[Paper Review] Expression of Interest in R&D towards a Neutrino Factory Based on a Storage Ring and a Muon Collider

Kirk T. McDonald|arXiv (Cornell University)|Nov 6, 1999

Neutrino Physics Research52 references13 citations

TL;DR

This paper proposes a neutrino factory based on a muon storage ring and a muon collider, utilizing a high-power proton beam to produce pions that decay into muons, which are then cooled, accelerated, and stored in a ring to generate intense, well-characterized neutrino beams. The key contribution is a comprehensive R&D roadmap for advancing accelerator physics toward a precision neutrino factory and muon collider, highlighting critical technical challenges and collaborative opportunities in the U.S. and Europe.

ABSTRACT

We are exploring the feasibility of a neutrino factory based on a muon storage ring. In this, beams of nu_mu and nubar_e arise from the decay of mu- particles (or alternatively, nubar_mu and nu_e from mu+). The muons come from the decay of low-energy pions produced by a megawatt proton beam incident on a nuclear target. The muons are captured into a magnetic channel, "cooled" by ionization in liquid hydrogen, accelerated to energy of order 50 GeV, and injected into a storage ring. A nonhorizontal ring can deliver neutrino beams to an on-site detector, as well as to two off-site detectors separated by global distances. Such a neutrino factory is a challenging extension of present accelerator technology. It is also a natural path to a muon collider, in that both facilities share many common elements upstream of their storage rings. Prior to a formal design study, R&D must be performed in several keys areas, such detailed simulations and actual targetry and cooling experiments. This in an excellent opportunity to advance the field of accelerator physics both at national laboratories and at universities.

Motivation & Objective

• To establish a new generation of accelerator-based neutrino experiments capable of precisely measuring neutrino masses, mixing angles, and CP violation.
• To address the challenge of producing intense, well-collimated neutrino beams using muons from pion decay in a high-intensity proton beam.
• To enable a path toward a muon collider by leveraging shared R&D infrastructure and technologies upstream of the storage ring.
• To identify and prioritize critical R&D areas—such as ionization cooling, rf cavities, and high-power targets—necessary for feasibility.
• To advocate for early funding support from the NSF to initiate university-based R&D in preparation for a formal MRE proposal.

Proposed method

• Use a megawatt-class proton beam to bombard a nuclear target, producing pions via spallation.
• Capture secondary muons in a magnetic channel and use ionization cooling in liquid hydrogen to reduce transverse emittance.
• Employ phase rotation and recirculating linacs to accelerate muons from 2 GeV to 50 GeV.
• Store 50 GeV muons in a non-horizontal storage ring to produce collimated neutrino beams via muon decay.
• Utilize the same accelerator chain for both a neutrino factory and a muon collider, with the storage ring as the common element.
• Conduct targeted R&D experiments in the U.S. and Europe, including pion production measurements, muon scattering in liquid hydrogen, and rf cavity radiation testing.

Experimental results

Research questions

• RQ1 Can ionization cooling in liquid hydrogen effectively reduce muon emittance to enable efficient storage and beam production?
• RQ2 What are the critical technical challenges in designing a high-power proton driver and target system capable of 4 MW or more?
• RQ3 How can rf cavities be made radiation- and field-resistant for use in pion capture and phase rotation systems?
• RQ4 What are the performance limits of muon cooling and acceleration systems under realistic radiation and magnetic field conditions?
• RQ5 Can a neutrino factory design be extended to a precision muon collider with sufficient technical and financial feasibility?

Key findings

• Neutrino oscillation data from atmospheric, solar, and accelerator sources indicate that neutrinos have mass and mix among three flavors, necessitating new high-precision experiments.
• A muon storage ring-based neutrino factory can produce well-characterized, high-intensity neutrino beams suitable for long-baseline measurements of mixing angles and CP violation.
• Ionization cooling in liquid hydrogen is a key enabling technology, but its performance depends on precise measurements of large-angle muon scattering in materials.
• R&D on high-power targets, radiation-hardened rf cavities, and pion production cross sections is essential for design validation and feasibility.
• The neutrino factory and muon collider share a common accelerator chain upstream of the storage ring, enabling shared R&D and cost efficiency.
• European and U.S. R&D programs are complementary, with planned experiments at CERN-PS and other facilities to address critical unknowns in muon beam dynamics and materials response.