Muon (g-2) Technical Design Report

Abstract

Executive SummaryThe Muon g − 2 Experiment, E989 at Fermilab, will measure the muon anomalous magnetic moment, a µ ≡ (g − 2)/2, to unprecedented precision: the goal is 0.14 parts per million (ppm). The worth of such an undertaking is coupled to the fact that the Standard Model (SM) prediction for a µ can also be determined to similar precision. As such, the comparison between experiment and theory provides one of the most sensitive tests of the completeness of the model. The Brookhaven-based E821 experiment, which completed data taking in 2001, determined a µ (Expt) to 0.54 ppm. Steady improvements in theory since that time have resulted in a present SM uncertainty on a µ (SM) of 0.42 ppm. The experimental measurement and SM predictions differ by 3.3 to 3.6 standard deviations, depending on which evaluation of the lowest-order hadronic contribution in the SM is used: ∆a µ (Expt − SM) = (286 ± 80) × 10 −11(1) = (260 ± 78) × 10 −11(2) (see Chapter 2 for details). This is a highly cited result, owing in part to the many natural SM extensions, from supersymmetry to dark photons, that could cause such an effect. The planned four-fold improvement in experimental precision compared to E821, could establish beyond doubt a signal for new physics-if the central value of the measurement remains unchanged. During the time it will require to mount, run and analyze the data, the SM hadronic predictions are expected to become even more precise; thus the comparison of experiment to theory will be quite powerful, no matter what final values are found. The Motivation for the new experiment and a detailed exposition on the SM theory is provided in Chapter 2 of this document.The original E989 Proposal, and the additional design work now completed in preparation of this Technical Design Report (TDR), outline a credible plan to achieve the experimental goal in a timely and cost-efficient manner. The approach is anchored by the re-use of the existing precision muon storage ring, an efficient and parasitic deployment of the Fermilab proton complex and beamlines, and strategic upgrades or replacements of outdated or underperforming components from E821. The experiment will be carried out by a collaboration of accelerator, atomic, nuclear and particle physicists, drawing from domestic and international universities and national laboratories. The collaboration retains a strong core of experienced participants from BNL E821, augmented by many new groups selected for their expertise in areas that are required to mount a next-generation experiment.In many ways, E989 is a unique, large-scale Project. Several core aspects involve proven elements from E821 that will be retained in whole or with minor upgrades. This is especially true for the storage ring elements and the magnetic field measuring tools, which will be relocated, re-assembled and restored to operation. Many of these items are well beyond a normal TDR stage in terms of design; indeed, they exist and often require no more than testing and minor repair. In contrast, several items hav...

Electron–positron annihilation into hadrons at the higher-loop levels

“…For example, Fig. 4D implies that at the five-loop level the maximum value of the fifth-order functionρ (5) 5 (σ ) is about three orders of magnitude less than the maximum value of the first-order functionρ (5) 1 (σ ). In turn, as it will be discussed in the next section, the fact that the functionρ ( ) j+1 (σ ) is subdominant toρ ( ) j (σ ) eventually results in an enhanced higher-loop stability of the proper expression for the R-ratio (10) at moderate and low energies with respect to both its naive form (12) and the commonly employed truncated reexpanded approximation (31).…”

“…(30) is large enough, then it can provide a rather accurate approximation of the strong correction to the R-ratio (10) for √ s/Λ > exp(π/2) 4.81. However, one usually truncates the re-expansion (30) at the order , thereby neglecting all the higher-order π 2 -terms, 5 which results in the following expression commonly employed in practical applications:…”

“…Additionally, the energy scales relevant to the foregoing strong interaction processes span from the infrared to ultraviolet domain, so that their theoretical investigation provides a native framework for a profound study of both perturbative and intrinsically nonperturbative aspects of hadron dynamics. It is worth noting also that a majority of the aforementioned processes are of a direct relevance to the physics at the currently designed Future Collider Projects, such as the Future Circular Collider FCC-ee [1], Circular ElectronPositron Collider CEPC (its first phase) [2], the International Linear Collider ILC [3], the Compact Linear Collider CLIC [4], as well as the E989 experiment at Fermilab [5], the E34 experiment at J-PARC [6], and others.…”

Measuring the leading hadronic contribution to the muon g-2 via the -e elastic scattering.

“…New experiments at Fermilab and J-PARC, aiming at measuring the muon g-2 to a a e-mail: umberto.marconi@bo.infn.it b e-mail: fulvio.piccinini@pv.infn.it precision of 1.6×10 −10 (0.14 ppm), are in preparation [12][13][14][15].…”

Hadronic Leading Order Contribution to the Muon g-2

“…This deviation may be due to a contribution from physics beyond the SM, which makes a µ extremely important. There are two experiments which aim to improve the experimental uncertainty by a factor of 4 [4,5], which further enhances the importance of this quantity.…”