[Paper Review] Feasibility Study for a Next-Generation Mu2e Experiment

K. Knoepfel, Pronskikh, V.+22 more|arXiv (Cornell University)|Jul 3, 2013

Muon and positron interactions and applications1 references18 citations

TL;DR

This paper investigates the feasibility of a next-generation Mu2e-II experiment using Project-X beams to achieve a factor-of-ten improvement in sensitivity over the currently planned Mu2e experiment. By reusing most of the existing Mu2e apparatus and leveraging Project-X's high-duty-factor, narrow-pulse, and low-extinction beams, the study shows that key backgrounds—including muon decay-in-orbit and radiative pion capture—can be kept under control, making a significant sensitivity enhancement both technically and economically viable.

ABSTRACT

We explore the feasibility of a next-generation Mu2e experiment that uses Project-X beams to achieve a sensitivity approximately a factor ten better than the currently planned Mu2e facility.

Motivation & Objective

• To assess the feasibility of a next-generation Mu2e experiment (Mu2e-II) that improves sensitivity by a factor of ten over the currently planned Mu2e facility.
• To evaluate whether the existing Mu2e apparatus can be reused with only modest upgrades to achieve this sensitivity gain using Project-X beams.
• To quantify background levels in the Mu2e-II scenario, particularly focusing on muon decay-in-orbit and radiative pion capture, which are the dominant background sources.
• To determine the beam parameters (energy, intensity, pulse structure) required to achieve a single-event sensitivity of 2×10⁻¹⁸ while keeping background rates manageable.
• To explore how material choices (e.g., aluminum, titanium, boronated polyethylene) and shielding modifications affect neutron and photon backgrounds.

Proposed method

• Used G4Beamline simulations to model beam transport and background generation for Project-X beams at 1 or 3 GeV, assuming 100–150 kW beam power.
• Assumed the same run time and proton pulse spacing as the original Mu2e experiment to maintain comparable exposure and sensitivity scaling.
• Estimated background yields for key processes: muon decay-in-orbit, radiative muon capture, late-arriving particles (e.g., muons, pions from decay-in-flight), beam electrons, and anti-proton-induced backgrounds.
• Evaluated the impact of detector upgrades, particularly reducing straw tracker wall thickness from 15 μm to 8 μm, to improve momentum resolution and suppress the muon decay-in-orbit background.
• Simulated shielding improvements, including using heavy concrete (barium or iron-doped), thicker concrete walls, and boronated/lithiated polyethylene, to reduce neutron and photon backgrounds.
• Compared background estimates across different stopping target materials (aluminum, titanium) and beam energies to identify optimal configurations.

Experimental results

Research questions

• RQ1 Can a factor-of-ten improvement in sensitivity for charged-lepton-flavor-violating muon-to-electron conversion be achieved using Project-X beams and minimal modifications to the existing Mu2e apparatus?
• RQ2 What are the dominant background sources in the Mu2e-II scenario, and can they be suppressed to acceptable levels through beam and detector design?
• RQ3 How do beam parameters—such as pulse width, extinction, and energy—impact background rates, particularly for radiative pion capture and late-arriving particles?
• RQ4 To what extent can existing Mu2e components, especially the tracker and shielding, be reused with only minor upgrades to meet the requirements of Mu2e-II?
• RQ5 What material choices (e.g., HDPE, stainless steel, boronated polyethylene) and shielding configurations minimize neutron and photon backgrounds without excessive cost or complexity?

Key findings

• A Mu2e-II experiment with a factor-of-ten sensitivity improvement over the original Mu2e is feasible using Project-X beams at 1 or 3 GeV and 100–150 kW beam power.
• The total background estimate for Mu2e-II is 0.46 events (for Al target, 1 GeV beam) or 1.40 events (for Ti target, 3 GeV beam), with the muon decay-in-orbit (DIO) background being the dominant contributor at 0.26–1.19 events.
• Reducing the straw tracker wall thickness from 15 μm to 8 μm improves momentum resolution and reduces the DIO background, making it feasible to keep this background below one event.
• Using heavy concrete (barium or iron-doped) in the CRV support walls reduces the neutron rate by a factor of about two, while doubling the concrete thickness reduces it by a factor of ten.
• The narrow pulse width and excellent intrinsic extinction of Project-X beams significantly suppress radiative pion capture and late-arriving particle backgrounds, including beam electrons and muons from decay-in-flight.
• The high duty factor of Project-X enables a ten-fold sensitivity gain over a reasonable timescale with only a 3- to 5-fold increase in instantaneous detector rates, supporting the reuse of the current Mu2e apparatus with minimal upgrades.