Session T11: Muon Physics

Measurement of the anomalous muon precession frequency in the Fermilab Muon g - 2 Experiment

The Muon g – 2 Experiment has measured the muon magnetic anomaly to unprecedented sensitivity. Comparison of a measurement of this quantity with the theoretical prediction, which includes contributions from all known particles and forces, provides a stringent test of the Standard Model. A significant difference between experimental measurement and theoretical prediction would be a strong indication of unknown particles or forces. By measuring the precession frequency of muons in a magnetic storage ring and the highly-uniform magnetic field, the Muon g-2  Experiment achieved a precision of 0.2 parts per million.  In this talk, I will describe the analysis of the precession frequency of the Run-2/3 dataset, outlining its major systematic uncertainties, and look ahead to what we can expect in the future.   

Precision Magnetic Field Measurement with NMR Probes at Fermilab Muon g-2 Experiment

The Fermilab Muon g-2 Experiment measured the muon magnetic moment anomaly to a precision of 200 parts per billion (ppb). This requires a high-precision measurement of the muon spin precession frequency, as well as the magnetic field experienced by the muons. In the experiment, the magnetic field is measured using Nuclear Magnetic Resonance (NMR) probes and is related to the Larmor frequency of protons shielded in a spherical water sample at a specific reference temperature. Absolutely calibrated NMR probes mounted on a survey trolley measure the magnetic field inside the muon storage ring, and fixed NMR probes located at various locations around the ring track the field over time. The measurements are synchronized and interpolated, then averaged over space and time, and weighted by the muon density. Additional transient corrections reduce the magnetic field uncertainty to below 60 ppb. The probe used for absolute calibration in the Fermilab Muon g-2 Experiment is cross-calibrated with NMR probes designed for the Muon g-2/EDM Experiment (E34) and the Muonium Spectroscopy Experiment Using Microwave (MuSEUM) at Japan Proton Accelerator Research Complex (J-PARC), using the Argonne National Laboratory (ANL) 4T Magnet Facility. While Fermilab Muon g-2 probes use pulsed NMR and the others use the continuous wave (CW) technique, the cross-calibration efforts aim to cross-check the probes with an uncertainty at a 35-ppb level.   

Beam dynamics corrections to the measurement of the muon magnetic anomaly in the Muon g-2 Experiment at Fermilab

The Muon g-2 Experiment at Fermilab aims to measure the muon magnetic anomaly to an unprecedented precision of 0.14 parts per million (ppm). In August 2023, the collaboration published its second result, which was based on the second and third years of collected data and pushed the experimental world average to a precision of 0.19 ppm. The muon magnetic anomaly is extracted from precision measurements of both the muon's anomalous spin precession and the uniform magnetic field in which the muons precess. Accounting for beam dynamics effects on the extraction of the muon's anomalous spin precession is critical to the measurement. In this talk, we describe corrections that arise from these beam dynamics effects and highlight the improvements we have made since our first result.   

Dark Matter Search in the Muon g-2 Experiment at Fermilab

Dark Matter (DM) is one of the most interesting research topics in physics. Many particle physicists are trying to identify it because we know that dark matter is likely to be a major component of a complete fundamental description of nature. The Muon g-2 Experiment at Fermilab measures the anomalous precession frequency of the muon. Oscillations of this precession frequency could be produced by DM coupling to muons. This talk will describe how we could observe DM signals in the Muon g-2 data. I will describe the analysis strategies throughout the DM mass range and explain how we determine the sensitivity. Finally, I will show laboratory limits for the DM coupling constant with muons in selected DM model-dependent scenarios.   

Computing the Hadronic Vacuum Polarization Contribution to the Muon Anomalous Magnetic Moment in Lattice QCD

The Fermilab muon g-2 experiment is expected to reach its precision goal of 120 ppb within the next two years. The uncertainty in Standard Model predictions of this quantity is dominated by hadronic corrections. I will report on the status of the Fermilab, HPQCD, and MILC Collaborations' multi-year project to compute the hadronic vacuum polarization contribution (HVP) in lattice QCD at sub-percent precision. In our newly-generated data set we employ noise reduction techniques to handle large statistical errors in contributions from light quarks, which comprise roughly 90% of the total HVP. I will present preliminary results on the light quark connected contribution to the HVP as well as other HVP observables obtained from this new data set.   

Short- and Intermediate-distance Windows Hadronic Vacuum Polarization Contributions to the Muon Anomalous Magnetic Moment

Precise measurement of the muon anomalous magnetic moment (g-2) presents an exciting opportunity to search for evidence of beyond the Standard Model physics. Precise calculations of the hadronic vacuum polarization (HVP) contribution are necessary to compare with experiment. We present recent results for short- and intermedate-distance windows in euclidean time using highly improved staggered quarks. Results for the connected light-quark contribution dominate, but subleading contributions such as from strange and charm quarks, disconnected diagrams, isospin breaking, and electromagnetic effects will also be discussed. (Some results may be blinded.)   

Using a Pile-up Simulation to Characterize Detector Resolution in Mu2e's Stopping Target Monitor

The Mu2e experiment will search for coherent neutrino-less conversion of muons to electrons in muonic aluminum. The Mu2e Stopping Target Monitor employs two detectors optimized to detect photons from 100 keV up to a few MeV. One is a high-purity Germanium diode (HPGe), and the other is a Lanthanum Bromide (LaBr) scintillator detector; they will measure x-rays and gamma rays ejected from the Mu2e stopping target to count the number of stopped muons. LaBr is used to mitigate the effects of pile-up in high-rate data, since HPGe is susceptible to over-saturation in a high-rate environment. In this presentation, I present the performance of LaBr in a pile-up simulation, characterizing how the resolution of a 662 keV Cs source is affected by how close successive pulses are from each other.   

Design, construction and qualification tests of the Mu2e crystal calorimeter

The Mu2e experiment at Fermilab will search for the charged-lepton flavour violating conversion of negative muons into electrons in the coulomb field of an Al nucleus, planning to reach a single event sensitivity of about 3x10⁻¹⁷, four orders of magnitude beyond the current best limit.

 

The conversion electron has a clear monoenergetic signature at 104.967 MeV, slightly below the muon rest mass, and will be identified by a complementary measurement carried out by a high-resolution tracker and an electromagnetic calorimeter (EMC). The calorimeter is composed of 1348 pure CsI crystals, each read by two custom UV-extended SiPMs, that are arranged in two annular disks. The EMC should achieve better than 10% energy resolution and 500 ps timing resolution for 100 MeV electrons and maintain extremely high levels of reliability and stability and in a harsh operating environment with high vacuum, 1 T B-field and radiation exposures up to 100 krad and 10¹² n_1MeVeq/cm².

 

The calorimeter technological choice, along with the design of the custom front-end electronics, cooling and mechanical systems were validated through an electron beam test on a large-scale 51-crystals prototype (Module-0) and extensive test campaigns that characterized and verified the performance of crystals, photodetectors, analogue and digital electronics. This included hardware stress tests and irradiation campaigns with neutrons, protons, and photons.

 

The production and QC phases of all calorimeter components is completed apart for the digital electronics that is being produced. A series of vertical slice tests with the final electronics was carried out on the Module-0 at LNF to implement and validate calibration procedures. The two calorimeter disks have been assembled at the moment of writing. Status of construction will be summarised, along with plans for commissioning and first calibration of the fully assembled disks.