Probes of physics beyond the Standard Model via nuclear physics at low energy

Introduction

Minute violations of the fundamental symmetries of nature can lead to measurable low-energy observables. For example, the CP violation mechanism necessary to explain the matter-antimatter asymmetry in the universe might well give rise to permanent electric dipole moments (EDM) far larger than predicted by the Standard Model. Similarly, the weak interaction between electrons and nucleons induces a feeble parity mixing in the electronic wavefunctions and the possible presence of interactions outside the standard V-A structure of the weak interaction will affect the angular correlations between emitted particles. These observables provide access to possible physics beyond the Standard Model and often probe energy ranges far beyond what can be reached by direct measurements at high energy. Many of these effects can be best measured in specific radioactive nuclei chosen to enhance or isolate the sought effects.

These precision measurements greatly benefit from an array of new tools such as optical or ion traps, developed in atomic and nuclear physics, to confine and study small samples of specific atoms or nuclei. The well controlled environment in which the samples are suspended allows very sensitive observation of minute effects such as the possible spinning of atoms in static electric fields due to their EDM. For many observables of interest, relativistic and shape effects enhance the signal expected for a given strength of the underlying beyond the Standard Model physical processes. The high-Z atoms with nuclei of the required shape are therefore particularly interesting for these studies since they can lead to many orders of magnitude enhanced sensitivity to new physics. The atoms with largest enhancement are, however, typically among the heaviest atoms which are also radioactive. A powerful production facility is therefore required to provide the strongest samples of these atoms and reach the highest sensitivity to new physics.

Our proposal is to create a joint facility with shared infrastructure to address this “Physics Beyond the Standard Model” research with radioactive isotopes, as well as, carry out some important engineering demonstrations related to Nuclear Energy and Transmutation of Radioactive Waste.

Beam Needed

The preferred beam for the Nuclear Energy applications is a CW (RF frequency) proton beam at 1 GeV and 1 mA. This beam is well suited also to be the basis of isotope production for the Standard Model research. The concept proposed is to share this beam 50/50 with the Nuclear Energy group on the time-scale of months, i.e. alternating a few months of beam for the Nuclear Energy demonstrations with a few months of beam for Standard Model research. A higher energy CW beam is also useful for the Standard Model research, but sharing the beam at a joint facility at the lower energy required by the Nuclear Energy applications is much more cost effective in terms of the infrastructure investment and operations.

Joint Nuclear Facility Layout

A schematic functional diagram of the proposed Joint Nuclear Facility is shown in figure 1 and a somewhat more detailed schematic layout in figure 2.

Figure 1. Simplified functional layout of the Joint Nuclear Facility

Figure 2. Schematic layout of the Joint Nuclear Facility

The proposed layouts indicated in figures 1 and 2 illustrate the concept of the joint facility in which the infrastructure, utilities, and radiological handling and shielding aspects are shared in a cost-effective manner between the two applications, the nuclear energy demonstration capabilities and the Isotope Production On-Line (ISOL) production module.

The basic building blocks required for the Standard Model research are the ISOL production module, a low energy beam line for delivery of specific, high atomic number, radioactive isotopes to the research area, and the research area containing the instruments such as the atom trap indicated below in figure 3. The ISOL production module is to be designed to accept 1 MW of 1-GeV protons. Thorium is the typical production target for isotopes of Rn, Fr, and Ra, for example. Several isotopes of these elements with half-lives in the minutes, hours and days range are especially suited to fundamental probes for physics beyond the Standard Model. The mode of operation depends on the half-life and physical/chemical properties of the specific isotopes. Isotopes such as 223Ra and 225Ra as discussed below have long half lives, 11 days and 15 days, respectively, so there are options for high power production targets that are not applicable for shorter-lived isotopes. Another important isotope for an EDM search is 211Fr with a lifetime of three minutes, which is also suitable for a high power production target. For a specific experiment the isotope of interest is produced, extracted thermally, ionized, mass separated magnetically, and delivered at low energy (~50 keV) to the instruments in the research area. These and other important isotopes of these elements can be produced and delivered at low energies for fundamental measurements at unprecedented intensities, in the range of 1012 to 1013

ions per second.

A second target station, labeled materials irradiation and isotope production, is a general purpose facility that can be used for engineering studies of materials under intense irradiation conditions, as well as, for production of longer lived isotopes for a variety of isotopes for practical applications such as for nuclear medicine.

Instruments

A fraction of the proton beam from Project X will be used for production of the required heavy radioactive ions in an isotope production target. For the specific cases of 225Ra that will be used for atomic EDM searches, and 211Fr for electron EDM searches, the optimum target material is a refractory compound of thorium. The target is maintained at high temperature so that the activity produced by the impinging protons on the target is efficiently released and migrates to an ion source which ionizes the species of interest. These ions form a low-energy beam which is mass separated and transported to an experimental area where the measurement system is located.

Possible specific measurement systems are shown schematically below, Figure 3 for the atomic EDM search in 225Ra and Figure 4 for the electron EDM search in 211Fr. The mass separated beams of 225Ra or 211Fr ions are implanted in an oven where they are neutralized and released as atomic beams of thermal atoms. These beams are focused transversely by laser beams and slowed down in a Zeeman slower for efficient capture in a magneto-optical trap (MOT) where the atoms are accumulated. The magneto-optical trap uses a combination of lasers and an anti-Helmholtz magnetic field configuration which provides a large confining volume for atom accumulation but is not suitable for the precision EDM measurement which requires a weak and very stable magnetic field combined with a strong and homogeneous electric field. Parallel plates enclosed in a magnetically shielded region provide the required conditions. For 225Ra the transfer of the atoms to this region is performed via a transfer of the atoms from the MOT to an adjacent optical dipole trap (ODT) and then to the measurement region in the strong electric field. For the electron EDM search a 211Fr atomic beam is launched into a vertical "fountain" by detuning the MOT laser beams to create a moving potential. The 211Fr atoms move through the measurement region where they are slowed by gravity, are turned around and fall out of the measurement region; this maximizes the time the atoms spend in the strong electric field.

In both EDM searches, the measurement itself then proceeds via a polarization of the sample by lasers, followed by monitoring the precession in the combined electric and magnetic fields also by lasers. The very small EDM effect is obtained, or constrained, by the difference in frequency when the electric field and magnetic field are aligned or anti-aligned.

The limits that can be put by such measurements are limited by the control of various systematics and the statistical uncertainty which scales as the square-root of the number of atoms in the sample. The four orders of magnitude or so increase in count rate available from this facility compared to a source based approach could yield a sensitivity gain of two orders of magnitude. This large gain provides great discovery potential.

Transversecooling

Oven:225Ra

Magneto-opticalTrap (MOT)

Optical dipoletrap (ODT)

EDMmeasurement

Figure 3. Schematic of the instrument designed to measure the atomic EDM of 225Ra

Figure 4. Schematic of the instrument designed to measure the electron EDM using a radioactive francium isotope.

Physics Opportunities

Permanent electric dipole moments (EDM) of fundamental particles like neutrons, electrons or neutral atoms, violate both parity (P) and time (T) reversal symmetry. Standard Model predictions for these EDMs lead to extremely small values - typically many orders of magnitude below current experimental limits. However, extensions of the Standard Model, e.g. Supersymmetric Models, are generally predicted to give rise to much larger EDMs, and the existing experimental EDM limits already place stringent constraints on these models. EDM experiments provide an ideal opportunity for searches of physics beyond the Standard Model with minimal Standard Model physics background. These searches go hand in hand with the work at the high energy frontier. If the LHC does not see evidence for new physics, this leaves a large window of opportunity for low-energy experiments to make discoveries; if the LHC observes new physics, the Standard Model has many new parameters and the low-energy experiments are necessary to help pin down these parameters.

Currently, low-energy experimental efforts strive to improve the sensitivity of EDM searches by employing new experimental techniques to reduce the influence of systematic uncertainties and by selecting nuclear or atomic systems with special properties enhancing the EDM signal. For example, experimental searches of nuclear EDMs currently explore high Z, octupole deformed nuclei like 225Ra or 223Rn which benefit from a two to three orders of magnitude enhanced sensitivity over the previously best measured case of 199Hg. However, those isotopes are radioactive and the availability of sufficient production yields will be a major issue in the future since the experiments will be largely limited by statistics. Likewise, electron EDM searches using laser cooled 211Fr atoms benefit from an order of magnitude improvement in EDM sensitivity and an order of magnitude reduction in systematic uncertainty over cesium (the heaviest stable atom suitable for laser trapping and cooling for electron EDM experiments). In parallel, next-generation neutron EDM experiments aim to employ new intense UCN sources to increase their statistical sensitivity. The accompanying refinement of control over systematic uncertainties has become the focal point of these measurements, however, with numerous different approaches currently being pursued.

The isotope production capability provided by Project X will allow one to obtain yields more than 1012 atoms per second for key isotopes such as 225Ra which is presently being pursued for an atomic EDM measurement using the setup shown in the previous section. The current measurement will be performed using 225Ra obtained from long-lived isotopes produced by irradiation in reactors. The resulting intensity is of the order of 107-108 atoms per second and the limit extracted from the current effort is expected to be statistics limited. No existing accelerator-based facility provides a significant improvement over the yield available from reactor-based sources. Likewise, for electron EDM searches using 211Fr, the intensity available from Project X is needed so that statistics do not limit the sensitivity of the measurement. Clearly, the availability of the isotope production facility at Project X is a game changer in this field. It could enable a gain in sensitivity of over two orders of magnitude, allowing one to probe essentially the full parameter space spanned by SUSY extensions of the Standard Model.

It is important to note that EDM measurements on different systems, i.e. neutron, electron and nuclei, are highly complementary. When a non-zero EDM is found in one system, measurements

on the other systems will be required to understand the sources of the underlying T-violating processes and the ability of a facility based at the proton driver to deliver the highest yields of candidates for these various searches will strongly enhance the physics reach.