The storage ring magnetic field is measured using nuclear magnetic resonance probes calibrated in terms of the equivalent proton spin precession frequencyω 0 p in a spherical water sample at 34.7☌. Intensity variation of high-energy positrons from muon decays directly encodes the difference frequency ω a between the spin-precession and cyclotron frequencies for polarized muons in a magnetic storage ring. The anomaly is determined from the precision measurements of two angular frequencies. We present the first results of the Fermilab National Accelerator Laboratory (FNAL) Muon g − 2 Experiment for the positive muon magnetic anomaly a μ ≡ ðg μ − 2Þ=2. A precise theoretical computation of the anomalous magnetic moment of the muon based on ab initio quantum chromodynamics and quantum electrodynamics calculations is presented, which favours the existing experimental values. Moreover, the methods used and developed in this work will enable further increased precision as more powerful computers become available. Our result favours the experimentally measured value over those obtained using the dispersion relation. We reach sufficient precision to discriminate between the measurement of the anomalous magnetic moment of the muon and the predictions of dispersive methods. To eliminate our reliance on these experiments, here we use ab initio quantum chromodynamics (QCD) and quantum electrodynamics simulations to compute the LO-HVP contribution.
The most precise, model-independent determinations so far rely on dispersive techniques, combined with measurements of the cross-section of electron–positron annihilation into hadrons3–6. For the upcoming measurements, it is essential to evaluate the prediction for this contribution with independent methods and to reduce its uncertainties. Theoretically, the dominant source of error is the leading-order hadronic vacuum polarization (LO-HVP) contribution. Today, theoretical and measurement errors are comparable however, ongoing and planned experiments aim to reduce the measurement error by a factor of four. Standard-model predictions1 exhibit disagreement with measurements2 that is tightly scattered around 3.7 standard deviations. One long-standing discrepancy concerns the anomalous magnetic moment of the muon, a measure of the magnetic field surrounding that particle. Any deviation from its predictions would be a sign of new, fundamental physics. The standard model of particle physics describes the vast majority of experiments and observations involving elementary particles. Recent observations of the angular correlation spectra in the decays $ $. We also provide an overview of possible future experiments probing pair production in the A=4 system at a number of candidate facilities. While electromagnetic interactions are treated to high orders in the chiral expansion, the interactions of the hypothetical boson with nucleons are modeled in leading-order χEFT (albeit, in some instances, selected subleading contributions are also accounted for). The ab initio calculations use exact hyperspherical-harmonics methods to describe the bound state and low-energy spectrum of the A=4 continuum, and they fully account for initial state interaction effects in the 3+1 clusters. We consider several possibilities, that this boson is either a scalar, pseudoscalar, vector, or axial particle. Next, we examine how the exchange of a hypothetical low-mass boson would impact the cross section for such a process. We first analyze the process as a purely electromagnetic one in the context of a state-of-the-art approach to nuclear strong-interaction dynamics and nuclear electromagnetic currents, derived from chiral effective field theory (χEFT).
The European Physical Journal.The present work deals with e+−e− pair production in the four-nucleon system.Hunting down the X17 boson at the CERN SPS down the X17 boson at the CERN SPS},Īuthor=,