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Sara Collins

Publications and source records attributed to Sara Collins.

At least 55 records · Page 3Linked to original sources
arXiv2017

The $ρ$-meson light-cone distribution amplitudes from lattice QCD

We present the results of a lattice study of the normalization constants and second moments of the light-cone distribution amplitudes of longitudinally and transversely polarized $ρ$ mesons. The calculation is performed using two flavors of dynamical clover fermions at lattice spacings between $0.060\,\text{fm}$ and $0.081\,\text{fm}$, different lattice volumes up to $m_πL = 6.7$ and pion masses down to $m_π=150\,\text{MeV}$. Bare lattice results are renormalized non-perturbatively using a variant of the RI'-MOM scheme and converted to the $\overline{\text{MS}}$ scheme. The necessary conversion coefficients, which are not available in the literature, are calculated. The chiral extrapolation for the relevant decay constants is worked out in detail. We obtain for the ratio of the tensor and vector coupling constants $f_ρ^T/f_ρ^{\vphantom{T}} = 0.629(8)$ and the values of the second Gegenbauer moments $a_2^\parallel = 0.132(27)$ and $a_2^\perp = 0.101(22)$ at the scale $μ= 2\,\text{GeV}$ for the longitudinally and transversely polarized $ρ$ mesons, respectively. The errors include the statistical uncertainty and estimates of the systematics arising from renormalization. Discretization errors cannot be estimated reliably and are not included. In this calculation the possibility of $ρ\toππ$ decay at the smaller pion masses is not taken into account.

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arXiv2017

Hadro-quarkonium from Lattice QCD

The hadro-quarkonium picture [S. Dubinskiy and M.B. Voloshin, Phys. Lett. B 666, 344 (2008)] provides one possible interpretation for the pentaquark candidates with hidden charm, recently reported by the LHCb Collaboration, as well as for some of the charmonium-like "X,Y,Z" states. In this picture, a heavy quarkonium core resides within a light hadron giving rise to four- or five-quark/antiquark bound states. We test this scenario in the heavy quark limit by investigating the modification of the potential between a static quark-antiquark pair induced by the presence of a hadron. Our lattice QCD simulations are performed on a Coordinated Lattice Simulations (CLS) ensemble with $N_f = 2+1$ flavours of non-perturbatively improved Wilson quarks at a pion mass of about 223 MeV and a lattice spacing of about 0.0854 fm. We study the static potential in the presence of a variety of light mesons as well as of octet and decuplet baryons. In all these cases, the resulting configurations are favoured energetically. The associated binding energies between the quarkonium in the heavy quark limit and the light hadron are found to be smaller than a few MeV, similar in strength to deuterium binding. It needs to be seen if the small attraction survives in the infinite volume limit and supports bound states or resonances.

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arXiv2017

Towards the continuum limit with improved Wilson fermions employing open boundary conditions

We present selected results obtained by RQCD from simulations of $N_f=2+1$ flavours of non-perturbatively $\mathcal{O}(a)$ improved Wilson fermions, employing open boundary conditions in time. The ensembles were created within the CLS (Coordinated Lattice Simulations) effort at five different values of the lattice spacing, ranging from 0.085fm down to below 0.04fm. Many quark mass combinations were realized, in particular along lines where the sum of the bare quark masses was kept fixed as well as trajectories of an approximately physical renormalized strange quark mass. Several key observables, including meson and baryon masses and the axial charge of the nucleon have been computed, and preliminary results are presented here. In some cases an accurate and controlled extrapolation to the continuum limit has become possible.

hep-lat↗
arXiv2017

Charmed pseudoscalar decay constants on three-flavour CLS ensembles with open boundaries

We determine the masses and pseudoscalar decay constants of D and D_s mesons employing lattice QCD with non-perturbatively O(a) improved Wilson quarks and a tree-level Symanzik-improved gauge action. Our analysis is based on the large-volume N_f=2+1 ensembles using open boundary conditions, generated within the CLS effort. The status of results presented here covers two lattice spacings, a ~ 0.0854 fm and a ~ 0.0644 fm, and pion masses varied from 420 to 200 MeV. We also report on our implementation of distance preconditioning for the calculation of heavy quark propagators and discuss the impact of the resulting accuracy improvements on the extraction of charmed meson masses and decay constants. This is part of a continuing analysis by the RQCD and ALPHA Collaborations, aiming at a stable continuum extrapolation using several lattice spacings. To extrapolate to the physical masses, we follow both, the (2*m_l+m_s)=const. and the m_s=const. line in parameter space.

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arXiv2016

Testing the hadro-quarkonium model on the lattice

Recently the LHCb experiment found evidence for the existence of two exotic resonances consisting of $c\bar{c}uud$ quarks. Among the possible interpretations is the hadro-charmonium model, in which charmonium is bound "within" a light hadron. We test this idea on CLS $N_f$=2+1 lattices using the static formulation for the heavy quarks. We find that the static potential is modified by the presence of a hadron such that it becomes more attractive. The effect is of the order of a few MeV.

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arXiv2016

Direct determinations of the nucleon and pion $σ$ terms at nearly physical quark masses

We present a high statistics study of the pion and nucleon light and strange quark sigma terms using $N_f=2$ dynamical non-perturbatively improved clover fermions with a range of pion masses down to $m_π\sim 150$ MeV and several volumes, $Lm_π=3.4$ up to $6.7$, and lattice spacings, $a=0.06-0.08$ fm, enabling a study of finite volume and discretisation effects for $m_π\gtrsim 260$ MeV. Systematics are found to be reasonably under control. For the nucleon we obtain $σ_{πN}=35(6)$ MeV and $σ_s=35(12)$ MeV, or equivalently in terms of the quark fractions, $f_{T_u}=0.021(4)$, $f_{T_d}=0.016(4)$ and $f_{T_s}=0.037(13)$, where the errors include estimates of both the systematic and statistical uncertainties. These values, together with perturbative matching in the heavy quark limit, lead to $f_{T_c}=0.075(4)$, $f_{T_b}=0.072(2)$ and $f_{T_t}=0.070(1)$. In addition, through the use of the (inverse) Feynman-Hellmann theorem our results for $σ_{πN}$ are shown to be consistent with the nucleon masses determined in the analysis. For the pion we implement a method which greatly reduces excited state contamination to the scalar matrix elements from states travelling across the temporal boundary. This enables us to demonstrate the Gell-Mann-Oakes-Renner expectation $σ_π=m_π/2$ over our range of pion masses.

hep-lat↗
arXiv2016

$ρ$ and $K^*$ resonances on the lattice at nearly physical quark masses and $N_f=2$

Working with a pion mass $m_π\approx 150$ MeV, we study $ππ$ and $Kπ$ scattering using two flavours of non-perturbatively improved Wilson fermions at a lattice spacing $a\approx 0.071$ fm. Employing two lattice volumes with linear spatial extents of $N_s=48$ and $N_s=64$ points and moving frames, we extract the phase shifts for p-wave $ππ$ and $Kπ$ scattering near the $ρ$ and $K^*$ resonances.Comparing our results to those of previous lattice studies, that used pion masses ranging from about 200 MeV up to 470 MeV, we find that the coupling $g_{ρππ}$ appears to be remarkably constant as a function of $m_π$.

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arXiv2016

Nucleon generalized form factors from lattice QCD with nearly physical quark masses

We determine generalized form factors of the nucleon from lattice simulations with $N_f = 2$ mass-degenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions down to a pion mass of 150 MeV. We also present the resulting isovector quark angular momentum. Possible excited-state contaminations are investigated with correlated simultaneous fits.

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arXiv2015

(Approximate) Low-Mode Averaging with a new Multigrid Eigensolver

We present a multigrid based eigensolver for computing low-modes of the Hermitian Wilson Dirac operator. For the non-Hermitian case multigrid methods have already replaced conventional Krylov subspace solvers in many lattice QCD computations. Since the $γ_5$-preserving aggregation based interpolation used in our multigrid method is valid for both, the Hermitian and the non-Hermitian case, inversions of very ill-conditioned shifted systems with the Hermitian operator become feasible. This enables the use of multigrid within shift-and-invert type eigensolvers. We show numerical results from our MPI-C implementation of a Rayleigh quotient iteration with multigrid. For state-of-the-art lattice sizes and moderate numbers of desired low-modes we achieve speed-ups of an order of magnitude and more over PARPACK. We show results and develop strategies how to make use of our eigensolver for calculating disconnected contributions to hadronic quantities that are noisy and still computationally challenging. Here, we explore the possible benefits, using our eigensolver for low-mode averaging and related methods with high and low accuracy eigenvectors. We develop a low-mode averaging type method using only a few of the smallest eigenvectors with low accuracy. This allows us to avoid expensive exact eigensolves, still benefitting from reduced statistical errors.

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arXiv2015

Charmed baryon spectroscopy and light flavour symmetry from lattice QCD

We determine the ground state and first excited state masses of singly and doubly charmed spin 1/2 and 3/2 baryons with positive and negative parity. Configurations with $N_f=2+1$ non-perturbatively improved Wilson-clover fermions were employed, with the same quark action also being used for the valence quarks, including the charm. The spectrum is calculated for pion masses in the range $M_π\sim 259-460$ MeV at a lattice spacing $a\sim 0.075$ fm. Finite volume effects are studied comparing lattices with two different linear spatial extents ($1.8\,{\rm fm}$ and $2.4\,{\rm fm}$). The physical point is approached from the SU(3) limit keeping the flavour averaged light quark mass fixed. The baryon masses are extrapolated using expansions in the strange-light quark mass difference. Most particles fall into the expected SU(3) multiplets with well constrained extrapolations, the exceptions having a possibly more complex internal structure. Overall agreement is found with experiment for the masses and splittings of the singly charmed baryons. As part of the calculation an analysis of the lower lying charmonium, $D$ and $D_s$ spectra was performed in order to assess discretisation errors. The gross spectra are reproduced, including the $D^*_{s0}$, $D_{s1}$ and $D_1$ mesons, while at this single lattice spacing hyperfine splittings come out $10-20$ MeV too low.

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arXiv2015

Nucleon isovector couplings from $N_f=2$ lattice QCD

We compute the axial, scalar, tensor and pseudoscalar isovector couplings of the nucleon as well as the induced tensor and pseudoscalar charges in lattice simulations with $N_f=2$ mass-degenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions. The simulations are carried out down to a pion mass of 150 MeV and linear spatial lattice extents of up to 4.6 fm at three different lattice spacings ranging from approximately 0.08 fm to 0.06 fm. Possible excited state contamination is carefully investigated and finite volume effects are studied. The couplings, determined at these lattice spacings, are extrapolated to the physical pion mass. In this limit we find agreement with experimental results, where these exist, with the exception of the magnetic moment. A proper continuum limit could not be performed, due to our limited range of lattice constants, but no significant lattice spacing dependence is detected. Upper limits on discretization effects are estimated and these dominate the error budget.

hep-lat↗
arXiv2015

$D_s \rightarrow η, η'$ semileptonic decay form factors with disconnected quark loop contributions

We calculate for the first time the form factors of the semi-leptonic decays of the $D_s$ meson to $η$ and $η^\prime$ using lattice techniques. As a by-product of the calculation we obtain the masses and leading distribution amplitudes of the $η$ and $η^\prime$ mesons. We use $N_f=2+1$ non-perturbatively improved clover fermions on configurations with a lattice spacing $a\sim 0.075$ fm. We are able to obtain clear signals for relevant matrix elements, using several noise reduction techniques, both for the connected and disconnected contributions. This includes a new method for reducing the variance of pseudoscalar disconnected two-point functions. At zero momentum transfer, we obtain for the scalar form factors, $|f_0^{D_s\to η}|=0.564(11)$ and $|f_0^{D_s\to η'}|=0.437(18)$ at $M_π\approx 470\, {\rm MeV}$, as well as $|f_0^{D_s\to η}|=0.542(13)$ and $|f_0^{D_s\to η'}|=0.404(25)$ at $M_π\approx 370\, {\rm MeV}$, where the errors are statistical only.

hep-lat↗
arXiv2014

The moment $\langle x\rangle_{u-d}$ of the nucleon from $N_f=2$ lattice QCD down to nearly physical quark masses

We present an update of our analysis [1] which includes additional ensembles at different quark masses, lattice spacings and volumes, all with high statistics. We use $N_f=2$ mass-degenerate quark flavours, employing the non-perturbatively improved clover action. The lattice matrix elements are converted to the $\overline{\rm MS}$ scheme via renormalization factors determined non-perturbatively in the RI$^\prime$-MOM scheme. We have systematically investigated excited state contributions, in particular, at the smallest, near physical, pion mass. While our results~(with much increased precision) are consistent with Ref.~[1], comparing with previous determinations we find that excited state contributions can be significant if the quark smearing is not suitably optimized, in agreement with other recent studies. The difference with respect to the value for $\langle x\rangle_{u-d}$ extracted from experimental data is reduced but not resolved. Using lattice sizes in the range $L m_π\sim 3.4-6.7$, no significant finite volume effects were observed. Performing a controlled continuum limit that may remove the discrepancy will require simulations at lattice spacings $a< 0.06$ fm.

hep-lat↗
arXiv2013

Nucleon structure from stochastic estimators

Using stochastic estimators for connected meson and baryon three-point functions has successfully been tried in the past years. Compared to the standard sequential source method we trade the freedom to compute the current-to-sink propagator independently of the hadron sink for additional stochastic noise in our observables. In the case of the nucleon we can use this freedom to compute many different sink-momentum/polarization combinations, which grants access to more virtualities. We will present preliminary results on the scalar, electro-magnetic and axial form factors of the nucleon in $N_f=2+1$ lattice QCD and contrast the performance of the stochastic method to the sequential source method. We find the stochastic method to be competitive in terms of errors at fixed cost.

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arXiv2013

Mesons in large-N QCD

We present the results of a systematic, first-principles study of the spectrum and decay constants of mesons for different numbers of color charges N, via lattice computations. We restrict our attention to states in the non-zero isospin sector, evaluating the masses associated with the ground-state and first excitation in the pseudoscalar, vector, scalar, and axial vector channels. Our results are based on a new set of simulations of four dimensional SU(N) Yang-Mills theories with the number of colors ranging from N=2 to N=17; the spectra and the decay constants are computed in the quenched approximation (which becomes exact in the 't Hooft limit) using Wilson fermions. After discussing the extrapolations to the chiral and large-N limits, we present a comparison of our results to some of the numerical computations and analytical predictions available in the literature - including, in particular, those from holographic computations.

hep-lat↗
arXiv2013

The meson spectrum in large-N QCD

We present lattice results on the meson spectrum and decay constants in large-N QCD. The results are obtained in the quenched approximation for N = 2, 3, 4, 5, 6, 7 and 17 and extrapolated to infinite N.

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arXiv2012

Charmed hadron spectroscopy on the lattice for $N_f=2+1$ flavours

We study the spectra of charmonia, charmed mesons, singly and doubly charmed baryons using Lattice QCD, with 2+1 flavours of fermions. In the case of mesons, we include higher spin states, while for baryons, both positive and negative parity channels were investigated. By means of the variational method, we were able to extract a clean signal from the correlation functions and information about the excited states.

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