MUSE experiment

MUSE experiment
Name	MUSE experiment
Collaboration	Paul Scherrer Institute, University of Basel, University of Glasgow, University of Liverpool, University of Mainz, University of Manchester
Institution	Paul Scherrer Institute
Location	Villigen, Switzerland
Detector	Pion beamline, Time Projection Chamber, Silicon tracker, Scintillating fiber hodoscopes
Accelerator	High Intensity Proton Accelerator
Energy	~100 MeV
Start	2018
End	Ongoing

MUSE experiment. The MUSE (MUon proton Scattering Experiment) is a precision particle physics investigation conducted at the Paul Scherrer Institute in Switzerland. It aims to resolve a long-standing discrepancy in the measurement of the proton's charge radius by simultaneously scattering both muons and electrons from a liquid hydrogen target. This dual-lepton approach is designed to directly control for systematic uncertainties that have plagued previous experiments, offering a potential solution to the "proton radius puzzle."

Overview

The experiment is situated at the world's most intense continuous pion beam facility, which is part of the High Intensity Proton Accelerator complex. By utilizing the decay of pions, MUSE creates mixed beams of positive and negative muons and electrons, allowing for concurrent measurements with all four particle types. This innovative method, championed by a collaboration of international institutions, directly tests whether the observed radius discrepancy stems from novel physics involving muons or from more conventional experimental systematics. The collaboration includes leading groups from the University of Basel, the University of Glasgow, and the University of Liverpool.

Scientific goals and design

The primary goal is to precisely determine the proton's charge radius via elastic scattering at low momentum transfer. A key design principle is the direct comparison of scattering cross-sections between leptons, specifically testing for possible violations of lepton universality as predicted by some theories beyond the Standard Model. The experiment is designed to measure with high precision the relative yields of muon-proton and electron-proton scattering, thereby isolating any potential difference attributable to the lepton type. This approach rigorously controls for uncertainties related to beam energy calibration and target effects that have affected prior measurements from experiments like those at the Mainz Microtron and the Jefferson Lab.

Experimental setup and components

The core apparatus is installed in the πM1 beamline at the Paul Scherrer Institute. The beamline delivers a mixed secondary beam containing muons and electrons of both charges, with momenta tunable around 210 MeV/c. A sophisticated beam analysis system, including Cherenkov detectors and scintillating fiber hodoscopes, identifies and tracks individual particle species before they reach the target. The central detector is a novel, high-precision Time Projection Chamber surrounded by a cylindrical silicon tracker, which measures the trajectories of scattered leptons and recoil protons from the liquid hydrogen target. This system is complemented by a dedicated trigger and data acquisition system to handle the high event rates.

Timeline and collaboration

Initial development and prototyping began around 2014, with the full apparatus being commissioned and installed starting in 2018. The collaboration, spearheaded by the Paul Scherrer Institute, brings together physicists from Europe and North America, including teams from the University of Mainz, the University of Manchester, and the College of William & Mary. Data-taking runs have been conducted in several campaigns, with analysis of the initial datasets ongoing. The collaboration regularly presents its progress and technical developments at major conferences such as the International Conference on High Energy Physics and the International Conference on the Structure of Baryons.

Results and findings

As of the latest updates, the experiment has successfully demonstrated the operation of its complex beamline and detector systems, collecting preliminary scattering data with all four lepton species. Early results focus on characterizing the beam composition, achieving precise tracking resolution with the Time Projection Chamber, and verifying the performance of the silicon tracker. The full statistical analysis required for a definitive charge radius extraction is still in progress. These technical milestones were reported in publications in journals like Nuclear Instruments and Methods in Physics Research and presentations at the APS Division of Nuclear Physics meeting.

Significance and future prospects

The final results are anticipated to provide the most direct test yet of whether the proton radius puzzle originates from physics beyond the Standard Model or from unidentified experimental systematics. A confirmation of a difference between muon-based and electron-based measurements would have profound implications, potentially indicating new interactions or particles that couple differently to muons, such as those proposed in certain dark photon models. Regardless of the outcome, the experiment's pioneering techniques in mixed-beam lepton scattering will inform future precision studies in hadron physics, possibly influencing next-generation projects at facilities like the Electron-Ion Collider and the Muon g-2 experiment at Fermilab.

Category:Particle physics experiments Category:Paul Scherrer Institute Category:Proton