arXiv subjects

S. Simula

Publications and source records attributed to S. Simula.

At least 73 records · Page 4Linked to original sources

arXiv2018

Hypercubic effects in semileptonic decays of heavy mesons, toward $B \to π\ell ν$, with $N_f=2+1+1$ Twisted fermions

We present a preliminary study toward a lattice determination of the vector and scalar form factors of the $B \to π\ell ν$ semileptonic decays. We compute the form factors relative to the transition between heavy-light pseudoscalar mesons, with masses above the physical D-mass, and the pion. We simulate heavy-quark masses in the range $m_c^{phys} < m_h < 2m_c^{phys}$. 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. We discuss the size of this breaking as the parent-meson mass increases. 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 at three different values of the lattice spacing and with pion masses as small as $210$ MeV.

hep-lat↗

arXiv2018

Tensor form factor of $D \to π(K) \ell ν$ and $D \to π(K) \ell \ell$ decays with $N_f=2+1+1$ twisted-mass fermions

We present the first lattice Nf=2+1+1 determination of the tensor form factor $f_T^{D π(K)}(q^2)$ corresponding to the semileptonic and rare $D \to π(K)$ decays as a function of the squared 4-momentum transfer $q^2$. Together with our recent determination of the vector and scalar form factors we complete the set of hadronic matrix elements regulating the semileptonic and rare $D \to π(K)$ transitions within and beyond the Standard Model, when a non-zero tensor coupling is possible. Our analysis is based on the gauge configurations produced by ETMC with Nf=2+1+1 flavors of dynamical quarks, which include in the sea, besides two light mass-degenerate quarks, also the strange and charm quarks with masses close to their physical values. We simulated at three different values of the lattice spacing and with pion masses as small as 220 MeV. The matrix elements of the tensor current are determined for plenty of kinematical conditions in which parent and child mesons are either moving or at rest. As in the case of the vector and scalar form factors, Lorentz symmetry breaking due to hypercubic effects is clearly observed also in the data for the tensor form factor 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 and infinite volume limits we determine the tensor form factor in the whole kinematical region accessible in the experiments. A set of synthetic data points, representing our results for $f_T^{D π(K)}(q^2)$ for several selected values of $q^2$, is provided and the corresponding covariance matrix is also available. At zero four-momentum transfer we get $f_T^{D π}(0) = 0.506 (79)$ and $f_T^{D K}(0) = 0.687 (54)$, which correspond to $f_T^{D π}(0)/f_+^{D π}(0) = 0.827 (114)$ and $f_T^{D K}(0)/f_+^{D K}(0)= 0.898 (50)$.

hep-lat↗

arXiv2018

Extraction of $|V_{cd}|$ and $|V_{cs}|$ from experimental decay rates using lattice QCD $D \to π(K) \ell ν$ form factors

We present a determination of the Cabibbo-Kobayashi-Maskawa matrix elements $|V_{cd}|$ and $|V_{cs}|$ obtained by combining the momentum dependence of the semileptonic vector form factors $f_+^{D \to π}(q^2)$ and $f_+^{D \to K}(q^2)$, recently determined from lattice QCD simulations, with the differential rates measured for the semileptonic $D \to π\ell ν$ and $D \to K \ell ν$ decays. Our analysis is based on the results for the semileptonic form factors produced by the European Twisted Mass Collaboration with $N_f = 2 + 1 + 1$ flavors of dynamical quarks in the whole range of values of the squared 4-momentum transfer accessible in the experiments. The statistical and systematic correlations between the lattice data as well as those present in the experimental data are properly taken into account. With respect to the standard procedure based on the use of only the vector form factor at zero 4-momentum transfer, we obtain more precise and consistent results: $|V_{cd} |= 0.2341 ~ (74)$ and $|V_{cs} |= 0.970 ~ (33)$. The second-row CKM unitarity is fulfilled within the current uncertainties: $|V_{cd}|^2 + |V_{cs}|^2 + |V_{cb}|^2 = 0.996 ~ (64)$. Moreover, using for the first time hadronic inputs determined from first principles, we have calculated the ratio of the semileptonic $D \to π(K)$ decay rates into muons and electrons, which represent a test of lepton universality within the SM, obtaining in the isospin-symmetric limit of QCD: ${\cal{R}}_{LU}^{Dπ} = 0.985~(2)$ and ${\cal{R}}_{LU}^{DK} = 0.975~(1)$.

hep-lat↗

arXiv2018

$K \to π$ matrix elements of the chromomagnetic operator on the lattice

We present the results of the first lattice QCD calculation of the $K \to π$ matrix elements of the chromomagnetic operator $O_{CM} = g\, \bar s\, σ_{μν} G_{μν} d$, which appears in the effective Hamiltonian describing $ΔS = 1$ transitions in and beyond the Standard Model. Having dimension 5, the chromomagnetic operator is characterized by a rich pattern of mixing with operators of equal and lower dimensionality. The multiplicative renormalization factor as well as the mixing coefficients with the operators of equal dimension have been computed at one loop in perturbation theory. The power divergent coefficients controlling the mixing with operators of lower dimension have been determined non-perturbatively, by imposing suitable subtraction conditions. The numerical simulations have been carried out using the gauge field configurations produced by the European Twisted Mass Collaboration with $N_f = 2+1+1$ dynamical quarks at three values of the lattice spacing. Our result for the B-parameter of the chromomagnetic operator at the physical pion and kaon point is $B_{CMO}^{K π} = 0.273 ~ (70)$, while in the SU(3) chiral limit we obtain $B_{CMO} = 0.072 ~ (22)$. Our findings are significantly smaller than the model-dependent estimate $B_{CMO} \sim 1 - 4$, currently used in phenomenological analyses, and improve the uncertainty on this important phenomenological quantity.

hep-lat↗

arXiv2017

First lattice calculation of the QED corrections to leptonic decay rates

The leading-order electromagnetic and strong isospin-breaking corrections to the ratio of $K_{μ2}$ and $π_{μ2}$ decay rates are evaluated for the first time on the lattice, following a method recently proposed. The lattice results are obtained using the gauge ensembles produced by the European Twisted Mass Collaboration with $N_f = 2 + 1 + 1$ dynamical quarks. Systematics effects are evaluated and the impact of the quenched QED approximation is estimated. Our result for the correction to the tree-level $K_{μ2} / π_{μ2}$ decay ratio is $-1.22\,(16) \%$ to be compared to the estimate $-1.12\,(21) \%$ based on Chiral Perturbation Theory and adopted by the Particle Data Group.

hep-lat↗

arXiv2017

The pion vector form factor from Lattice QCD at the physical point

We present an investigation of the electromagnetic pion form factor, $F_π(Q^2)$, at small values of the four-momentum transfer $Q^2$ ($\lesssim 0.25$ GeV$^2$), based on the gauge configurations generated by European Twisted Mass Collaboration with $N_f = 2$ twisted-mass quarks at maximal twist including a clover term. Momentum is injected using non-periodic boundary conditions and the calculations are carried out at a fixed lattice spacing ($a \simeq 0.09$ fm) and with pion masses equal to its physical value, 240 MeV and 340 MeV. Our data are successfully analyzed using Chiral Perturbation Theory at next-to-leading order in the light-quark mass. For each pion mass two different lattice volumes are used to take care of finite size effects. Our final result for the squared charge radius is $\langle r^2 \rangle_π= 0.443~(29)$ fm$^2$, where the error includes several sources of systematic errors except the uncertainty related to discretization effects. The corresponding value of the SU(2) chiral low-energy constant $\overline{\ell}_6$ is equal to $\overline{\ell}_6 = 16.2 ~ (1.0)$.

hep-lat↗

arXiv2017

Leading isospin-breaking corrections to meson masses on the lattice

We present a study of the isospin-breaking (IB) corrections to pseudoscalar (PS) meson masses using the gauge configurations produced by the ETM Collaboration with $N_f=2+1+1$ dynamical quarks at three lattice spacings varying from 0.089 to 0.062 fm. Our method is based on a combined expansion of the path integral in powers of the small parameters $(\widehat{m}_d - \widehat{m}_u)/Λ_{QCD}$ and $α_{em}$, where $\widehat{m}_f$ is the renormalized quark mass and $α_{em}$ the renormalized fine structure constant. We obtain results for the pion, kaon and $D$-meson mass splitting; for the Dashen's theorem violation parameters $ε_γ(\overline{\mathrm{MS}}, 2~\mbox{GeV})$, $ε_{π^0}$, $ε_{K^0}(\overline{\mathrm{MS}}, 2~\mbox{GeV})$; for the light quark masses $(\widehat{m}_d - \widehat{m}_u)(\overline{\mathrm{MS}}, 2~\mbox{GeV})$, $(\widehat{m}_u / \widehat{m}_d)(\overline{\mathrm{MS}}, 2~\mbox{GeV})$; for the flavour symmetry breaking parameters $R(\overline{\mathrm{MS}}, 2~\mbox{GeV})$ and $Q(\overline{\mathrm{MS}}, 2~\mbox{GeV})$ and for the strong IB effects on the kaon decay constants.

hep-lat↗

arXiv2017

Strange and charm HVP contributions to the muon ($g - 2)$ including QED corrections with twisted-mass fermions

We present a lattice calculation of the Hadronic Vacuum Polarization (HVP) contribution of the strange and charm quarks to the anomalous magnetic moment of the muon including leading-order electromagnetic corrections. We employ the 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 ($a \simeq 0.062, 0.082, 0.089$ fm) with pion masses in the range $M_π\simeq 210 - 450$ MeV. The strange and charm quark masses are tuned at their physical values. Neglecting disconnected diagrams and after the extrapolations to the physical pion mass and to the continuum limit we obtain: $a_μ^s(α_{em}^2) = (53.1 \pm 2.5) \cdot 10^{-10}$, $a_μ^s(α_{em}^3) = (-0.018 \pm 0.011) \cdot 10^{-10}$ and $a_μ^c(α_{em}^2) = (14.75 \pm 0.56) \cdot 10^{-10}$, $a_μ^c(α_{em}^3) = (-0.030 \pm 0.013) \cdot 10^{-10}$ for the strange and charm contributions, respectively.

hep-lat↗

arXiv2017

HVP contributions to the muon ($g - 2$) including QED corrections with twisted-mass fermions

We present a lattice calculation of the Hadronic Vacuum Polarization (HVP) contribution of the strange and charm quarks to the anomalous magnetic moment of the muon including leading-order electromagnetic (e.m.) corrections. We employ the 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 ($a \simeq 0.062, 0.082, 0.089$ fm) with pion masses in the range $M_π\simeq 210 - 450$ MeV. The strange and charm quark masses are tuned at their physical values. Neglecting disconnected diagrams and after the extrapolations to the physical pion mass and to the continuum limit we obtain: $a_μ^s(α_{em}^2) = (53.1 \pm 2.5) \cdot 10^{-10}$, $a_μ^s(α_{em}^3) = (-0.018 \pm 0.011) \cdot 10^{-10}$ and $a_μ^c(α_{em}^2) = (14.75 \pm 0.56) \cdot 10^{-10}$, $a_μ^c(α_{em}^3) = (-0.030 \pm 0.013) \cdot 10^{-10}$ for the strange and charm contributions, respectively.

hep-lat↗

arXiv2017

Leading isospin-breaking corrections to pion, kaon and charmed-meson masses with Twisted-Mass fermions

We present a lattice computation of the isospin-breaking corrections to pseudoscalar meson masses using the gauge configurations produced by the European Twisted Mass collaboration with $N_f = 2 + 1 + 1$ dynamical quarks at three values of the lattice spacing ($a \simeq 0.062, 0.082$ and $0.089$ fm) with pion masses in the range $M_π\simeq 210 - 450$ MeV. The strange and charm quark masses are tuned at their physical values. We adopt the RM123 method based on the combined expansion of the path integral in powers of the $d$- and $u$-quark mass difference ($\widehat{m}_d - \widehat{m}_u$) and of the electromagnetic coupling $α_{em}$. Within the quenched QED approximation, which neglects the effects of the sea-quark charges, and after the extrapolations to the physical pion mass and to the continuum and infinite volume limits, we provide results for the pion, kaon and (for the first time) charmed-meson mass splittings, for the prescription-dependent parameters $ε_{π^0}$, $ε_γ(\overline{MS}, 2~\mbox{GeV})$, $ε_{K^0}(\overline{MS}, 2~\mbox{GeV})$, related to the violations of the Dashen's theorem, and for the light quark mass difference $(\widehat{m}_d - \widehat{m}_u)(\overline{MS}, 2~\mbox{GeV})$.

hep-lat↗

arXiv2017

Extraction of HQE parameters from unquenched lattice data on pseudoscalar and vector heavy-light meson masses

We present a precise lattice computation of pseudoscalar and vector heavy-light meson masses for heavy-quark masses ranging from the physical charm mass up to $\simeq 4$ times the physical b-quark mass. We employ the 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 ($a \simeq 0.062, 0.082, 0.089$ fm) with pion masses in the range $M_π\simeq 210 - 450$ MeV. The heavy-quark mass is simulated directly on the lattice up to $\simeq 3$ times the physical charm mass. The interpolation to the physical $b$-quark mass is performed using the ETMC ratio method, based on ratios of the meson masses computed at nearby heavy-quark masses, and adopting the kinetic mass scheme. The extrapolation to the physical pion mass and to the continuum limit yields $m_b^{\rm kin}(1~\mbox{GeV}) = 4.61 (20)$ GeV, which corresponds to $\overline{m}_b(\overline{m}_b) = 4.26 (18)$ GeV in the $\overline{MS}$ scheme. The lattice data are analyzed in terms of the Heavy Quark Expansion (HQE) and the matrix elements of dimension-4 and dimension-5 operators are extracted with a good precision, namely: $\overlineΛ = 0.552 (26)$ GeV, $μ_π^2 = 0.321 (32)$ GeV$^2$ and $μ_G^2(m_b) = 0.253 (25)$ GeV$^2$. The data also allow for a rough estimate of the dimension-6 operator matrix elements. As the HQE parameters play a crucial role in the inclusive determination of the Cabibbo-Kobayashi-Maskawa matrix elements $V_{ub}$ and $V_{cb}$, their precise determination on the lattice may eventually validate and improve the analyses based on fits to the semileptonic moments.

hep-lat↗

arXiv2017

$|V_{us}|$ from $K$ decays in theory

Leptonic and semileptonic kaon decays represent till now the golden modes for the extraction of the Cabibbo angle from experiments, provided the relevant hadronic quantities, namely the ratio of the kaon and pion leptonic decay constants, $f_K / f_π$, and the semileptonic vector form factor at zero four-momentum transfer, $f_+(0)$, are computed accurately from QCD. In the last years, using large-scale lattice QCD simulations, the determination of both $f_K / f_π$ and $f_+(0)$ has reached an impressive level of precision ($\approx 0.3 \%$), which is expected to be further improved in the near future. However, at a permille level of precision both electromagnetic and strong isospin-breaking effects cannot be neglected anymore. In this contribution a new lattice strategy aiming at determining QCD+QED effects in the inclusive leptonic decay rates of charged pseudoscalar mesons is briefly illustrated, and the preliminary results obtained in the case of $π_{\ell 2}$ and $K_{\ell 2}$ decays are presented. As for the semileptonic $K_{\ell 3}$ decays, it is pointed out that the determination of the relevant form factors in the full kinematical range covered by the experiments is necessary for a more stringent test of the Standard Model. In this respect the perspectives based on the only existing results from the European Twisted Mass Collaboration (ETMC) are quite encouraging.

hep-lat↗

arXiv2016

Finite-Volume QED Corrections to Decay Amplitudes in Lattice QCD

We demonstrate that the leading and next-to-leading finite-volume effects in the evaluation of leptonic decay widths of pseudoscalar mesons at $O(α)$ are universal, i.e. they are independent of the structure of the meson. This is analogous to a similar result for the spectrum but with some fundamental differences, most notably the presence of infrared divergences in decay amplitudes. The leading non-universal, structure-dependent terms are of $O(1/L^2)$ (compared to the $O(1/L^3)$ leading non-universal corrections in the spectrum). We calculate the universal finite-volume effects, which requires an extension of previously developed techniques to include a dependence on an external three-momentum (in our case, the momentum of the final state lepton). The result can be included in the strategy proposed in Ref.\,\cite{Carrasco:2015xwa} for using lattice simulations to compute the decay widths at $O(α)$, with the remaining finite-volume effects starting at order $O(1/L^2)$. The methods developed in this paper can be generalised to other decay processes, most notably to semileptonic decays, and hence open the possibility of a new era in precision flavour physics.

hep-lat↗

arXiv2016

Masses and decay constants of $D_{(s)}^$ and $B_{(s)}^$ mesons in Lattice QCD with $N_f = 2 + 1 + 1$ twisted-mass fermions

We present a lattice calculation of the decay constants and masses of $D_{(s)}^*$ and $B_{(s)}^*$ mesons using the gauge configurations produced by the European Twisted Mass Collaboration (ETMC) with $N_f = 2 + 1 + 1$ dynamical quarks and at three values of the lattice spacing $a \sim 0.06 - 0.09$ fm. Pion masses are simulated in the range $m_π \sim 210 - 450$ MeV, while the strange and charm quark masses are close to their physical values. We computed the ratios of vector to pseudoscalar decay constants or masses for various values of the heavy-quark mass $m_h$ in the range $0.7 m_c^{phys} \lesssim m_h \lesssim 3 m_c^{phys}$. In order to reach the physical b-quark mass, we exploited the HQET prediction that, in the static limit of infinite heavy-quark mass, all the considered ratios are equal to one. We obtain: $ f_{D^*}/f_{D} = 1.078(36),$ $m_{D^*}/m_{D} = 1.0769(79)$, $f_{D^*_{s}}/f_{D_{s}} = 1.087(20)$, $m_{D^*_{s}}m_{D_{s}} = 1.0751(56)$, $f_{B^*}/f_{B} = 0.958(22)$, $m_{B^*}/m_{B} = 1.0078(15)$, $f_{B^*_{s}}/f_{B_{s}} = 0.974(10)$ and $m_{B^*_{s}}/m_{B_{s}} = 1.0083(10)$. Combining them with the corresponding experimental masses from the PDG and the pseudoscalar decay constants calculated by ETMC, we get: $f_{D^*} = 223.5(8.4)~\mathrm{MeV}$, $m_{D^*} = 2013(14)~\mathrm{MeV}$, $f_{D^*_{s}} = 268.8(6.6)~\mathrm{MeV}$, $ m_{D^*_{s}} = 2116(11)~\mathrm{MeV}$, $f_{B^*} = 185.9(7.2)~\mathrm{MeV}$, $m_{B^*} = 5320.5(7.6)~\mathrm{MeV}$, $f_{B^*_{s}} = 223.1(5.4)~\mathrm{MeV}$ and $m_{B^*_{s}}= 5411.36(5.3)~\mathrm{MeV}$.

hep-lat↗

arXiv2016

Hypercubic Effects in semileptonic $D \to π$ decays on the lattice

We present a lattice determination of the vector and scalar form factors of the semileptonic $D \to π\ell ν$ decays, which are relevant for the extraction of the CKM matrix element $\lvert V_{cd} \rvert$ 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. Quark momenta are injected on the lattice using non-periodic boundary conditions. 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. Our preliminary estimate for the vector form factor at zero 4-momentum transfer is $f_+^{D \to π}(0) = 0.631\,(40)$, which can be compared with the latest FLAG average $f_+^{D \to π}(0) = 0.666\,(29)$ available only at $N_f = 2 + 1$.

hep-lat↗

arXiv2016

Electromagnetic corrections to the leptonic decay rates of charged pseudoscalar mesons: lattice results

Electromagnetic effects in the leptonic decay rates $π^+ \to μ^+ ν$ and $K^+ \to μ^+ ν$ are evaluated for the first time on the lattice. Following a method recently proposed in Ref. [1] the emission of virtual photons at leading order in the electromagnetic coupling is evaluated on the lattice and the infrared divergence computed for a point-like meson at finite lattice volume is subtracted. The physical decay rate is then obtained by adding the emission of real and virtual photons regularised with a photon mass. Using the gauge ensembles produced by the European Twisted Mass Collaboration with $N_f = 2 + 1 + 1$ dynamical quarks the feasibility of our approach is demonstrated. Preliminary results for the electromagnetic corrections to charged (neutral) pion and kaon masses as well as to the leptonic decay rates of charged pions and kaons are presented.

hep-lat↗

arXiv2016

Review of lattice results concerning low-energy particle physics

We review lattice results related to pion, kaon, D- and B-meson physics with the aim of making them easily accessible to the particle physics community. More specifically, we report on the determination of the light-quark masses, the form factor f+(0), arising in the semileptonic K -> pi transition at zero momentum transfer, as well as the decay constant ratio fK/fpi and its consequences for the CKM matrix elements Vus and Vud. Furthermore, we describe the results obtained on the lattice for some of the low-energy constants of SU(2)LxSU(2)R and SU(3)LxSU(3)R Chiral Perturbation Theory. We review the determination of the BK parameter of neutral kaon mixing as well as the additional four B parameters that arise in theories of physics beyond the Standard Model. The latter quantities are an addition compared to the previous review. For the heavy-quark sector, we provide results for mc and mb (also new compared to the previous review), as well as those for D- and B-meson decay constants, form factors, and mixing parameters. These are the heavy-quark quantities most relevant for the determination of CKM matrix elements and the global CKM unitarity-triangle fit. Finally, we review the status of lattice determinations of the strong coupling constant alpha_s.

hep-lat↗

arXiv2016

Mass of the b-quark and B-decay constants from Nf=2+1+1 twisted-mass Lattice QCD

We present precise lattice computations for the b-quark mass, the quark mass ratios mb/mc and mb/ms as well as the leptonic B-decay constants. We employ gauge configurations with four dynamical quark flavors, up/down, strange and charm, at three values of the lattice spacing (a ~ 0.06 - 0.09 fm) and for pion masses as low as 210 MeV. Interpolation in the heavy quark mass to the bottom quark point is performed using ratios of physical quantities computed at nearby quark masses exploiting the fact that these ratios are exactly known in the static quark mass limit. Our results are also extrapolated to the physical pion mass and to the continuum limit and read: mb(MSbar, mb) = 4.26(10) GeV, mb/mc = 4.42(8), mb/ms = 51.4(1.4), fBs = 229(5) MeV, fB = 193(6) MeV, fBs/fB = 1.184(25) and (fBs/fB)/(fK/fpi) = 0.997(17).

hep-lat↗