arXiv subjects

Hector Gisbert

Publications and source records attributed to Hector Gisbert.

26 records · Page 2Linked to original sources

arXiv2021

Theoretical and experimental status of rare charm decays

Rare charm decays offer the unique possibility to explore flavour-changing neutral-currents in the up-sector within the Standard Model and beyond. Due to the lack of effective methods to reliably describe its low energy dynamics, rare charm decays have been considered as less promising for long. However, this lack does not exclude the possibility to perform promising searches for New Physics per se, but a different philosophy of work is required. Exact or approximate symmetries of the Standard Model allow to construct clean null-test observables, yielding an excellent road to the discovery of New Physics, complementing the existing studies in the down-sector. In this review, we summarize the theoretical and experimental status of rare charm $|Δc|=|Δu|=1$ transitions, as well as opportunities for current and future experiments such as LHCb, Belle II, BES III, the FCC-ee and proposed tau-charm factories. We also use the most recent experimental results to report updated limits on lepton-flavour conserving and lepton-flavour violating Wilson coefficients.

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arXiv2021

Rare charm $\boldsymbol{c\to u\,ν\barν}$ dineutrino null tests for $\boldsymbol{e^+e^-}$-machines

Rare $|Δc|=|Δu|=1$ transitions into dineutrinos are strongly GIM-suppressed and constitute excellent null tests of the standard model. While branching ratios of $D \to P \,ν\bar ν$, $D \to P^+ P^- \, ν\bar ν$, $P=π, K$, baryonic $Λ_c^+ \to p \,ν\bar ν$, and $Ξ_c^+ \to Σ^+ \, ν\bar ν$ and inclusive $D \to X ν\bar ν$ decays are experimentally unconstrained, signals of new physics can be just around the corner. We provide model-independent upper limits on branching ratios reaching few $\times 10^{-5}$ in the most general case of arbitrary lepton flavor structure, $\sim 10^{-5}$ for scenarios with charged lepton conservation and few $\times 10^{-6}$ assuming lepton universality. We also give upper limits in $Z^\prime$ and leptoquark models. The presence of light right-handed neutrinos can affect these limits, a possibility that can occur for lepton number violation at a TeV, and that can be excluded with an improved bound on $\mathcal{B}(D^0 \to \text{invisibles})$ at the level of $\sim 10^{-6}$, about two orders of magnitude better than the present one. Signatures of $c \to u ν\bar ν$ modes contain missing energy and are suited for experimental searches at $e^+ e^-$-facilities, such as BES III, Belle II and future $e^+ e^-$-colliders, such as the FCC-ee running at the $Z$.

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arXiv2020

Nonleptonic $K\to 2\,π$ decay dynamics

Using Chiral Perturbation Theory to properly account for the dynamics of nonleptonic $K\to 2\,π$ decays, we found the Standard Model prediction for the CP violating ratio $\mbox{Re}\left(\varepsilon'/\varepsilon\right)=\left(14\pm 5\right) \times 10^{-4}$, where isospin breaking effects are included, in perfect agreement with the current experimental world average. Similar results have been reported by a recent release of improved lattice data.

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arXiv2020

Improved bounds on heavy quark electric dipole moments

New bounds on the electric dipole moment (EDM) of charm and bottom quarks are derived using the stringent limits on their chromo-EDMs. The new limits, $|d_c|<1.5\times10^{-21}\:e\,\text{cm}$ and $|d_b|< 1.2\times 10^{-20}\:e\,\text{cm}$, improve the previous ones by about three orders of magnitude. These indirect bounds have implications for different models of new physics, including two-Higgs-doublet, leptoquarks, and supersymmetry models.

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arXiv2020

Exploiting CP-asymmetries in rare charm decays

We analyze patterns from CP-violating new physics (NP) in hadronic and semileptonic rare charm $\vert Δc \vert=\vert Δu\vert=1$ transitions. Observation of direct CP-violation in hadronic decays, as in $ΔA_{\text{CP}}$, provides opportunities for $c \to u \,\ell^+ \ell^-$, $\ell=e,μ$ transitions, and vice versa. For the concrete case of flavorful, anomaly-free $Z^\prime$-models a NP-interpretation of $ΔA_{\text{CP}}$ suggests measurable CP-asymmetries in semileptonic decays such as $D \to π\, \ell^+ \ell^-$ or $D \to ππ\, \ell^+ \ell^-$. Conversely, an observation of CP-violation in $c \to u\, e^+ e^-$ or $c \to u\, μ^+ μ^-$ decays supports a NP--interpretation of $ΔA_{\text{CP}}$. Flavorful $U(1)^\prime$-extensions provide explicit U--spin and isospin breaking which can be probed in patterns of hadronic decays of charm mesons. We work out signatures for CP-asymmetries in $D^0 \to π^+ π^-$, $D^0 \to K^+ K^-$ and $D^0 \to π^0 π^0$, $D^+ \to π^+ π^0$ decays, which can be probed in the future at LHCb and Belle II and provide further informative cross checks.

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arXiv2018

Current status of $\varepsilon'/\varepsilon$ in the Standard Model

A recent lattice prediction of $\varepsilon'/\varepsilon$, with a 2.1 $σ$ deviation from the experimental value, has triggered several studies of possible contributions from physics beyond the Standard Model. We have recently updated the SM prediction, including all known short-distance and long-distance contributions, our result $\mbox{Re}\left(\varepsilon'/\varepsilon\right) = (15 \pm 7)\cdot 10^{-4}$ is in complete agreement with the experimental measurement. In addition, we emphasize on the importance of the long-distance re-scattering of the final pions in $K \to ππ$ for a correct prediction of $\varepsilon'/\varepsilon$.

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arXiv2018

Updated Standard Model Prediction for $\varepsilon'/\varepsilon$

A recent lattice evaluation of $\varepsilon'/\varepsilon$, finding a 2.1 $σ$ deviation from the experimental value, has revived the old debate about a possible $\varepsilon'/\varepsilon$ anomaly. The unfounded claims of a too low Standard Model prediction are based on incorrect estimates that neglect the long-distance re-scattering of the final pions in $K\rightarrow 2π$. In view of the current situation, we have recently updated the Standard Model calculation, including all known short- and long-distance contributions. Our result, $\text{Re}\left(\varepsilon'/\varepsilon\right) = (15 \pm 7)\cdot 10^{-4}$, is in complete agreement with the experimental measurement.

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arXiv2017

Direct CP violation in $K^0\toππ$: Standard Model Status

In 1988 the NA31 experiment presented the first evidence of direct CP violation in the $K^0\toππ$ decay amplitudes. A clear signal with a $7.2\,σ$ statistical significance was later established with the full data samples from the NA31, E731, NA48 and KTeV experiments, confirming that CP violation is associated with a $ΔS=1$ quark transition, as predicted by the Standard Model. However, the theoretical prediction for the measured ratio $\varepsilon'/\varepsilon$ has been a subject of strong controversy along the years. Although the underlying physics was already clarified in 2001, the recent release of improved lattice data has revived again the theoretical debate. We review the current status, discussing in detail the different ingredients that enter into the calculation of this observable and the reasons why seemingly contradictory predictions were obtained in the past by several groups. An update of the Standard Model prediction is presented and the prospects for future improvements are analysed. Taking into account all known short-distance and long-distance contributions, one obtains $\mbox{Re}\left(\varepsilon'/\varepsilon\right) = (15 \pm 7)\cdot 10^{-4}$, in good agreement with the experimental measurement.

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