arXiv subjects

Giancarlo D'Ambrosio

Publications and source records attributed to Giancarlo D'Ambrosio.

At least 37 records · Page 2Linked to original sources

arXiv2016

Probing lepton flavour (universality) violation at NA62 and future kaon experiments

Recent results from the LHC's first run have revealed intriguing departures from lepton flavour universality in the semi-leptonic decays of $B$-mesons. We discuss the complementary role that rare kaon decays can provide in testing new physics explanations of these flavour anomalies. In the framework of minimal flavour violation, we relate the chiral low-energy constants involved in $K\toπ\ell\ell'$ and $K\to\ell\ell'$ ($\ell = μ\mbox{ or } e$) with the new physics Wilson coefficients of the $b\to s$ effective Hamiltonian. We comment on the determination of these low-energy constants at NA62 and future kaon experiments, as well as the required improvements in sensitivity necessary to test the $B$-physics anomalies in the kaon sector.

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arXiv2016

Lepton flavor (universality) violation in rare kaon decays

Recent anomalies in the decays of $B$ mesons and the Higgs boson provide hints towards lepton flavor (universality) violating physics beyond the Standard Model. We observe that four-fermion operators which can explain the $B$-physics anomalies have corresponding analogs in the kaon sector, and we analyze their impact on $K\toπ\ell \ell'$ and $K\to\ell \ell'$ decays $(\ell=μ,e)$. For these processes, we note the corresponding physics opportunities at the NA62 experiment. In particular, assuming minimal flavor violation, we comment on the required improvements in sensitivity necessary to test the $B$-physics anomalies in the kaon sector.

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arXiv2015

Assuming Regge trajectories in holographic QCD: from OPE to Chiral Perturbation Theory

The soft wall model in holographic QCD has Regge trajectories but wrong operator product expansion (OPE) for the two-point vectorial QCD Green function. We modify the dilaton potential to comply OPE. We study also the axial two-point function using the same modified dilaton field and an additional scalar field to address chiral symmetry breaking. OPE is recovered adding a boundary term and low energy chiral parameters, $F_π$ and $L_{10}$, are well described analytically by the model in terms of Regge spacing and QCD condensates. The model nicely supports and extends previous theoretical analyses advocating Digamma function to study QCD two-point functions in different momentum regions.

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arXiv2015

Explaining the LHC flavour anomalies

The LHC observed deviations from the Standard Model (SM) in the flavour sector: LHCb found a $4.3\,σ$ discrepancy compared to the SM in $b\to sμμ$ transitions and CMS reported a non-zero measurement of $h\toμτ$ with a significance of $2.4\,σ$. Here we discuss how these deviations from the SM can be explained, focusing on two models with gauged $L_μ-L_τ$ symmetry. The first model contains two scalar doublets and vector-like quarks while the second one employs three scalar doublets but does not require vector-like fermions. In both models, interesting correlations between $b\to sμμ$ transitions, $h\toμτ$, and $τ\to3μ$ arise.

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arXiv2015

Addressing the LHC flavour anomalies with horizontal gauge symmetries

We study the impact of an additional $U(1)'$ gauge symmetry with flavour-dependent charges for quarks and leptons on the LHC flavour anomalies observed in $B \to K^* μ^+μ^-$, $R(K) = B \to K μ^+μ^-/B \to K e^+e^-$, and $h \to μτ$. In its minimal version with two scalar doublets, the resulting model naturally explains the deviations from the Standard Model observed in $B \to K^* μ^+μ^-$ and $R(K)$. The CMS access in $h \to μτ$ can be explained by introducing a third scalar doublet, which gives rise to a prediction for $τ\to 3μ$. We investigate constraints from flavour observables and direct LHC searches for $pp \to Z' \to μ^+μ^-$. Our model successfully generates the measured fermion-mixing matrices and does not require vector-like fermions, unlike previous attempts to explain these anomalies.

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arXiv2015

Explaining $h\toμ^\pmτ^\mp$, $B\to K^* μ^+μ^-$ and $B\to K μ^+μ^-/B\to K e^+e^-$ in a two-Higgs-doublet model with gauged $L_μ-L_τ$

The LHC observed so far three deviations from the Standard Model (SM) predictions in flavour observables: LHCb reported anomalies in $B\to K^* μ^+μ^-$ and $R(K)=B\to K μ^+μ^-/B\to K e^+e^-$ while CMS found an excess in $h\toμτ$. We show, for the first time, how these deviations from the SM can be explained within a single well-motivated model: a two-Higgs-doublet model with gauged $L_μ-L_τ$ symmetry. We find that, despite the constraints from $τ\toμμμ$ and $B_s$--$\overline{B}_s$ mixing, one can explain $h \toμτ$, $B\to K^* μ^+μ^-$ and $R(K)$ simultaneously, obtaining interesting correlations among the observables.

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arXiv2013

Standard Model and New Physics contributions to $K_L$ and $K_S$ into four leptons

We study the $K_L$ and $K_S$ decays into four leptons ($e\bar{e}e\bar{e}, \, μ\barμμ\barμ,\, e\bar{e}μ\barμ$) where we use a form factor motivated by vector meson dominance, and show the dependence of the branching ratios and spectra from the slopes. A precise determination of short distance contribution to $K_L\to μμ$ is affected by our ignorance on the sign of the amplitude $\mathcal{A}(K_L\to γγ)$ but we show a possibility to measure the sign of this amplitude by studying $K_L$ and $K_S$ decays in four leptons. We also investigate the effect of New Physics contributions for these decays.

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arXiv2013

Nonstandard Higgs Couplings from Angular Distributions in $h\to Z \ell^+\ell^-$

We compute the fully differential rate for the Higgs-boson decay $h\to Z\ell^+\ell^-$, with $Z\to\ell^{'+}\ell^{'-}$. For these processes we assume the most general matrix elements within an effective Lagrangian framework. The electroweak chiral Lagrangian we employ assumes minimal particle content and Standard Model gauge symmetries, but is otherwise completely general. We discuss how information on new physics in the decay form factors may be obtained that is inaccessible in the dilepton-mass spectrum integrated over angular variables. The form factors are related to the coefficients of the effective Lagrangian, which are used to estimate the potential size of new-physics effects.

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arXiv2013

Standard Model prediction and new physics tests for D0 -> h+h-l+l- (h=π,K; l=e,μ)

Motivated by the recent evidence for direct CP-violation in D0 -> h+h- decays, we provide an exhaustive study of both Cabibbo-favored and Cabibbo-suppressed (singly and doubly) D0 -> h1+h2-l+l- decays. In particular, we study the Dalitz plot for the long-distance contributions in the (m_{ll}^2,m_{hh}^2) parameter space. We find that near-resonant effects, i.e., D0 -> V(h1+h2-)l+l- with V=ρ,K*,ϕ, are sizeable and even dominant (over Bremsstrahlung) for the μ+μ- decay modes, bringing the branching ratios close to the LHCb reach. We also provide a detailed study of the angular asymmetries for such decays and identify signatures for new physics detection. In particular, new physics signals can be neatly isolated in asymmetries involving the semileptonic operator Q_{10}, where for typical new physics scenarios the effects can be as sizeable as O(1%) for the doubly Cabibbo-suppressed modes.

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arXiv2013

Rare Kaon decays

Rare Kaon decays and CP violation are important tools to test the Standard Model and its extensions. We describe kaon physics observables testing CP violation and more generally short distance physics. Channels under consideration will be K -> pi nu nubar, K -> pi l+ l-, K+- > pi 3 pi, K+- -> pi+- pi gamma, KS -> mu mu and K+- -> pi+- pi0 e+ e-

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arXiv2012

Status of CP violation in Kaon systems

CP violation is an important tool to test the Standard Model and his extensions. We describe kaon physics observables testing CP violation and more in general short distance physics. Channels under consideration will be K -> pi nu nu, K -> pi l+ l-, K+- -> 3 pi, K+- -> pi +- pi gamma and K+- -> pi +- pi0 e+ e-

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arXiv2011

K^+ -> pi^+pi^0e^+e^-: a novel short-distance probe

We study the decay K^+ -> pi^+ pi^0 e^+ e^-, currently under analysis by the NA62 Collaboration at CERN. In particular, we provide a detailed analysis of the Dalitz plot for the long-distance, gamma^*-mediated, contributions (Bremsstrahlung, direct emission and its interference). We also examine a set of asymmetries to isolate genuine short-distance effects. While we show that charge asymmetries are not required to test short distances, they provide the best environment for its detection. This constitutes by itself a strong motivation for NA62 to study K^- decays in the future. We therefore provide a detailed study of different charge asymmetries and the corresponding estimated signals. Whenever possible, we make contact with the related processes K^+ -> pi^+ pi^0 gamma and K_L -> pi^+ pi^- e^+ e^- and discuss the advantages of K^+ -> pi^+ pi^0 e^+ e^- over them.

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arXiv2011

A holographic approach to low-energy weak interactions of hadrons

We apply the double-trace formalism to incorporate nonleptonic weak interactions of hadrons into holographic models of the strong interactions. We focus our attention upon $ΔS=1$ nonleptonic kaon decays. By working with a Yang-Mills--Chern-Simons 5-dimensional action, we explicitly show how, at low energies, one recovers the $ΔS=1$ weak chiral Lagrangian for both the anomalous and nonanomalous sectors. We provide definite predictions for the low energy coefficients in terms of the AdS metric and argue that the double-trace formalism is a 5-dimensional avatar of the Weak Deformation Model introduced long ago by Ecker et al. As a significant phenomenological application, we reassess the $K\to 3π$ decays in the light of the holographic model. Previous models found a fine-tuned cancellation of resonance exchange in these decays, which was both conceptually puzzling and quantitatively in disagreement with experimental results. The holographic model we build is an illustrative counterexample showing that the cancellation encountered in the literature is not generic but a model-dependent statement and that agreement with experiment can be obtained.

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arXiv2011

The hadronic light by light contribution to the $(g-2)_μ$ with holographic models of QCD

We study the anomalous electromagnetic pion form factor $F_{π^0γ^*γ^*}$ with a set of holographic models. By comparing with the measured value of the linear slope, some of these models can be ruled out. From the remaining models we obtain predictions for the low-energy quadratic slope parameters of $F_{π^0γ^*γ^*}$, currently out of experimental reach but testable in the near future. We find it particularly useful to encode this low-energy information in a form factor able to satisfy also QCD short-distance constraints. We choose the form factor introduced by D'Ambrosio, Isidori and Portoles in kaon decays, which has the right short distance for a particular value of the quadratic slope, which is later shown to be compatible with our holographic predictions. We then turn to a determination of the (dominant) pion exchange diagram in the hadronic light by light scattering contribution to the muon anomalous magnetic moment. We quantify the theoretical uncertainty in $(g-2)_μ$ coming from the different input we use: QCD short distances, experimental input and low-energy holographic predictions. We also test the pion-pole approximation. Our final result is $a_μ^{π^0}=6.54(25)\cdot 10^{-10}$, where the error is driven by the linear slope of $F_{π^0γ^*γ^*}$, soon to be measured with precision at KLOE-2. Our numerical analysis also indicates that large values of the magnetic susceptibility $χ_0$ are disfavored, therefore pointing at a mild effect from the pion off-shellness. However, in the absence of stronger bounds on $χ_0$, an additional $(10-15)%$ systematic uncertainty on the previous value for $a_μ^{π^0}$ cannot be excluded.

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arXiv2010

Antisymmetric tensors in holographic approaches to QCD

We study real (massive) antisymmetric tensors of rank two in holographic models of QCD based on the gauge/string duality. Our aim is to understand in detail how the AdS/CFT correspondence describes correlators with tensor currents in QCD. To this end we study a set of bootstrapped correlators with spin-1 vector and tensor currents, imposing matching to QCD at the partonic level. We show that a consistent description of this set of correlators yields a very predictive picture. For instance, it imposes strong constraints on infrared boundary conditions and precludes the introduction of dilatonic backgrounds as a mechanism to achieve linear confinement. Additionally, correlators with tensor currents turn out to be especially sensitive to chiral symmetry breaking, thus offering an ideal testing ground for genuine QCD effects. Several phenomenological consequences are explored, such as the nontrivial interplay between $1^{+-}$ states and conventional $1^{--}$ vector mesons.

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arXiv2010

On the Evaluation of Gluon Condensate Effects in the Holographic Approach to QCD

In holographic QCD the effects of gluonic condensate can be encoded in a suitable deformation of the 5D metric. We develop two different methods for the evaluation of first order perturbative corrections to masses and decay constants of vector resonances in 5D Hard-Wall models of QCD due to small deformations of the metric. They are extracted either from a novel compact form for the first order correction to the vector two-point function, or from perturbation theory for vector bound-state eigenfunctions: the equivalence of the two methods is shown. Our procedures are then applied to flat and to AdS 5D Hard-Wall models; we complement results of existing literature evaluating the corrections to vector decay constant and to two-pion-one-vector couplings: this is particularly relevant to satisfy the sum rules. We concentrate our attention on the effects for the Gasser-Leutwyler coefficients; we show that, as in the Chiral Quark model, the addition of the gluonic condensate improves the consistency, the understanding and the agreement with phenomenology of the holographic model.

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arXiv2007

Form factor in K+ --> pi+ pi0 gamma: interference versus direct emission

We analyze the effect of a form factor in the magnetic contribution to K+ --> pi+ pi0 gamma. We emphasize how this can show up experimentally: in particular we try to explore the difference between a possible interference contribution and a form factor in the magnetic part. The form factor used for K+ --> pi+ pi0 gamma is analogous to the one for KL --> pi+ pi- gamma, experimentally well established.

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arXiv2006

Vector meson decays from the Extended Chiral Quark Model

We derive the the effective lagrangian that describes the interactions among vector, axial-vector mesons and pseudoscalars starting from the extended chiral quark model (ECQM). The results for the low-energy constants of this effective lagrangian have a parametric resemblance with existing predictions based on the Nambu-Jona-Lasinio model (except for some overall signs that we correct), but are numerically different. Therefore a precise measurement of these decay constants can shed some light on the way chiral symmetry breaking is modelled in QCD. Although most of the constants are poorly measured, comparison with phenomenology allows us to determine one of the parameters of the ECQM that could not be fully determined in previous analyses.

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