The Muon $g-2$ in Progress

Abstract

Two next generation muon g − 2 experiments at Fermilab in the US and at J-PARC in Japan have been designed to reach a four times better precision from 0.54 ppm to 0.14 ppm and the challenge for the theory side is to keep up in precision as far as possible. This has triggered a lot of new research activities. The main motivation is the persisting 3 to 4 σ deviation between standard theory and experiment. As Standard Model predictions almost without exception match perfectly all other experimental information, the deviation in one of the most precisely measured quantities in particle physics remains a mystery and inspires the imagination of model builders. Plenty of speculations are aiming to explain what beyond the Standard Model effects could fill what seems to be missing. Here very high precision experiments are competing with searches for new physics at the high energy frontier lead by the Large Hadron Collider at CERN. Actually, the tension is increasing steadily as no new states are found which could accommodate the g µ − 2 discrepancy. With the new muon g − 2 experiments this discrepancy would go up at least to 6 σ, in case the central values do not move, up to 10 σ could be reached if the present theory error could be reduced by a factor of two. Interestingly, the new α from Berkeley by R. H. Parker et al. Science 360, 191 (2018): α −1 (Cs18) = 137.035999046 (27) gives an a e prediction a e = 0.00115965218157(23) such that a exp e − a the e = (−84 ± 36) × 10 −14 shows a −2.3 σ deviation now.

Influence of an inert charged Higgs boson on the muon g−2 and radiative neutrino masses in a scotogenic model

“…However, even so, the τ → µγ decay may play an important role in constraining the parameters, where from Eqs. (23) and (25), the related parameters are y * R2 y R3 , y * R2 ξ L3 , and ξ * L2 y R3 . If we take the limit with y R3 = 0, the BR for τ → µγ does not vanish due to the y * R2 ξ L3 effect.…”

“…From Eqs. (23) and (25), it can be seen that even when using ξ L1 = 0, the BR for µ → eγ is still dictated by y * R1 ξ L2 ; that is, the µ → eγ also gives a strict constraint on the y R1 parameter. In order to understand how BR(µ → eγ) is sensitive to ξ L1 , BR(µ → eγ) (in units of 10 −13 ) as a function of |ξ L2 | is shown in Fig.…”

“…Accordingly, resolutions to the muon g − 2 excess have been broadly studied in the literature [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. A detailed review of the muon g − 2 can be found in [20][21][22][23].…”

Opportunities for Lattice QCD in quark and lepton flavor physics

“…In addition, USQCD is devoting substantial human effort and computing resources towards the task of bringing the hadronic uncertainties on a µ to the level needed by the Fermilab E989 and J-PARC E34 experiments. where the uncertainties from experiment [176,177], the fine-structure-constant [173], and the Standard-Model theory calculation [174] are given separately. The theory uncertainty stems in approximately equal parts from the hadronic contributions and from the five-loop QED contribution calculated in Ref.…”

Pseudoscalar-Meson Contributions to $g−2$ via Schwinger’s Sum Rule

“…Presently, there is about 4 σ discrepancy between the experimental value of the muon anomalous magnetic moment (AMM), a µ = 1 /2(g − 2) µ , from BNL-E821 [1] and the Standard Model (SM) prediction [2]: where the theory uncertainty is dominated by the hadronic vacuum polarization (HVP) and hadronic light-by-light scattering (HLbL) contributions, see Fig. 1 (a) The ongoing experiments at Fermilab [5,6] and J-PARC [7] are expected to improve the experimental precision by a factor of 4, as well as to provide an important cross check of the previous experiment and of each other.…”