Speaker
Description
The muon anomaly, $a_\mu=(g_{\mu}-2)/2$, is a low-energy observable which can be both measured and computed to high precision, making it a sensitive test of the Standard Model (SM) and a probe for new physics. The current discrepancy between the experimental value and the Standard Model calculation from the Muon $g-2$ Theory Initiative [T. Aoyama et al. - Phys. Rep. 887, 1 (2020)] is $a_{\mu}^{exp}-a_{\mu}^{SM}=(251\pm59)\cdot10^{-11}$, with a significance of $4.2\,\sigma$.
The Fermilab E989 experiment aims, with the full statistical power, to measure $a_{\mu}$ with a precision of $140\,$parts per billion (ppb), a four-fold improvement with respect to the previous measurement at the Brookhaven E821 experiment (1997-2001).
In April 2021 the FNAL E989 collaboration published their first result, based on the first year of data taking (2018 campaign) [B. Abi et al. (Muon $g-2$ Collaboration), Phys. Rev. Lett. 126, 141801 (2021)], and this year a new result is expected to be published, based on the datasets collected during Run-2 and Run-3 (2019 and 2020 campaigns).
A $3.1$-GeV spin-polarized beam of muons is injected into a storage ring of $14\,$m of diameter, in the presence of a $1.45\,$T magnetic field. The anomaly $a_\mu$ can be extracted by accurately measuring the anomalous muon spin precession frequency, $\omega_a$, and the magnetic field environment using Nuclear Magnetic Resonance techniques. The measurement of $\omega_a$ is based on the arrival time distribution of decay positrons in the high-energy tail of the spectrum, observed by $24$ electromagnetic calorimeters that are placed around the inner circumference of the storage ring. The histogram of positron counts is fitted with a function that takes into account detector and beam dynamics effects.
This talk will present details about the improvements and upgrades to the positron reconstruction and to the $\omega_a$ analysis since the 2021 results, and it will describe the final statistical and systematic sources of uncertainty in the new result.
