arXiv subjects

Stefan Meinel

Publications and source records attributed to Stefan Meinel.

At least 73 records · Page 4Linked to original sources

arXiv2018

Calculating the $ρ$ radiative decay width with lattice QCD

We present the results of our lattice QCD study of the $πγ\toππ$ process, where the $ρ$ resonance appears as an enhancement in the transition amplitude. We use $N_f=2+1$ clover fermions on a lattice of $L=3.6$ fm and a pion mass of $320$ MeV. Using a combination of forward, stochastic, and sequential propagators, we calculate the two-point and three-point functions that allow us to determine the $πγ\toππ$ matrix elements for several values of the invariant mass $s$ and momentum transfer $q^2$. To fit the $q^2$ and $s$ dependence of the $πγ\toππ$ amplitude, we explore a set of general parametrizations based on a Taylor expansion. By analytic continuation to the complex pole corresponding to the $ρ$ resonance, we determine the resonant form factors and calculate the radiative decay width of the $ρ$.

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arXiv2018

Nucleon electromagnetic form factors at high $Q^2$ from Wilson-clover fermions

We present results on the nucleon electromagnetic form factors from Lattice QCD at momentum transfer up to about $12$~GeV$^2$. We analyze two gauge ensembles with the Wilson-clover fermion action, a lattice spacing of $a\approx 0.09$~fm and pion masses $m_π\approx 170$~MeV and $m_π\approx 280$~MeV. In our analysis we employ momentum smearing as well as a set of techniques to investigate excited state effects. Good agreement with experiment and phenomenology is found for the ratios $G_E/G_M$ and $F_2/F_1$, whereas discrepancies are observed for the individual form factors $F_1$ and $F_2$. We discuss various systematics that may affect our calculation.

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arXiv2018

A Lattice QCD study of the $ρ$ resonance

We present a lattice QCD study of the $ρ$ resonance with $N_f=2+1$ clover fermions at a pion mass of approximately $320$ MeV and lattice size $3.6$ fm. We consider two processes involving the $ρ$. The first process is elastic scattering of two pions in P-wave with isospin $1$. Using the Lüscher method we determine the scattering phase shift, from which we obtain the $ρ$ resonance mass and decay width $Γ(ρ\toππ)$. The second process is the radiative transition $πγ\toππ$, where we follow the Briceño-Hansen-Walker-Loud approach to determine the transition amplitude in the invariant mass region near the $ρ$ resonance and for both space- and time-like photon momentum. This allows us to determine the coupling between the $ρ$, the pion and the photon, and the resulting $ρ$ radiative decay width.

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arXiv2017

Computing the nucleon charge and axial radii directly at $Q^2=0$ in lattice QCD

We describe a procedure for extracting momentum derivatives of nucleon matrix elements on the lattice directly at $Q^2=0$. This is based on the Rome method for computing momentum derivatives of quark propagators. We apply this procedure to extract the nucleon isovector magnetic moment and charge radius as well as the isovector induced pseudoscalar form factor at $Q^2=0$ and the axial radius. For comparison, we also determine these quantities with the traditional approach of computing the corresponding form factors, i.e. $G^v_E(Q^2)$ and $G_M^v(Q^2)$ for the case of the vector current and $G_P^v(Q^2)$ and $G_A^v(Q^2)$ for the axial current, at multiple $Q^2$ values followed by $z$-expansion fits. We perform our calculations at the physical pion mass using a 2HEX-smeared Wilson-clover action. To control the effects of excited-state contamination, the calculations were done at three source-sink separations and the summation method was used. The derivative method produces results consistent with those from the traditional approach but with larger statistical uncertainties especially for the isovector charge and axial radii.

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arXiv2017

Phenomenology of $Λ_b \to Λ_c τ\barν_τ$ using lattice QCD calculations

In a recent paper we studied the effect of new-physics operators with different Lorentz structures on the semileptonic $Λ_b \to Λ_c τ\barν_τ$ decay. This decay is of interest in light of the $R({D^{(*)}})$ puzzle in the semileptonic $\bar{B} \to D^{(*)} τ{\barν}_τ$ decays. In this work we add tensor operators to extend our previous results and consider both model-independent new physics (NP) and specific classes of models proposed to address the $R({D^{(*)}})$ puzzle. We show that a measurement of $R(Λ_c) = {\cal B}[Λ_b \to Λ_c τ\barν_τ] / {\cal B}[Λ_b \to Λ_c \ell \barν_{\ell}]$ can strongly constrain the NP parameters of models discussed for the $R({D^{(*)}})$ puzzle. We use form factors from lattice QCD to calculate all $Λ_b \to Λ_c τ\barν_τ$ observables. The $Λ_b \to Λ_c$ tensor form factors had not previously been determined in lattice QCD, and we present new lattice results for these form factors here.

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arXiv2017

$P$-wave $ππ$ scattering and the $ρ$ resonance from lattice QCD

We calculate the parameters describing elastic $I=1$, $P$-wave $ππ$ scattering using lattice QCD with $2+1$ flavors of clover fermions. Our calculation is performed with a pion mass of $m_π\approx 320\:\:{\rm MeV}$ and a lattice size of $L\approx 3.6$ fm. We construct the two-point correlation matrices with both quark-antiquark and two-hadron interpolating fields using a combination of smeared forward, sequential and stochastic propagators. The spectra in all relevant irreducible representations for total momenta $|\vec{P}| \leq \sqrt{3} \frac{2π}{L}$ are extracted with two alternative methods: a variational analysis as well as multi-exponential matrix fits. We perform an analysis using Lüscher's formalism for the energies below the inelastic thresholds, and investigate several phase shift models, including possible nonresonant contributions. We find that our data are well described by the minimal Breit-Wigner form, with no statistically significant nonresonant component. In determining the $ρ$ resonance mass and coupling we compare two different approaches: fitting the individually extracted phase shifts versus fitting the $t$-matrix model directly to the energy spectrum. We find that both methods give consistent results, and at a pion mass of $am_π=0.18295(36)_{stat}$ obtain $g_{ρππ} = 5.69(13)_{stat}(16)_{sys}$, $am_ρ= 0.4609(16)_{stat}(14)_{sys}$, and $am_ρ/am_{N} = 0.7476(38)_{stat}(23)_{sys} $, where the first uncertainty is statistical and the second is the systematic uncertainty due to the choice of fit ranges.

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arXiv2017

Up, down, and strange nucleon axial form factors from lattice QCD

We report a calculation of the nucleon axial form factors $G_A^q(Q^2)$ and $G_P^q(Q^2)$ for all three light quark flavors $q\in\{u,d,s\}$ in the range $0\leq Q^2\lesssim 1.2\text{ GeV}^2$ using lattice QCD. This work was done using a single ensemble with pion mass 317 MeV and made use of the hierarchical probing technique to efficiently evaluate the required disconnected loops. We perform nonperturbative renormalization of the axial current, including a nonperturbative treatment of the mixing between light and strange currents due to the singlet-nonsinglet difference caused by the axial anomaly. The form factor shapes are fit using the model-independent $z$ expansion. From $G_A^q(Q^2)$, we determine the quark contributions to the nucleon spin and axial radii. By extrapolating the isovector $G_P^{u-d}(Q^2)$, we obtain the induced pseudoscalar coupling relevant for ordinary muon capture and the pion-nucleon coupling constant. We find that the disconnected contributions to $G_P$ form factors are large, and give an interpretation based on the dominant influence of the pseudoscalar poles in these form factors.

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arXiv2017

Lattice QCD calculation of form factors for $Λ_b \to Λ(1520) \ell^+ \ell^-$ decays

Experimental results for mesonic $b \to s μ^+ μ^-$ decays show a pattern of deviations from Standard-Model predictions, which could be due to new fundamental physics or due to an insufficient understanding of hadronic effects. Additional information on the $b \to s μ^+ μ^-$ transition can be obtained from $Λ_b$ decays. This was recently done using the process $Λ_b \to Λμ^+ μ^-$, where the $Λ$ is the lightest strange baryon. A further interesting channel is $Λ_b \to p^+ K^- μ^+ μ^-$, where the $p^+ K^-$ final state receives contributions from multiple higher-mass $Λ$ resonances. The narrowest and most prominent of these is the $Λ(1520)$, which has $J^P=\frac32^-$. Here we present an ongoing lattice QCD calculation of the relevant $Λ_b \to Λ(1520)$ form factors. We discuss the choice of interpolating field for the $Λ(1520)$, and explain our method for extracting the fourteen $Λ_b \to Λ(1520)$ helicity form factors from correlation functions that are computed in the $Λ(1520)$ rest frame. We present preliminary numerical results at a pion mass of 340 MeV and a lattice spacing of 0.11 fm. This calculation uses a domain-wall action for the $u$, $d$, and $s$ quarks and a relativistic heavy-quark action for the $b$ quark, and is based on gauge-field configurations generated by the RBC and UKQCD Collaborations.

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arXiv2017

$Λ_c \to Λ\ell^+ ν_\ell$ form factors and decay rates from lattice QCD with physical quark masses

The first lattice QCD calculation of the form factors governing $Λ_c \to Λ\ell^+ ν_\ell$ decays is reported. The calculation was performed with two different lattice spacings and includes one ensemble with a pion mass of 139(2) MeV. The resulting predictions for the $Λ_c \to Λe^+ ν_e$ and $Λ_c \to Λμ^+ ν_μ$ decay rates divided by $|V_{cs}|^2$ are $0.2007(71)(74)\:{\rm ps}^{-1}$ and $0.1945(69)(72)\:{\rm ps}^{-1}$, respectively, where the two uncertainties are statistical and systematic. Taking the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cs}|$ from a global fit and the $Λ_c$ lifetime from experiments, this translates to branching fractions of $\mathcal{B}(Λ_c\toΛe^+ν_e)=0.0380(19)_{\rm LQCD\:\:}(11)_{τ_{Λ_c}}$ and $\mathcal{B}(Λ_c\toΛμ^+ν_μ)=0.0369(19)_{\rm LQCD\:\:}(11)_{τ_{Λ_c}}$. These results are consistent with, and two times more precise than, the measurements performed recently by the BESIII Collaboration. Using instead the measured branching fractions together with the lattice calculation to determine the CKM matrix element gives $|V_{cs}|= 0.949(24)_{\rm LQCD\:\:}(14)_{τ_{Λ_c}}(49)_{\mathcal{B}}$.

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arXiv2016

Computing the nucleon Dirac radius directly at $Q^2=0$

We describe a lattice approach for directly computing momentum derivatives of nucleon matrix elements using the Rome method, which we apply to obtain the isovector magnetic moment and Dirac radius. We present preliminary results calculated at the physical pion mass using a 2HEX-smeared Wilson-clover action. For removing the effects of excited-state contamination, the calculations were done at three source-sink separations and the summation method was used.

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arXiv2016

A study of the radiative transition $ππ\to πγ^{*}$ with lattice QCD

Lattice QCD calculations of radiative transitions between hadrons have in the past been limited to processes of hadrons stable under the strong interaction. Recently developed methods for $1\to2$ transition matrix elements in a finite volume now enable the determination of radiative decay rates of strongly unstable particles. Our lattice QCD study focuses on the process $ππ\to πγ^{*}$, where the $ρ$ meson is present as an enhancement in the cross-section. We use $2+1$ flavors of clover fermions at a pion mass of approximately $320$ MeV and a lattice size of approximately $3.6$ fm. The required $2$-point and $3$-point correlation functions are constructed from a set of forward, sequential and stochastic light quark propagators. In addition to determining the $ρ$ meson resonance parameters via the Lüscher method, the scattering phase shift is used in conjunction with the $1\to2$ transition matrix element formalism of Briceño, Hansen and Walker-Loud to compute the $ππ\toπγ^{*}$ amplitude at several values of the momentum transfer and $ππ$ invariant mass.

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arXiv2016

Lattice QCD study of heavy-heavy-light-light tetraquark candidates

We investigate heavy-light four-quark systems $ud\bar b \bar b$ with bottom quarks of finite mass which are treated in the framework of NRQCD. We focus on $I(J^P)=0(1^+)$, where we recently found evidence for the existence of a tetraquark state using static bottom quarks. Furthermore, we report on an investigation of the $u \bar d b \bar b$ four-quark system with quantum numbers $I(J^P)=1(1^+)$ again using static bottom quarks.

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arXiv2016

Using $Λ_b\to Λμ^+μ^-$ data within a Bayesian analysis of $|ΔB| = |ΔS| = 1$ decays

We study the impact of including the baryonic decay $Λ_b\to Λ(\to p π^-)μ^+μ^-$ in a Bayesian analysis of $|ΔB | = |ΔS| = 1$ transitions. We perform fits of the Wilson coefficients $C_{9}$, $C_{9'}$, $C_{10}$ and $C_{10'}$, in addition to the relevant nuisance parameters. Our analysis combines data for the differential branching fraction and three angular observables of $Λ_b\to Λ(\to p π^-)μ^+μ^-$ with data for the branching ratios of $B_s \to μ^+μ^-$ and inclusive $b \to s\ell^+\ell^-$ decays. Newly available precise lattice QCD results for the full set of $Λ_b \to Λ$ form factors are used to evaluate the observables of the baryonic decay. Our fits prefer shifts to $C_{9}$ that are opposite in sign compared to those found in global fits of only mesonic decays, and the posterior odds show no evidence of physics beyond the Standard Model. We investigate a possible hadronic origin of the observed tensions between theory and experiment.

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arXiv2015

$Λ_b \to p \ell^- \barν_\ell$ and $Λ_b \to Λ_c \ell^- \barν_\ell$ form factors from lattice QCD with relativistic heavy quarks

Measurements of the $Λ_b \to p \ell^- \barν_\ell$ and $Λ_b \to Λ_c \ell^- \barν_\ell$ decay rates can be used to determine the magnitudes of the CKM matrix elements $V_{ub}$ and $V_{cb}$, provided that the relevant hadronic form factors are known. Here we present a precise calculation of these form factors using lattice QCD with 2+1 flavors of dynamical domain-wall fermions. The $b$ and $c$ quarks are implemented with relativistic heavy-quark actions, allowing us to work directly at the physical heavy-quark masses. The lattice computation is performed for six different pion masses and two different lattice spacings, using gauge-field configurations generated by the RBC and UKQCD collaborations. The $b \to u$ and $b \to c$ currents are renormalized with a mostly nonperturbative method. We extrapolate the form factor results to the physical pion mass and the continuum limit, parametrizing the $q^2$-dependence using $z$-expansions. The form factors are presented in such a way as to enable the correlated propagation of both statistical and systematic uncertainties into derived quantities such as differential decay rates and asymmetries. Using these form factors, we present predictions for the $Λ_b \to p \ell^- \barν_\ell$ and $Λ_b \to Λ_c \ell^- \barν_\ell$ differential and integrated decay rates. Combined with experimental data, our results enable determinations of $|V_{ub}|$, $|V_{cb}|$, and $|V_{ub}/V_{cb}|$ with theory uncertainties of 4.4%, 2.2%, and 4.9%, respectively.

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arXiv2015

High-precision calculation of the strange nucleon electromagnetic form factors

We report a direct lattice QCD calculation of the strange nucleon electromagnetic form factors $G_E^s$ and $G_M^s$ in the kinematic range $0 \leq Q^2 \lesssim 1.2\: {\rm GeV}^2$. For the first time, both $G_E^s$ and $G_M^s$ are shown to be nonzero with high significance. This work uses closer-to-physical lattice parameters than previous calculations, and achieves an unprecedented statistical precision by implementing a recently proposed variance reduction technique called hierarchical probing. We perform model-independent fits of the form factor shapes using the $z$-expansion and determine the strange electric and magnetic radii and magnetic moment. We compare our results to parity-violating electron-proton scattering data and to other theoretical studies.

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arXiv2015

Flavor physics with $Λ_b$ baryons

At the LHC, bottom baryons are being produced in unprecedented quantities, which opens up a new field for flavor physics. For example, the decay $Λ_b \to p μ^- \barν$ can be used to obtain a novel determination of the CKM matrix element $|V_{ub}|$, and the decay $Λ_b \to Λμ^+ μ^-$ probes the weak interactions at the loop level. The first lattice calculations of the relevant $Λ_b \to p$ and $Λ_b \to Λ$ form factors have recently been performed using domain-wall light quarks and static $b$ quarks. To further reduce the theoretical uncertainty, one has to go beyond the static approximation. Here I present new calculations of $Λ_b \to p$, $Λ_b \to Λ$, and $Λ_b \to Λ_c$ form factors using a relativistic heavy-quark action.

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arXiv2014

Charmed bottom baryon spectroscopy from lattice QCD

We calculate the masses of baryons containing one, two, or three heavy quarks using lattice QCD. We consider all possible combinations of charm and bottom quarks, and compute a total of 36 different states with $J^P = \frac12^+$ and $J^P = \frac32^+$. We use domain-wall fermions for the up, down, and strange quarks, a relativistic heavy-quark action for the charm quarks, and nonrelativistic QCD for the bottom quarks. Our analysis includes results from two different lattice spacings and seven different pion masses. We perform extrapolations of the baryon masses to the continuum limit and to the physical pion mass using $SU(4|2)$ heavy-hadron chiral perturbation theory including $1/m_Q$ and finite-volume effects. For the 14 singly heavy baryons that have already been observed, our results agree with the experimental values within the uncertainties. We compare our predictions for the hitherto unobserved states with other lattice calculations and quark-model studies.

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arXiv2014

Calculation of $B^0 \to K^{*0} μ^+ μ^-$ and $B_s^0 \to ϕμ^+ μ^-$ observables using form factors from lattice QCD

We calculate the differential branching fractions and angular distributions of the rare decays $B^0 \to K^{*0} μ^+ μ^-$ and $B_s^0 \to ϕμ^+ μ^-$, using for the first time form factors from unquenched lattice QCD. We focus on the kinematic region where the $K^*$ or $ϕ$ recoils softly; there the newly available form factors are most precise and the nonlocal matrix elements can be included via an operator product expansion. Our results for the differential branching fractions calculated in the Standard Model are higher than the experimental data. We consider the possibility that the deviations are caused by new physics, and perform a fit of the Wilson coefficients $C_9$ and $C_9^\prime$ to the experimental data for multiple $B^0 \to K^{*0} μ^+ μ^-$ and $B_s^0 \to ϕμ^+ μ^-$ observables. In agreement with recent results from complementary studies, we obtain $C_9-C_9^{SM} = -1.0 \pm 0.6$ and $C_9^\prime = 1.2 \pm 1.0$, whose deviations from zero would indicate the presence of non-standard fundamental interactions.

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