arXiv subjects

Francisco Albergaria

Publications and source records attributed to Francisco Albergaria.

8 recordsLinked to original sources

arXiv2026

Feasible Deviations from Unitarity with Vector-Like Quark Singlets

We deduce pertinent relations between the elements of the CKM matrix, and find that not all of these are totally compatible with experiment and/or the assumption of the SM $3 \times 3$ unitarity. We identify relations between moduli and complex phases of the CKM-elements which may signal deviations from unitary (DUs). Recent analyses of $|V_{cs}|$ appear to be in tension with the SM assumption of CKM unitarity in the second row, similar to previous hints of DUs in the first row from previous $|V_{ud,s}|$ results. Here, we explore DUs induced in VLQ iso-singlet models, in particular the possibility of having significant DUs of the first and second rows of the CKM matrix. We perform a extensive analysis of $n\leq 5$ VLQ iso-singlet models and identify a useful set of parametrizations with the intention of coherently exploring parameter space. We assess the feasibility of each model and confront it with constraints from key flavor observables, with particular attention to the neutral kaon and $D^0$-meson sectors, especially $ε_K$ and $x_D$. We find that smaller $n\leq 2$ VLQ-models cannot accommodate the new $|V_{cs}|$ results in the DUs of the second CKM-row. In contrast, larger models extending both up- and down-type sectors, can indeed produce significant second-row DUs, e.g. in a model with 3 up- and 2 down-type VLQs, even reaching the order of the Cabibbo angle. To our knowledge, the phenomenology of extensions combining both up- and down-type VLQs has yet to be thoroughly studied. Finally, we find that VLQ-singlet models may induce very large enhancements in the rephasing invariant phase $β_K$, entering in the Cabibbo sector.

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arXiv2024

The oblique parameters from arbitrary new fermions

We compute the six oblique parameters $S, T, U, V, W, X$ in a New Physics Model with an arbitrary number of new fermions, in arbitrary representations of $SU(2) \times U(1)$, and mixing arbitrarily among themselves. We show that $S$ and $U$ are automatically finite, but $T$ is finite only if there is a specific relation between the masses of the new fermions and the representations of $SU(2) \times U(1)$ that they sit in. We apply our general computation to two illustrative cases.

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arXiv2023

Unitarity Relations in the Presence of Vector-Like Quarks

We study, in a systematic way, the $V_{\text{CKM}}$ unitarity relations which arise in extensions of the three generations Standard Model (3gSM) involving the addition of vector-like quarks (VLQ). In particular, we emphasize the effect of the presence of VLQ on $V_{\text{CKM}}$ moduli differences, as well on the size of the imaginary parts of rephasing invariant $V_{\text{CKM}}$ quartets. We consider the special case where an up-type VLQ is used to attempt at solving the unitarity problem in the first line of $V_{\text{CKM}}$.

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arXiv2023

Oblique corrections from leptoquarks

We present general formulas for the oblique-correction parameters $S$, $T$, $U$, $V$, $W$, and $X$ in a model of New Physics having arbitrary numbers of scalar leptoquarks of the five permissible types. We allow for a general mixing among the scalars of the various electric charges, $\textit{viz.}$ $-4/3$, $-1/3$, $2/3$, and $5/3$. We then extend the formulas to the case of a New Physics model with additional scalars in any representations of the gauge group $SU(2) \times U(1)$, mixing arbitrarily among themselves.

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arXiv2023

CP-odd and CP-even Weak-Basis Invariants in the Presence of Vector-Like Quarks

We propose a minimal set of weak-basis invariants in an extension of the SM where one up-type isosinglet vector-like quark is introduced, which allows us to obtain all the physical content of the CKM matrix. We present CP-odd invariants of lower order in mass than the one in the SM, which may have important consequences for Baryogenesis. We study the extreme chiral limit, where the two lightest generations have vanishing mass, showing that in this extension, contrary to the SM, CP violation can be observed in collisions much above the electroweak scale.

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arXiv2023

Does Quark Mixing play a role in the Lepton Sector?

We suggest a simple relation between the quark and the lepton mixing within the framework of type-I seesaw mechanism. We show that within our ansatz the empirical King-Mohapatra-Smirnov relation, which suggests a connection between the CKM and PMNS mixing where $|V^\text{PMNS}_{13}|\approx \frac{1}{\sqrt{2}}\sin(θ_C) $, can be derived. This is possible within a restricted region of the Dirac and Majorana mass parameters.

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arXiv2023

Oblique corrections from triplet quarks

We present general formulas for the oblique-correction parameters $S$, $T$, $U$, $V$, $W$, and $X$ in an extension of the Standard Model having arbitrary numbers of singlet, doublet, and triplet quarks with electric charges $-4/3$, $-1/3$, $2/3$, and $5/3$ that mix with the standard quarks of the same charge.

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arXiv2021

Prescription for finite oblique parameters $S$ and $U$ in extensions of the SM with $m_W \neq m_Z \cos{θ_W}$

We consider extensions of the Standard Model with neutral scalars in multiplets of $SU(2)$ larger than doublets. When those scalars acquire vacuum expectation values, the resulting masses of the gauge bosons $W^\pm$ and $Z^0$ are not related by $m_W = m_Z \cos{θ_W}$. In those extensions of the Standard Model the oblique parameters $S$ and $U$, when computed at the one-loop level, turn out to be either gauge-dependent or divergent. We show that one may eliminate this problem by modifying the Feynman rules of the Standard Model for some vertices containing the Higgs boson; the modifying factors are equal to $1$ in the limit $m_W = m_Z \cos{θ_W}$. We give the result for $S$ in a model with arbitrary numbers of scalar $SU(2)$ triplets with weak hypercharges either $0$ or $1$.

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