arXiv subjects

Stefan Meinel

Publications and source records attributed to Stefan Meinel.

At least 91 records · Page 5Linked to original sources

arXiv2014

Lattice QCD calculation of form factors describing the rare decays $B \to K^* \ell^+ \ell^-$ and $B_s \to ϕ\ell^+ \ell^-$

The rare decays $B^0 \to K^{*0} μ^+ μ^-$ and $B_s \to ϕμ^+ μ^-$ are now being observed with enough precision to test Standard Model predictions. A full understanding of these decays requires accurate determinations of the corresponding hadronic form factors. Here we present results of lattice QCD calculations of the $B \to K^*$ and $B_s \to ϕ$ form factors; we also determine the form factors relevant for the tree-level decays $B_s \to K^* \ell ν$. We use full-QCD configurations including 2+1 flavors of sea quarks using an improved staggered action, and we employ lattice non-relativistic QCD to describe the bottom quark.

hep-lat↗

arXiv2013

Nucleon form factors with light Wilson quarks

We present nucleon observables - primarily isovector vector form factors - from calculations using 2+1 flavors of Wilson quarks. One ensemble is used for a dedicated high-precision study of excited-state effects using five source-sink separations between 0.7 and 1.6 fm. We also present results from a larger set of calculations that include an ensemble with pion mass 149 MeV and box size 5.6 fm, which nearly eliminates the uncertainty associated with extrapolation to the physical pion mass. The results show agreement with experiment for the vector form factors, which occurs only when excited-state contributions are reduced. Finally, we show results from a subset of ensembles that have pion mass 254 MeV with varying temporal and spatial box sizes, which we use for a controlled study of finite-volume effects and a test of the "$m_πL=4$" rule of thumb.

hep-lat↗

arXiv2013

$Λ_b \to p l^- \barν$ form factors from lattice QCD with static b quarks

We present a lattice QCD calculation of form factors for the decay $Λ_b \to p μ^- \barν$, which is a promising channel for determining the CKM matrix element $|V_{ub}|$ at the Large Hadron Collider. In this initial study we work in the limit of static b quarks, where the number of independent form factors reduces to two. We use dynamical domain-wall fermions for the light quarks, and perform the calculation at two different lattice spacings and at multiple values of the light-quark masses in a single large volume. Using our form factor results, we calculate the $Λ_b \to p μ^- \barν$ differential decay rate in the range $14 GeV^2 \leq q^2 \leq q^2_{max}$, and obtain the integral $\int_{14 GeV^2}^{q^2_{max}} [dΓ/dq^2] dq^2 / |V_{ub}|^2 = 15.3 \pm 4.2 ps^{-1}$. Combined with future experimental data, this will give a novel determination of $|V_{ub}|$ with about 15\% theoretical uncertainty. The uncertainty is dominated by the use of the static approximation for the b quark, and can be reduced further by performing the lattice calculation with a more sophisticated heavy-quark action.

hep-lat↗

arXiv2013

Omega_bbb excited-state spectroscopy from lattice QCD

Triply heavy baryons are very interesting systems analogous to heavy quarkonia, but are difficult to access experimentally. Lattice QCD can provide precise predictions for these systems, which can be compared to other theoretical approaches. In this work, the spectrum of excited states of the Omega_bbb baryon is calculated using lattice NRQCD for the b quarks, and using a domain-wall action for the u, d and s sea quarks. The calculations are done for multiple values of the sea-quark masses, and for two different lattice spacings. The energies of states with angular momentum up to J=7/2 are calculated, and the effects of rotational symmetry breaking by the lattice are analyzed. Precise results are obtained even for the small spin-dependent energy splittings, and the contributions of individual NRQCD interactions to these energy splittings are studied. The results are compared to potential-model calculations.

hep-lat↗

arXiv2012

Form factors for Lambda_b -> Lambda transitions from lattice QCD

The rare baryonic decays $Λ_b \to Λμ^+ μ^-$ and $Λ_b \to Λγ$ can complement rare $B$ meson decays in constraining models of new physics. In this work, we calculate the relevant $Λ_b \to Λ$ transition form factors at leading order in heavy-quark effective theory using lattice QCD. Our analysis is based on RBC/UKQCD gauge field ensembles with 2+1 flavors of domain-wall fermions, and with lattice spacings of $a\approx 0.11$ fm and $a\approx 0.08$ fm. We compute appropriate ratios of three-point and two-point correlation functions for a wide range of source-sink separations, and extrapolate to infinite separation in order to eliminate excited-state contamination. We then extrapolate the form factors to the continuum limit and to the physical values of the light-quark masses.

hep-lat↗

arXiv2012

Lambda_b -> Lambda l+ l- form factors and differential branching fraction from lattice QCD

We present the first lattice QCD determination of the $Λ_b \to Λ$ transition form factors that govern the rare baryonic decays $Λ_b \to Λl^+ l^-$ at leading order in heavy-quark effective theory. Our calculations are performed with 2+1 flavors of domain-wall fermions, at two lattice spacings and with pion masses down to 227 MeV. Three-point functions with a wide range of source-sink separations are used to extract the ground-state contributions. The form factors are extrapolated to the physical values of the light-quark masses and to the continuum limit. We use our results to calculate the differential branching fractions for $Λ_b \to Λl^+ l^-$ with $l=e,μ,τ$ within the standard model. We find agreement with a recent CDF measurement of the $Λ_b \to Λμ^+ μ^-$ differential branching fraction.

hep-lat↗

arXiv2012

Calculation of the heavy-hadron axial couplings g_1, g_2, and g_3 using lattice QCD

In a recent letter [Phys. Rev. Lett. 108, 172003 (2012), arXiv:1109.2480] we have reported on a lattice QCD calculation of the heavy-hadron axial couplings $g_1$, $g_2$, and $g_3$. These quantities are low-energy constants of heavy-hadron chiral perturbation theory (HH$χ$PT) and are related to the $B^*Bπ$, $Σ_b^*Σ_bπ$, and $Σ_b^{(*)}Λ_bπ$ couplings. In the following, we discuss important details of the calculation and give further results. To determine the axial couplings, we explicitly match the matrix elements of the axial current in QCD with the corresponding matrix elements in HH$χ$PT. We construct the ratios of correlation functions used to calculate the matrix elements in lattice QCD, and study the contributions from excited states. We present the complete numerical results and discuss the data analysis in depth. In particular, we demonstrate the convergence of $SU(4|2)$ HH$χ$PT for the axial-current matrix elements at pion masses up to about 400 MeV and show the impact of the nonanalytic loop contributions. Finally, we present additional predictions for strong and radiative decay widths of charm and bottom baryons.

hep-lat↗

arXiv2012

Excited-state spectroscopy of triply-bottom baryons from lattice QCD

The spectrum of baryons containing three b quarks is calculated in nonperturbative QCD, using the lattice regularization. The energies of ten excited bbb states with J^P = 1/2^+, 3/2^+, 5/2^+, 7/2^+, 1/2^-, and 3/2^- are determined with high precision. A domain-wall action is used for the up-, down- and strange quarks, and the bottom quarks are implemented with NRQCD. The computations are done at lattice spacings of a \approx 0.11 fm and a \approx 0.08 fm, and the results demonstrate the improvement of rotational symmetry as a is reduced. A large lattice volume of (2.7 fm)^3 is used, and extrapolations of the bbb spectrum to realistic values of the light sea-quark masses are performed. All spin-dependent energy splittings are resolved with total uncertainties of order 1 MeV, and the dependence of these splittings on the couplings in the NRQCD action is analyzed.

hep-lat↗

arXiv2012

Axial couplings and strong decay widths of heavy hadrons

We calculate the axial couplings of mesons and baryons containing a heavy quark in the static limit using lattice QCD. These couplings determine the leading interactions in heavy hadron chiral perturbation theory and are central quantities in heavy quark physics, as they control strong decay widths and the light-quark mass dependence of heavy hadron observables. Our analysis makes use of lattice data at six different pion masses, 227 MeV < m_π< 352 MeV, two lattice spacings, a=0.085, 0.112 fm, and a volume of (2.7 fm)^3. Our results for the axial couplings are g_1=0.449(51), g_2=0.84(20), and g_3=0.71(13), where g_1 governs the interaction between heavy-light mesons and pions and g_{2,3} are similar couplings between heavy-light baryons and pions. Using our lattice result for g_3, and constraining 1/m_Q corrections in the strong decay widths with experimental data for Σ_c^{(*)} decays, we obtain Γ[Σ_b^{(*)} \to Λ_b π^\pm] = 4.2(1.0), 4.8(1.1), 7.3(1.6), 7.8(1.8) MeV for the Σ_b^+, Σ_b^-, Σ_b^{*+}, Σ_b^{*-} initial states, respectively. We also derive upper bounds on the widths of the Ξ_b^{\prime(*)} baryons.

hep-lat↗

arXiv2012

Axial couplings of heavy hadrons from domain-wall lattice QCD

We calculate matrix elements of the axial current for static-light mesons and baryons in lattice QCD with dynamical domain wall fermions. We use partially quenched heavy hadron chiral perturbation theory in a finite volume to extract the axial couplings g_1, g_2, and g_3 from the data. These axial couplings allow the prediction of strong decay rates and enter chiral extrapolations of most lattice results in the b sector. Our calculations are performed with two lattice spacings and with pion masses down to 227 MeV.

hep-lat↗

arXiv2011

Axial couplings in heavy hadron chiral perturbation theory at the next-to-leading order

We present calculations of axial-current matrix elements between various heavy-meson and heavy-baryon states to the next-to-leading order in heavy hadron chiral perturbation theory in the p-regime. When compared with data from lattice computations or experiments, these results can be used to determine the axial couplings in the chiral Lagrangian. Our calculation is performed in partially-quenched chiral perturbation theory for both SU(4|2) and SU(6|3). We incorporate finite-size effects arising from a single Goldstone meson wrapping around the spatial volume. Results for full QCD with two and three flavours can be obtained straightforwardly by taking the sea-quark masses to be equal to the valence-quark masses. To illustrate the impact of our chiral perturbation theory calculation on lattice computations, we analyse the SU(2) full QCD results in detail. We also study one-loop contributions relevant to the heavy hadron strong-decay amplitudes involving final-state Goldstone bosons, and demonstrate that the quark-mass dependence of these amplitudes can be significantly different from that of the axial current matrix elements containing only single hadron external states.

hep-lat↗

arXiv2011

A lattice calculation of B -> K(*) form factors

Lattice QCD can contribute to the search for new physics in b -> s decays by providing first-principle calculations of B -> K(*) form factors. Preliminary results are presented here which complement sum rule determinations by being done at large q^2 and which improve upon previous lattice calculations by working directly in the physical b sector on unquenched gauge field configurations.

hep-ph↗

arXiv2010

Bottomonium spectrum at order v^6 from domain-wall lattice QCD: precise results for hyperfine splittings

The bottomonium spectrum is computed in dynamical 2+1 flavor lattice QCD, using NRQCD for the b quarks. The main calculations in this work are based on gauge field ensembles generated by the RBC and UKQCD collaborations with the Iwasaki action for the gluons and a domain-wall action for the sea quarks. Lattice spacing values of approximately 0.08 fm and 0.11 fm are used, and simultaneous chiral extrapolations to the physical pion mass are performed. As a test for gluon discretization errors, the calculations are repeated on two ensembles generated by the MILC collaboration with the Luscher-Weisz gauge action. Gluon discretization errors are also studied in a lattice potential model using perturbation theory for four different gauge actions. The nonperturbative lattice QCD results for the radial and orbital bottomonium energy splittings obtained from the RBC/UKQCD ensembles are found to be in excellent agreement with experiment. To get accurate results for spin splittings, the spin-dependent order-v^6 terms are included in the NRQCD action, and suitable ratios are calculated such that most of the unknown radiative corrections cancel. The cancellation of radiative corrections is verified explicitly by repeating the calculations with different values of the couplings in the NRQCD action. Using the lattice ratios of the S-wave hyperfine and the 1P tensor splitting, and the experimental result for the 1P tensor splitting, the 1S hyperfine splitting is found to be 60.3+-5.5(stat)+-5.0(syst)+-2.1(exp) MeV, and the 2S hyperfine splitting is predicted to be 23.5+-4.1(stat)+-2.1(syst)+-0.8(exp) MeV.

hep-lat↗

arXiv2010

Prediction of the Omega_bbb mass from lattice QCD

The mass of the triply heavy baryon Omega_bbb is calculated in lattice QCD with 2+1 flavors of light sea quarks. The b quark is implemented with improved lattice NRQCD. Gauge field ensembles from both the RBC/UKQCD and MILC collaborations with lattice spacings in the range from 0.08 fm to 0.12 fm are used. The final result for the Omega_bbb mass, which includes an electrostatic correction, is 14.371+-0.004(stat)+-0.011(syst)+-0.001(exp) GeV. The hyperfine splitting between the physical J=3/2 state and a fictitious J=1/2 state is also calculated.

hep-lat↗

arXiv2010

Form factors for rare B decays: strategy, methodology, and numerical study

We investigate the combined use of moving NRQCD and stochastic sources in lattice calculations of form factors describing rare B and B_s decays. Moving NRQCD leads to a reduction of discretisation errors compared to standard NRQCD. Stochastic sources are tested for reduction of statistical errors.

hep-lat↗

arXiv2009

Bottom hadrons from lattice QCD with domain wall and NRQCD fermions

Dynamical 2+1 flavor lattice QCD is used to calculate the masses of bottom hadrons, including B mesons, singly and doubly bottom baryons, and for the first time also the triply-bottom baryon Omega_bbb. The domain wall action is used for the up-, down-, and strange quarks (both valence and sea), while the bottom quark is implemented with non-relativistic QCD. A calculation of the bottomonium spectrum is also presented.

hep-lat↗

arXiv2009

Radiative corrections to the m(oving)NRQCD action and heavy-light operators

Rare decays of B mesons, such as B \to K^*γand B\to K^{(*)}\ell^+\ell^- are loop suppressed in the Standard Model and sensitive to new physics. The final state meson in heavy-light decays at large recoil has sizeable momentum in the rest frame of the decaying meson. To reduce the resulting discretization errors we formulate the nonrelativistic heavy quark action in a moving frame. We discuss the perturbative renormalization of the leading order heavy-light operators in the resulting theory which is known as m(oving)NRQCD. We also present radiative corrections to the NRQCD action computed using automated lattice perturbation theory. By combining this technique with high-beta simulations in the weak coupling regime of the theory higher order loop corrections can be calculated very efficiently.

hep-lat↗

arXiv2009

The bottomonium spectrum from lattice QCD with 2+1 flavors of domain wall fermions

Recently, realistic lattice QCD calculations with 2+1 flavors of domain wall fermions and the Iwasaki gauge action have been performed by the RBC and UKQCD collaborations. Here, results for the bottomonium spectrum computed on their gauge configurations of size 24^3x64 with a lattice spacing of approximately 0.11 fm and four different values for the light quark mass are presented. Improved lattice NRQCD is used to treat the b quarks inside the bottomonium. The results for the radial and orbital energy splittings are found to be in good agreement with experimental measurements, indicating that systematic errors are small. The calculation of the Upsilon(2S)-Upsilon(1S) energy splitting provides an independent determination of the lattice spacing. For the most physical ensemble it is found to be a^{-1}=1.740(25)(19) GeV, where the first error is statistical/fitting and the second error is an estimate of the systematic errors due to the lattice NRQCD action.

hep-lat↗