arXiv subjects

Sara Collins

Publications and source records attributed to Sara Collins.

At least 37 records · Page 2Linked to original sources

arXiv2022

Leading order mesonic and baryonic SU(3) low energy constants from $N_f = 3$ lattice QCD

We determine the leading order mesonic~($B_0$ and $F_0$) and baryonic~($m_0$, $D$ and $F$) SU(3) chiral perturbation theory low energy constants from lattice QCD. We employ gauge ensembles with $N_f=3$ (i.e., $m_u=m_d=m_s$) non-perturbatively improved Wilson fermions at six distinct values of the lattice spacing in the range $a\approx (0.039 - 0.098)$ fm, which constitute a subset of the Coordinated Lattice Simulations (CLS) gauge ensembles. The pseudoscalar meson mass $M_π$ ranges from around $430$ MeV down to $240$ MeV and the linear spatial lattice extent $L$ from $6.4\,M_π^{-1}$ to $3.3\,M_π^{-1}$, where $ L M_π\geq 4$ for the majority of the ensembles. This allows us to perform a controlled extrapolation of all the low energy constants to the chiral, infinite volume and continuum limits. We find the SU(3) chiral condensate and $F_0$ to be smaller than their SU(2) counterparts while the Gell-Mann--Oakes--Renner parameters $B_0\approx B$ are similar. Regarding baryonic LECs, we obtain $F/D = 0.612^{(14)}_{(12)}$.

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arXiv2021

Scale Setting for CLS 2+1 Simulations

We present an update of the scale setting for $N_f=2+1$ flavor QCD using gradient flow scales and pseudo-scalar decay constants. We analyze the latest ensembles with $2+1$ flavors of non-perturbatively improved Wilson fermions generated by CLS for improved precision. Special care is taken to correct for mistuning by measuring directly the mass derivatives of the various observables. We determine $t_0$ with input taken from a combination of leptonic decay rates of the Pion and the Kaon.

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arXiv2021

Towards the determination of sigma terms for the baryon octet on $N_\mathrm{f} = 2+1$ CLS ensembles

A lot of progress has been made in the determination of nucleon sigma terms. In this work we consider the sigma terms of the other octet baryons as well. These are determined on CLS gauge field ensembles employing the Lüscher-Weisz gluon action and the Sheikholeslami-Wohlert fermion action with $N_\mathrm{f} = 2 + 1$. The ensembles have pion masses ranging from ${410}\,\mathrm{MeV}$ down to the physical value and lattice spacings covering a range between ${0.09}\,\mathrm{fm}$ and ${0.04}\,\mathrm{fm}$. We present some preliminary results for $a\approx 0.06$ fm along a trajectory where the sum of the sea quark masses is kept constant, focusing on the quark mass dependence. We discuss multi-state fits to tackle the well-known problem of excited state contamination and detail how we analyse connected and disconnected contributions.

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arXiv2021

Nucleon Charges and Sigma Terms from $N_{f}=2+1$ QCD

We report on recent progress of our analysis of the nucleon sigma terms, as well as the singlet scalar, axial and tensor nucleon charges. These are determined employing the CLS gauge ensembles, which are generated using the Lüscher-Weisz gluon action and the non-perturbatively improved Sheikholeslami-Wohlert fermion action with $N_{f}=2+1$ dynamical fermions. For the ensembles analysed thus far, the pion masses range from 200 MeV up to 410 MeV, and the lattice spacings take five values between 0.09~fm and 0.04~fm. We have employed a variety of methods to determine the relevant correlation functions, including the sequential source method for connected contributions and the truncated solver method for disconnected contributions.

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arXiv2021

Properties of the $η$ and $η^{\prime}$ mesons: Masses, decay constants and gluonic matrix elements

We present results for the $η$ and $η^\prime$ masses and their four independent decay constants at the physical point as well as their anomalous gluonic matrix elements $a_{η^{(\prime)}}$. The chiral and continuum limit extrapolation is performed on twenty-one $N_f = 2+1$ Coordinated Lattice Simulations (CLS) ensembles with non-perturbatively improved Wilson fermions at four different lattice spacings and along two trajectories in the quark mass plane, including one ensemble very close to physical quark masses. For the first time the decay constants are determined directly from the axialvector matrix elements without model assumptions. This allows us to study their QCD scale dependence and to determine all low-energy constants contributing at next-to-leading order in large-$N_c$ ChPT at a well defined QCD renormalization scale. We also discuss higher excited states in the $1400\,\mathrm{MeV}$ region.

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arXiv2021

Investigating $\mathbf{N \to Nπ}$ axial matrix elements

Excited state contamination is one of the most challenging sources of systematics to tackle in the determination of nucleon matrix elements and form factors. The signal-to-noise problem prevents one from considering large source-sink time separations for the three-point functions to ensure ground state dominance. Instead, relevant analyses consider multi-state fits. Excited state contributions are particularly significant in the axial channel. In this work, we confront the problem directly. Since the major source of contamination is understood to be related to pion production, we consider three-point correlation functions with a nucleon operator at the source and a nucleon-pion interpolating operator at the sink, which allows studies of $N \to Nπ$ matrix elements. We discuss the construction of these three-point correlation functions and we solve the generalized eigenvalue problem (GEVP) using different sets of nucleon and nucleon-pion interpolators. The analysis is performed on the CLS ensemble A653 with $m_π\approx 420$ MeV. Results were generated with valence quark masses corresponding to $m_π\approx 1750$ MeV and $m_π\approx 420$ MeV.

hep-lat↗

arXiv2021

Masses and decay constants of the $η$ and $η^\prime$ mesons from lattice QCD

We determine the masses, the singlet and octet decay constants as well as the anomalous matrix elements of the $η$ and $η^\prime$ mesons in $N_f=2+1$ QCD\@. The results are obtained using twenty-one CLS ensembles of non-perturbatively improved Wilson fermions that span four lattice spacings ranging from $a\approx 0.086\,$fm down to $a\approx 0.050\,$fm. The pion masses vary from $M_π=420\,$MeV to $126\,$MeV and the spatial lattice extents $L_s$ are such that $L_sM_π\gtrsim 4$, avoiding significant finite volume effects. The quark mass dependence of the data is tightly constrained by employing two trajectories in the quark mass plane, enabling a thorough investigation of U($3$) large-$N_c$ chiral perturbation theory (ChPT). The continuum limit extrapolated data turn out to be reasonably well described by the next-to-leading order ChPT parametrization and the respective low energy constants are determined. The data are shown to be consistent with the singlet axial Ward identity and, for the first time, also the matrix elements with the topological charge density are computed. We also derive the corresponding next-to-leading order large-$N_{c}$ ChPT formulae. We find $F^8 = 115.0(2.8)~\text{MeV}$, $θ_{8} = -25.8(2.3)^{\circ}$, $θ_0 = -8.1(1.8)^{\circ}$ and, in the $\overline{\mathrm{MS}}$ scheme for $N_f=3$, $F^{0}(μ= 2\,\mathrm{GeV}) = 100.1(3.0)~\text{MeV}$, where the decay constants read $F^8_η=F^8\cos θ_8$, $F^8_{η^\prime}=F^8\sin θ_8$, $F^0_η=-F^0\sin θ_0$ and $F^0_{η^\prime}=F^0\cos θ_0$. For the gluonic matrix elements, we obtain $a_η(μ= 2\,\mathrm{GeV}) = 0.0170(10)\,\mathrm{GeV}^{3}$ and $a_{η^{\prime}}(μ= 2\,\mathrm{GeV}) = 0.0381(84)\,\mathrm{GeV}^{3}$, where statistical and all systematic errors are added in quadrature.

hep-lat↗

arXiv2020

Nucleon axial structure from lattice QCD

We present a new analysis method that allows one to understand and model excited state contributions in observables that are dominated by a pion pole. We apply this method to extract axial and (induced) pseudoscalar nucleon isovector form factors, which satisfy the constraints due to the partial conservation of the axial current up to expected discretization effects. Effective field theory predicts that the leading contribution to the (induced) pseudoscalar form factor originates from an exchange of a virtual pion, and thus exhibits pion pole dominance. Using our new method, we can recover this behavior directly from lattice data. The numerical analysis is based on a large set of ensembles generated by the CLS effort, including physical pion masses, large volumes (with up to $96^3 \times 192$ sites and $L m_π= 6.4$), and lattice spacings down to $0.039 \, \text{fm}$, which allows us to take all the relevant limits. We find that some observables are much more sensitive to the choice of parametrization of the form factors than others. On the one hand, the $z$-expansion leads to significantly smaller values for the axial dipole mass than the dipole ansatz ($M_A^{\text{$z$-exp}}=1.02(10) \, \text{GeV}$ versus $M_A^{\text{dipole}} = 1.31(8) \, \text{GeV}$). On the other hand, we find that the result for the induced pseudoscalar coupling at the muon capture point is almost independent of the choice of parametrization ($g_P^{\star \ \text{$z$-exp}} = 8.68(45)$ and $g_P^{\star \ \text{dipole}} = 8.30(24)$), and is in good agreement with both, chiral perturbation theory predictions and experimental measurement via ordinary muon capture. We also determine the axial coupling constant $g_A$.

hep-lat↗

arXiv2019

Hyperon couplings from $N_f = 2 + 1$ lattice QCD

We compute various (generalized) isovector charges of the octet baryons. These include $g_A$, $g_T$ and $g_S$ as well as the unpolarized, polarized and transversity parton distribution function (PDF) momentum fractions $\langle x\rangle_{u^+-d^+}$, $\langle x\rangle_{Δu^--Δd^-}$ and $\langle x\rangle_{δu^+-δ^+}$. The simulations are carried out on a subset of the (isospin symmetric) $N_f=2+1$ flavour Coordinated Lattice Simulations (CLS) gauge ensembles with lattice spacings ranging from $a\approx 0.086\,$fm down to $a\approx 0.050\,$fm. First results on the breaking of flavour symmetry and the low energy constants $F$ and $D$ are presented. While SU(3) flavour symmetry violations are found to be sizeable for $g_A=\langle 1\rangle_{Δu^+-Δd^+}$, these are quite small for $g_T=\langle 1\rangle_{δu^--δd^-}$ and $\langle x\rangle_{u^+-d^+}$.

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arXiv2019

Nucleon generalized form factors from two-flavor lattice QCD

We determine the generalized form factors, which correspond to the second Mellin moment (i.e., the first $x$-moment) of the generalized parton distributions of the nucleon at leading twist. The results are obtained using lattice QCD with $N_f=2$ nonperturbatively improved Wilson fermions, employing a range of quark masses down to an almost physical value with a pion mass of about 150 MeV. We also present results for the isovector quark angular momentum and for the first $x$-moment of the transverse quark spin density. We compare two different fit strategies and find that directly fitting the ground state matrix elements to the functional form expected from Lorentz invariance and parametrized in terms of form factors yields comparable, and usually more stable results than the traditional approach where the form factors are determined from an overdetermined linear system based on the fitted matrix elements.

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arXiv2019

Light-cone distribution amplitudes of octet baryons from lattice QCD

We present lattice QCD results for the wave function normalization constants and the first moments of the distribution amplitudes for the lowest-lying baryon octet. The analysis is based on a large number of $N_f=2+1$ ensembles comprising multiple trajectories in the quark mass plane including physical pion (and kaon) masses, large volumes, and, most importantly, five different lattice spacings down to $a=0.039\,\mathrm{fm}$. This allows us to perform a controlled extrapolation to the continuum and infinite volume limits by a simultaneous fit to all available data. We demonstrate that the formerly observed violation of flavor symmetry breaking constraints can, indeed, be attributed to discretization effects that vanish in the continuum limit.

hep-lat↗

arXiv2019

Identifying spin and parity of charmonia in flight with lattice QCD

The spectrum of charmonium resonances contains a number of unanticipated states along with several conventional quark-model excitations. The hadrons of different quantum numbers $J^P$ appear in a fairly narrow energy band, where $J^P$ refers to the spin-parity of a hadron at rest. This poses a challenge for Lattice QCD studies of (coupled-channel) meson-meson scattering aimed at the determination of scattering amplitudes and resonance pole positions. A wealth of information for this purpose can be obtained from the lattice spectra in frames with nonzero total momentum. These are particularly dense since hadrons with different $J^P$ contribute to any given lattice irreducible representation. This is because $J^P$ is not a good quantum number in flight, and also because the continuum symmetry is reduced on the lattice. In this paper we address the assignment of the underlying continuum $J^P$ quantum numbers to charmonia in flight using a $N_f = 2 + 1$ CLS ensemble. As a first step, we apply the single-hadron approach, where only interpolating fields of quark-antiquark type are used. The approach follows techniques previously applied to the light meson spectrum by the Hadron Spectrum Collaboration. The resulting spectra of charmonia with assigned $J^P$ will provide valuable information for the parameterization of (resonant) amplitudes in future determinations of resonance properties with lattice QCD.

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arXiv2018

Charmonium resonances from 2+1 flavor CLS lattices

Many exotic charmonium resonances have been identified recently in experiment, however their nature and properties are mostly unknown. Algorithmic and theoretical progress in lattice calculations has enabled reliable numerical investigation of the spectrum below the strong decay threshold, while the study of charmonium resonances remains an open challenge. The main difficulty to overcome is the presence of many open decay channels which are coupled together, resulting in a complex finite volume quantization condition. We report on our recent progress towards the determination of single-channel and coupled-channel scattering matrices in the scalar and vector channels on CLS ensembles. We also present an update concerning the study of the charmonium spectrum in moving frames.

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arXiv2018

Parton distributions and lattice QCD calculations: a community white paper

In the framework of quantum chromodynamics (QCD), parton distribution functions (PDFs) quantify how the momentum and spin of a hadron are divided among its quark and gluon constituents. Two main approaches exist to determine PDFs. The first approach, based on QCD factorization theorems, realizes a QCD analysis of a suitable set of hard-scattering measurements, often using a variety of hadronic observables. The second approach, based on first-principle operator definitions of PDFs, uses lattice QCD to compute directly some PDF-related quantities, such as their moments. Motivated by recent progress in both approaches, in this document we present an overview of lattice-QCD and global-analysis techniques used to determine unpolarized and polarized proton PDFs and their moments. We provide benchmark numbers to validate present and future lattice-QCD calculations and we illustrate how they could be used to reduce the PDF uncertainties in current unpolarized and polarized global analyses. This document represents a first step towards establishing a common language between the two communities, to foster dialogue and to further improve our knowledge of PDFs.

hep-ph↗

arXiv2017

$η$ and $η^\prime$ masses and decay constants

We present preliminary results for the masses and decay constants of the $η$ and $η^\prime$ mesons using CLS $N_f = 2+1$ ensembles. One of the major challenges in these calculations are the large statistical fluctuations due to disconnected quark loops. We tackle these by employing a combination of noise reduction techniques which are tuned to minimize the statistical error at a fixed cost. On the analysis side we carefully assess excited states contributions by using a direct fit approach.

hep-lat↗

arXiv2017

Leptonic decay constants for D-mesons from 3-flavour CLS ensembles

We report on the status of an ongoing effort by the RQCD and ALPHA Collaborations, aimed at determining leptonic decay constants of charmed mesons. Our analysis is based on large-volume ensembles generated within the CLS effort, employing N_f=2+1 non-perturbatively O(a) improved Wilson quarks, tree-level Symanzik-improved gauge action and open boundary conditions. The ensembles cover lattice spacings from a ~ 0.09 fm to a ~ 0.05 fm, with pion masses varied from 420 to 200 MeV. To extrapolate to the physical masses, we follow both the 2m_l+m_s=const. and the m_s=const. lines in parameter space.

hep-lat↗

arXiv2017

Baryonic and mesonic 3-point functions with open spin indices

We have implemented a new way of computing three-point correlation functions. It is based on a factorization of the entire correlation function into two parts which are evaluated with open spin- (and to some extent flavor-) indices. This allows us to estimate the two contributions simultaneously for many different initial and final states and momenta, with little computational overhead. We explain this factorization as well as its efficient implementation in a new library which has been written to provide the necessary functionality on modern parallel architectures and on CPUs, including Intel's Xeon Phi series.

hep-lat↗

arXiv2017

Masses and decay constants of the $D_{s0}^*(2317)$ and $D_{s1}(2460)$ from $N_f=2$ lattice QCD close to the physical point

We perform a high statistics study of the $J^{P}=0^{+}$ and $1^{+}$ charmed-strange mesons, $D_{s0}^*(2317)$ and $D_{s1}(2460)$, respectively. The effects of the nearby $DK$ and $D^{*}K$ thresholds are taken into account by employing the corresponding four quark operators. Six ensembles with $N_f=2$ non-perturbatively ${\cal O}(a)$ improved clover Wilson sea quarks at $a=0.07$ fm are employed, covering different spatial volumes and pion masses: linear lattice extents $L/a=24,32,40,64$, equivalent to 1.7 fm to 4.5 fm, are realised for $m_π=290$ MeV and $L/a=48,64$ or 3.4 fm and 4.5 fm for an almost physical pion mass of $150$ MeV. Through a phase shift analysis and the effective range approximation we determine the scattering lengths, couplings to the thresholds and the infinite volume masses. Differences relative to the experimental values are observed for these masses, however, this is likely to be due to discretisation effects as spin-averaged quantities and splittings are reasonably compatible with experiment. We also compute the weak decay constants of the scalar and axialvector and find $f_V^{0^+}=114(2)(0)(+5)(10)$ MeV and $f_A^{1^+}=194(3)(4)(+5)(10)$ MeV, where the errors are due to statistics, renormalisation, finite volume and lattice spacing effects.

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