arXiv subjects

Stefan Meinel

Publications and source records attributed to Stefan Meinel.

At least 55 records · Page 3Linked to original sources

arXiv2021

$Λ_b \to Λ_c^*(2595,2625)\ell^-\barν$ form factors from lattice QCD

We present the first lattice-QCD determination of the form factors describing the semileptonic decays $Λ_b \to Λ_c^*(2595)\ell^-\barν$ and $Λ_b \to Λ_c^*(2625)\ell^-\barν$, where the $Λ_c^*(2595)$ and $Λ_c^*(2625)$ are the lightest charm baryons with $J^P=\frac12^-$ and $J^P=\frac32^-$, respectively. These decay modes provide new opportunities to test lepton flavor universality and also play an important role in global analyses of the strong interactions in $b\to c$ semileptonic decays. We determine the full set of vector, axial vector, and tensor form factors for both decays, but only in a small kinematic region near the zero-recoil point. The lattice calculation uses three different ensembles of gauge-field configurations with $2+1$ flavors of domain-wall fermions, and we perform extrapolations of the form factors to the continuum limit and physical pion mass. We present Standard-Model predictions for the differential decay rates and angular observables. In the kinematic region considered, the differential decay rate for the $\frac12^-$ final state is found to be approximately 2.5 times larger than the rate for the $\frac32^-$ final state. We also test the compatibility of our form-factor results with zero-recoil sum rules.

hep-lat↗

arXiv2021

P-wave nucleon-pion scattering amplitude in the $Δ(1232)$ channel from lattice QCD

We determine the $Δ(1232)$ resonance parameters using lattice QCD and the Lüscher method. The resonance occurs in elastic pion-nucleon scattering with $J^P=3/2^+$ in the isospin $I = 3/2$, $P$-wave channel. Our calculation is performed with $N_f=2+1$ flavors of clover fermions on a lattice with $L\approx 2.8$ fm. The pion and nucleon masses are $m_π=255.4(1.6)$ MeV and $m_N=1073(5)$ MeV, and the strong decay channel $Δ\rightarrow πN$ is found to be above the threshold. To thoroughly map out the energy-dependence of the nucleon-pion scattering amplitude, we compute the spectra in all relevant irreducible representations of the lattice symmetry groups for total momenta up to $\vec{P}=\frac{2π}{L}(1,1,1)$, including irreps that mix $S$ and $P$ waves. We perform global fits of the amplitude parameters to up to 21 energy levels, using a Breit-Wigner model for the $P$-wave phase shift and the effective-range expansion for the $S$-wave phase shift. From the location of the pole in the $P$-wave scattering amplitude, we obtain the resonance mass $m_Δ=1378(7)(9)$ MeV and the coupling $g_{Δ\text{-}πN}=23.8(2.7)(0.9)$.

hep-lat↗

arXiv2021

$Λ_b \to Λ^*(1520)\ell^+\ell^-$ form factors from lattice QCD

We present the first lattice QCD determination of the $Λ_b \to Λ^*(1520)$ vector, axial vector, and tensor form factors that are relevant for the rare decays $Λ_b \to Λ^*(1520)\ell^+\ell^-$. The lattice calculation is performed in the $Λ^*(1520)$ rest frame with nonzero $Λ_b$ momenta, and is limited to the high-$q^2$ region. An interpolating field with covariant derivatives is used to obtain good overlap with the $Λ^*(1520)$. The analysis treats the $Λ^*(1520)$ as a stable particle, which is expected to be a reasonable approximation for this narrow resonance. A domain-wall action is used for the light and strange quarks, while the $b$ quark is implemented with an anisotropic clover action with coefficients tuned to produce the correct $B_s$ kinetic mass, rest mass, and hyperfine splitting. We use three different ensembles of lattice gauge-field configurations generated by the RBC and UKQCD collaborations, and perform extrapolations of the form factors to the continuum limit and physical pion mass. We give Standard-Model predictions for the $Λ_b \to Λ^*(1520)\ell^+\ell^-$ differential branching fraction and angular observables in the high-$q^2$ region.

hep-lat↗

arXiv2021

$I=1/2$ $S$-wave and $P$-wave $Kπ$ scattering and the $κ$ and $K^*$ resonances from lattice QCD

We present a lattice-QCD determination of the elastic isospin-$1/2$ $S$-wave and $P$-wave $Kπ$ scattering amplitudes as a function of the center-of-mass energy using Lüscher's method. We perform global fits of $K$-matrix parametrizations to the finite-volume energy spectra for all irreducible representations with total momenta up to $\sqrt{3}\frac{2π}{L}$; this includes irreps that mix the $S$- and $P$-waves. Several different parametrizations for the energy dependence of the $K$-matrix are considered. We also determine the positions of the nearest poles in the scattering amplitudes, which correspond to the broad $κ$ resonance in the $S$-wave and the narrow $K^*(892)$ resonance in the $P$-wave. Our calculations are performed with $2+1$ dynamical clover fermions for two different pion masses of $317.2(2.2)$ and $175.9(1.8)$ MeV. Our preferred $S$-wave parametrization is based on a conformal map and includes an Adler zero; for the $P$-wave we use a standard pole parametrization including Blatt-Weisskopf barrier factors. The $S$-wave $κ$-resonance pole positions are found to be $\left[0.86(12) - 0.309(50)\,i\right]\:{\rm GeV}$ at the heavier pion mass and $\left[0.499(55)- 0.379(66)\,i\right]\:{\rm GeV}$ at the lighter pion mass. The $P$-wave $K^*$-resonance pole positions are found to be $\left[ 0.8951(64) - 0.00250(21)\,i \right]\:{\rm GeV}$ at the heavier pion mass and $\left[0.8718(82) - 0.0130(11)\,i\right]\:{\rm GeV}$ at the lighter pion mass, which corresponds to couplings of $g_{K^* Kπ}=5.02(26)$ and $g_{K^* Kπ}=4.99(22)$, respectively.

hep-lat↗

arXiv2020

Bethe-Salpeter amplitudes of Upsilons

Based on lattice non-relativistic QCD (NRQCD) studies we present results for Bethe-Salpeter amplitudes for $Υ(1S)$, $Υ(2S)$ and $Υ(3S)$ in vacuum as well as in quark-gluon plasma. Our study is based on 2+1 flavor $48^3 \times 12$ lattices generated using the Highly Improved Staggered Quark (HISQ) action and with a pion mass of $161$ MeV. At zero temperature the Bethe-Salpeter amplitudes follow the expectations based on non-relativistic potential models. At non-zero temperatures, the interpretation of Bethe-Salpeter amplitudes turns out to be more nuanced, but consistent with our previous lattice QCD study of excited Upsilons in quark-gluon plasma.

hep-lat↗

arXiv2020

Investigation of Doubly Heavy Tetraquark Systems using Lattice QCD

We search for possibly existent bound states in the heavy-light tetraquark channels with quark content $ \bar{b}\bar{b}ud $, $ \bar{b}\bar{b}us $ and $ \bar{b}\bar{c}ud $ using lattice QCD. We carry out calculations on several gauge link ensembles with $ N_f=2+1 $ flavours of domain-wall fermions and consider a basis of local and non-local interpolators. Besides extracting the energy spectrum from the correlation matrices, we also perform a Lüscher analysis to extrapolate our results to infinite volume.

hep-lat↗

arXiv2020

Quarkonium at non-zero isospin density

We calculate the energies of quarkonium bound states in the presence of a medium of nonzero isospin density using lattice QCD. The medium, created using a canonical (fixed isospin charge) approach, induces a reduction of the quarkonium energies. As the isospin density increases, the energy shifts first increase and then saturate. The saturation occurs at an isospin density close to that where previously a qualitative change in the behaviour of the energy density of the medium has been observed, which was conjectured to correspond to a transition from a pion gas to a Bose-Einstein condensed phase. The reduction of the quarkonium energies becomes more pronounced as the heavy-quark mass is decreased, similar to the behaviour seen in two-colour QCD at non-zero quark chemical potential. In the process of our analysis, the $η_b$-$π$ and $Υ$-$π$ scattering phase shifts are determined at low momentum. An interpolation of the scattering lengths to the physical pion mass gives $a_{η_b,π} = 0.0025(8)(6)$ fm and $a_{Υ,π} = 0.0030(9)(7)$ fm.

hep-lat↗

arXiv2020

Excited bottomonia in quark-gluon plasma from lattice QCD

We present the first lattice QCD study of up to $3S$ and $2P$ bottomonia at non-zero temperatures. Correlation functions of bottomonia were computed using novel bottomonium operators and a variational technique, within the lattice non-relativistic QCD framework. We analyzed the bottomonium correlation functions based on simple physically-motivated spectral functions. We found evidence of sequential in-medium modifications, in accordance with the sizes of the bottomonium states.

hep-lat↗

arXiv2020

Thermal Broadening of Bottomonia: Lattice Non-Relativistic QCD with Extended Operators

We present lattice non-relativistic QCD calculations of bottomonium correlation functions at temperatures $T \simeq 150-350$ MeV. The correlation functions were computed using extended bottomonium operators, and on background gauge-field configurations for 2+1-flavor QCD having physical kaon and nearly-physical pion masses. We analyzed these correlation functions based on simple theoretically-motivated parameterizations of the corresponding spectral functions. The results of our analyses are compatible with significant in-medium thermal broadening of the ground state S- and P-wave bottomonia.

hep-lat↗

arXiv2019

Bayesian analysis of $b\to sμ^+μ^-$ Wilson coefficients using the full angular distribution of $Λ_b\to Λ(\to p\, π^-)μ^+μ^-$ decays

Following updated and extended measurements of the full angular distribution of the decay $Λ_b\to Λ(\to p\,π^-)μ^+μ^-$ by the LHCb collaborations, as well as a new measurement of the $Λ\to p π^-$ decay asymmetry parameter by the BESIII collaboration, we study the impact of these results on searches for non-standard effects in exclusive $b\to sμ^+μ^-$ decays. To this end, we constrain the Wilson coefficients $\mathcal{C}_{9}$ and $\mathcal{C}_{10}$ of the numerically leading dimension-six operators in the weak effective Hamiltonian, in addition to the relevant nuisance parameters. In stark contrast to previous analyses of this decay mode, the changes in the updated experimental results lead us to find very good compatibility with both the Standard Model and with the $b\to sμ^+μ^-$ anomalies observed in rare $B$-meson decays. We provide a detailed analysis of the impact of the partial angular distribution, the full angular distribution, and the $Λ_b\to Λμ^+μ^-$ branching fraction on the Wilson coefficients. In this process, we are also able to constrain the size of the production polarization of the $Λ_b$ baryon at LHCb.

hep-ph↗

arXiv2019

Opportunities for lattice QCD in quark and lepton flavor physics

This document is one of a series of whitepapers from the USQCD collaboration. Here, we discuss opportunities for lattice QCD in quark and lepton flavor physics. New data generated at Belle II, LHCb, BES III, NA62, KOTO, and Fermilab E989, combined with precise calculations of the relevant hadronic physics, may reveal what lies beyond the Standard Model. We outline a path toward improvements of the precision of existing lattice-QCD calculations and discuss groundbreaking new methods that allow lattice QCD to access new observables.

hep-lat↗

arXiv2019

Radiative leptonic decays on the lattice

Adding a hard photon to the final state of a leptonic pseudoscalar-meson decay lifts the helicity suppression and can provide sensitivity to a larger set of operators in the weak effective Hamiltonian. Furthermore, radiative leptonic $B$ decays at high photon energy are well suited to constrain the first inverse moment of the $B$-meson light-cone distribution amplitude, an important parameter in the theory of nonleptonic $B$ decays. We demonstrate that the calculation of radiative leptonic decays is possible using Euclidean lattice QCD, and present preliminary numerical results for $D_s^+ \to \ell^+ νγ$ and $K^- \to \ell^-\barνγ$.

hep-lat↗

arXiv2019

Excited-state effects in nucleon structure on the lattice using hybrid interpolators

It would be very useful to find a way of reducing excited-state effects in lattice QCD calculations of nucleon structure that has a low computational cost. We explore the use of hybrid interpolators, which contain a nontrivial gluonic excitation, in a variational basis together with the standard interpolator with tuned smearing width. Using the clover discretization of the field strength tensor, a calculation using a fixed linear combination of standard and hybrid interpolators can be done using the same number of quark propagators as a standard calculation, making this a cost-effective option. We find that such an interpolator, optimized by solving a generalized eigenvalue problem, reduces excited-state contributions in two-point correlators. However, the effect in three-point correlators, which are needed for computing nucleon matrix elements, is mixed: for some matrix elements such as the tensor charge, excited-state effects are suppressed, whereas for others such as the axial charge, they are enhanced. The results illustrate that the variational method is not guaranteed to reduce the net contribution from excited states except in its asymptotic regime, and suggest that it may be important to use a large basis of interpolators capable of isolating all of the relevant low-lying states.

hep-lat↗

arXiv2019

Lattice QCD investigation of a doubly-bottom $\bar{b} \bar{b} u d$ tetraquark with quantum numbers $I(J^P) = 0(1^+)$

We use lattice QCD to investigate the spectrum of the $\bar{b} \bar{b} u d$ four-quark system with quantum numbers $I(J^P) = 0(1^+)$. We use five different gauge-link ensembles with $2+1$ flavors of domain-wall fermions, including one at the physical pion mass, and treat the heavy $\bar{b}$ quark within the framework of lattice nonrelativistic QCD. Our work improves upon previous similar computations by considering in addition to local four-quark interpolators also nonlocal two-meson interpolators and by performing a Lüscher analysis to extrapolate our results to infinite volume. We obtain a binding energy of $(-128 \pm 24 \pm 10) \, \textrm{MeV}$, corresponding to the mass $(10476 \pm 24 \pm 10) \, \textrm{MeV}$, which confirms the existence of a $\bar{b} \bar{b} u d$ tetraquark that is stable with respect to the strong and electromagnetic interactions.

hep-lat↗

arXiv2019

Nucleon axial, scalar, and tensor charges using lattice QCD at the physical pion mass

We report on lattice QCD calculations of the nucleon isovector axial, scalar, and tensor charges. Our calculations are performed on two 2+1-flavor ensembles generated using a 2-HEX-smeared Wilson-clover action at the physical pion mass and lattice spacings $a\approx$ 0.116 and 0.093 fm. We use a wide range of source-sink separations - eight values ranging from roughly 0.4 to 1.4 fm on the coarse ensemble and three values from 0.9 to 1.5 fm on the fine ensemble - which allows us to perform an extensive study of excited-state effects using different analysis and fit strategies. To determine the renormalization factors, we use the nonperturbative Rome-Southampton approach and compare RI'-MOM and RI-SMOM intermediate schemes to estimate the systematic uncertainties. Our final results are computed in the MS-bar scheme at scale 2 GeV. The tensor and axial charges have uncertainties of roughly 4%, $g_T=0.972(41)$ and $g_A=1.265(49)$. The resulting scalar charge, $g_S=0.927(303)$, has a much larger uncertainty due to a stronger dependence on the choice of intermediate renormalization scheme and on the lattice spacing.

hep-lat↗

arXiv2019

Hadrons and Nuclei

This document is one of a series of whitepapers from the USQCD collaboration. Here, we discuss opportunities for lattice QCD calculations related to the structure and spectroscopy of hadrons and nuclei. An overview of recent lattice calculations of the structure of the proton and other hadrons is presented along with prospects for future extensions. Progress and prospects of hadronic spectroscopy and the study of resonances in the light, strange and heavy quark sectors is summarized. Finally, recent advances in the study of light nuclei from lattice QCD are addressed, and the scope of future investigations that are currently envisioned is outlined.

hep-lat↗

arXiv2019

$K π$ scattering and the $K^*(892)$ resonance in 2+1 flavor QCD

In this project, we will compute the form factors relevant for $B \to K^*(\to K π)\ell^+\ell^-$ decays. To map the finite-volume matrix elements computed on the lattice to the infinite-volume $B \to K π$ matrix elements, the $K π$ scattering amplitude needs to be determined using Lüscher's method. Here we present preliminary results from our calculations with $2+1$ flavors of dynamical clover fermions. We extract the $P$-wave scattering phase shifts and determine the $K^*$ resonance mass and the $K^* K π$ coupling for two different ensembles with pion masses of $317(2)$ and $178(2)$ MeV.

hep-lat↗

arXiv2018

Towards the P-wave nucleon-pion scattering amplitude in the $Δ(1232)$ channel

We use lattice QCD and the Lüscher method to study elastic pion-nucleon scattering in the isospin $I = 3/2$ channel, which couples to the $Δ(1232)$ resonance. Our $N_f=2+1$ flavor lattice setup features a pion mass of $m_π\approx 250$ MeV, such that the strong decay channel $Δ\rightarrow πN$ is close to the threshold. We present our method for constructing the required lattice correlation functions from single- and two-hadron interpolating fields and their projection to irreducible representations of the relevant symmetry group of the lattice. We show preliminary results for the energy spectra in selected moving frames and irreducible representations, and extract the scattering phase shifts. Using a Breit-Wigner fit, we also determine the resonance mass $m_Δ$ and the $g_{Δ-πN}$ coupling.

hep-lat↗