Abstract
The new measurement of the muon's anomalous magnetic moment released by the Muon g-2 experiment at Fermilab sets strong constraints on the properties of many new particles. Using an effective field theory approach to the interactions of higher-spin fields, we evaluate the contribution of an electrically neutral and colour singlet spin-3/2 fermion to (g−2)μ and derive the corresponding constraints on its mass and couplings. These constraints are then compared with the ones on spin-1/2 fermions, such as the vector-like leptons that are predicted by various extensions of the Standard Model, the excited leptons which appear in composite models, as well as the charginos and neutralinos of supersymmetric theories. Unlike these new spin-1/2 fermions, the spin-3/2 particles generate only small contributions to the muon anomalous magnetic moment unless the effective new physics scale is very close to the electroweak scale.
Highlights
The Muon g − 2 collaboration at Fermilab has released [1,2] the long awaited new measurement of the anomalous magnetic moment of the muon, aμ = ( g − 2)μ.The result of the previous Brookhaven E821 muon−2 experiment was [3] aEμ821116592089(63) × 10−11, and had a deviation of about 3.7σ, aEμ821 = aEμ821 − aSμM = (279 ± 76) × 10−11 when compared with the recent worldwide consensus of the Standard Model (SM) contribution [4]
These values are at the boundary of validity of the effective field theory (EFT) and, in addition, lead to a ψ3/2 cross section at the Large Hadron Collider (LHC) that is large enough to rule out this possibility
We have computed the contribution of a generic massive SM singlet spin-3/2 fermion to the muon anomalous magnetic moment (g − 2)μ and to the muon electric dipole moment dμ
Summary
The Muon g − 2 collaboration at Fermilab has released [1,2] the long awaited new measurement of the anomalous magnetic moment of the muon, aμ. We present relevant details of the effective theory for a generic spin-3/2 fermion, perform the calculation for its contribution to the muon anomalous magnetic moment and confront the result with the new measurement. The contribution to (g − 2)μ from the higher-spin field as a function of its mass m3/2 and for different values of cγ is shown in the left-panel of Fig. 2 for a renormalization scale μ = 250 GeV and a new physics scale = 1 TeV. One can obtain an anomalous ψ3/2 contribution close to the measured (g − 2)μ value if both the effective scale and the mass m3/2 are close to the weak scale, O(300 GeV) These values are at the boundary of validity of the EFT and, in addition, lead to a ψ3/2 cross section at the LHC that is large enough to rule out this possibility. Current observations allow for O(1) imaginary parts of cμW , cγμ but can be mildly constraining in the case of the electron
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