1,721,179 research outputs found
Electromagnetic Form Factor of the Pion from Twisted-mass Lattice QCD at Nf=2
No full textWe present a lattice calculation of the electromagnetic form factor of the pion obtained using the tree-level Symanzik improved gauge action with two flavors of dynamical twisted Wilson quarks. The simulated pion masses range approximately from 260 to 580 MeV, and the lattice box sizes are chosen in order to guarantee that Mpi*L>~4. Accurate results for the form factor are obtained using all-to-all quark propagators evaluated by a stochastic procedure. The momentum dependence of the pion form factor is investigated up to values of the squared four-momentum transfer Q^2~=0.8 GeV^2 and, thanks to the use of twisted boundary conditions, down to Q^2~=0.05 GeV^2. Volume and discretization effects on the form factor appear to be within the statistical errors. Our results for the pion mass, decay constant and form factor are analyzed using (continuum) chiral perturbation theory at next-to-next-to-leading order. The extrapolated value of the pion charge radius is <r2>_phys=0.456±0.030(stat)±0.024(syst) in nice agreement with the experimental result. The extrapolated values of the pion form factor agree very well with the experimental data up to Q^2~=0.8 GeV^2 within uncertainties which become competitive with the experimental errors for Q^2>~0.3 GeV^2. The relevant low-energy constants appearing in the chiral expansion of the pion form factor are extracted from our lattice data, which come essentially from a single lattice spacing, adding the experimental value of the pion scalar radius in the fitting procedure. Our findings are in nice agreement with the available results of chiral perturbation theory analyses of pi-pi scattering data as well as with other analyses of our collaboration
Neutron electric dipole moment on the lattice: a theoretical reappraisal
No full textWe present a strategy for a lattice evaluation of the neutron electric dipole moment induced by the strong CP violating term of the QCD lagrangian. Our strategy is based on the standard definition of the electric dipole moment, involving the charge density operator J(0), in case of three flavors with non-degenerate masses. We present a complete diagrammatic analysis showing how the axial chiral Ward identities can be used to replace the topological charge operator with the flavor-singlet pseudoscalar density P-S. Our final result is characterized only by disconnected diagrams, where the disconnected part can be either the single insertion of P-S or the separate insertions of both P-S and J(0). The applicability of our strategy to the case of lattice formulations that explicitly break chiral symmetry, like the Wilson and Clover actions, is discussed
Light-quark contribution to the leading hadronic vacuum polarization term of the muon g-2 from twisted-mass fermions
No full textWe present a lattice calculation of the leading hadronic vacuum polarization (HVP) contribution of the light u- and d-quarks to the anomalous magnetic moment of the muon, aμHVP(ud), adopting the gauge configurations generated by the European Twisted Mass Collaboration (ETMC) with Nf=2+1+1 dynamical quarks at three values of the lattice spacing (a≃0.062,0.082,0.089 fm) with pion masses in the range Mπ≃210-450 MeV. Thanks to several lattices at fixed values of the light-quark mass and scale but with different sizes we perform a careful investigation of finite-volume effects (FVEs), which represent one of main source of uncertainty in modern lattice calculations of aμHVP(ud). In order to remove FVEs we develop an analytic representation of the vector correlator, which describes the lattice data for time distances larger than ≃0.2 fm. The representation is based on quark-hadron duality at small and intermediate time distances and on the two-pion contributions in a finite box at larger time distances. After removing FVEs we extrapolate the corrected lattice data to the physical pion point and to the continuum limit taking into account the chiral logs predicted by Chiral Perturbation Theory (ChPT). We obtain aμHVP(ud)=619.0(17.8)×10-10. Adding the contribution of strange and charm quarks, obtained by ETMC, and an estimate of the isospin-breaking corrections and quark-disconnected diagrams from the literature we get aμHVP(udsc)=683(19)×10-10, which is consistent with recent results based on dispersive analyses of the experimental cross section data for e+e- annihilation into hadrons. Using our analytic representation of the vector correlator, taken at the physical pion mass in the continuum and infinite volume limits, we provide the first eleven moments of the polarization function and we compare them with recent results of the dispersive analysis of the π+π- channels. We estimate also the light-quark contribution to the missing part of aμHVP not covered in the MUonE experiment
Masses and decay constants of D*(s) and B*(s) mesons with Nf=2+1+1 twisted mass fermions
No full textWe present a lattice calculation of the masses and decay constants of D*(s) and B*(s) mesons using the gauge configurations produced by the European Twisted Mass Collaboration (ETMC) with Nf=2+1+1 dynamical quarks at three values of the lattice spacing a~(0.06-0.09) fm. Pion masses are simulated in the range Mpi=(210-450) MeV, while the strange and charm sea-quark masses are close to their physical values. We compute the ratios of vector to pseudoscalar masses and decay constants for various values of the heavy-quark mass mh in the range 0.7mcphysmh3mcphys. In order to reach the physical b-quark mass, we exploit the heavy quark effective theory prediction that, in the static limit of infinite heavy-quark mass, the considered ratios are equal to one. At the physical point our results are MD*/MD=1.0769(79), MD*sMDs=1.0751(56), fD*/fD=1.078(36), fD*s/fDs=1.087(20), MB*/MB=1.0078(15), MB*s/MBs=1.0083(10), fB*/fB=0.958(22) and fB*s/fBs=0.974(10). Combining them with the experimental values of the pseudoscalar meson masses (used as input to fix the quark masses) and the values of the pseudoscalar decay constants calculated by ETMC, we get MD*=2013(14), MD*s=2116(11), fD*=223.5(8.4), fD*s=268.8(6.6), MB*=5320.5(7.6), MB*s=5411.36(5.3), fB*=185.9(7.2) and fB*s=223.1(5.4) MeV
K ->pi l nu semileptonic form factors from two-flavor lattice QCD
Full text linkWe present new lattice results of the K ->pi l nu semileptonic form factors obtained from simulations with two flavors of dynamicaltwisted-mass fermions, using pion masses as light as 260 MeV. Our main result is f(+)(0)=0.9560(84), which, combined with the latest experimental data for Kl3 decays, leads to |Vus|=0.2267(5)(exp)(20)(f+(0)). Using the PDG(2008) determinations of|Vud| and |Vub| our result implies for the unitarity relation |Vud|^2+|Vus|^2+|Vub|^2=1.0004(15). For the O(p^6) term of the chiral expansion of f+(0) we get Delta f = f+(0)-1-f2=-0.0214(84)
Electromagnetic and strong isospin-breaking corrections to the muon g-2 from lattice QCD+QED
Full text linkWe present a lattice calculation of the leading-order electromagnetic and strong isospin-breaking corrections to the hadronic vacuum polarization (HVP) contribution to the anomalous magnetic moment of the muon. We employ the gauge configurations generated by the European Twisted Mass Collaboration with Nf=2+1+1 dynamical quarks at three values of the lattice spacing (a≃0.062,0.082,0.089 fm) with pion masses between ≃210 and ≃450 MeV. The results are obtained by adopting the RM123 approach in the quenched-QED approximation, which neglects the charges of the sea quarks. Quark disconnected diagrams are not included. After the extrapolations to the physical pion mass and to the continuum and infinite-volume limits the contributions of the light, strange, and charm quarks are, respectively, equal to δaμHVP(ud)=7.1(2.5)×10-10, δaμHVP(s)=-0.0053(33)×10-10, and δaμHVP(c)=0.0182(36)×10-10. At leading order in αem and (md-mu)/ΛQCD we obtain δaμHVP(udsc)=7.1(2.9)×10-10, which is currently the most accurate determination of the isospin-breaking corrections to aμHVP
Tensor form factor of D →π (K) ν and D →π (K) decays with Nf=2+1+1 twisted-mass fermions
No full textWe present the first lattice Nf=2+1+1 determination of the tensor form factor fTDπ(K)(q2) corresponding to the semileptonic D→π(K) nu; and rare D→π(K) decays as a function of the squared four-momentum transfer q2. Together with our recent determination of the vector f+Dπ(K)(q2) and scalar f0Dπ(K)(q2) form factors, we complete the set of hadronic matrix elements regulating the semileptonic D→π(K) ν and rare D→π(K) transitions within and beyond the standard model, when a nonzero tensor coupling is possible. Our analysis is based on the gauge configurations produced by the European Twisted Mass Collaboration 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 and with the valence heavy quark in the mass range from ≃0.7mcphys to ≃1.2mcphys. The matrix elements 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 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 from q2=0 up to qmax2=(MD-Mπ(K))2 accessible in the experiments. A set of synthetic data points, representing our results for fTDπ(K)(q2) for several selected values of q2, is provided and the corresponding covariance matrix is also available. At zero four-momentum transfer, we get fTDπ(0)=0.506 (79) and fTDK(0)=0.687 (54), which correspond to fTDπ(0)/f+Dπ(0)=0.827 (114) and fTDK(0)/f+DK(0)=0.898 (50)
Improved analysis of the scalar and vector form factors of kaon semileptonic decays with Nf = 2 twisted-mass fermions
No full textWe investigate the vector and scalar form factors relevant for Kl3 semileptonic decays using maximally twisted-mass fermions with two flavors of dynamical quarks (Nf = 2). The simulations cover pion masses as light as 260 MeV and four values of the lattice spacing, ranging from ~0.05 up to ~0.1 fm, which allow to compute directly, for the first time, the continuum limit for the vector form factor at zero-momentum transfer, f_+(0). The preliminary result is f_+(0) = 0.9544(68), where the error is statistical only. We also extrapolate both form factors to the physical point and study their momentum dependence. Our results are in good agreement with those obtained from a dispersion analyses of the experimental data. Together with the form factors, we analyze the ratio of the leptonic decay constants f_K / f_pi, by imposing the constraint coming from the Callan-Treiman theorem, obtaining at the physical point f_K / f_pi = 1.190(8). Combining our results for f_+(0) and f_K / f_pi with the experimental measurements of the leptonic and semilpetonic decay rates, and using the determination of |V_ud| from nuclear beta decays, we determine the values of the Cabibbo angle |V_us| from both Kl3 and Kl2 decays, obtaining |V_us|^{Kl3} = 0.2266(17) and $|V_us|^{Kl2} = 0.2258(16)
Pseudoscalar meson decay constants fK, fD and fDs, from Nf = 2 twisted mass Lattice QCD
No full textWe present the results of a lattice QCD calculation of the pseudoscalar meson decay constants f_K, f_D and f_Ds, performed with N_f=2 dynamical fermions. The simulation is carried out with the tree-level improved Symanzik gauge action and with the twisted mass fermionic action at maximal twist. With respect to our previous study (0709.4574 [hep-lat]), here we have analysed data at three values of the lattice spacing (a=0.10 fm, 0.09 fm, 0.07 fm) and performed the continuum limit, and we have included at a=0.09 fm data with a lighter quark mass (m_pi = 260 MeV) and a larger volume (L = 2.7 fm), thus having at each lattice spacing L >= 2.4 fm and m_pi*L >= 3.6. Our result for the kaon decay constant is f_K=(157.5 +- 0.8|_{stat.} +- 3.3|_{syst.}) MeV and for the ratio f_K/f_pi=1.205 +- 0.006|_{stat.} +- 0.025|_{syst.}, in good agreement with the other N_f=2 and N_f=2+1 lattice calculations. For the D and D_s meson decay constants we obtain f_D=(205 +- 7|_{stat.} +- 7|_{syst.}) MeV, in good agreement with the CLEO-c experimental measurement and with other recent N_f=2 and N_f=2+1 lattice calculations, and f_{Ds}=(248 +- 3|_{stat.} +- 8|_{syst.}) MeV that, instead, is 2.3 sigma below the CLEO-c/BABAR experimental average, confirming the present tension between lattice calculations and experimental measurements
- …
