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Register a new userExperimental measurement of muon (g-2)
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F.E. Gray
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The muon (g-2) experiment at Brookhaven National Laboratory has measured the anomalous magnetic moment of the positive muon with a precision of 0.7 ppm. This paper presents that result, concentrating on some of the important experimental issues that arise in extracting the anomalous precession frequency from the data.
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The Muon g-2 Experiment at Fermi National Accelerator Laboratory (FNAL) has measured the muon anomalous precession frequency $omega_a$ to an uncertainty of 434 parts per billion (ppb), statistical, and 56 ppb, systematic, with data collected in four storage ring configurations during its first physics run in 2018. When combined with a precision measurement of the magnetic field of the experiments muon storage ring, the precession frequency measurement determines a muon magnetic anomaly of $a_{mu}({rm FNAL}) = 116,592,040(54) times 10^{-11}$ (0.46 ppm). This article describes the multiple techniques employed in the reconstruction, analysis and fitting of the data to measure the precession frequency. It also presents the averaging of the results from the eleven separate determinations of omega_a, and the systematic uncertainties on the result.
I report on the progress of two new muon anomalous magnetic moment experiments, which are in advanced design and construction phases. The goal of Fermilab E989 is to reduce the experimental uncertainty of $a_mu$ from Brookhaven E821 by a factor of 4; that is, $delta a_mu sim 16 times 10^{-11}$, a relative uncertainty of 140~ppb. The method follows the same magic-momentum storage ring concept used at BNL, and pioneered previously at CERN, but muon beam preparation, storage ring internal hardware, field measuring equipment, and detector and electronics systems are all new or upgraded significantly. In contrast, J-PARC E34 will employ a novel approach based on injection of an ultra-cold, low-energy, muon beam injected into a small, but highly uniform magnet. Only a small magnetic focusing field is needed to maintain storage, which distinguishes it from CERN, BNL and Fermilab. E34 aims to roughly match the previous BNL precision in their Phase~1 installation.
The Fermi National Accelerator Laboratory has measured the anomalous precession frequency $a^{}_mu = (g^{}_mu-2)/2$ of the muon to a combined precision of 0.46 parts per million with data collected during its first physics run in 2018. This paper documents the measurement of the magnetic field in the muon storage ring. The magnetic field is monitored by nuclear magnetic resonance systems and calibrated in terms of the equivalent proton spin precession frequency in a spherical water sample at 34.7$^circ$C. The measured field is weighted by the muon distribution resulting in $tilde{omega}^{}_p$, the denominator in the ratio $omega^{}_a$/$tilde{omega}^{}_p$ that together with known fundamental constants yields $a^{}_mu$. The reported uncertainty on $tilde{omega}^{}_p$ for the Run-1 data set is 114 ppb consisting of uncertainty contributions from frequency extraction, calibration, mapping, tracking, and averaging of 56 ppb, and contributions from fast transient fields of 99 ppb.
Precision measurements of fundamental quantities have played a key role in pointing the way forward in developing our understanding of the universe. Though the enormously successful Standard Model (SM) describes the breadth of both historical and modern experimental particle physics data, it is necessarily incomplete. The muon $g-2$ experiment executed at Brookhaven concluded in 2001 and measured a discrepancy of more than three standard deviations compared to the Standard Model calculation. Arguably, this remains the strongest hint of physics beyond the SM. A new initiative at Fermilab is under construction to improve the experimental accuracy four-fold. The current status is presented here.
The Muon g-2 experiment at Fermilab will measure the anomalous magnetic moment of the muon to a precision of 140 parts per billion, which is a factor of four improvement over the previous E821 measurement at Brookhaven. The experiment will also extend the search for the muon electric dipole moment (EDM) by approximately two orders of magnitude. Both of these measurements are made by combining a precise measurement of the 1.45T storage ring magnetic field with an analysis of the modulation of the decay rate of the higher-energy positrons from the (anti-)muon decays recorded by 24 calorimeters and 3 straw tracking detectors. The current status of the experiment as well as results from the initial beam delivery and commissioning run in the summer of 2017 will be discussed.
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