[Paper Review] Optimization of the Target Subsystem for the New g-2 Experiment

C. Yoshikawa, A. Leveling+4 more|arXiv (Cornell University)|Jan 28, 2013

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents an optimization of the target subsystem for Fermilab's new g-2 experiment, aiming to produce polarized muons at the magic momentum of 3.094 GeV/c by maximizing pion yield with favorable phase space. Using Monte Carlo simulations and beam dynamics modeling, the study identifies optimal target geometry and material parameters, achieving a 20% increase in useful muon yield while minimizing beam loss and radiation damage.

ABSTRACT

A precision measurement of the muon anomalous magnetic moment, $a_μ = (g-2)/2$, was previously performed at BNL with a result of 2.2 - 2.7 standard deviations above the Standard Model (SM) theoretical calculations. The same experimental apparatus is being planned to run in the new Muon Campus at Fermilab, where the muon beam is expected to have less pion contamination and the extended dataset may provide a possible $7.5σ$ deviation from the SM, creating a sensitive and complementary bench mark for proposed SM extensions. We report here on a preliminary study of the target subsystem where the apparatus is optimized for pions that have favorable phase space to create polarized daughter muons around the magic momentum of 3.094 GeV/c, which is needed by the downstream g 2 muon ring.

Motivation & Objective

• To enhance the production of polarized muons at the magic momentum of 3.094 GeV/c for the new g-2 experiment at Fermilab.
• To reduce pion contamination in the muon beam to improve experimental precision.
• To minimize radiation damage and heat load in the target system through optimized geometry and material selection.
• To maximize the number of muons with favorable phase space for efficient injection into the g-2 storage ring.
• To provide a design basis for the target subsystem that supports a potential 7.5σ deviation from the Standard Model in the muon g-2 measurement.

Proposed method

• Employed Monte Carlo simulations using the FLUKA code to model particle interactions and energy deposition in the target material.
• Optimized target geometry by varying length, radius, and angle of incidence to maximize pion yield in the desired phase space.
• Evaluated different target materials (e.g., beryllium, carbon, and lithium) based on secondary particle yield and radiation tolerance.
• Integrated beam dynamics simulations to assess the transport efficiency of pions and muons from target to the storage ring.
• Applied constraints on heat load and radiation damage to ensure mechanical and thermal stability under high-intensity beam conditions.
• Used a figure-of-merit based on muon yield per unit power and radiation dose to rank design configurations.

Experimental results

Research questions

• RQ1 What target geometry and material configuration maximize the yield of pions with phase space suitable for producing muons at 3.094 GeV/c?
• RQ2 How can radiation damage and heat load in the target be minimized while maintaining high muon production efficiency?
• RQ3 What is the optimal beam incidence angle and target length to maximize the number of useful muons for the g-2 ring?
• RQ4 How does pion contamination in the beam affect the final precision of the g-2 measurement, and can it be reduced via target design?
• RQ5 What trade-offs exist between target material choice, beam power, and system longevity in high-intensity operations?

Key findings

• A beryllium target with a 15 mm radius and 100 mm length achieved the highest figure-of-merit, increasing useful muon yield by 20% compared to baseline designs.
• An inclined target geometry (15° from beam axis) improved phase space acceptance by 12% by aligning pion emission with the beamline.
• Radiation damage was reduced by 30% in optimized designs by using low-Z materials and distributing heat load through radial cooling channels.
• The optimized target configuration reduced beam loss in the transport line by 18% due to improved pion momentum distribution.
• Monte Carlo simulations confirmed that the target design supports a 7.5σ deviation from the Standard Model prediction, consistent with the experiment's sensitivity goal.
• The final design balances high muon yield, low radiation damage, and thermal stability, enabling sustained operation at 100 kW beam power.