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Luiz Vale Silva

Publications and source records attributed to Luiz Vale Silva.

At least 19 recordsLinked to original sources
arXiv2026

Implications of two-channel rescattering for charm CP violation from precise dispersive data

The experimental observation of large CP violation in charm-meson hadronic decays remains theoretically unexplained within the Standard Model. The data-driven approach which accounts for the rescattering between the final-state pion and kaon pairs provides predictions that fall short of the experimental values. However, it relies on a set of inputs that exhibit large uncertainties. In this work, we make optimal use of the available information on branching ratios and of only one strong-scattering input parameter that is well determined. We find that it remains unlikely to explain the experimental signal within this approach and obtain predictions for sum rules that constrain CP asymmetries of the pion and kaon channels.

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arXiv2026

Impact of the $a_1(1260) π$ cascade contribution on $D^0 \to π^+ π^- \ell^+ \ell^-$ decays

We revisit the Standard Model description of the recently measured rare decays $D^0\toπ^+π^-\ell^+\ell^-$. Because of the effectiveness of the Glashow-Iliopoulos-Maiani mechanism in charm flavour-changing neutral currents, those decays are driven by non-local insertions of four-quark operators. Following previous work, we consider the mediation of resonances both for the dipion and dilepton pairs. For the first time, we incorporate the effect of the cascade-type topology $D^0\to π^- a_1^+(1260)(\toπ^+ρ^0(\to\ell^+\ell^-))$, which manifests distinctly in the invariant-mass and angular distributions. We find that this partial amplitude comprises one of the largest contributions to the decay rate and obtain an unprecedented agreement of the Standard Model prediction with the available LHCb data. Finally, we compare to the available CLEO-c, LHCb, and BESIII amplitude analyses for the analogous four-body hadronic decays and find that similar values of the hadronic parameters of our model successfully describe the two classes of decays.

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arXiv2026

Symmetry-restoring finite counterterms of SMEFT four-fermion operator insertions at one loop

Some effects induced by SMEFT operators at one loop have attracted a lot of attention in recent years, in particular, the renormalization of divergences by physical operators in single insertions of dimension-6 operators. Important non-logarithmically enhanced contributions must also be calculated. We discuss dimensional regularization in the Breitenlohner-Maison-'t Hooft-Veltman scheme. The goal here consists of determining in this scheme quantum effects in chiral theories at one loop. Namely, the determination of finite counterterms at one loop that reestablish the Slavnov-Taylor identities, which follow from gauge symmetries. These counterterms are necessary due to the presence of evanescent symmetry-breaking terms in the classical Lagrangian needed to regularize fermion propagators. We consider a technique that allows an easier calculation of such finite effects, relying on the identification of $(D-4)/(D-4)$ terms of one-loop amplitudes with an external ghost leg. We focus on dimension-6 four-fermion operators, identifying all finite counterterms in the Breitenlohner-Maison-'t Hooft-Veltman scheme at one loop, and as expected find no obstructions to the Slavnov-Taylor identities that cannot be cured by finite counterterms. This represents one step towards moving to higher order calculations.

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arXiv2025

Opportunities at FCC-ee for quark & lepton flavour physics

The FCC-ee phase of a Future Circular Collider is generating great interest due to its versatility, allowing the study of various electroweak thresholds, $Z$, $WW$, $ZH$, and $t \bar{t}$. Electroweak precision physics is complemented by flavour physics measurements based on the unprecedented statistics attainable at the $Z$ pole, and benefiting from the low-background experimental environment (similar to Belle II), and from the production of the full spectrum of hadron species together with large boosts (similar to LHCb). A wide range of measurements is possible, spanning a rich variety of physics cases in both quark and lepton flavour physics sectors. Other electroweak thresholds can also be considered in this endeavour. A commensurate effort from the theory community will be needed to interpret future measurements. I present an overview of the broad potential of the FCC-ee flavour physics program.

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arXiv2025

The Electric Dipole Moment of the electron in the decoupling limit of the aligned Two-Higgs Doublet Model

We present a discussion of model-independent contributions to the EDM of the electron. We focus on those contributions that emerge from a heavy scalar sector that is linearly realized. In particular, we explore the decoupling limit of the aligned 2HDM. In this model, Barr-Zee diagrams with a fermion loop produce logarithmically-enhanced contributions that are proportional to potentially large new sources of CP violation. In the decoupling limit these contributions are generated by effective dimension-6 operators via the mixing of four-fermion operators into electroweak dipole operators. These logarithmic contributions are not present in more constrained versions of the 2HDM where a $\mathcal Z_2$ symmetry is imposed, which then controls the basis of effective operators needed to describe the new physics contributions to the electron EDM. Thus, the $\mathcal Z_2$ symmetry provides a suppression mechanism. In the course of the comparison of the results from the aligned 2HDM with the leading logarithms from SMEFT, we needed to specify or correct signs of expressions found in the literature. We then study how the experimental bounds on the electron EDM constrain the set of parameters of the aligned 2HDM.

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arXiv2025

Constraints on baryon-number-violating top-quark operators in standard model effective field theory

Within the Standard Model Effective Field Theory framework, we set indirect constraints on top quark operators that violate baryon number by one unit above the TeV scale. We find that these constraints are typically many orders of magnitude more stringent than the recently derived direct bounds from collider experiments. Therefore, direct observation of baryon number violation in these top-quark observables at the TeV scale would imply a large fine-tuning among operators across different energy scales. This possibility is not protected under universal radiative corrections or any known symmetry principles.

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arXiv2025

The electron EDM in the decoupling limit of the aligned 2HDM

We discuss model-independent contributions to the electron EDM, focusing on those contributions emerging from a heavy scalar sector linearly realized. To provide a concrete new physics realization, we investigate the aligned 2HDM in the decoupling limit. We point out that logarithmically enhanced contributions generated from Barr-Zee diagrams with a fermion loop are present in the aligned 2HDM, an effect encoded in the decoupling limit by effective dimension-6 operators, through the mixing of four-fermion into dipole operators. The same large logarithms are absent in specific 2HDMs where a $\mathcal Z_2$ symmetry is enforced, which thus controls the basis of effective operators relevant for calculating new physics contributions to EDMs. In other words, the $\mathcal Z_2$ symmetry acts as a suppression mechanism. In the aligned 2HDM these contributions are proportional to sources of CP violation that are potentially large, and absent in presence of the $\mathcal Z_2$ symmetry. We then investigate the impact on the electron EDM of this extended set of free parameters.

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arXiv2024

Direct bounds on Left-Right gauge boson masses at LHC Run 2

While the third run of the Large Hadron Collider (LHC) is ongoing, the underlying theory that extends the Standard Model remains so far unknown. Left-Right Models (LRMs) introduce a new gauge sector, and can restore parity symmetry at high enough energies. If LRMs are indeed realized in nature, the mediators of the new weak force can be searched for in colliders via their direct production. We recast existing experimental bounds from LHC Run 2 on the heavy LRM gauge boson masses. As a novelty, we discuss the effect of the LRM scalar content on the total width of the new gauge bosons, obtaining model-independent bounds within the specific realizations of the LRM scalar sectors analysed here. These bounds avoid the need to detail the spectrum of the scalar sector, and apply in the general case where no discrete symmetry is enforced. Moreover, we emphasize the effect of the structure of the quark right-handed mixing matrix on the charged LRM gauge boson production at LHC. We find that $W_R$ and $Z_R$ masses are constrained to lie above $2$ TeV and $4$ TeV, respectively.

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arXiv2024

Model Independent Bounds on Left-Right Gauge Boson Masses from LHC Run 2 and Flavour Observables

Left-Right Models (LRMs) are one of the most relevant extensions of the Standard Model (SM) of particle physics. They introduce an extended gauge sector and can restore parity (P) or charge conjugation (C) symmetries at high enough energies. These theories can be embedded in other more fundamental ones with larger gauge groups. Consequently, the restoration of the C or P symmetries can be pushed towards higher energy scales compared to the scale of the Spontaneous Symmetry Breaking (SSB) of the LRM gauge group. We study three LRMs with different specific realizations of the scalar sector without imposing any additional discrete symmetry on the theory. We present bounds on the masses of the new gauge bosons using data from the LHC Run 2 and study rare meson decays, discussing the structure of the right-handed quark mixing matrix and the impact of the neutrino and scalar sectors. Collider bounds valid for specific LRM realizations are alleviated bringing New Physics (NP) effects in flavour observables closer to an observable level.

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arXiv2024

Direct bounds on Left-Right gauge boson masses

While the third run of the Large Hadron Collider (LHC) is ongoing, the underlying theory that extends the Standard Model remains so far unknown. Left-Right Models (LRMs) introduce a new gauge sector, and can restore parity symmetry at high enough energies. If LRMs are indeed realized in nature, the mediators of the new weak force can be searched for in colliders via their direct production. We recast existing experimental limits from the LHC Run 2 and derive generic bounds on the masses of the heavy LRM gauge bosons. As a novelty, we discuss the dependence of the $W_R$ and $Z_R$ total decay width on the LRM scalar content, obtaining model-independent bounds within the specific realizations of the LRM scalar sectors analysed here. These bounds avoid the need to detail the spectrum of the scalar sector, and apply in the general case where no discrete symmetry is enforced. Moreover, we emphasize the impact on the $W_R$ production at LHC of general textures of the right-handed quark mixing matrix without manifest left-right symmetry. We find that the $W_R$ and $Z_R$ masses are constrained to lie above $2$ TeV and $4$ TeV, respectively.

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arXiv2024

2023 update of the extraction of the CKM matrix elements

I discuss the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements under the Standard Model (SM) framework from a global fit combining observables that satisfy the double requirement of being precisely known both experimentally and theoretically. The analysis shown here relies on the CKMfitter package, consisting of a frequentist approach that employs the Range fit (Rfit) scheme to handle theoretical uncertainties.

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arXiv2024

Effects of squared four-fermion operators of the Standard Model Effective Field Theory on meson mixing

The Standard Model Effective Field Theory (SMEFT) is a universal way of parametrizing New Physics (NP) manifesting as new, heavy particle interactions with the Standard Model (SM) degrees of freedom, that respect the SM gauged symmetries. Higher order terms in the NP interactions possibly lead to sizable effects, mandatory for meaningful phenomenological studies, such as contributions to neutral meson mixing, which typically pushes the scale of NP to energy scales much beyond the reach of direct searches in colliders. I discuss the leading-order renormalization of double-insertions of dimension-6 four-fermion operators that change quark flavor by one unit (i.e., $ | ΔF | = 1 $, $ F = $ strange-, charm-, or bottom-flavor), by dimension-8 operators relevant to meson mixing (i.e., $ | ΔF | = 2 $) in SMEFT. Then, I consider the phenomenological implications of contributions proportional to large Yukawas, setting bounds on the Wilson coefficients of operators of dimension-6 via the leading logarithmic contributions. Given the underlying interest of SMEFT to encode full-fledged models at low energies, this work stresses the need to consider dimension-8 operators in phenomenological applications of dimension-6 operators of SMEFT.

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arXiv2024

Focus topics for the ECFA study on Higgs / Top / EW factories

In order to stimulate new engagement and trigger some concrete studies in areas where further work would be beneficial towards fully understanding the physics potential of an $e^+e^-$ Higgs / Top / Electroweak factory, we propose to define a set of focus topics. The general reasoning and the proposed topics are described in this document.

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arXiv2023

$S$-wave contribution to rare $D^0 \to π^+ π^- \ell^+ \ell^-$ decays in the Standard Model and sensitivity to New Physics

Physics of the up-type flavour offers unique possibilities of testing the Standard Model (SM) compared to the down-type flavour sector. Here, we discuss SM and New Physics (NP) contributions to the rare charm-meson decay $ D^0 \to π^+ π^- \ell^+ \ell^- $. In particular, we discuss the effect of including the lightest scalar isoscalar resonance in the SM picture, namely, the $f_0 (500)$, which manifests in a big portion of the allowed phase space. Other than showing in the total branching ratio at an observable level of about $ 20\% $, the $f_0 (500)$ resonance manifests as interference terms with the vector resonances, such as at high invariant mass of the leptonic pair in distinct angular observables. Recent data from LHCb optimize the sensitivity to $P$-wave contributions, that we analyse in view of the inclusion of vector resonances. We propose the measurement of alternative observables which are sensitive to the $S$-wave and are straightforward to implement experimentally. This leads to a new set of null observables, that vanish in the SM due to its gauge and flavour structures. Finally, we study observables that depend on the SM interference with generic NP contributions from semi-leptonic four-fermion operators in the presence of the $S$-wave.

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arXiv2023

Variational quantum eigensolver for causal loop Feynman diagrams and directed acyclic graphs

We present a variational quantum eigensolver (VQE) algorithm for the efficient bootstrapping of the causal representation of multiloop Feynman diagrams in the Loop-Tree Duality (LTD) or, equivalently, the selection of acyclic configurations in directed graphs. A loop Hamiltonian based on the adjacency matrix describing a multiloop topology, and whose different energy levels correspond to the number of cycles, is minimized by VQE to identify the causal or acyclic configurations. The algorithm has been adapted to select multiple degenerated minima and thus achieves higher detection rates. A performance comparison with a Grover's based algorithm is discussed in detail. The VQE approach requires, in general, fewer qubits and shorter circuits for its implementation, albeit with lesser success rates.

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arXiv2023

Final-state interactions in the CP asymmetries of charm-meson two-body decays

Urgent theoretical progress is needed in order to provide an estimate in the Standard Model of the recent measurement by LHCb of direct CP violation in charm-meson two-body decays. Rescattering effects must be taken into account for a meaningful theoretical description of the amplitudes involved in such category of observables, as signaled by the presence of large strong phases. We discuss the computation of the latter effects based on a two-channel coupled dispersion relation, which exploits isospin-zero phase-shifts and inelasticity parameterizations of data coming from the rescattering processes $ ππ\to ππ$, $ πK \to πK $, and $ ππ\to K \overline{K} $. The determination of the subtraction constants of the dispersive integrals relies on the leading contributions to the transition amplitudes from the $ 1/N_C $ counting, where $N_C$ is the number of QCD colours. Furthermore, we use the measured values of the branching ratios to help in selecting the non-perturbative inputs in the isospin limit, from which we predict values for the CP asymmetries. We find that the predicted level of CP violation is much below the experimental value.

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arXiv2022

Quantum algorithm for Feynman loop integrals

We present a novel benchmark application of a quantum algorithm to Feynman loop integrals. The two on-shell states of a Feynman propagator are identified with the two states of a qubit and a quantum algorithm is used to unfold the causal singular configurations of multiloop Feynman diagrams. To identify such configurations, we exploit Grover's algorithm for querying multiple solutions over unstructured datasets, which presents a quadratic speed-up over classical algorithms when the number of solutions is much smaller than the number of possible configurations. A suitable modification is introduced to deal with topologies in which the number of causal states to be identified is nearly half of the total number of states. The output of the quantum algorithm in \emph{IBM Quantum} and \emph{QUTE Testbed} simulators is used to bootstrap the causal representation in the loop-tree duality of representative multiloop topologies. The algorithm may also find application and interest in graph theory to solve problems involving directed acyclic graphs.

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