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Register a new userLeptoquarks and Matter Unification: Flavor Anomalies and the Muon $g-2$
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We discuss the minimal theory for quark-lepton unification at the low scale. In this context, the quarks and leptons are unified in the same representations and neutrino masses are generated through the inverse seesaw mechanism. The properties of the leptoquarks predicted in this theory are discussed in detail and we investigate the predictions for the leptonic and semi-leptonic decays of mesons. We study the possibility to explain the current value of $mathcal{R}_K$ reported by the LHCb collaboration and the value of the muon anomalous magnetic moment reported by the Muon $g-2$ experiment at Fermilab.
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In the light of the recent result of the Muon g-2 experiment and the update on the test of lepton flavour universality $R_K$ published by the LHCb collaboration, we systematically build and discuss a set of models with minimal field content that can simultaneously give: (i) a thermal Dark Matter candidate; (ii) large loop contributions to $bto sellell$ processes able to address $R_K$ and the other $B$ anomalies; (iii) a natural solution to the muon $g-2$ discrepancy through chirally-enhanced contributions.
We perform a phenomenological analysis of simplified models of light, feebly interacting particles (FIPs) that can provide a combined explanation of the anomalies in $bto s l^+ l ^-$ transitions at LHCb and the anomalous magnetic moment of the muon. Different scenarios are categorised according to the explicit momentum dependence of the FIP coupling to the $b-s$ and $mu-mu$ vector currents and they are subject to several constraints from flavour and precision physics. We show that a phenomenologically viable combined solution to the muon $g-2$ and flavour anomalies always exists if a vector with mass larger than $4 ,textrm{GeV}$ is exchanged. Interestingly, the LHC has the potential to probe this region of the parameter space by increasing the precision of the $Zto 4mu$ cross-section measurement. Conversely, we find that solutions based on the exchange of a lighter vector, in the $m_V < 1,textrm{GeV}$ range, are essentially excluded by a combination of $Bto K +textrm{invisible}$ and $W$-decay precision bounds.
Data from the Muon g-2 experiment and measurements of the fine structure constant suggest that the anomalous magnetic moments of the muon and electron are at odds with standard model expectations. We survey the ability of axion-like-particles, two-Higgs-doublet models and leptoquarks to explain the discrepancies. We find that accounting for other constraints, all scenarios except the Type-I, Type-II and Type-Y two-Higgs-doublet models fit the data well.
We present a model of radiative neutrino masses which also resolves anomalies reported in $B$-meson decays, $R_{D^{(star)}}$ and $R_{K^{(star)}}$, as well as in muon $g-2$ measurement, $Delta a_mu$. Neutrino masses arise in the model through loop diagrams involving TeV-scale leptoquark (LQ) scalars $R_2$ and $S_3$. Fits to neutrino oscillation parameters are obtained satisfying all flavor constraints which also explain the anomalies in $R_{D^{(star)}}$, $R_{K^{(star)}}$ and $Delta a_mu$ within $1, sigma$. An isospin-3/2 Higgs quadruplet plays a crucial role in generating neutrino masses; we point out that the doubly-charged scalar contained therein can be produced in the decays of the $S_3$ LQ, which enhances its reach to 1.1 (6.2) TeV at $sqrt s=14$ TeV high-luminosity LHC ($sqrt s=100$ TeV FCC-hh). We also present flavor-dependent upper limits on the Yukawa couplings of the LQs to the first two family fermions, arising from non-resonant dilepton ($pp rightarrow ell^+ ell^-$) processes mediated by $t$-channel LQ exchange, which for 1 TeV LQ mass, are found to be in the range $(0.15 - 0.36)$. These limits preclude any explanation of $R_{D^{(star)}}$ through LQ-mediated $B$-meson decays involving $ u_e$ or $ u_mu$ in the final state. We also find that the same Yukawa couplings responsible for the chirally-enhanced contribution to $Delta a_mu$ give rise to new contributions to the SM Higgs decays to muon and tau pairs, with the modifications to the corresponding branching ratios being at (2-6)% level, which could be tested at future hadron colliders, such as HL-LHC and FCC-hh.
According to the FNAL+BNL measurements for the muon $g-2$ and the Berkeley $^{133}$Cs measurement for the electron $g-2$, the SM prediction for the muon (electron) $g-2$ is $4.2sigma$ ($2.4sigma$) below (above) the experimental value. A joint explanation requires a positive contribution to the muon $g-2$ and a negative contribution to the electron $g-2$. In this work we explore the possibility of such a joint explanation in the minimal supersymmetric standard model (MSSM). Assuming no universality between smuon and selectron soft masses, we find out a part of parameter space for a joint explanation of muon and electron $g-2$ anomalies at $2sigma$ level. This part of parameter space can survive the LHC and LEP constraints, but gives an over-abundance for the dark matter if the bino-like lightest neutralino is assumed to be the dark matter candidate. With the assumption that the dark matter candidate is a superWIMP (say a pseudo-goldstino in multi-sector SUSY breaking scenarios, whose mass can be as light as GeV and produced from the late-dacay of the thermally freeze-out lightest neutralino), the dark matter problem can be avoided. So, the MSSM may give a joint explanation for the muon and electron $g-2$ anomalies at $2sigma$ level (the muon $g-2$ anomaly can be ameliorated to $1sigma$).
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