arXiv subjects

Carlos A. Ramirez

Publications and source records attributed to Carlos A. Ramirez.

10 recordsLinked to original sources

arXiv2026

CP Violation in $D \to KK$ Decays: A Comparative Analysis of Triplet and Sextet Diquarks

Recent measurements of the CP asymmetry in the decay $D^0 \rightarrow K_S^0 K_S^0$ by the CMS collaboration, $A_{CP}(K_S^0 K_S^0) = (6.2 \pm 3.0 \pm 0.2 \pm 0.8)\%$, and by LHCb, $A_{CP}(D^0 \to K_S^0 K_S^0) = (1.86 \pm 1.04 \pm 0.41)\%$, suggest possible deviations from Standard Model (SM) expectations, which predict asymmetries below the percent level. This singly Cabibbo-suppressed decay is particularly sensitive to new physics, as the leading amplitudes vanish in the exact U-spin symmetry limit and the process is dominated by W-exchange topologies. We investigate scalar diquark contributions to this decay, comparing color-sextet and color-triplet representations. We find that the color-sextet diquark, characterized by a symmetric color structure $(C_1^{\mathrm{NP}} = C_2^{\mathrm{NP}})$, avoids color suppression and can generate CP asymmetries in the range $0.5\%$--$1.5\%$ for a diquark mass of order 1~TeV. In contrast, the color-triplet contribution is strongly suppressed due to destructive interference from its antisymmetric color structure. We further show that a flavor hierarchy in the sextet couplings, with $λ_{ud} > λ_{us}$, can simultaneously account for the observed deviation from the U-spin sum rule in $D^0 \to K^+ K^-$ and $D^0 \to π^+ π^-$ and the measured CP asymmetry in $D^0 \to K_S^0 K_S^0$. These results identify color-sextet scalar diquarks as viable candidates for explaining enhanced CP violation in charm decays.

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arXiv2021

Exploring new physics contributions to $CP$ violation in $τ^- \to K^-π^0ν_τ $

A general analysis of possible violation of CP in processes like $τ\to Kπν$, for unpolarized $τ$ is presented. In this paper, we derive the new contributions to the effective Hamiltonian governs $\vertΔS \vert=1$ semileptonic tau decays in the framework of two Higgs doublet model with generic Yukawa structure and Leptoquarks models. Within these models, we list all operators, in the effective Hamiltonian and provide analytical expression for their corresponding Wilson coefficients. Moreover, we analyze the role of the different contributions, originating from the scalar, vecor and tensor hadronic currents, in generating direct CP asymmetry in the decay rate of $τ^-\to K^-π^0ν_τ$. We show that non vanishing direct CP asymmetry in the decay rate of $τ^-\to K^-π^0ν_τ$ can be generated due to the presence of both, the weak phase in the Wilson coefficient corresponding to the tensor operator and the strong phase difference resulting from the interference between the form factors expressing the matrix elements of the vector and tensor hadronic currents. After taking into account all relevant constraints, we find that the generated direct CP asymmetry is of order $10^{-8}$ which is several orders of magnitude larger than the standard model prediction. We show also that, in two Higgs doublet model with generic Yukawa structure , direct local or non integrated CP violation can be as large as $0.3$ % not far from experimental possibilities. This kind of asymmetry can be generated due to the interference between vector and scalar contributions with different weak phases which is not the case in the SM.

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arXiv2019

New Physics signature in $D^0 (\bar{D}^0)\to f$ effective width asymmetries

Violation of charge conjugation-parity ($\rm CP$) symmetry plays a major rule in the dominance of matter in our universe. A kind of $\rm CP$ violation results from the asymmetry of the life time measured in $M^0$ and $\bar M^0$, here $M$ is a heavy meson, decays to final states which is referred in the literature as $A_Γ^f$. In this paper, we give an estimation of the upper bound on $|A_Γ^f|$ for the Cabibbo Favored $D^0 \rightarrow K^- π^+$ decay process in different models. We show that in the standard model, $|A_Γ^f| \lesssim\mathcal{O} (10^{-10})$. Recently a bound on $A_Γ^f$ has been obtained: $(A^f_Γ)^{Exp.}= (1.6 \pm 1)\times 10^{-4}$. This result motivates further studies on $A_Γ^f$ in beyond standard model physics. In the framework of two Higgs doublet model with generic Yukawa structure, we show that $|A_Γ^{f}|\lesssim \mathcal{O} (10^{-7})$ which is several orders of magnitude smaller than the current experimental value. Finally, in the framework of left-right symmetric models in which the mixing between the left and the right gauge bosons is allowed and the left-right symmetry is not manifest at unification scale, we find that $A_Γ^f$ can be as large as $|A_Γ^f|\lesssim\mathcal{O} (10^{-5})$ which is one order of magnitude smaller than the experimentally measured value by LHCb collaborators.

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arXiv2019

Direct CP violation in $D^+ \to K^0(\bar K^0) π^+$ decays as a probe for new physics

In this paper we investigate CP violation in charged decays of $D$ meson. Particularly, we study the direct CP asymmetry of the Cabibbo favored non-leptonic $D^+ \rightarrow \bar K^0 π^+$ and the doubly Cabibbo-suppressed decay mode $D^+ \rightarrow K^0 π^+$ within standard model, two Higgs doublet model with generic Yukawa structure and left right symmetric models. In the standard model, we first derive the contributions from box and di-penguin diagrams contributing to their amplitudes which are relevant to the generation of the weak phases essential for non-vanishing direct CP violation. Then, we show that these phases are so tiny leading to a direct CP asymmetry of order $10^{-11}$ in both decay modes. Regarding the two Higgs doublet model with generic Yukawa structure and after taking into account all constraints on the parameter space of the model, we show that the enhanced direct CP asymmetries can be 6 and 7 orders of magnitudes larger than the standard model prediction for $D^+ \rightarrow \bar K^0 π^+$ and $D^+ \rightarrow K^0 π^+$ respectively. Finally, within left right symmetric models, we find that sizable direct CP asymmetry of ${\mathcal O } (10^{-3})$ can be obtained for the decay mode $D^+ \rightarrow \bar K^0 π^+$ after respecting all relevant constraints.

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arXiv2017

CP violation in $D^0 \to K^+ π^- $

In this paper we study the direct CP asymmetry of the doubly Cabibbo-suppressed decay mode $D^0 \to K^+ π^- $ within standard model and two Higgs doublet model with generic Yukawa structure. In the standard model we derive the corrections to the tree level amplitude, generated from the box and di-penguin diagrams, required for generating the weak CP violating phases. We show that these phases are so tiny leading to a direct CP asymmetry of order $10^{-9}$. Regarding the two Higgs doublet model with generic Yukawa structure we derive the Wilson coefficients relevant to $D^0 \to K^+ π^- $. After taking into account all constraints on the parameter space of the model we show that charged Higgs couplings to quarks can lead to a direct CP asymmetry of order $10^{-3}$ which is $6$ orders of magnitude larger than the standard model prediction.

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arXiv2014

New Physics effects in D^+ \rightarrow K^- π^+ π^+

In this paper, we study the Cabibbo favored three body non-leptonic D^+ \rightarrow K^- π^+ π^+ decay. We show that the corresponding direct CP asymmetry is so tiny in the framework of the Standard Model and is out of the experimental range. Motivated by this result we extend the study of the CP asymmetry to include a toy model with CP violating weak phase equals 20 degrees in a_2, a model with extra gauge bosons within Left-Right Grand Unification models and a model with charged Higgs boson. We show that the toy model can strongly improve the SM prediction of the CP asymmetry to be about 30%. The largest CP asymmetry can be achieved in the non-manifest Left-Right models where a CP asymmetry up to 25% can be reached. For the two Higgs doublets models the CP asymmetry is of order 10^{-3}.

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arXiv2012

Observation of CP violation in $D^0 \rightarrow K^- π^+ $ as a smoking gun for New Physics

In this paper, we study the Cabibbo favored non-leptonic $D^0$ decays into $K^- π^+$ decays. First we show that, within the Standard Model, the corresponding CP asymmetry is strongly suppressed and out of the experimental range even taking into account the large strong phases coming from final state Interactions. We show also that although new physics models with extra sequential generation can enhance the CP asymmetry by few orders of magnitude however the resulting CP asymmetry is still far from experimental range. The most sensitive New Physics Models to this CP asymmetry comes from no-manifest Left-Right models where a CP asymmetry up to 10% can be reached and general two Higgs models extension of SM where a CP asymmetry of order $10^{-2}$ can be obtained without being in contradiction with the experimental constraints on these models.

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arXiv2008

A Consistent Scenario for B to PS Decays

We consider B to PS decays where P stands for pseudoscalar and S for a heavy (~1500 MeV) scalar meson. We achieve agreement with available experimental data -- which includes a two orders of magnitude hierarchy -- assuming the scalars mesons are two quark states. The contribution of the dipolar penguin operator O_{11} is quantified.

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arXiv2005

Annihilation contribution and $B\to a_0 π, f_0 K$ decays

We analyze the decays $B^0 \to a^\pm_0 π^\mp$ and $B^{-,0} \to f_0 K^{-,0}$ and show that within the factorization approximation a phenomenological consistent picture can be obtained. We show that in this approach the $O_6$ operator provides the dominant contributions to the suppressed channel $B^0 \to a^+_0 π^-$. When the $a_0$ is considered a two quark state, evaluation of the annihilation form factor using Perturbative $QCD$ implies that this contribution is not negligible, and furthermore it can interfere constructively or destructively with other penguin contributions. As a consequence of this ambiguity, the positive identification of $B^0 \to π^+ a_0^-$ can not distinguish between the two or four quark assignment of the $a_0$. According to our calculation, a best candidate to distinguish the nature of $a_0$ scalar is $Br(B^-\to π^0a_0^-)$ since the predictions for a four quark model is one order of magnitude smaller than for the two quark assignment. When the scalars are seen as two quarks states, simple theoretical assumptions based on SU(2) isospin symmetry provide relations between different B decays involving one scalar and one pseudoscalar meson.

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arXiv2000

Nonrelativistic Quark Model Calculation of the S parameter

A critical review is presented of the attempts to estimate the Strong Interactions contributions to the parameter $S$ ($L_{10}$ in the QCD Chiral Version). In particular it is discussed why the estimations done for Technicolor are unreliable. $S$ is calculated for heavy doublets of Techniquarks using the Nonrelativistic Quark Model and keeping $v\simeq 0.25$ TeV fixed. It is found that heavy Techniquarks decouple, so it is possible to obtain values for $S$ in agreement with present experimental data.

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