Measurement of the anomalous precession frequency of the muon in the Fermilab Muon g − 2 Experiment

With the long-standing tension between experiment and Standard-Model (SM) prediction in the anomalous magnetic moment of the muon aμ recently reaffirmed by the Fermilab experiment, the crucial question becomes which other observables could be sensitive to the underlying physics beyond the SM to which aμ may be pointing. While from the effective field theory (EFT) point of view no direct correlations exist, this changes in specific new physics models. In particular, in the case of explanations involving heavy new particles above the electroweak (EW) scale with chiral enhancement, which are preferred to evade exclusion limits from direct searches, correlations with other observables sensitive to EW symmetry breaking are expected. Such scenarios can be classified according to the SU(2)L representations and the hypercharges of the new particles. We match the resulting class of models with heavy new scalars and fermions onto SMEFT and study the resulting correlations with h → μμ and Z → μμ decays, where, via SU(2)L symmetry, the latter process is related to Z → νν and modified W-μ-ν couplings.

Section: Introductionsupporting

confidence: 70%

Consequences of chirally enhanced explanations of (g − 2)μ for h → μμ and Z → μμ

Crivellin

Hoferichter

2021

“…Furthermore, dark sectors coupling to muons can explain a longstanding tension between theory and experiment in the determination of the muon anomalous magnetic moment [38][39][40][41]. This tension has recently increased to 4.2σ, 1 due to the measurement by the g − 2 collaboration at Fermilab [43][44][45][46], which is in agreement with the previous measurement at Brookhaven [39]. This discrepancy will be further tested by the J-PARC g − 2 experiment [47].…”

Section: Introductionsupporting

confidence: 78%

“…In the first two windows this scenario can explain the discrepancy between the observed value of (g − 2) µ and the SM prediction [38][39][40][41][43][44][45][46]. As noted in the introduction, the available parameter space for forbidden annihilations into the SM is reminiscent of WIMP DM in the MSSM.…”

Section: Jhep06(2021)103mentioning

confidence: 99%

Forbidden dark matter annihilations into Standard Model particles

D’Agnolo

Liu

Ruderman

et al. 2021

We present kinematically forbidden dark matter annihilations into Standard Model leptons. This mechanism precisely selects the dark matter mass that gives the observed relic abundance. This is qualitatively different from existing models of thermal dark matter, where fixing the relic density typically leaves open orders of magnitude of viable dark matter masses. Forbidden annihilations require the dark matter to be close in mass to the particles that dominate its annihilation rate. We show examples where the dark matter mass is close to the muon mass, the tau mass, or the average of the tau and muon masses. We find that most of the relevant parameter space can be covered by the next generation of proposed beam-dump experiments and future high-luminosity electron positron colliders. Forbidden dark matter predicts large couplings to the Standard Model that can explain the observed value of (g − 2)μ.

“…They can thus lead to relevant quantum corrections to leptonic precision observables and generate lepton flavour violating decays of leptons that are extremely suppressed in the SM since they vanish in the limit of massless neutrinos. The (g − 2) µ discrepancy [17,18], recently reinforced by the g − 2 experiment at Fermilab [19][20][21][22], with a tension of 4.2 σ compared to the SM prediction [23], can be explained with a Z boson heavier than the electroweak (EW) scale if it couples flavour violatingly to the second and third lepton generation [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43].…”

Section: Jhep06(2021)068mentioning

confidence: 99%

“…∆a µ is extracted from the measurement of refs. [17,18] and from the recent results from the Fermilab Muon g − 2 experiment [19][20][21][22]. The theory consensus is taken from ref.…”

Section: Anomalous Magnetic Moments and Electric Dipole Momentsmentioning

confidence: 99%

Global analysis of leptophilic Z′ bosons

Buras

Crivellin

Kirk

et al. 2021

New neutral heavy gauge bosons (Z′) are predicted within many extensions of the Standard Model. While in case they couple to quarks the LHC bounds are very stringent, leptophilic Z′ bosons (even with sizable couplings) can be much lighter and therefore lead to interesting quantum effects in precision observables (like (g − 2)μ) and generate flavour violating decays of charged leptons. In particular, $$ \mathrm{\ell}\to \mathrm{\ell}^{\prime }v\overline{v} $$ ℓ → ℓ ′ v v ¯ decays, anomalous magnetic moments of charged leptons, ℓ → ℓ′γ and ℓ → 3ℓ′ decays place stringent limits on leptophilic Z′ bosons. Furthermore, in case of mixing Z′ with the SM Z, Z pole observables are affected. In light of these many observables we perform a global fit to leptophilic Z′ models with the main goal of finding the bounds for the Z′ couplings to leptons. To this end we consider a number of scenarios for these couplings. While in generic scenarios correlations are weak, this changes once additional constraints on the couplings are imposed. In particular, if one considers an Lμ− Lτ symmetry broken only by left-handed rotations, or considers the case of τ − μ couplings only. In the latter setup, on can explain the (g − 2)μ anomaly and the hint for lepton flavour universality violation in $$ \tau \to \mu v\overline{v}/\tau \to ev\overline{v} $$ τ → μv v ¯ / τ → ev v ¯ without violating bounds from electroweak precision observables.