Fermilab Steering Group Report

Chapter 4
Physics at the
Intensity Frontier

Precision physics

Ultraprecise experiments using high-intensity sources of muons and quarks provide unique discovery potential. These experiments would complement those at the LHC as well as those in the worldwide program of neutrino science and precision physics. Results from these experiments would provide essential clues for interpreting discoveries and their implications for the great questions of particle physics.

Muons

Lepton flavor violation was discovered in neutrino experiments, where the three flavors of neutrinos are observed to morph, or oscillate, into one another. Physicists do not know why LFV occurs or if it is related to the flavor violation seen with quarks or to new phenomena at the Terascale. A key question is whether LFV also occurs with the charged leptons: electron, muon and tau. Theoretical models that incorporate ideas such as unification, supersymmetry or heavy-neutrino mixing predict charged LFV at rates that could be within reach of new experiments. Combined with results from neutrinos and the LHC, these experiments could point the way to leptogenesis or unification.

illustration
μ-to-e conversion sensitivity
Comparison of the sensitivity to lepton flavor violation of the MEG (μ→eγ ) experiment at a transition rate of 10-13 and a μ-to-e conversion experiment with Fermilab Booster at the rate of 10-17. Project X could reach the rate of 10-18. See details in Appendix E.

A new experiment could search for the direct coherent conversion of muons into electrons in the field of a nucleus. This muon-to-electron conversion experiment could detect LFV decays even if they occur at 10-17 the rate of standard muon processes. It would probe several distinct LFV processes. If a signal is detected, a μ→e conversion experiment could zero in on the new physics by repeated measurements with different nuclear targets. This experiment would have sensitivity to very high energy scales, beyond the direct reach of colliders.

The Muon-to-Electron-Gamma experiment at the Paul Scherrer Institute will soon begin to look for the LFV process μ→eγ, with predicted sensitivity at the 10-13 level. A μ→e conversion experiment at the 10-17 level would have greater sensitivity to the μ→eγ transition than MEG, and orders of magnitude better sensitivity for more general LFV processes. Other approaches to LFV using taus are not expected to have comparable sensitivity, due to the available flux of taus, which is much less than that of muons, and to the greater cleanliness of the muon experiment.

A μ→e conversion experiment at Fermilab could be 10,000 times more sensitive than previous experiments. An intense 8 GeV proton beam and the Accumulator and Debuncher rings, available after the end of antiproton production for the Tevatron collider program, would make this LFV search possible. The SNuMI accelerator upgrades would increase the total proton flux at 8 GeV, allowing a modest increase in beam power for the muon program while also increasing the beam power available to the neutrino program. Project X could increase the beam power available to the muon program by a factor of 10. Exploiting this increased intensity and a reoptimized muon beam (e.g. decreased energy spread and transverse beam size) has the potential to further improve sensitivity beyond that possible with the SNuMI upgrades.

Kaons

Theories of Terascale physics typically predict new contributions to flavor-violating processes involving quarks. New particles predicted by Terascale physics are expected to have flavor- violating and CP-violating couplings. Experiments at B factories or elsewhere have unexpectedly found no clear signals of such contributions. These results favor theoretical models with minimal flavor violation. The data suggest a strategy of concentrating on rare processes that are as theoretically and experimentally clean as possible, to maximize the sensitivity to small contributions from new physics.

illustration
K→πνv sensitivity
Project X experiments based on 1000 Standard Model events could probe Terascale physics with greater than fi ve sigma sensitivity.

The ultrarare process K→πνv is the most promising opportunity for implementing this new strategy. The neutral K→πνv decay is extremely suppressed in the Standard Model and has not yet been observed. It is a clean, purely CP-violating process, with a Standard Model theoretical uncertainty no larger than two percent. A phased program at KEK and then J-PARC may eventually detect about 100 of these rare decays. The physics reviewed above shows the importance of a new experiment with the ultimate capability to detect about 1000 neutral decays, achieving a statistical error that approaches the theoretical uncertainty. Such an experiment would be even more powerful if combined with a precision measurement of charged K→πνv decays, which are also highly suppressed in the Standard Model and have a modest theoretical uncertainty.

Such experiments would be sensitive to new sources of CP violation involving quarks. They would also be sensitive to flavor-violating effects from new particles, even in cases where the only source of CP violation is the CKM phase of the Standard Model. Either way, rare kaon decays offer a unique window on these phenomena. For example, if superpartner particles are discovered at the LHC, kaon experiments could address such fundamental questions as distinguishing among different mechanisms for the breaking of supersymmetry.

Other rare kaon-decay modes offer opportunities for major surprises. They include possible detection of the lepton-flavor-violating decays K→πμe or K→μe, and exotic decays of kaons into axions or gravitons.

The high-intensity 8 GeV proton facilities and the Tevatron Stretcher concept described in the next chapter represent a potential for a breakthrough in ultrarare kaon-decay experiments. They would provide kaon beams at Fermilab of unprecedented purity and intensity. Discovery sensitivities would benefit from increased kaon beam power. Project X's ability to optimize kaon beam characteristics would simplify the experiments and reduce technical risk.

Charm and hyperon physics with antiprotons

Fermilab operates the world's most intense antiproton source, a distinction it will continue to hold even after the planned 2014 startup of the Facility for Antiproton and Ion Research in Germany. The anticipated shutdown of the Tevatron collider program presents the opportunity for a world-leading low- and medium-energy antiproton program capable of studying a range of physics questions with unequaled sensitivity: hyperon CP violation and rare decays, charm mixing, the charmonium spectrum and recently discovered nearby states, and CPT and antimatter-gravity tests with antihydrogen.