Fermilab A Plan for Discovery

Chapter 3
The Intensity Frontier

The Intensity Frontier

Experiments at the Intensity Frontier search for new physics phenomena at energies many orders of magnitude higher than in experiments at particle colliders. At this frontier, scientists address central questions in particle physics that may not be addressed for many years at the Energy Frontier. Over the next twenty years, Fermilab's Intensity Frontier program will use protons to produce intense beams of neutrinos, muons and kaons, and high yields of heavy nuclei. Experiments using these beams and nuclei will study rare processes more precisely and with more sensitivity than ever before.

Throughout history many new phenomena in physics were first discovered through the study of rare processes, including the first observation of the weak interactions, the indirect detection of the neutrino and the prediction of the charm quark mass. Major scientific goals at today's Intensity Frontier include the search for CP violation in neutrinos, which may provide the critical information needed to solve the puzzle of the excess of matter over antimatter in the universe. Intensity Frontier experiments will investigate the mysterious new physics of leptons to understand precisely how and why lepton flavor is not conserved. Experiments to observe and measure ultra-rare kaon decays and search for electric dipole moments of the muon and electron may reveal new forces and new physics.

The initial phase of Fermilab's new program at the Intensity Frontier program is already well underway and builds upon the unique and extensive Fermilab infrastructure to provide a powerful and dramatic suite of experiments with neutrinos and muons. The second phase includes construction of the Long-Baseline Neutrino Experiment and Project X. LBNE's critical scientific mission is to determine if leptons experience CP violation.

Project X will be the world's most intense and flexible particle accelerator, and is the centerpiece of Fermilab's long-term strategy to develop the world's leading Intensity Frontier physics program. Project X will enhance and permit extremely detailed studies of neutrinos at LBNE, enable the study of the rarest processes involving muons and kaons and permit the creation of heavy nuclei that provide unprecedented sensitivity to the detailed properties of electrons, neutrons and nuclei themselves.

Theoretical Physics at the Intensity Frontier

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The NOνA near detector

A scientist conducts tests on the NOνA near detector, which also acts as a prototype for the 15,000-ton far detector in Ash River, Minnesota.

The Fermilab theory group plays a major role in shaping the Intensity Frontier physics program. Fermilab theorists will continue to mentor the Fermilab neutrino program, from helping to define the physics case for NOνA and LBNE to proposing novel physics searches in short-baseline experiments. Theorists actively consult with experimentalists on all aspects of Intensity Frontier physics at all stages of the Fermilab program: what aspects of physics are investigated by the experiments, and how to place limits and define new physics search strategies.

Flavor physics has been a cornerstone of Fermilab's lattice gauge theory effort. Fermilab theorists have helped craft the case for measuring rare-kaon-decay rates in the LHC era, and are focusing on the constraints these imply for TeV-scale physics. Fermilab's Lattice QCD calculations are important for Intensity Frontier physics, for example by driving the evolution of the precision of quark-flavor physics results, or by determining the hadronic contributions to the magnetic moment of the muon as required by the Muon g-2 experiment.

Neutrino Physics

Experiments in neutrino physics address three fundamental goals. The first goal comprises two parts: measuring θ13, the mixing angle between first- and third-generation neutrinos without which there can be no CP violation; and determining whether the neutrino mass hierarchy is like that of quarks or if it is inverted. The second goal is to observe CP violation in neutrinos, such as an asymmetry between the oscillation rates of νμ→νe and νμνe. The third goal is to determine the nature of neutrino mass. Fermilab's long-baseline neutrino program is ideally poised for precision measurements that address the first two goals.

Near-Term Neutrino Program

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Sensitivity to sin213

Colored bands show for what fraction of CP-violating phases a given value of sin213 can be distinguished from zero at the 3σ confidence level, assuming a normal mass hierarchy.

Fermilab's near-term long-baseline neutrino-oscillation experiments—MINOS, which is currently operating; MINOS+, its next phase; and NOνA, currently under construction— provide a powerful evolutionary progression in neutrino measurement capability and are also sensitive to relatively large new physics phenomena. The 735-kilometer-baseline MINOS experiment has already provided the world's most precise measurement of Δm232, as well as highly competitive constraints on many other phenomena in neutrino physics. MINOS+ will search for sterile neutrinos and for non-standard neutrino interactions that could point to new physics. NOνA, with an 810-kilometer baseline, will measure θ23, |Δm232| and θ13 with greatly increased precision. NOνA will also measure the sign of Δm232 (the neutrino mass hierarchy) provided θ13 is relatively large, as has been hinted by recent results from the T2K, MINOS and Double CHOOZ experiments.

The short-baseline experiments MiniBooNE, MINERνA and MicroBooNE provide a rich ensemble of complementary experiments in this decade. They measure the cross sections and kinematics of neutrino processes to high accuracy, offer a unique discovery potential for new phenomena, and can significantly advance our understanding of the underlying dynamics of neutrino-nucleus interactions, vital to the future worldwide neutrino program.

LBNE

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Sensitivity to mass hierarchy

Colored bands show for which sin213 values and for what fraction of possible values of the CP-violating phase the Fermilab neutrino program can distinguish the normal and inverted mass hierarchies at the 3σ confidence level, assuming the actual hierarchy is normal.

The LBNE experiment will have an unprecedentedly long baseline of approximately 1300 kilometers. This will significantly enhance the experiment's sensitivity to θ23, Δm232 and θ13. As its main goal, LBNE will search for a CP-violating asymmetry in the oscillation of muon neutrinos and antineutrinos to electron neutrinos and antineutrinos. LBNE will be the first experiment capable of discovering CP violation in the neutrino sector over a large range of possible θ13 values. LBNE's capability to measure θ13 and determine the mass hierarchy will greatly exceed that of NOνA even for small θ13.

An important aspect of LBNE will be its versatile and massive far detector. This will be either a water Cerenkov detector with a mass of up to 200 kilotons or a liquid-argon time-projection chamber of several tens of kilotons. Such massive detectors are crucial for collecting sufficient event samples over such long distances. Liquid-argon detectors have not yet been realized on such large scales, but Fermilab is playing an active role in the development of this detector technology. The upcoming MicroBooNE experiment will serve as a demonstration of the technology, which is also under active study for dark-matter and neutrinoless-double-beta-decay detection.

Both detector technologies allow for a rich physics program beyond neutrino oscillation studies. This includes a high-sensitivity search for proton decay, which probes the grand unification scale of order 1016 GeV, and high-sensitivity studies of neutrinos coming from supernovae within our galaxy.

Neutrino Program with Project X

Project X provides a significant upgrade path for the neutrino program. Project X supplies a neutrino beam power that is more than three times larger than the Fermilab accelerator complex will produce for NOνA. Its flexible beam energy and power enhance the sensitivity of LBNE, particularly to leptonic CP violation, turning a 3σ measurement of CP violation with NOνA's 700 kW beam into a 5σ measurement with Project X. The physics achievable by LBNE in 10 years with Project X would take more than 30 years without it.

Project X will also drive new short-baseline programs to study the physics of neutrino interactions with superior precision. Enhanced short-baseline detectors driven by a Project X beam could become discovery machines for new physics.

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(click image for larger version)

Project X

Project X will be the world's most intense and flexible proton accelerator. The key to Fermilab's long-term world leadership at the Intensity Frontier, it will simultaneously provide 2.3 MW of proton beam power at 60—120 GeV; up to 200 kW at 8 GeV; and 2.9 MW at 3 GeV. Fermilab is pursuing the design and construction of this unique facility in collaboration with national and international partners.

Experiments at Project X

Project X will produce intense beams of neutrinos, kaons and muons, and will produce copious quantities of heavy nuclei. Starting in the 2020s, experiments using Project X beams will be essential to break through to a deeper understanding of nature and the origins of matter, either discovering new phenomena or providing the critical clues that explain new physics discovered elsewhere.

Neutrinos:

Long- and short-baseline neutrino experiments will search for leptonic CP violation and study the physics of neutrino interactions with unmatched precision

Muons:

Experiments could discover unexpected lepton-flavor violations or subtle new physics at the quantumloop level

Kaons:

Ultra-rare kaon-decay experiments will either test the Standard Model with uniquely high precision or discover new physics at the 1000-TeV scale

Nuclei:

Ultra-sensitive electric dipole moment searches will approach limits of 10-31 e-cm or discover a new type of CP violation in the strong interaction

The Neutrino Factory

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Sensitivity to CP violation

Sensitivity of the Fermilab neutrino program for detecting CP violation at a 3σ confidence level. NOνA is unable to detect CP violation at this level of significance over the plotted range of θ13.

The ultimate stage of Fermilab's neutrino physics program is the construction of a Neutrino Factory. Such a facility would use the high-intensity proton beam from Project X to produce a high-quality muon beam that can be accelerated and circulated in a storage ring. The muons would decay, yielding superb, background-free beams of electron and muon neutrinos. Neutrino detectors would be located at distances of up to several thousand kilometers, making the Neutrino Factory an ideal tool to study perturbations of the oscillation pattern induced by the matter through which the neutrinos travel.

Small values of θ13 require a high-energy Neutrino Factory. For large θ13 a low-energy Neutrino Factory is optimal, as the high-energy facility would suVer from larger backgrounds due to CP-conserving electron-muon oscillations. Fermilab is studying both a high-energy Neutrino Factory with a 25 GeV muon beam and a low-energy option with a 5 GeV muon beam. The Neutrino Factory would provide high-intensity beams of both electron and muon neutrinos, which would enable superior measurements of the parameters of the neutrino mass and mixing matrices. The Neutrino Factory would study up to 12 of the 18 known three-flavor oscillation channels, thus exhaustively covering the landscape of both standard and non-standard neutrino physics. The low-background environment of a Neutrino Factory will also provide enhanced sensitivity to subtle new physics effects and will cover a much larger range of possible phenomena than any other existing or currently proposed neutrino facility.

Muon Physics

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μ-to-e conversion sensitivity

Comparison of the sensitivity to charged LFV of the MEG (μ→eγ) experiment at the Paul Scherrer Institute at a transition rate of 10-13 and the Mu2e experiment using the Fermilab Booster at the rate of 10-16 to 10-17.

High-intensity sources of muons enable ultra-precise measurements of their fundamental physical parameters, as well as searches for new physics phenomena in rare processes involving muons. Such measurements could yield significant discoveries, such as the unexpected violation of lepton-flavor symmetries or subtle new physics entering at the quantum-loop level. The anomalous magnetic moment of the muon, g-2, measured by the E821 experiment at Brookhaven National Laboratory with a precision of 0.54 parts per million, remains discrepant with the Standard Model and is a possible indication of new physics. A new Muon g-2 experiment at Fermilab, which will reuse the Brookhaven muon storage ring with the former Fermilab antiproton source, will achieve a fourfold improvement in precision. Combined with improved theoretical analysis, the new g-2 measurement will provide an important constraint on new physics phenomena.

The observation of an electric dipole moment for any elementary particle would constitute a major discovery of a new source of CP violation. The current 95% CL muon EDM limit is 10-19. The Muon g-2 experiment will also search for a muon EDM with a sensitivity 100 times better than current experiments. A next-generation experiment with Project X could further improve the sensitivity by another three orders of magnitude.

Rare decays of muons, if observed, could be harbingers of new physics phenomena. Starting late this decade the Mu2e experiment will search for one such rare-decay process, the conversion of a muon to an electron. This process, known as charged lepton flavor violation, is predicted by many theories that include new physics to occur at rates that could be within reach of Mu2e. Such studies provide sensitivity to mass scales of new physics phenomena that may lie at thousands of TeV for certain scenarios, significantly beyond the reach of the LHC.

Mu2e will use the 8 GeV proton beam from Fermilab's Booster and the beamline from the former antiproton source to search for the muon-to-electron conversion process with sensitivity at the 10-16 to 10-17 level, 1,000 to 10,000 times better than previous experiments. Project X would increase the beam power to the experiment by more than a factor of 10, further improving the sensitivity to the 10-19 level if the first phase of the Mu2e experiment does not discover charged lepton flavor violation. If a discovery is made in the first phase, a Project X-era experiment would offer the unique capability of distinguishing the underlying new physics by measuring the muon-to-electron conversion rate using a variety of nuclear targets.

Kaon Physics

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K→πνν sensitivity

Project X experiments based on 1000 Standard Model events can probe new physics phenomena with greater than 5σ sensitivity.

A global suite of experiments seeks to measure ultra-rare decays of kaons, either to test the Standard Model with uniquely high precision or to discover new physics phenomena at the 103 TeV scale. The Project X era at Fermilab will offer unmatched sensitivity to ultrarare decays of kaons.

The rare kaon decay processes, K+→π+νν and KL→πOνν, provide physicists with the opportunity to test the Standard Model with high precision or discover new physics, since both modes are extremely suppressed in the Standard Model. The KL→πOνν is a purely CP-violating process, with a hadronic matrix element that is known theoretically at the one percent level of precision. The observation and precise measurement of this rare process will constitute a major triumph in kaon physics.

Fully exploiting the opportunities of kaon physics requires experiments capable of detecting about 1000 decays in each mode, thus achieving a statistical error that approaches the theoretical uncertainty. The high-intensity proton beam of Project X would readily enable experiments at the 1000-event level. The continuous-wave linac technology proposed for Project X would provide ideal conditions for these experiments, which would yield simplifications of the experimental apparatus and reduced technical risks. Both neutral and charged kaon-decay measurements would reach precisions of a few percent, comparable to the uncertainty on the Standard Model prediction, thus offering greatly increased sensitivity for new physics phenomena in these processes. Kaon experiments at Project X would also be sensitive to a variety of other rare decays involving possible exotic final states.

Search for New Physics with Heavy Nuclei

Project X can be used to produce large quantities of heavy short-lived nuclei, such as radon, radium, americium and francium, which offer significantly amplified sensitivity to the electron EDM. The current 90% CL limit on the electron EDM of 10-27 e-cm already rules out generic models of supersymmetry. Project X-era experiments using heavy nuclei and a combination of nuclear shape and relativistic effects could approach limits of 10-31 e-cm, or discover a new type of CP violation in the strong interaction.

The target station of a facility to produce heavy nuclei with Project X could also include an ultra-cold neutron capability, the development of isotope-production techniques for beta-beams, biological studies with radiotherapeutic (alpha-emitting) isotopes, and materials-science studies with implanted radioisotopes.

Clean Nuclear Energy Applications of Project X

In addition to its broad program in fundamental science, Project X could support R&D towards the destruction of spent fuel from conventional nuclear reactors, and the development of accelerator-driven subcritical systems for safe and abundant nuclear energy production.

The issue of high radio-toxicity and the long lifetime of conventional spent nuclear fuel is a global challenge. Accelerator-driven systems can be used to transmute spent nuclear fuel, significantly reducing the lifetime and toxicity of nuclear waste. Accelerators can also drive fission reactors that incorporate advanced fuels, such as those based on thorium. This has advantages over conventional reactor fuels, including a fuel supply that can meet global demand for 100 to 1000 times longer; less long-lived nuclear waste; reduced possibility of nuclear weapons production; and relatively safer operation. Fermilab will not establish a full-scale accelerator-driven nuclear reactor development program, but key elements for the future of accelerator-driven nuclear energy can be studied and developed with Project X.