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Fulvia De Fazio

Publications and source records attributed to Fulvia De Fazio.

At least 37 records · Page 2Linked to original sources
arXiv2018

Scrutinizing $\bar B \to D^*(D π) \ell^- \bar ν_\ell$ and $\bar B \to D^*(D γ) \ell^- \bar ν_\ell$ in search of new physics footprints

Besides being important to determine Standard Model parameters such as the CKM matrix elements $|V_{cb}|$ and $|V_{ub}|$, semileptonic $B$ decays seem also promising to reveal new physics (NP) phenomena, in particular in connection with the possibility of uncovering lepton flavour universality (LFU) violating effects. In this view, it could be natural to connect the tensions in the inclusive versus exclusive determinations of $|V_{cb}|$ to the anomalies in the ratios $R(D^{(*)})$ of decay rates into $τ$ vs $μ, e$. However, the question has been raised about the role of the parametrization of the hadronic $B \to D^{(*)}$ form factors in exclusive $B$ decay modes. We focus on the fully differential angular distributions of $\bar B \to D^* \ell^-{\bar ν}_\ell$ with $D^* \to D π$ or $D^* \to D γ$, the latter mode being important in the case of $B_s \to D_s^*$ decays. We show that the angular coefficients in the distributions can be used to scrutinize the role of the form factor parametrization and to pin down deviations from SM. As an example of a NP scenario, we include a tensor operator in the $b \to c$ semileptonic effective Hamiltonian, and discuss how the angular coefficients allow to construct observables sensitive to this structure, also defining ratios useful to test LFU.

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arXiv2017

Theory overview of tree-level $B$ decays

I describe the theoretical progress in the study of semileptonic tree-level B decays, and its interplay with recent experimental results. In particular, I focus on two anomalies: the ratios $R(D^{(*)})=\displaystyle\frac{{\cal B}(B \to D^{(*)} τ\bar ν_τ)}{{\cal B}(B \to D^{(*)} \ell \bar ν_\ell)}$ and the inclusive versus exclusive determination of $|V_{cb}|$. I review a few explanations proposed for such anomalies, and discuss tests to shed light on their origin.

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arXiv2017

Tension in the inclusive versus exclusive determinations of $|V_{cb}|$: a possible role of new physics

We reconsider the possibility that the tension in the $|V_{cb}|$ determinations from inclusive and exclusive $B$ decay modes is due to a new physics effect. We modify the Standard Model effective Hamiltonian for semileptonic $b \to c$ transitions including a tensor operator with a lepton flavour dependent coupling $ε_T^\ell$, and investigate separately the muon and electron modes. The interference term between SM and NP, proportional to the lepton mass, has different impact in the inclusive and exclusive $B$ modes to muon. Moreover, even when the lepton mass is small as for the electron, the NP effect is different in inclusive and exclusive $B$ channels. For both $μ$ and $e$ we find a region of $ε_T^{μ,\,e}$ where the constraints from $B^- \to D^{(*)0} \ell^- {\bar ν}_\ell$ and $B \to X_c \, \ell \, \bar ν_\ell$ are satisfied for the same $|V_{cb}|$.

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arXiv2016

On Exclusive $h \to V \ell^+ \ell^-$ Decays

We study a set of exclusive decay modes of the Standard Model Higgs boson into a vector meson and a dilepton pair: $h\to V \ell^+ \ell^-$, with $V=Υ, J/ψ,ϕ$, and $\ell=μ, τ$, determining the decay rates, the dilepton mass spectra and the $V$ longitudinal helicity fraction distributions. In the same framework, we analyze the exclusive modes into neutrino pairs $h\to V ν\bar ν$. We also discuss the implications of the recent CMS and ATLAS results for the lepton flavor-changing process $h\to τ^+ μ^-$ on the $h\to V τ^+ μ^-$ decay modes.

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arXiv2016

Role of nonlocal probes of thermalization for a strongly interacting non-Abelian plasma

We use a holographic method to investigate thermalization of a boost-invariant strongly interacting non-Abelian plasma. Boundary sourcing, a distorsion of the boundary metric, is employed to drive the system far from equilibrium. Thermalization is analyzed through nonlocal probes: the equal-time two-point correlation function of large conformal dimension operators in the boundary theory, and Wilson loops of different shapes. We study the dependence of the thermalization time on the size of the probes, and compare the results to the ones obtained using local observables: the onset of thermalization is first observed at short distances.

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arXiv2016

331 Models Facing the Tensions in $ΔF=2$ Processes with the Impact on $\varepsilon^\prime/\varepsilon$, $B_s\toμ^+μ^-$ and $B\to K^*μ^+μ^-$

Motivated by the recently improved lattice QCD results on the hadronic matrix elements entering $ΔM_{s,d}$ in $B_{s,d}^0-\bar B_{s,d}^0$ mixings and the resulting increased tensions between $ΔM_{s,d}$ and $\varepsilon_K$ in the Standard Model and CMFV models, we demonstrate that these tensions can be removed in 331 models based on the gauge group $SU(3)_C\times SU(3)_L\times U(1)_X$ both for $M_{Z^\prime}$ in the LHC reach and well beyond it. But the implied new physics (NP) patterns in $ΔF=1$ observables depend sensitively on the value of $|V_{cb}|$. Concentrating the analysis on three 331 models that have been selected by us previously on the basis of their performance in electroweak precision tests and $\varepsilon^\prime/\varepsilon$ we illustrate this for $|V_{cb}|=0.042$ and $|V_{cb}|=0.040$. We find that these new lattice data still allow for positive shifts in $\varepsilon^\prime/\varepsilon$ up to $6\times 10^{-4}$ for $M_{Z^\prime}=3~TeV$ for both values of $$|V_{cb}|$ but for $M_{Z^\prime}=10~TeV$ only for $|V_{cb}|=0.040$ such shifts can be obtained. NP effects in $B_s\toμ^+μ^-$ and in the Wilson coefficient $C_9$ are significantly larger in all three models for the case of $|V_{cb}|=0.040$. In particular in two models the rate for $B_s\toμ^+μ^-$ can be reduced by NP by $20\%$ for $M_{Z^\prime}=3~TeV$ resulting in values in the ballpark of central values from CMS and LHCb. In the third model a shift in $C_9$ up to $C_9^\text{NP}=-0.5$ is possible. We also consider the simplest 331 model, analyzed recently in the literature, in which $X=Y$, the usual hypercharge. We find that in this model NP effects in flavour observables are much smaller than in the three models with $X\not=Y$, in particular NP contributions to the ratio $\varepsilon^\prime/\varepsilon$ are very strongly suppressed.

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arXiv2016

$\varepsilon'/\varepsilon$ in 331 Models

Motivated by the recent findings that the ratio $\varepsilon'/\varepsilon$ in the Standard Model (SM) appears to be significantly below the data we investigate whether the necessary enhancement of this ratio can be obtained in 331 models, in which new physics (NP) contributions to $\varepsilon'/\varepsilon$ and other flavour observables are dominated by tree-level exchanges of a $Z^\prime$. NP contributions to $\varepsilon'/\varepsilon$ in these models are governed by the electroweak operator $Q_8$ for which the hadronic matrix element is rather well known so that our analysis of NP contributions is subject to much smaller uncertainties than within the SM. We consider seven 331 models selected in our earlier analysis on the basis of electroweak precision data. Imposing the constraints from $ΔF=2$ transitions we find that only three of these models can provide a significant positive shift in $\varepsilon'/\varepsilon$ up to $6\times 10^{-4}$ for $M_{Z^\prime}=3$ TeV. Two of them allow simultaneously a supression of ${\cal B}(B_{s}\to μ^+μ^-)$ by $20\%$, bringing the theory closer to the data without any significant impact on the Wilson coefficient $C_9$. The third one provides also the shift $ΔC_9=-0.6$, softening the anomalies in $B\to K^*μ^+μ^-$, without any significant impact on $B_{s}\to μ^+μ^-$. NP effects in rare $K$ decays and in $B\to K(K^*)ν\barν$ turn out to be small. The flavour structure of 331 models implies that even for $M_{Z^\prime}=30$ Tev a shift of $\varepsilon'/\varepsilon$ up to $8\times 10^{-4}$ and a significant shift in $\varepsilon_K$ can be obtained, while the effects in other flavour observables are small.

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arXiv2015

Thermalization of a boost-invariant non-Abelian plasma: Holographic approach with boundary sourcing

In a holographic approach, the evolution of a 4D strongly coupled non-Abelian plasma towards equilibrium can be studied investigating a 5D gravitational dual. The process driving the plasma out-of-equilibrium can be described by boundary sourcing, a deformation of the boundary metric; the analysis of the late-time dynamics allows to understand how the hydrodynamic regime settles in. We apply the method to a boost-invariant case, considering the effects of different quenches, solving the Einstein equations in the bulk and studying the time-dependence of observables such as the effective temperature, the energy density and the pressures. The main outcome is that, if the effective temperature of the system when the quench is switched off is $T_{eff}(τ^*)=500$ MeV, thermalization is reached within a time of ${\cal O}$(1 fm/c), an important information if the case of the QCD plasma produced in relativistic heavy ion collisions is considered.

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arXiv2015

NP models with extended gauge groups and extra dimensions: Impact on flavour observables in RS$_c$

Deviations with respect to Standard Model predictions have recently shown up in angular distributions of the FCNC induced mode $B^0 \to K^{*0} μ^+ μ^-$. Within New Physics models, such tensions might be explained by new contributions to the Wilson coefficients of the effective Hamiltonian governing this decay. I discuss the issue in the framework of the Randall-Sundrum model with custodial protection (RS$_c$), giving also predictions for other rare $B$ decays.

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arXiv2014

New Physics Scenarios in $b \to c \ell {\bar ν}_\ell$ decays

The latest BaBar measurements of the ratios ${\cal R}(D^{(*)})=\displaystyle{\frac{{\cal B}(B \to D^{(*)} τ{\bar ν}_τ)}{{\cal B}(B \to D^{(*)} μ{\bar ν}_μ)}}$ deviate from the Standard Model predictions at the global level of 3.4$σ$. A possibility to reproduce these experimental ratios without affecting other modes which do not show similar deviations is to consider new physics scenarios producing an additional tensor operator in the effective weak Hamiltonian. I describe the impact of such an operator in semileptonic $B \to D^{(*)}$ modes and in semileptonic $B$ and $B_s$ decays to excited positive parity charmed mesons. In particular, I discuss the most effective observables able to discriminate new physics from the Standard Model.

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arXiv2014

Exclusive $b \to s ν\bar ν$ induced transitions in RS$_c$ model

We study a set of exclusive $B$ and $B_s$ decay modes induced by the rare $b \to s ν\bar ν$ transition in the RS$_c$ model, an extra-dimensional extension of the standard model with warped 5D metric and extended gauge group. We emphasize the role of correlations among the observables, and their importance for detecting the predicted small deviations from the standard model expectations.

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arXiv2014

Rare semileptonic $B\to K^* \ell^+ \ell^- $ decays in RS$_c$ model

Recent LHCb measurements show small discrepancies with respect to the Standard Model (SM) predictions in selected angular distributions of the mode $B^0 \to K^{*0} μ^+ μ^-$. The possibility of explaining such tensions within theories beyond the SM crucially depends on the size of the deviations of the Wilson coefficients of the effective Hamiltonian for this mode, in comparison to their SM values. We analyse this issue in the framework of the Randall Sundrum model with custodial protection (RS$_c$); in our study we also consider the mode with $τ$ leptons in the final state. We discuss the small deviations of RS$_c$ results from SM ones, found scanning the parameter space of the model.

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arXiv2014

Z-Z' Mixing and Z-Mediated FCNCs in SU(3)_C x SU(3)_L x U(1)_X Models

Most of the existing analyses of FCNC processes in the 331 models, based on the gauge group SU(3)_C x SU(3)_L x U(1)_X, take only into account tree-level exchanges of a new heavy neutral gauge boson Z'. However due to the Z-Z' mixing also corresponding contributions from Z boson are present that are usually neglected. We calculate the impact of these contributions on Delta F=2 processes and rare K, B_s and B_d decays for different values of a parameter beta, which distinguishes between various 331 models and for different fermion representations under the SU(3)_L group. We find a general expression for the Z-Z' mixing in terms beta, M_Z, M_Z' and tan(bar{beta}), familiar from 2 Higgs Doublet models, that differs from the one quoted in the literature. We study in particular the models with beta=+-n/sqrt{3} with n=1,2 which have recently been investigated by us in the context of new data on B_{s,d}->mu^+ mu^- and B_d->K^*(K)mu^+ mu^-. We find that these new contributions can indeed be neglected in the case of Delta F=2 transitions and decays, like B_d->K^*mu^+mu^-, where they are suppressed by the small vectorial Z coupling to charged leptons. However the contributions of tree-level Z exchanges to decays sensitive to axial-vector couplings, like B_{s,d}->mu^+ mu^- and B_d->K mu^+ mu^-, and those with neutrinos in the final state, like b->s nu bar{nu} transitions, K^+->pi^+ nu bar{nu} and K_L->pi^0 nu bar{nu} cannot be generally neglected with size of Z contributions depending on beta, tan(bar{beta}) and M_Z'. We analyze for the first time the ratio epsilon'/epsilon in these models including both Z' and Z contributions. Our analysis of electroweak precision observables within 331 models demonstrates transparently that the interplay of NP effects in electroweak precision observables and those in flavour observables could allow in the future to identify the favourite 331 model.

hep-ph↗
arXiv2014

Delta I=1/2 Rule, epsilon'/epsilon and K -> pi nu nubar in Z'(Z) and G' Models with FCNC Quark Couplings

The experimental value for the isospin amplitude Re(A_2) in K->pi pi decays has been successfully explained within the Standard Model, both within large N approach to QCD and by QCD lattice calculations. On the other hand in both approaches the theoretical values of Re(A_0) are by at least 30% below the data so that the Delta I=1/2 rule in K->pi pi decays is not fully explained. While this deficit could be the result of present theoretical uncertainties in both approaches, it cannot be excluded that the missing piece in Re(A_0) comes from New Physics. We demonstrate that this deficit can be significantly softened by tree-level FCNC transitions mediated by a heavy colourless Z' gauge boson with particularly chosen flavour violating couplings assuring negligible NP contributions to Re(A_2) and significantly enhancing the contribution of the leading QCD penguin operator Q_6 to Re(A_0). A large fraction of the missing piece in the Delta I=1/2 rule can be explained in this manner for M_{Z'} in the reach of the LHC, while satisfying constraints from epsilon_K, epsilon'/epsilon, Delta M_K, LEP-II and the LHC. We identify quartic correlation between Z' contributions to Re(A_0), epsilon'/epsilon, epsilon_K and Delta M_K. We present correlations between epsilon'/epsilon and rare decays K->pi nu bar{nu} with and without the Delta I=1/2 rule constraint and generalize the whole analysis to Z' with colour (G') and Z with FCNC couplings. In the latter case no improvement on Re(A_0) can be achieved without destroying the agreement of the SM with the data on Re(A_2). Moreover, this scenario is very tightly constrained by epsilon'/epsilon. On the other hand in the context of the Delta I=1/2 rule G' is even more effective than Z': it provides the missing piece in Re(A_0) for M_{G'}=(3.5-4.0) TeV.

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arXiv2013

331 models facing new b -> s mu^+ mu^- data

We investigate how the 331 models face new data on B_{s,d}->mu^+mu^- and B_d->K^*mu^+mu^- taking into account present constraints from Delta F=2 observables, low energy precision measurements, LEP-II and the LHC data. In these models new flavour violating interactions of ordinary quarks and leptons are dominated by a new heavy Z^prime gauge boson with the strength of the relevant couplings governed by four new parameters in the quark sector and the parameter beta. We study the implications of these models for beta=+-n/sqrt{3} with n=1,2,3. The case beta=-sqrt{3} leading to Landau singularities for M_{Z'}approx 4Tev can be ruled out when the present constraints on Z^prime couplings, in particular from LEP-II, are taken into account. For n=1,2 interesting results are found for M_{Z^prime}< 4Tev with largest NP effects for beta<0 in B_d->K^*mu^+mu^- and the ones in B_{s,d}->mu^+mu^- for beta>0. As Re(C_9^NP) can reach the values -0.8 and -0.4 for n=2 and n=1, respectively the B_d-> K^*mu^+mu^- anomalies can be softened with the size depending on Delta M_s/(Delta M_s)_SM and the CP-asymmetry S_{psiphi}. A correlation between Re(C_9^NP) and BR(B_s->mu^+mu^-), identified for beta<0, implies for negative Re(C_9^NP) uniquely suppression of BR(B_s->mu^+mu^-) relative to its SM value which is favoured by the data. In turn also S_{psiphi}< S_{psiphi}^SM is favoured having dominantly opposite sign to S_{psiphi}^SM. We find that the absence of B_d-> K^*mu^+mu^- anomalies in the future data and confirmation of the suppression of BR(B_s->mu^+mu^-) relative to its SM value would favour beta=1/sqrt{3} and M_{Z'}approx 3Tev. Assuming lepton universality, we find an upper bound |C_9^NP|<1.1(1.4) from LEP-II data for all Z' models with only left-handed flavour violating couplings to quarks when NP contributions to Delta M_s at the level of 10%(15%) are allowed.

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arXiv2013

The Anatomy of Neutral Scalars with FCNCs in the Flavour Precision Era

In many extensions of the Standard Model flavour changing neutral current processes can be mediated by tree-level heavy neutral scalars and/or pseudo-scalars H^0(A^0). This generally introduces new sources of flavour violation and CP violation as well as left-handed and right-handed scalar currents. These new physics contributions imply a pattern of deviations from SM expectations for FCNC processes that depends only on the couplings of H^0(A^0) to fermions and on their masses. In situations in which a single H^0 or A^0 dominates NP contributions stringent correlations between Delta F=2 and Delta F=1 observables exist. Anticipating the Flavour Precision Era ahead of us we illustrate this by searching for allowed oases in the landscape of a given model assuming significantly smaller uncertainties in CKM and hadronic parameters than presently available. To this end we analyze Delta F=2 observables in B^0_{s,d}-bar B^0_{s,d} and K^0-bar K^0 systems and rare B and K decays with charged leptons in the final state including both left-handed and right-handed scalar couplings of H^0 and A^0 to quarks in various combinations. We identify a number of correlations between various flavour observables that could test and distinguish these different scenarios. The prominent role of the decays B_{s,d}-> mu^+ mu^- in these studies is emphasized. Imposing the existing flavour constraints, a rich pattern of deviations from the SM expectations in rare B_{s,d} decays emerges provided M_H< 1 TeV. NP effects in rare K decays are very small. Neutral SM Higgs contributions to rare B and K decays turn out to be negligible once the constraints from Delta F=2 processes are taken into account. Finally, we point out striking differences between the correlations found here and in scenarios in which tree-level FCNC are mediated by a new neutral gauge boson Z'.

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arXiv2013

On the anomalous enhancement observed in $B \to D^{(*)} \, τ\, {\bar ν}_τ$ decays

The BaBar measurements of the ratios ${\cal R}(D^{(*)})=\frac{{\cal B}(B \to D^{(*)} τ{\bar ν}_τ)}{{\cal B}(B \to D^{(*)} μ{\bar ν}_μ)}$ deviate from the standard model expectation, while new results on the purely leptonic $B \to τ{\bar ν}_τ$ mode show a better consistency with the standard model, within the uncertainties. In a new physics scenario, one possibility to accomodate these two experimental facts consists in considering an additional tensor operator in the effective weak hamiltonian. We study the effects of such an operator in a set of observables, in semileptonic $B \to D^{(*)}$ modes as well as in semileptonic $B$ and $B_s$ decays to excited positive parity charmed mesons.

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