-dependent scattering from thick targets of carbon. Ed Stephenson is leading a team to develop extremely sensitive techniques for making such polarization measurements at the COSY electron-cooled synchrotron located in Jülich, Germany.
Nuclear Physics Completed Projects
The Cooler CSB experiments at IUCF were concerned with the d+d -> 4He + πo reaction. Several searches for the d+d -> 4He + πo reaction have produced only upper limits. Ed Stephenson chose to look just above the reaction threshold at 225.5 MeV to avoid other pion-producing channels and to take advantage of the clean experimental conditions afforded by the IUCF electron-cooled storage ring. A 6 degree bend located in one section of the ring provided a site where 4He nuclei could be separated from the circulating deuteron beam. A gas jet target was positioned just upstream and a large solid scattering angle was covered by two arrays of Pb-glass detectors. The data from these experiments provided the first unambiguous measurements of the d+d -> 4He + πo cross section as well as that for the competing double radiative capture process.
The Polarized Internal Target Experiments (PINTEX) group, headed by Hans Otto-Meyer, studied proton-proton and proton-deuteron scattering, and reactions between 100 and 500 MeV at the IUCF. Some experiments made use of electron-cooled polarized proton or deuteron beams orbiting in the IUCF electron-cooled storage ring. Others utilized a polarized atomic-beam target of hydrogen or deuterium in the path of the stored beam. The collaboration involved researchers from several Midwestern universities, as well as a number of European institutions. The PINTEX program ended when the Indiana Cooler was shut down in August 2002.
