arXiv subjects

Sara Collins

Publications and source records attributed to Sara Collins.

At least 19 recordsLinked to original sources

arXiv2026

Nucleon-sigma terms at $m_\pi=222 ~\rm MeV$ with a variational analysis from lattice QCD

Nucleon sigma terms are important for the decomposition of the nucleon mass and for searches for new physics beyond the Standard Model involving scalar interactions. A persistent tension between lattice QCD and phenomenological determinations may be due to uncontrolled excited-state contamination in lattice QCD analyses. In previous work at $m_\pi=429~\rm MeV$, we showed that this contamination is dominated by $N\sigma$ states and can be strongly suppressed with a variational analysis using $N$ and $N\sigma$ operators. In this talk, we present preliminary results at $m_\pi=222~\rm MeV$, where the $\sigma$ becomes unstable and decays into $\pi\pi$. We investigate whether the same small variational basis can remove the expected $N\pi\pi$ contamination through the overlap of the $N\sigma$ operator with these states.

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arXiv2026

Strange partner of $T_{cc}^+$ from lattice QCD in $D^{()}D_s^{()}$ scattering

We report on our lattice QCD study of coupled $DD_s^* - D^*D_s$ scattering in the $J^P=1^+$ channel and elastic $DD_s$ scattering in the $J^P=0^+$ channel, aimed at investigating the possible existence of $cc\bar{u}\bar{s}$ tetraquarks near threshold. The calculation uses CLS ensembles with $m_\pi \approx 280$ MeV, lattice spacing $a \approx 0.09$ fm, and spatial extents $L/a=24, 32$. Finite-volume spectra are obtained from a variational analysis of two-point correlation matrices constructed from two-meson operator bases using distillation. The $l=0$ partial-wave scattering amplitudes are determined from the lattice spectra in multiple moving frames using L\"uscher's formalism as well as a finite-volume implementation of the Lippmann-Schwinger equation. In both channels we observe small but nonzero shifts relative to the noninteracting spectrum, indicating weak meson interactions. The extracted physically plausible $S$-wave amplitudes show no pole structures near threshold.

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arXiv2026

Electromagnetic form factors and structure of the $T_{bb}$ tetraquark

We present the first lattice QCD calculation of electromagnetic form factors of a tetraquark, focusing on the $T_{bb} = bb\bar u \bar d$ with quantum numbers $I(J^P) = 0(1^+)$. The electromagnetic current probes the charge monopole, magnetic dipole and the electric quadrupole distributions within the tetraquark. From it, we find evidence that its structure consists of a compact heavy diquark $[bb]$ in spin one, color-antitriplet configuration, and a light antidiquark $[\bar u \bar d]$ in spin zero, color-triplet configuration. The computations were performed on a single CLS ensemble with $N_f = 2+1$ dynamical quarks at a lattice spacing $a\approx 0.064$ fm and with a pion mass $m_\pi \approx 290$ MeV.

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arXiv2025

Electromagnetic form factors and structure of the $T_{bb}$ tetraquark from lattice QCD

We present the first lattice QCD determination of the electromagnetic form factors of the exotic tetraquark $T_{bb} \ (bb \bar u \bar d)$ with quantum numbers $I( J^P ) = 0( 1^+ )$. The extracted form factors encode information about its internal structure, including the charge distribution and the magnetic dipole moments, determined separately for the light and heavy quarks. Our results provide evidence in favor of it being a bound state consisting of a compact heavy diquark $[bb]$ in a color-antitriplet with spin one, and a light antidiquark $[\bar u \bar d]$ in a color-triplet with spin zero. The charge radius of $T_{bb}$ is found to be significantly smaller than the combined charge radii of $B$ and $B^*$ mesons. These two comprise the lowest-lying threshold $BB^*$ in the channel we are considering, and their electric charge form factors are also determined. The computations were performed on a single CLS ensemble with $N_f = 2+1$ dynamical quarks and a lattice spacing of approximately $a \approx0.064 \ \mathrm{fm}$ at the pion mass $m_\pi \approx 290 \ \mathrm{MeV}$.

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arXiv2025

Lattice study of $cc\bar u\bar s$ tetraquark channel in $D^{()}D^{()}_s$ scattering

We present the first lattice QCD determination of coupled $DD_s^*$ and $D^*D_s$ scattering amplitudes in the $J^{P}=1^{+}$ channel and elastic $DD_s$ scattering amplitude in the $J^{P}=0^{+}$ channel. The aim is to investigate whether tetraquarks with flavor $cc\bar u\bar s$ exist in the region near threshold. Lattice QCD ensembles from the CLS consortium with $m_{\pi} \sim 280$ MeV, $a\sim0.09$ fm and $L/a = 24, 32$ are utilized. Finite-volume spectra are determined via variational analysis of two-point correlation matrices, computed using large bases of operators resembling bilocal two-meson structures within the distillation framework. The scattering matrix for partial wave $l=0$ is determined using lattice eigenenergies from multiple inertial frames following L\"uscher's formalism as well as following the solutions of Lippmann-Schwinger Equation in the finite-volume on a plane-wave basis. We observe small nonzero energy shifts in the simulated spectra from the noninteracting scenario in both the channels studied, which points to rather weak nontrivial interactions between the mesons involved. Despite the nonzero energy shifts, the lattice-extracted $S$-wave amplitudes do not carry signatures of any hadron pole features in the physical amplitudes in the energy region near the threshold.

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arXiv2024

Nucleon sigma terms with a variational analysis from Lattice QCD

We determine the nucleon-sigma terms from lattice QCD. We find that the dominant excited state contamination in the nucleon three-point function with a scalar current is due to the transition between the nucleon and a S-wave scattering state of a nucleon and a scalar (sigma) meson. In this proof-of-concept study, we analyse a single $N_f=3$ ensemble with the unphysically large pion mass $M_\pi=429$ MeV. Excited state contamination is substantially reduced compared to the standard method when employing nucleon-sigma type interpolating operators within a generalised eigenvector analysis.

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arXiv2024

$T_{cc}^+$ via the plane wave approach and including diquark-antidiquark operators

The determination of the $DD^{*}$ scattering amplitude from lattice QCD is complicated by long-range interactions. In particular, the L\"uscher method is no longer applicable in the kinematical region close to the left-hand cut. We tackle this problem by adopting plane-wave and effective-field-theoretic methods, which also address partial wave mixing. In addition, we incorporate a diquark-antidiquark interpolator in the operator basis (along with the relevant scattering operators) in order to achieve a better resolution of the energy spectrum. Results show that inclusion of it already has some impact at physical charm quark mass, although it is more significant for larger heavy quark masses, in line with expectations.

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arXiv2024

Charmoniumlike Channels $1^{+}$ with Isospin $1$ from Lattice and Effective Field Theory

Many exotic charmoniumlike mesons have already been discovered experimentally, of which the $Z_c$ mesons with $I=1$ are prominent examples. We investigate $J^{PC}=1^{+\pm}$ states with flavor $\bar cc\bar qq$ ($q=u,d$) in $I=1$ using lattice QCD. This is the first study of these mesons employing more than one volume and involving frames with nonzero total momentum. We utilize two $N_f=2+1$ CLS ensembles with $m_{\pi}\simeq 280\,$MeV. The simulations are performed with unphysical light quark masses at a single lattice spacing of $a\simeq 0.086\,$fm and omit $\psi(2S)\pi$, $\psi(3770)\pi$ and three-particle decay channels, so our results provide only qualitative insights. Resulting eigenenergies are compatible or just slightly shifted down with respect to noninteracting energies, where the most significant shifts occur for certain $D\bar D^*$ states. Both channels $1^{+\pm}$ have a virtual pole slightly below the threshold if $D\bar D^*$ is assumed to be decoupled from other channels. In addition, we perform a coupled channel analysis of $J/\psi\pi$ and $D\bar D^*$ scattering with $J^{PC}=1^{+-}$ within an effective field theory framework. The $J/\psi\pi$ and $D\bar D^*$ line shapes from BESIII and finite-volume energies from several lattice QCD simulations, including this work, are fitted simultaneously. All fits yield two poles relatively close to the $D\bar D^*$ threshold and reasonably reproduce the experimental $Z_c$ peaks. They also reproduce lattice energies up to slightly above the $D\bar D^*$ threshold, while reproduction at even higher energies is better for fits that put more weight on the lattice data. Our findings suggest that the employed EFT can reasonably reconcile the peaks in the experimental line shapes and the lattice energies, although those lie close to noninteracting energies. We also study $J/\psi\pi$ scattering in s wave and place upper bounds on the phase shift.

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arXiv2024

Progress on nucleon transition matrix elements with a lattice QCD variational analysis

Nucleon weak matrix elements can be extracted from nucleon correlation functions with lattice QCD simulations. The signal-to-noise ratio prohibits the analysis at large source-sink separations and as a consequence, excited state contamination affects the extraction of the nucleon matrix elements. Chiral perturbation theory (ChPT) suggests that the dominant contamination in some of these channels is due to $N\pi$ states where the pion carries the same momentum of the current. In this talk, we report updates on the variational analysis with $qqq$-operators (nucleon-like) and $(qqq)(\bar{q}q)$-operators (nucleon-pion-like) where we report for the first time some preliminary results of $\langle N\pi| \mathcal{J}| N \rangle $, modulo some kinematic and volume factors, and we compare the results against ChPT. This pilot study is performed on a CLS ensemble with $N_f=3$, $m_\pi \approx 420~\mathrm{MeV}$, $a\approx 0.1~\mathrm{fm}$ and $T=2L\approx 4.8~\mathrm{fm}$.

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arXiv2024

$\mathrm{D}$ and $\mathrm{D_s}$ decay constants in $N_{\rm f}=2+1$ QCD with Wilson fermions

We present results for the leptonic decay constants of the D and D$_{\rm s}$ mesons from $N_{\rm f}=2+1$ lattice QCD. We employ a set of 49 high statistics gauge ensembles generated by the Coordinated Lattice Simulations (CLS) effort utilising non-perturbatively improved Wilson fermions and the tree-level Symanzik improved gauge action at six values of the lattice spacing in the range $a = 0.098\,$fm down to $a = 0.039\,$fm, with pion masses varying from around $420\,$MeV down to below the physical point. The ensembles lie on three trajectories in the quark mass plane, two trajectories intersecting close to the physical quark mass point and the third one approaching the SU(3) chiral limit, enabling tight control of the light and strange quark mass dependence. We obtain $f_{\mathrm{D_s}}=246.8(1.3)\,$MeV, $f_\mathrm{D}=208.4(1.5)\,$MeV and $f_{\mathrm{D_s}}/f_\mathrm{D}=1.1842(36)$, where the precision of our results is mostly limited by the determination of the scale.

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arXiv2024

Towards the quark mass dependence of $T_{cc}^+$ from lattice QCD

The $DD^*$ scattering phase shifts in the $T_{cc}^+=cc\bar{u}\bar{d}$ channel are extracted from lattice QCD for five different charm quark masses and a fixed light-quark mass corresponding to $m_\pi\simeq 280$~MeV. The phase shifts are analysed employing two approaches: effective range expansion and Lippmann--Schwinger equation derived in the effective field theory. In the latter case, the results imply an attraction at short range parametrised by contact terms and a slight repulsion at long range mediated by one-pion exchange with $m_\pi >m_{D^*}-m_D$. The poles in the amplitude across the complex energy plane are extracted and their trajectories are discussed as the charm quark mass is varied. Two complex conjugate poles corresponding to a resonance below threshold are found for $m_c$ close to the physical value. They turn into a pair of virtual states at the largest $m_c$ studied. With further increasing $m_c$, one virtual pole representing $T_{cc}^+$ is expected to move towards the two-body threshold and turn into a bound state. The light-quark mass dependence of the $T_{cc}^+$ pole is briefly discussed using the data on $DD^*$ scattering from other lattice collaborations.

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arXiv2023

Doubly charmed tetraquark: isospin channels and diquark-antidiquark interpolators

We perform a lattice simulation to investigate the doubly charmed tetraquark $T^+_{cc}$ observed by the LHCb collaboration, slightly below the $D^{*+}D^0$ threshold, with flavor content $cc\bar{u}\bar{d}$ and isospin-$0$. Two-meson interpolators are implemented to explore the isospin quantum numbers $I=0$ and $I=1$. We observe attraction near the $DD^*$ threshold for $I=0$ and repulsion for $I=1$. Moreover, we also include diquark-antidiquark interpolators to study their effect on the energy spectrum. There is no significant shift in the ground state energy when adding diquark-antidiquark interpolators to the interpolator basis when the heavy quark mass is close to the physical charm quark mass. However, we observe a non-negligible shift in the second energy level. This effect has to be taken into account to extract the scattering amplitude of the $T^+_{cc}$. Finally, with a higher mass (close to the bottom quark), the ground state is shifted down significantly. The simulation is performed on $N_f=2+1$ CLS ensembles with $m_\pi \simeq 280$ MeV.

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arXiv2023

Octet baryon isovector charges from $N_f = 2 + 1$ lattice QCD

We determine the axial, scalar and tensor isovector charges of the nucleon, sigma and cascade baryons as well as the difference between the up and down quark masses, $m_u-m_d$. We employ gauge ensembles with $N_f=2+1$ non-perturbatively improved Wilson fermions at six values of the lattice spacing in the range $a\approx (0.039 - 0.098) \,$fm, generated by the Coordinated Lattice Simulations (CLS) effort. The pion mass $M_\pi$ ranges from around $430 \, $MeV down to a near physical value of $130 \, $MeV and the linear spatial lattice extent $L$ varies from $6.5\,M_{\pi}^{-1}$ to $3.0\,M_{\pi}^{-1}$, where $L M_\pi \geq 4$ for the majority of the ensembles. This allows us to perform a controlled interpolation/extrapolation of the charges to the physical mass point in the infinite volume and continuum limit. Investigating SU(3) flavour symmetry, we find moderate symmetry breaking effects for the axial charges at the physical quark mass point, while no significant effects are found for the other charges within current uncertainties.

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arXiv2023

Sigma terms of the baryon octet in $N_\mathrm{f} = 2+1$ QCD with Wilson quarks

A lot of progress has been made in the direct determination of nucleon sigma terms. Using similar methods, we consider the sigma terms of the other octet baryons as well. These are determined on CLS gauge field ensembles employing the L\"uscher-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 ${0.098}\,\mathrm{fm}$ and ${0.039}\,\mathrm{fm}$. We present some preliminary results for the pion and strange sigma terms and compare to indirect determinations. To do so, we discuss multi-state fits to tackle the well-known problem of excited state contamination comparing the ratio and summation methods also including priors.

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arXiv2022

Charmonium-like states with $J^{P}=1^{+}$ and isospin 1

Many mesons with properties incompatible with a $\bar cc$ structure have already been discovered, e.g. the $Z_c$ mesons with isospin 1. We investigate the spectrum of exotic charmonium-like mesons using lattice QCD. The focus is on $\bar cc \bar qq$ states with $J^{PC}=1^{+\pm}$ and isospin 1. This is the first study of four-quark states with these quantum numbers, a non-zero total momentum and two different lattice volumes. We extract the energy levels and determine the scattering length for $D\bar D^*$ scattering close to the threshold using L\"uscher's formalism. Our preliminary results show that the energy shifts for eigenstates dominated by $D\bar{D}^*$ are very small in the $1^{++}$ channel and consistent with zero in the $1^{+-}$ channel.

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arXiv2022

Toward $N$ to $N\pi$ matrix elements from lattice QCD

QCD matrix elements of axial and vector currents between nucleons are required for the Monte Carlo reconstruction of the energy of neutrinos that are detected in long baseline oscillation experiments in the quasi-elastic regime. The cleanest approach for determining the axial matrix elements is lattice QCD. However, the extraction of these from the corresponding correlation functions is complicated by very large excited state contributions, that are related to transitions from the nucleon to a nucleon-pion pair. In this pilot study with a pion mass $m_\pi = 429~ \mathrm{MeV}$, we demonstrate for the first time that these contributions can be removed by including five-(anti)quark operators into the basis of interpolators used to create the nucleon. The same techniques will be needed to compute transition matrix elements between the nucleon and nucleon-pion scattering states that are relevant in the resonance production regime.

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arXiv2022

Scale setting and the light baryon spectrum in $N_f=2+1$ QCD with Wilson fermions

We determine the light baryon spectrum on ensembles generated by the Coordinated Lattice Simulations (CLS) effort, employing $N_f=2+1$ flavours of non-perturbatively improved Wilson fermions. The hadron masses are interpolated and extrapolated within the quark mass plane, utilizing three distinct trajectories, two of which intersect close to the physical quark mass point and the third one approaching the SU(3) chiral limit. The results are extrapolated to the continuum limit, utilizing six different lattice spacings ranging from $a\approx 0.10\,$fm down to below $0.04\,$fm. The light pion mass varies from $M_{\pi}\approx 429\,$MeV down to $127\,$MeV. In general, the spatial extent is kept larger than four times the inverse pion mass and larger than $2.3\,$fm, with additional small and large volume ensembles to investigate finite size effects. We determine the Wilson flow scales $\sqrt{t_{0,{\rm ph}}}=0.1449^{(7)}_{(9)}\,$fm and $t_0^*\approx t_{0,{\rm ph}}$ from the octet cascade ($\Xi$ baryon). Determining the light baryon spectrum in the continuum limit, we find the nucleon mass $m_N=941.7^{(6.5)}_{(7.6)}\,$MeV and the other stable baryon masses to agree with their experimental values within sub-percent level uncertainties. Moreover, we determine SU(3) and SU(2) chiral perturbation theory low energy constants, including the octet and the $\Omega$ baryon sigma~terms $\sigma_{\pi N}=43.9(4.7)\,$MeV, $\sigma_{\pi\Lambda}=28.2^{(4.3)}_{(5.4)}\,$MeV, $\sigma_{\pi\Sigma}=25.9^{(3.8)}_{(6.1)}\,$MeV, $\sigma_{\pi\Xi}=11.2^{(4.5)}_{(6.4)}\,$MeV and $\sigma_{\pi\Omega}=6.9^{(5.3)}_{(4.3)}\,$MeV, as well as various parameters, renormalization factors and improvement coefficients that are relevant for simulations with our lattice action.

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arXiv2022

Determination of $m_c$ from $N_f = 2+1$ QCD with Wilson fermions

We present preliminary results for the charm quark mass in the $N_f=4$ RGI scheme. These were obtained using $N_f=2+1$ CLS ensembles with $\mathcal{O}(a)$ non-perturbatively improved Wilson fermions. We employed five different lattice spacings, ranging down to $a\lesssim 0.04$ fm and realized approximately physical pion and kaon masses, with ensembles spread out along three different trajectories in the quark mass plane, enabling a thorough study of the dependence on the lattice spacing and the light and strange sea quark masses. We sketch our analysis strategy and find that the dominant errors at present are due to the renormalization and scale setting uncertainties.

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