arXiv subjects

Cecilia Tarantino

Publications and source records attributed to Cecilia Tarantino.

At least 19 recordsLinked to original sources

arXiv2022

New UTfit Analysis of the Unitarity Triangle in the Cabibbo-Kobayashi-Maskawa scheme

Flavour mixing and CP violation as measured in weak decays and mixing of neutral mesons are a fundamental tool to test the Standard Model (SM) and to search for new physics. New analyses performed at the LHC experiment open an unprecedented insight into the Cabibbo-Kobayashi-Maskawa (CKM) metrology and new evidence for rare decays. Important progress has also been achieved in theoretical calculations of several hadronic quantities with a remarkable reduction of the uncertainties. This improvement is essential since previous studies of the Unitarity Triangle did show that possible contributions from new physics, if any, must be tiny and could easily be hidden by theoretical and experimental errors. Thanks to the experimental and theoretical advances, the CKM picture provides very precise SM predictions through global analyses. We present here the results of the latest global SM analysis performed by the UTfit collaboration including all the most updated inputs from experiments, lattice QCD and phenomenological calculations.

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arXiv2018

HVP contribution of the light quarks to the muon $(g - 2)$ including isospin-breaking corrections with Twisted-Mass fermions

We present a preliminary lattice calculation of the leading-order electromagnetic and strong isospin-breaking corrections to the Hadronic Vacuum Polarization (HVP) contribution of the light quarks to the anomalous magnetic moment of the muon. The results are obtained in the quenched-$QED$ approximation using the $QCD$ gauge configurations generated by the European Twisted Mass Collaboration (ETMC) with $N_f = 2 + 1 + 1$ dynamical quarks, at three values of the lattice spacing varying from $0.089$ to $0.062 ~ \mbox{fm}$, at several lattice volumes and with pion masses in the range $M_π\simeq 220 ÷490 ~ \mbox{MeV}$.

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arXiv2017

Unitarity Triangle Analysis in the Standard Model and Beyond

Flavour physics represents a unique test bench for the Standard Model (SM). New analyses performed at the LHC experiments are now providing unprecedented insights into CKM metrology and new evidences for rare decays. The CKM picture can provide very precise SM predictions through global analyses. We present here the results of the latest global SM analysis performed by the UTfit collaboration including all the most updated inputs from experiments, lattice QCD and phenomenological calculations. In addition, the Unitarity Triangle (UT) analysis can be used to constrain the parameter space in possible new physics (NP) scenarios. We update here also the UT analysis beyond the SM by the UTfit collaboration. All of the available experimental and theoretical information on $ΔF=2$ processes is reinterpreted including a model-independent NP parametrisation. We determine the allowed NP contributions in the kaon, $D$, $B_d$, and $B_s$ sectors and, in various NP scenarios, we translate them into bounds for the NP scale as a function of NP couplings.

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arXiv2017

Tensor form factor for the $D \to π(K)$ transitions with Twisted Mass fermions

We present a preliminary lattice calculation of the $D \to π$ and $D \to K$ tensor form factors $f_T(q^2)$ as a function of the squared 4-momentum transfer $q^2$. ETMC recently computed the vector and scalar form factors $f_+(q^2)$ and $f_0(q^2)$ describing $D \to π(K) \ell ν$ semileptonic decays analyzing the vector current and the scalar density. The study of the weak tensor current, which is directly related to the tensor form factor, completes the set of hadronic matrix element regulating the transition between these two pseudoscalar mesons within and beyond the Standard Model where a non-zero tensor coupling is possible. Our analysis is based on the gauge configurations produced by the European Twisted Mass Collaboration with $N_f = 2 + 1 + 1$ flavors of dynamical quarks. We simulated at three different values of the lattice spacing and with pion masses as small as 210 MeV and with the valence heavy quark in the mass range from $\simeq 0.7\, m_c$ to $\simeq 1.2\, m_c$. The matrix element of the tensor current are determined for a plethora of kinematical conditions in which parent and child mesons are either moving or at rest. As for the vector and scalar form factors, Lorentz symmetry breaking due to hypercubic effects is clearly observed in the data. We will present preliminary results on the removal of such hypercubic lattice effects.

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arXiv2017

$D\toπ$ and $D\to K$ semileptonic form factors with $N_f=2+1+1$ twisted mass fermions

We present a lattice determination of the vector and scalar form factors of the $D\toπ(K)\ellν$ semileptonic decays, which are relevant for the extraction of the CKM matrix elements $|V_{cd}|$ and $|V_{cs}|$ from experimental data. Our analysis is based on the gauge configurations produced by the European Twisted Mass Collaboration with $N_f=2+1+1$ flavors of dynamical quarks. We simulated at three different values of the lattice spacing and with pion masses as small as 210 MeV. The matrix elements of both vector and scalar currents are determined for a plenty of kinematical conditions in which parent and child mesons are either moving or at rest. Lorentz symmetry breaking due to hypercubic effects is clearly observed in the data and included in the decomposition of the current matrix elements in terms of additional form factors. After the extrapolations to the physical pion mass and to the continuum limit the vector and scalar form factors are determined in the whole kinematical region from $q^2 = 0$ up to $q^2_{\rm max} = (M_D - M_{π(K)})^2$ accessible in the experiments, obtaining a good overall agreement with experiments, except in the region at high values of $q^2$ where some deviations are visible.

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arXiv2014

Standard Model updates and new physics analysis with the Unitarity Triangle fit

We present here the update of the Unitarity Triangle (UT) analysis performed by the UTfit Collaboration within the Standard Model (SM) and beyond. Continuously updated flavour results contribute to improving the precision of several constraints and through the global fit of the CKM parameters and the SM predictions. We also extend the UT analysis to investigate new physics (NP) effects on $ΔF=2$ processes. Finally, based on the NP constraints, we derive upper bounds on the coefficients of the most general $ΔF=2$ effective Hamiltonian. These upper bounds can be translated into lower bounds on the scale of NP that contributes to these low-energy effective interactions.

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arXiv2014

K -> pi vector form factor with N_f=2+1+1 Twisted Mass fermions

We present a lattice QCD determination of the vector form factor of the kaon semileptonic decay K -> pi l nu which is relevant for the extraction of the CKM matrix element |V_{us}| from experimental data. Our result is based on the gauge configurations produced by the European Twisted Mass Collaboration with N_f=2+1+1 dynamical fermions. We simulated at three different values of the lattice spacing and with pion masses as small as 210 MeV. Our preliminary estimate for the vector form factor at zero momentum transfer is f_+(0)=0.9683(65), where the uncertainty is both statistical and systematic. By combining our result with the experimental value of f_+(0)|V_{us}| we obtain |V_{us}|=0.2234(16), which satisfies the unitarity constraint of the Standard Model at the permille level.

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arXiv2014

Kaon semileptonic vector form factor with $N_f=2+1+1$ Twisted Mass fermions

We investigate the vector form factor relevant for the $K_{\ell 3}$ semileptonic decay using maximally twisted-mass fermions with 4 dynamical flavours ($N_f=2+1+1$). Our simulations feature pion masses ranging from $210$ MeV to approximately $450$ MeV and lattice spacing values as small as $0.06$fm. Our main result for the vector form factor at zero 4-momentum transfer is $f_+(0)=0.9683(65)$ where the uncertainty is both statistical and systematic. By combining our result with the experimental value of $f_+(0)|V_{us}|$ we obtain $|V_{us}|=0.2234(16)$, which satisfies the unitarity constraint of the Standard Model at the permille level.

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arXiv2013

$K$ and $D$ oscillations in the Standard Model and its extensions from $N_f=2+1+1$ Twisted Mass LQCD

We present the first $N_f=2+1+1$ results for the matrix elements of the operators describing neutral $K$ and $D$ mixing in the Standard Model and its extensions. The combination of maximally twisted sea quarks and Osterwalder-Seiler valence quarks ensures $\mathcal{O}(a)$-improvement and continuum like renormalization pattern. We have used the $N_f=2+1+1$ dynamical quark gauge configurations generated by ETMC. Simulations include three lattice spacings in the interval $[0.06:0.09]$ fm and pseudoscalar meson masses in the range $[230:500]$ MeV. Our results are extrapolated to the continuum limit and to the physical quark masses. The calculation of the renormalization constants has been performed non-perturbatively in the RI-MOM scheme.

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arXiv2012

Flavor Lattice QCD in the Precision Era

I discuss the important role that Lattice QCD plays in testing the Flavor sector of the Standard Model (SM) and in indirect searches of New Physics. I review in particular the Unitarity Triangle Analysis performed by the UTfit collaboration within and beyond the SM, presenting the recent accurate lattice results that enter the analyses. I conclude with a tentative outlook to the further progresses that we can expect in the next years from Flavor Lattice QCD.

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arXiv2012

D-meson decay constants and a check of factorization in non-leptonic B-decays

We compute the vector meson decay constants fD*, fDs* from the simulation of twisted mass QCD on the lattice with Nf = 2 dynamical quarks. When combining their values with the pseudoscalar D(s)-meson decay constants, we were able (i) to show that the heavy quark spin symmetry breaking effects with the charm quark are large, fDs*/fDs = 1.26(3), and (ii) to check the factorization approximation in a few specific B-meson non-leptonic decay modes. Besides our main results, fD* = 278 \pm 13 \pm 10 MeV, and fDs* = 311 \pm 9 MeV, other phenomenologically interesting results of this paper are: fDs*/fD* = 1.16 \pm 0.02 \pm 0.06, fDs*/fD = 1.46 \pm 0.05 \pm 0.06, and fDs/fD* = 0.89 \pm 0.02 \pm 0.03. Finally, we correct the value for B(B0 \rightarrow D+ pi-) quoted by PDG, and find B(B0 \rightarrow D+ pi-) = (7.8 \pm 1.4) \times 10-7. Alternatively, by using the ratios discussed in this paper, we obtain B(B0 \rightarrow D+ pi-) = (8.3 \pm 1.0 \pm 0.8)\times10-7.

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arXiv2010

FCNC Processes in the Littlest Higgs Model with T-Parity: an Update

We update our 2006-2007 results for FCNC processes in the Littlest Higgs model with T-parity (LHT). The removal of the logarithmic UV cutoff dependence in our previous results through a new contribution to the Z^0-penguin diagrams identified by Goto et al. and del Aguila et al., while making the deviations from the SM expectations in the quark sector less spectacular, still allows for sizable new physics effects in K -> pi nu anti-nu and K_L -> pi^0 l^+ l^- decays and in the CP-asymmetry S_{psi phi} with the latter unaffected by the new contribution. We extend our analysis by a study of the fine-tuning required to fit the data on epsilon_K and by the inclusion of the decay K_L -> mu^+ mu^-. A number of correlations can distinguish this model from the custodially protected Randall-Sundrum model analysed recently. We also reconsider lepton flavour violating decays, including now a discussion of fine-tuning. While the l_i -> l_j gamma decays are unaffected by the removal of the logarithmic cutoff dependence, the branching ratios for decays with three leptons in the final state, like mu -> 3 e are lowered by almost an order of magnitude. In spite of this, the pattern of lepton flavour violation in the LHT model can still be distinguished from the one in supersymmetric models.

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arXiv2010

The Littlest Higgs Model with T-Parity Facing CP-Violation in B_s - anti-B_s Mixing

The non-minimal flavour violating interactions of mirror quarks and new heavy gauge bosons in the Littlest Higgs model with T-parity (LHT) give rise to naturally large CP-violating effects in the B_s system. In view of a large new CP phase in B_s - anti-B_s mixing hinted by the CDF and D0 data and the recent UTfit analysis, we update our 2006 analysis of particle-antiparticle mixing and rare K and B decays in the LHT model, using the most recent values of a number of input parameters and performing a more careful error analysis. We find that the CP-asymmetry S_{psi phi} can easily reach values ~ 0.15-0.20, compared to the SM value ~ 0.04, while higher values are rather unlikely though not excluded. Large enhancements are also possible in the branching ratios for K_L -> pi0 nu anti-nu, K+ -> pi+ nu anti-nu and K_L -> pi0 l+ l- with much more modest effects in B_{s,d} -> mu+ mu-. We perform a detailed study of correlations between the latter decays and S_{psi phi} as well as of the correlation between S_{psi phi} and S_{psi K_S}. We also point out that the possible tension between epsilon_K and the tree level CKM determination recently hinted by various analyses can easily be resolved in the LHT model.

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arXiv2010

Littlest Higgs Model with T-Parity Confronting the New Data on D^0-\bar D^0 Mixing

Motivated by the first experimental evidence of meson oscillations in the D system, we study D^0 - \bar D^0 mixing in the Littlest Higgs model with T-parity, we investigate its role in constraining the model parameters and its impact on the most interesting flavour observables. We find that the experimental data are potentially strongly constraining but at present limited by large theoretical uncertainties in the long-distance Standard Model contribution to D^0 - \bar D^0 mixing.

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arXiv2010

Rare and CP-Violating K and B Decays in the Littlest Higgs Model with T-Parity

We calculate the most interesting rare and CP-violating K and B decays in the Littlest Higgs model with T-parity. We give a collection of Feynman rules including v^2/f^2 contributions that are presented here for the first time and could turn out to be useful also for applications outside flavour physics. We adopt a model-independent parameterization of rare decays in terms of the gauge independent functions X_i,Y_i,Z_i (i=K,d,s), which is in particular useful for the study of the breaking of the universality between K, B_d and B_s systems through non-MFV interactions. Performing the calculation in the unitary and 't Hooft-Feynman gauge, we find that the final result contains a divergence which signals some sensitivity to the ultraviolet completion of the theory. Including an estimate of this contribution, we calculate the branching ratios for the decays $K^+\toπ^+ν\barν$, $K_L\toπ^0ν\barν$, $B_{s,d}\to μ^+μ^-$, $B\to X_{s,d}ν\barν$, $K_L \toπ^0\ell^+\ell^-$ and $B\to X_{s,d}\ell^+\ell^-$, paying particular attention to non-MFV contributions present in the model. Imposing all available constraints we find that the decay rates for $B_{s,d} \to μ^+ μ^-$ and $B \to X_{s,d} ν\bar ν$ can be enhanced by at most 50% and 35% relative to the SM values, while $Br(K^+\toπ^+ν\barν)$ and $Br(K_L\toπ^0ν\barν)$ can be both as high as $5 \cdot 10^{-10}$. Significant enhancements of the decay rates $K_L \toπ^0\ell^+\ell^-$ are also possible. Simultaneously, the CP-asymmetries $S_{ψϕ}$ and $A^s_\text{SL}$ can be enhanced by an order of magnitude, while the electroweak penguin effects in $B \to πK$ turn out to be small, in agreement with the recent data.

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arXiv2010

Charged Lepton Flavour Violation and (g-2)_mu in the Littlest Higgs Model with T-Parity: a clear Distinction from Supersymmetry

We calculate the rates for the charged lepton flavour violating decays l_i -> l_j gamma, tau -> l pi, tau -> l eta, tau -> l eta', mu^- -> e^- e^+ e^-, the six three body leptonic decays tau^- -> l_i^- l_j^+ l_k^- and the rate for mu-e conversion in nuclei in the Littlest Higgs model with T-Parity (LHT). We also calculate the rates for K_{L,S} -> mu e, K_{L,S} -> pi^0 mu e and B_{d,s} -> l_i l_j. We find that the relative effects of mirror leptons in these transitions are by many orders of magnitude larger than analogous mirror quark effects in rare K and B decays analyzed recently. In particular, in order to suppress the mu -> e gamma and mu^- -> e^- e^+ e^- decay rates and the mu-e conversion rate below the experimental upper bounds, the relevant mixing matrix in the mirror lepton sector V_Hl must be rather hierarchical, unless the spectrum of mirror leptons is quasi-degenerate. We find that the pattern of the LFV branching ratios in the LHT model differs significantly from the one encountered in the MSSM, allowing in a transparent manner to distinguish these two models with the help of LFV processes. We also calculate (g-2)_mu and find the new contributions to a_mu below 10^{-10} and consequently negligible. We compare our results with those present in the literature.

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arXiv2010

Correlations between epsilon'/epsilon and Rare K Decays in the Littlest Higgs Model with T-Parity

We calculate the CP-violating ratio epsilon'/epsilon in the Littlest Higgs model with T-parity (LHT) and investigate its correlations with the branching ratios for K_L -> pi^0 nu {bar nu}, K_L -> pi^0 l^+ l^- and K^+ -> pi^+ nu {bar nu}. The resulting correlations are rather strong in the case of K_L decays, but less pronounced in the case of K^+ -> pi^+ nu {bar nu}. Unfortunately, they are subject to large hadronic uncertainties present in epsilon'/epsilon, whose theoretical prediction in the Standard Model (SM) is reviewed and updated here. With the matrix elements of Q_6 (gluon penguin) and Q_8 (electroweak penguin) evaluated in the large-N limit and m_s^MS(2 GeV) = 100 MeV from lattice QCD, (epsilon'/epsilon)_SM turns out to be close to the data so that significant departures of Br(K_L -> pi^0 nu {bar nu}) and Br(K_L -> pi^0 l^+ l^-) from the SM expectations are unlikely, while Br(K^+ -> pi^+ nu {bar nu}) can be enhanced even by a factor 5. On the other hand, modest departures of the relevant hadronic matrix elements from their large-N values allow for a consistent description of epsilon'/epsilon within the LHT model accompanied by large enhancements of Br(K_L -> pi^0 nu {bar nu}) and Br(K_L -> pi^0 l^+ l^-), but only modest enhancements of Br(K^+ -> pi^+ nu {bar nu}).

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arXiv2009

FCNC Processes in the LHT Model: a 2009 Look

We present a summary of our recent analysis update of flavour changing neutral current (FCNC) processes in the Littlest Higgs model with T-parity (LHT). The essential novelties of our update are: the removal of the logarithmic UV cutoff dependence by the inclusion of the new contribution to the Z^0-penguin diagrams identified by Goto et al. and by del Aguila et al., the study of the decay K_L -> mu^+ mu^-, a discussion of fine-tuning in epsilon_K and Br(mu -> e gamma).

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