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Register a new userThe New Muon g-2 Experiment at Fermilab
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J. Grange
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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.
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There is a long standing discrepancy between the Standard Model prediction for the muon g-2 and the value measured by the Brookhaven E821 Experiment. At present the discrepancy stands at about three standard deviations, with a comparable accuracy between experiment and theory. Two new proposals -- at Fermilab and J-PARC -- plan to improve the experimental uncertainty by a factor of 4, and it is expected that there will be a significant reduction in the uncertainty of the Standard Model prediction. I will review the status of the planned experiment at Fermilab, E989, which will analyse 21 times more muons than the BNL experiment and discuss how the systematic uncertainty will be reduced by a factor of 3 such that a precision of 0.14 ppm can be achieved.
A new experiment at Fermilab will measure the anomalous magnetic moment of the muon with a precision of 140 parts per billion (ppb). This measurement is motivated by the results of the Brookhaven E821 experiment that were first released more than a decade ago, which reached a precision of 540 ppb. As the corresponding Standard Model predictions have been refined, the experimental and theoretical values have persistently differed by about 3 standard deviations. If the Brookhaven result is confirmed at Fermilab with this improved precision, it will constitute definitive evidence for physics beyond the Standard Model. The experiment observes the muon spin precession frequency in flight in a well-calibrated magnetic field; the improvement in precision will require both 20 times as many recorded muon decay events as in E821 and a reduction by a factor of 3 in the systematic uncertainties. This paper describes the current experimental status as well as the plans for the upgraded magnet, detector and storage ring systems that are being prepared for the start of beam data collection in 2017.
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.
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.
Starting this summer, Fermilab will host a key experiment dedicated to the search for signals of new physics: The Fermilab Muon g-2 Experiment. Its aim is to precisely measure the anomalous magnetic moment of the muon. In full operation, in order to avoid contamination, the newly born secondary beam is injected into a 505 m long Delivery Ring (DR) wherein it makes several revolutions before being sent to the experiment. Part of the commissioning scenario will execute a running mode wherein the passage from the DR will be skipped. With the aid of numerical simulations, we provide estimates of the expected performance.
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