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Register a new userThe Natural Explanation of the Muon Anomalous Magnetic Moment via the Electroweak Supersymmetry from the GmSUGRA in the MSSM
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The Fermi-Lab Collaboration has announced the results for the measurement of muon anomalous magnetic moment. Combining with the previous results by the BNL experiment, we have $4.2 sigma$ deviation from the Standard Model (SM), which strongly implies the new physics around 1 TeV. To explain the muon anomalous magnetic moment naturally, we analyze the corresponding five Feynman diagrams in the Supersymetric SMs (SSMs), and show that the Electroweak Supersymmetry (EWSUSY) is definitely needed. We realize the EWSUSY in the Minimal SSM (MSSM) with Genernalized Mininal Supergravity (GmSUGRA). We find large viable parameter space, which is consistent with all the current experimental constraints. In particular, the Lightest Supersymmetric Particle (LSP) neutralino can be at least as heavy as 550 GeV. Most of the viable parameter space can be probed at the future HL-LHC, while we do need the future HE-LHC to probe some viable parameter space. However, it might still be challenge if R-parity is violated.
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The recent measurement of the muon anomalous magnetic moment a_muequiv (g-2)_mu/2 by the Fermilab Muon g-2 experiment sharpens an earlier discrepancy between theory and the BNL E821 experiment. We examine the predicted Delta a_muequiv a_mu(exp)-a_mu(th) in the context of supersymmetry with low electroweak naturalness (restricting to models which give a plausible explanation for the magnitude of the weak scale). A global analysis including LHC Higgs mass and sparticle search limits points to interpretation within the normal scalar mass hierarchy (NSMH) SUSY model wherein first/second generation matter scalars are much lighter than third generation scalars. We present a benchmark model for a viable NSMH point which is natural, obeys LHC Higgs and sparticle mass constraints and explains the muon magnetic anomaly. Aside from NSMH models, then we find the (g-2)_mu anomaly cannot be explained within the context of natural SUSY, where a variety of data point to decoupled first/second generation scalars. The situation is worse within the string landscape where first/second generation matter scalars are pulled to values in the 10-50 TeV range. An alternative interpretation for SUSY models with decoupled scalar masses is that perhaps the recent lattice evaluation of the hadronic vacuum polarization could be confirmed which leads to a Standard Model theory-experiment agreement in which case there is no anomaly.
We review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant $alpha$ and is broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including $mathcal{O}(alpha^5)$ with negligible numerical uncertainty. The electroweak contribution is suppressed by $(m_mu/M_W)^2$ and only shows up at the level of the seventh significant digit. It has been evaluated up to two loops and is known to better than one percent. Hadronic contributions are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. The leading hadronic contribution appears at $mathcal{O}(alpha^2)$ and is due to hadronic vacuum polarization, whereas at $mathcal{O}(alpha^3)$ the hadronic light-by-light scattering contribution appears. Given the low characteristic scale of this observable, these contributions have to be calculated with nonperturbative methods, in particular, dispersion relations and the lattice approach to QCD. The largest part of this review is dedicated to a detailed account of recent efforts to improve the calculation of these two contributions with either a data-driven, dispersive approach, or a first-principle, lattice-QCD approach. The final result reads $a_mu^text{SM}=116,591,810(43)times 10^{-11}$ and is smaller than the Brookhaven measurement by 3.7$sigma$. The experimental uncertainty will soon be reduced by up to a factor four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment. This and the prospects to further reduce the theoretical uncertainty in the near future-which are also discussed here-make this quantity one of the most promising places to look for evidence of new physics.
We analyse the low energy predictions of the minimal supersymmetric standard model (MSSM) arising from a GUT scale Pati-Salam gauge group further constrained by an $A_4 times Z_5$ family symmetry, resulting in four soft scalar masses at the GUT scale: one left-handed soft mass $m_0$ and three right-handed soft masses $m_1,m_2,m_3$, one for each generation. We demonstrate that this model, which was initially developed to describe the neutrino sector, can explain collider and non-collider measurements such as the dark matter relic density, the Higgs boson mass and, in particular, the anomalous magnetic moment of the muon $(g-2)_mu$. Since about two decades, $(g-2)_mu$ suffers a puzzling about 3$,sigma$ excess of the experimentally measured value over the theoretical prediction, which our model is able to fully resolve. As the consequence of this resolution, our model predicts specific regions of the parameter space with the specific properties including light smuons and neutralinos, which could also potentially explain di-lepton excesses observed by CMS and ATLAS.
A new QCD sum rule determination of the leading order hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon, $a_{mu}^{rm hvp}$, is proposed. This approach combines data on $e^{+}e^{-}$ annihilation into hadrons, perturbative QCD and lattice QCD results for the first derivative of the electromagnetic current correlator at zero momentum transfer, $Pi_{rm EM}^prime(0)$. The idea is based on the observation that, in the relevant kinematic domain, the integration kernel $K(s)$, entering the formula relating $a_{mu}^{rm hvp}$ to $e^{+}e^{-}$ annihilation data, behaves like $1/s$ times a very smooth function of $s$, the squared energy. We find an expression for $a_{mu}$ in terms of $Pi_{rm EM}^prime(0)$, which can be calculated in lattice QCD. Using recent lattice results we find a good approximation for $a_{mu}^{rm hvp}$, but the precision is not yet sufficient to resolve the discrepancy between the $R(s)$ data-based results and the experimentally measured value.
We compute the leading hadronic contribution to the anomalous magnetic moment of the muon a_mu^HLO using two dynamical flavours of non-perturbatively O(a) improved Wilson fermions. By applying partially twisted boundary conditions we are able to improve the momentum resolution of the vacuum polarisation, an important ingredient for the determination of the leading hadronic contribution. We check systematic uncertainties by studying several ensembles, which allows us to discuss finite size effects and lattice artefacts. The chiral behavior of a_mu^HLO turns out to be non-trivial, especially for small pion masses.
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