Fundamental Neutron Physics Beamline (FNPB)

The fundamental physics beamline showing the "cold neutron" area inside the SNS experimental hall and the external UCN facility. For scale, the existing n+ p → d + γ apparatus is shown in the "cold beam" position, and the proposed neutron electric dipole moment (EDM) apparatus is shown in the external building. Click image for a larger version.

The Fundamental Neutron Physics Beamline at the SNS will exploit the special characteristics of a pulsed spallation source to study the detailed nature of the interactions elementary particle. Of particular interest is the study of fundamental symmetries such as parity and time reversal invariance and the manner in which they are violated in elementary particle interactions. The experiments proposed for this beamline will address important questions in nuclear and particle physics as well as astrophysics and cosmology. These experiments include precise measurements of the parameters describing neutron beta decay, studies of the quark-quark Weak interaction, and the search for a neutron electric dipole moment.

Fundamental Physics with Cold and Ultra-Cold Neutrons at the SNS

Cold neutrons and ultracold neutrons (UCNs) have been employed in a wide variety of investigations that shed light on important issues in nuclear, particle, and astrophysics in the determination of fundamental constants and in the study of fundamental symmetry violation. In many cases, these experiments provide information not available from existing accelerator-based nuclear physics facilities or high-energy accelerators.

The scientific issues addressed by most current and proposed experiments in cold-neutron fundamental physics can be placed into three categories as follows:

1. Measurement of the parameters that describe neutron beta decay as (1) a detailed probe of the nature of the electroweak theory, (2) a test of the unitarity of the Cabibbo-Kobayashi-Moskawa (CKM) matrix, (3) a probe of physics beyond the Standard Model (SM), and (4) an important input to the theory of Big Bang Nucleosynthesis (BBN).
2. The study of the nature of the weak interaction between hadrons via the measurement of parity non-conserving (PNC) effects in simple two-particle systems such as n-p, n-d, and n-α.
3. The study of the nature of time reversal non-invariance and the origin of the cosmological baryon asymmetry through a search for a non-zero neutron electric dipole moment.

The first category involves the accurate determination of the parameters that describe neutron ß-decay (lifetime and correlation coefficients). Comparison of these results with nuclear and high-energy data can provide important information regarding the completeness of the three-family picture of the SM through a test of the unitarity of the CKM matrix. Neutron decay can be used to determine the CKM matrix element V_ud with high precision in a fashion that is relatively free of theoretical uncertainties. We note that neutron decay is the only system that offers the prospective of a significant improvement in the direct determination of V_ud. Such a measurement can be used to test whether the weak interaction in the charged-current sector is purely vector- axial vector (V-A) (as in the SM) or has right-handed or other components. Neutron ß-decay also dictates the time scale for Big Bang nucleosynthesis, and the neutron lifetime remains the most uncertain nuclear parameter in cosmological models that predict the cosmic ⁴He abundance.

The second category involves the study of the weak interaction between quarks in the strangeness-conserving sector. This study is very difficult because of the overwhelming direct effects of the strong interaction. As a result, the effective weak couplings in the usual meson-exchange model of the process are poorly known. In fact, current experiments yield somewhat contradictory results for the dominant weak hadronic coupling f_Π. Sensitive experiments using polarized cold neutrons to determine parity violation (an unambiguous tag for the weak interaction) in the n-p, n-d, and n-α systems provide an opportunity to measure nucleon-nucleon (NN) weak interactions in simple systems that are not complicated by nuclear structure effects. Several different PNC experiments have been suggested: (1) measurement of the PNC gamma asymmetry in n+p→d+γ, (2) measurement of the PNC neutron spin rotation in liquid Helium, (3) measurement of the PNC gamma asymmetry in n+d→t+γ, and (4) measurement of the PNC neutron spin rotation in liquid para-hydrogen. Because the observable in each of the above experiments depends upon a different linear combination of the π, ρ, and ω couplings, a determination of the complete set allows the extraction of not only f_π, but other couplings as well. Knowledge of these interactions is required to understand parity-violating (PV) phenomena in nuclei, such as the recently observed nuclear anapole moment, and can be used to gain information on quantum chromodynamics (QCD) in the strongly interacting limit.

The third category, which lies at the heart of modern cosmology and particle physics, involves the search for the neutron electric dipole moment (EDM). Among the important issues that are addressed by this experiment are whether or not the baryon asymmetry of the universe is directly related to fundamental T-violation, and whether or not the magnitude of T-violation is consistent with the predictions of the SM. In particular, Big Bang cosmology and the observed baryon asymmetry of the universe appear to require significantly more T-violation among quarks than is predicted by the SM. The next generation of neutron EDM searches will possess enough sensitivity to probe this issue.

While these three categories describe the specific agenda that has already been suggested for the proposed beamline, it is important to note that this field has historically included a wide variety of other investigations that have emerged intermittently as significant scientific opportunities arose. These include other studies of parity and time reversal non-conservation, the determination of fundamental constants, the measurement of nuclear cross sections of interest to astrophysics, investigations involving matter wave interferometry, limits on the neutrality of matter, and tests of baryon non-conservation, among others. While we note that there are no specific, mature, proposals for new experiments, it has been characteristic of this field that such experiments emerge from time to time and provide an intriguing addition to the base program described above.

The Spallation Neutron Source (SNS) offers the United States an extraordinary opportunity to establish leadership in fundamental neutron physics (FNP). In the past, measurements in this field have been significantly limited by statistical and systematic effects. The SNS offers significant gains in both areas. It will have, by far, the highest peak neutron source intensity in the world. The proposed beam will be the most intense pulsed beam in the world for fundament neutron physics. The fact that the SNS will be a pulsed source offers profound advantages for the reduction of systematic effects. The time-averaged neutron fluence from our proposed beamline at the SNS will be greater than that at any continuous neutron source in the United States, including the fundamental physics beamline at the National Institute of Standards and Technology (NIST). When the SNS reaches its final design goal of ~2MW, the flux and fluence will be within a factor of ~3–4 times that at the highest flux beam at the Institut Laue-Langevin (ILL).

Very significantly, the SNS, as a new facility, provides an exceptional opportunity to fully optimize the design of the beamlines, based upon the specific experience at existing facilities. This is especially important in the reduction of backgrounds and the minimization of magnetic interference, which have proven to be serious problems at other neutron facilities.

In general, the advantages of the SNS in reducing systematic errors lie in four main areas:

1. Utilizing the time structure of the beam to analyze background and to separate the signal from parasitic effects that have different velocity dependence (important for the experiments that study the weak NN interaction via gamma asymmetry measurements and neutron spin rotation).
2. Utilizing the time structure of the beam to make both precise and accurate determinations of neutron beam polarization with polarized ³He gas cells (important for the beta asymmetry measurements of the A and B correlation coefficients in neutron decay).
3. Utilizing developments in neutron guide technology, particularly curved "benders" to transport the beam far away from other equipment and experiments without significant loss of flux, thereby reducing gamma-ray and neutron backgrounds. The proposed external UCN facility will be far from other instruments, which will significantly reduce magnetic interference. We note that the SNS has made a commitment to the establishment of rigorous limits on stray magnetic fields.
4. Finally, the design of an independent external experimental facility allows the opportunity to address seismic/vibration noise that is particularly important for some experiments with UCNs.

We have identified five specific experiments as being particularly well suited to a cold-neutron program at the SNS. They relate to the three categories of experiments mentioned. They may be viewed as the projected, initial research program at the SNS fundamental physics facility. We note that the SNS fundamental neutron physics IDT executive committee includes representation from the leadership of all five of these experiments and that it is the express intention of the experimental collaborations to mount their experiments at the SNS.

The experiments are

1. precise measurement of the neutron lifetime, using magnetically trapped UCNs
2. determination of the gamma-ray asymmetry in the capture of polarized neutrons on light nuclei
3. precise measurement of a complete set of beta asymmetry parameters in polarized neutron decay
4. determination of the parity non-conserving neutron spin rotation in light nuclei
5. search for a nonzero neutron EDM, using UCNs and superfluid helium

Finally, in addition to the experiments for which there is an explicit intention to use the SNS, several other cold neutron fundamental physics experiments are ongoing or in the planning and development stage. Because the SNS will have the highest neutron fluence of any source in the United States, it is likely that the SNS will be the source of choice for many, if not all, future cold beam experiments.

For additional information:

Fundamental Neutron Physics at the Spallation Neutron Source

Instrument Development Team