We present a nonperturbative lattice calculation of the form factors which contribute to the amplitudes for the radiative decays P → ℓνℓγ, where P is a pseudoscalar meson and ℓ is a charged lepton. ...Together with the nonperturbative determination of the corrections to the processes P → ℓνℓ due to the exchange of a virtual photon, this allows accurate predictions at O(αem) to be made for leptonic decay rates for pseudoscalar mesons ranging from the pion to the Ds meson. We are able to separate unambiguously and nonpertubatively the pointlike contribution, from the structure-dependent, infrared-safe, terms in the amplitude. The fully nonperturbative O(a) improved calculation of the inclusive leptonic decay rates will lead to the determination of the corresponding Cabibbo-Kobayashi-Maskawa matrix elements also at O(αem). Prospects for a precise evaluation of leptonic decay rates with emission of a hard photon are also very interesting, especially for the decays of heavy D and B mesons for which currently only model-dependent predictions are available to compare with existing experimental data.
We present results on the light, strange and charm nucleon scalar and tensor charges from lattice QCD, using simulations with Nf=2 flavors of twisted mass clover-improved fermions with a physical ...value of the pion mass. Both connected and disconnected contributions are included, enabling us to extract the isoscalar, strange and charm charges for the first time directly at the physical point. Furthermore, the renormalization is computed nonperturbatively for both isovector and isoscalar quantities. We investigate excited state effects by analyzing several sink-source time separations and by employing a set of methods to probe ground state dominance. Our final results for the scalar charges are gSu=5.20(42)(15)(12), gSd=4.27(26)(15)(12), gSs=0.33(7)(1)(4), and gSc=0.062(13)(3)(5) and for the tensor charges gTu=0.794(16)(2)(13), gTd=−0.210(10)(2)(13), gTs=0.00032(24)(0), and gTc=0.00062(85)(0) in the MS¯ scheme at 2 GeV. The first error is statistical, the second is the systematic error due to the renormalization and the third the systematic arising from estimating the contamination due to the excited states, when our data are precise enough to probe the first excited state.
A
bstract
We present a lattice QCD computation of the
b
-quark mass, the
B
and
B
s
decay constants, the
B
-mixing bag prameters for the full four-fermion operator basis as well as determinations for
...ξ
and
f
Bq
extrapolated to the continuum limit and to the physical pion mass. We used
N
f
= 2 twisted mass Wilson fermions at four values of the lattice spacing with pion masses ranging from 280 to 500 MeV. Extrapolation in the heavy quark mass from the charm to the bottom quark region has been carried out on ratios of physical quantities computed at nearby quark masses, exploiting the fact that they have an exactly known infinite mass limit. Our results are
m
b
(
m
b
,
) = 4
.
29(12) GeV,
f
Bs
= 228(8) MeV,
f
B
= 189(8) MeV and
f
Bs
/f
B
= 1
.
206(24). Moreover with our results for the bag-parameters we find
ξ
= 1
.
225(31),
= 1
.
01(2),
f
Bd
= 216(10) MeV and 1
f
Bs
= 262(10) MeV. We also computed the bag parameters for the complete basis of the four-fermion operators which are required in beyond the SM theories. By using these results for the bag parameters we are able to provide a refined Unitarity Triangle analysis in the presence of New Physics, improving the bounds coming from
B
(
s
)
−
mixing.
We present a lattice QCD calculation of the up, down, strange and charm quark masses performed using the gauge configurations produced by the European Twisted Mass Collaboration with Nf=2+1+1 ...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. The simulations are based on a unitary setup for the two light quarks and on a mixed action approach for the strange and charm quarks. The analysis uses data at three values of the lattice spacing and pion masses in the range 210–450 MeV, allowing for accurate continuum limit and controlled chiral extrapolation. The quark mass renormalization is carried out non-perturbatively using the RI′-MOM method. The results for the quark masses converted to the MS¯ scheme are: mud(2 GeV)=3.70(17) MeV, ms(2 GeV)=99.6(4.3) MeV and mc(mc)=1.348(46) GeV. We obtain also the quark mass ratios ms/mud=26.66(32) and mc/ms=11.62(16). By studying the mass splitting between the neutral and charged kaons and using available lattice results for the electromagnetic contributions, we evaluate mu/md=0.470(56), leading to mu=2.36(24) MeV and md=5.03(26) MeV.
We present a determination of the ratio of kaon and pion leptonic decay constants in isosymmetric QCD (ISOQCD), fK/fπ, making use of the gauge ensembles produced by the Extended Twisted Mass ...Collaboration with Nf = 2 + 1 + 1 flavors of Wilson-clover twisted-mass quarks, including configurations close to the physical point for all dynamical flavors. The simulations are carried out at three values of the lattice spacing ranging from ∼ 0.068 to ∼ 0.092 fm with linear lattice size up to L ~ 5.5 fm. The scale is set by the particle data group (PDG) value of the pion decay constant, ... , at the ISOQCD pion point, ... , obtaining for the gradient-flow scales the values w 0 = 0.17383 (63) fm, ... and t0/w0 = 0.11969 (62) fm. The data are analyzed within the framework of SU(2) chiral perturbation theory without resorting to the use of renormalized quark masses. At the ISOQCD kaon point ... we get (fK/fπ) ISO QCD = 1.1995 (44), where the error includes both statistical and systematic uncertainties. Implications for the Cabibbo-Kobayashi-Maskawa matrix element |Vus| and for the first-row Cabibbo-Kobayashi-Maskawa unitarity are discussed.(ProQuest: … denotes formulae omitted.)
We present a comparison of existing experimental data for the radiative leptonic decays P → ℓνℓγ, where P = K or π and ℓ = e or μ, from the KLOE, PIBETA, E787, ISTRA+ and OKA collaborations with ...theoretical predictions based on the recent non-perturbative determinations of the structure-dependent vector and axial-vector form factors, FV and FA respectively. These were obtained using lattice QCD + QED simulations at order O(αem) in the electromagnetic coupling. We find good agreement with the KLOE data on K → eνeγ decays from which the form factor F+ = FV + FA can be determined. For K → μνμγ decays we observe differences of up to –34 standard deviations at large photon energies between the theoretical predictions and the data from the E787, ISTRA + and OKA experiments and similar discrepancies in some kinematical regions with the PIBETA experiment on radiative pion decays. A global study of all the kaon-decay data within the Standard Model results in a poor fit, largely because at large photon energies the KLOE and E787 data cannot be reproduced simultaneously in terms of the same form factor F+. The discrepancy between the theoretical and experimental values of the form factor F− = FV − FA is even more pronounced. These observations motivate future improvements of both the theoretical and experimental determinations of the structure-dependent form factors F+ and F−, as well as further theoretical investigations of models of "new physics" which might for example, include possible flavor changing interactions beyond V − A and/or nonuniversal corrections to the lepton couplings.
We present results for the moments of nucleon isovector vector and axial generalized parton distribution functions computed within lattice QCD. Three ensembles of maximally twisted mass ...clover-improved fermions simulated with a physical value of the pion mass are analyzed. Two of these ensembles are generated using two degenerate light quarks. A third ensemble is used having, in addition to the light quarks, strange and charm quarks in the sea. A careful analysis of the convergence to the ground state is carried out that is shown to be essential for extracting the correct nucleon matrix elements. This allows a controlled determination of the unpolarized, helicity, and tensor second Mellin moments. The vector and axial-vector generalized form factors are also computed as a function of the momentum transfer square up to about 1 GeV2. The three ensembles allow us to check for unquenching effects and to assess lattice finite volume effects.
We evaluate by means of lattice QCD calculations the low-energy constant ℓ7 which parametrizes strong isospin effects at next-to-leading order (NLO) in SU(2) chiral perturbation theory. Among all ...low-energy constants at NLO, ℓ7 is the one known less precisely, and its uncertainty is currently larger than 50%. Our strategy is based on the RM123 approach in which the lattice path-integral is expanded in powers of the isospin breaking parameter Δm = (md − mu)/2. In order to evaluate the relevant lattice correlators we make use of the recently proposed rotated twisted-mass (RTM) scheme. Within the RM123 approach, it is possible to cleanly extract the value of ℓ7 from either the pion mass splitting Mπ+ − Mπ0 induced by strong isospin breaking at order O((Δm)2) (mass method), or from the coupling of the neutral pion π 0 to the isoscalar operator .../... at order O ( Δ m ) (matrix element method). In this pilot study we limit the analysis to a single ensemble generated by the Extended Twisted Mass Collaboration (ETMC) with Nf = 2 + 1 + 1 dynamical quark flavors, which corresponds to a lattice spacing a ≃ 0.095 fm and to a pion mass Mπ ≃ 260 MeV. We find that the matrix element method outperforms the mass method in terms of resulting statistical accuracy. Our determination, ℓ7 = 2.5 (1.4) × 10−3, is in agreement and improves previous calculations.(ProQuest: … denotes formulae omitted.)
We present an investigation of the electromagnetic pion form factor, Fπ(Q2), at small values of the four-momentum transfer Q2 (≲0.25 GeV2), based on the gauge configurations generated by the ...European Twisted Mass Collaboration with Nf=2 twisted-mass quarks at maximal twist including a clover term. Momentum is injected using nonperiodic boundary conditions and the calculations are carried out at a fixed lattice spacing (a≃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 ⟨r2⟩π=0.443(29) fm2, 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 ℓ¯6 is equal to ℓ¯6=16.2(1.0).
We present results for the isospin-0 ππ s-wave scattering length calculated with Osterwalder-Seiler valence quarks on Wilson twisted mass gauge configurations. We use three Nf=2 ensembles with ...unitary (valence) pion mass at its physical value (250 MeV), at 240 MeV (320 MeV) and at 330 MeV (400 MeV), respectively. By using the stochastic Laplacian Heaviside quark smearing method, all quark propagation diagrams contributing to the isospin-0 ππ correlation function are computed with sufficient precision. The chiral extrapolation is performed to obtain the scattering length at the physical pion mass. Our result Mπa0I=0=0.198(9)(6) agrees reasonably well with various experimental measurements and theoretical predictions. Since we only use one lattice spacing, certain systematics uncertainties, especially those arising from unitary breaking, are not controlled in our result.
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