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Register a new userVector Leptoquarks Beyond Tree Level III: Vector-like Fermions and Flavor-Changing Transitions
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Extending previous work on this subject, we evaluate the impact of vector-like fermions at next-to-leading order accuracy in models with a massive vector leptoquark embedded in the $SU(4)times SU(3)^primetimes SU(2)_Ltimes U(1)_X$ gauge group. Vector-like fermions induce new sources of flavor symmetry breaking, resulting in tree-level flavor-changing couplings for the leptoquark not present in the minimal version of the model. These, in turn, lead to a series of non-vanishing flavor-changing neutral-current amplitudes at the loop level. We systematically analyze these effects in semileptonic, dipole and $Delta F=2$ operators. The impact of these corrections in $bto s u u$ and $bto ctau u$ observables are discussed in detail. In particular, we show that, in the parameter region providing a good fit to the $B$-physics anomalies, the model predicts a $10%$ to $50%$ enhancement of $mathcal{B}(Bto K^{(*)} u u)$.
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Models with massive vector leptoquarks, resulting from an $SU(4)$ gauge symmetry spontaneously broken at the TeV scale, are of great phenomenological interest given the current anomalies in semileptonic $B$ decays. We analyze the relations between low- and high-energy observables in such class of models to next-to-leading order accuracy in the $SU(4)$ gauge coupling $g_4$. For large values of $g_4$, motivated by recent $B$-physics data, one-loop corrections are sizeable. The main effect is an enhanced contribution at low-energy, at fixed on-shell couplings. This result has important implications for current and future high-energy searches of vector leptoquark models.
We clarify open issues in relating low- and high-energy observables, at next-to-leading order accuracy, in models with a massive leptoquark embedded in a flavor non-universal $SU(4) times SU(3) times SU(2) times U(1)$ gauge group. Extending previous work on this subject, we present a complete analysis of the $mathcal{O}(alpha_s)$ corrections to the matching conditions of semileptonic operators at the high scale. These corrections are not negligible, but they do not exceed the 10% level and are subleading compared to the $mathcal{O}(alpha_4)$ corrections proportional to the leading leptoquark coupling, which is expected to be much larger than the QCD coupling in the parameter space region of phenomenological interest. We further analyze the impact of radial modes, both at $mathcal{O}(alpha_4)$ and at $mathcal{O}(alpha_s)$ accuracy, highlighting their role in the renormalization of the theory.
We propose a 2-Higgs doublet model (2HDM) with a global non-Abelian flavor symmetry $mathcal{Q}_6timesmathcal{Z}_2$. This discrete group accounts for the observed pattern of fermion masses and mixing angles after spontaneous symmetry breaking. In this scenario only the third generation of fermions get their masses as in the Standard Model (SM). The masses of the remaining fermions are generated through a seesaw-like mechanism. To that end, the matter content of the 2HDM is enlarged by introducing electrically charged vector-like fermions (VLFs), right handed Majorana neutrinos and several SM scalar singlets. Here we study the processes involving VLFs that are within the reach of the Large Hadron Collider (LHC). We perform collider studies for vector-like leptons (VLLs) and vector-like quarks (VLQs), focusing on double production channels for both cases, while for VLLs single production topologies are also included. Utilizing genetic algorithms for neural network optimization, we determine the statistical significance for a hypothetical discovery at future LHC runs. In particular, we show that we can not safely exclude VLLs for masses greater than $200~mathrm{GeV}$. For VLQs in our model, we show that we can probe their masses up to 3.8 TeV, if we take only into account the high-luminosity phase of the LHC. Considering Run-III luminosities, we can also exclude VLQs for masses up to $3.4~mathrm{TeV}$. We also show how the model with predicted VLL masses accommodates the muon anomalous magnetic moment.
We investigate collider signatures of standard model extensions featuring vector-like leptons and a flavorful scalar sector. Such a framework arises naturally within asymptotically safe model building, which tames the UV behavior of the standard model towards the Planck scale and beyond. We focus on values of Yukawa couplings and masses which allow to explain the present data on the muon and electron anomalous magnetic moments. Using a CMS search based on $77.4 , rm{fb}^{-1}$ at the $sqrt{s}=13$ TeV LHC we find that flavorful vector-like leptons are excluded for masses below around $300$ GeV if they are singlets under $SU(2)_L$, and around $800$ GeV if they are doublets. Exploiting the flavor-violating-like decays of the scalars, we design novel null test observables based on opposite sign opposite flavor invariant masses. These multi-lepton distributions allow to signal new physics and to extract mass hierarchies in reach of near-future searches at the LHC and the HL-LHC.
The Nelson-Barr (NB) mechanism to solve the strong CP problem assumes CP conservation, arranges vanishing $bar{theta}$ at tree-level and requires vector-like quarks (VLQs) to transmit the CP breaking to the SM. We analyze the flavor constraints coming from the presence of one such down type VLQ of NB type by performing a global fit on the relevant flavor observables. A comparison is made to the case of one generic VLQ. We find that the allowed parameter space for the VLQ Yukawa couplings and the mixing to the SM are confined to a region much smaller than in the generic case, making the NB case falsifiable in principle.
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