arXiv subjects

Nicolas Lang

Publications and source records attributed to Nicolas Lang.

7 recordsLinked to original sources

arXiv2026

Analytic decomposition of two-body electroweak processes with left-hand cuts

We derive an on-shell representation for electroweak $2+J\to2$ transition amplitudes involving two hadrons in both the initial and final state for systems where there are left-hand singularities generated by light-particle exchange, e.g. one-pion exchange. The derivation treats the insertion of the electroweak current perturbatively, keeping only the leading-order contribution in the external field, while the hadronic interactions are treated to all orders, including one-particle exchange effects. We find that, in such processes, the amplitude can have logarithmic singularities, as well as one-particle pole singularities, due to these exchanges. We isolate those contributions, as well as previously identified triangle singularities. The result is expressed in terms of the purely hadronic amplitude, exchange-current kernels, triangle functions, and a class of short-distance transition functions that are real and smooth below unaccounted-for thresholds. While we consider only transitions with spinless particles, this work is an important step toward constraining form factors of systems involving nucleons or vector mesons in the heavy quark sector, where the one-pion-exchange singularity is quite close to threshold.

hep-lat↗

arXiv2025

$D_1$ and $D_2$ resonances in coupled-channel scattering amplitudes from lattice QCD

Isospin-1/2 charmed axial-vector $D^*\pi-D^*\eta-D^*_s\bar{K}$ scattering amplitudes are computed, along with interactions in several other $I=1/2$ $J^P$ channels. Using lattice QCD, we work at a light-quark mass corresponding to $m_\pi\approx 391$ MeV, where the lowest three-hadron threshold ($D\pi\pi$) lies high enough to enable a rigorous treatment of this system considering only two-hadron scattering channels. At this light-quark mass, an axial-vector $D_1$ bound state is observed just below $D^*\pi$ threshold, that is strongly coupled to $D^*\pi$ in a relative $S$-wave and influences a wide energy region up to the $D^*\eta$ threshold. An axial-vector $D_1^\prime$ resonance is observed in the elastic $D^*\pi$ energy-region, which is coupled more strongly to $D$-wave $D^*\pi$. A single narrow tensor state is seen in $J^P=2^+$ coupled to both $D\pi$ and $D^*\pi$. In the region where $D^*\eta$ and $D^*_s\bar{K}$ are kinematically open, the available energy levels indicate significant $S$-wave interactions. Upon searching this region for poles, several possibilities exist with large uncertainties. One additional state consistently arises, predominantly coupled to the $S$-wave $D^*\pi-D^*\eta-D^*_s\bar{K}$ amplitudes around the upper energy limit of this analysis.

hep-lat↗

arXiv2024

Optimising stochastic algorithms for hadron correlation function computations in lattice QCD using a localised distillation basis

Distillation is a quark-smearing method for the construction of a broad class of hadron operators useful in lattice QCD computations and defined via a projection operator into a vector space of smooth gauge-covariant fields. A new orthonormal basis for this space is constructed which builds in locality. This basis is useful for the construction of stochastic methods to estimate the correlation functions computed in Monte Carlo calculations relevant for hadronic physics.

hep-lat↗

arXiv2022

Towards efficient structure prediction and pre-compensation in multi-photon lithography

Microscale 3D printing technologies have been of increasing interest in industry and research for several years. Unfortunately, the fabricated structures always deviate from the respective expectations, often caused by the physico-chemical properties during and after the printing process. Here, we show first steps towards a simple, fast and easy to implement algorithm to predict the final structure topography for multi-photon lithography - also known as Direct Laser Writing (DLW). The three main steps of DLW, (i) exposure of a photo resin, (ii) cross-linking of the resin, and (iii) subsequent shrinkage are approximated by mathematical operations, showing promising results in coincidence with experimental observations. E.g., the root-mean-square error (rmse) between the unmodified 3D print of a radial-symmetrically chirped topography and our predicted topography is only 0.46 $\mu$m, whereas the rmse between this 3D print and its target is 1.49 $\mu$m. Thus, our robust predictions can be used prior to the printing process to minimize undesired deviations between the target structure and the final 3D printed structure. Using a Downhill-Simplex algorithm for identifying the optimal prediction parameters, we were able to reduce the rmse from 4.04 $\mu$m to 0.33 $\mu$m by only two correction loops in our best-case scenario (rmse = 0.72 $\mu$m after one loop). Consequently, this approach can eliminate the need for many structural optimization loops to produce highly conformal and high quality micro structures in the future.

physics.optics↗

arXiv2022

Axial-vector $D_1$ hadrons in $D^\ast\pi$ scattering from QCD

We present $I=1/2$ $D^\ast\pi$ scattering amplitudes from lattice QCD and determine two low-lying $J^P=1^+$ axial-vector $D_1$ states and a $J^P=2^+$ tensor $D_2^\ast$. Computing finite-volume spectra at a light-quark mass corresponding to $m_\pi=391$ MeV, for the first time, we are able to constrain coupled $J^P=1^+$ $D^\ast\pi$ amplitudes with $^{2S+1}\ell_J\,=\,^3S_1$ and $^3\!D_1$ as well as coupled $J^P=2^+$ $D\pi\{^1\!D_2\}$ and $D^\ast\pi \{^3\!D_2\}$ amplitudes via L\"uscher's quantization condition. Analyzing the scattering amplitudes for poles we find a near-threshold bound state, producing a broad feature in $D^\ast\pi\{^3\!S_1\}$. A narrow bump occurs in $D^\ast\pi\{^3\!D_1\}$ due to a $D_1$ resonance. A single resonance is found in $J^P=2^+$ coupled to $D\pi$ and $D^\ast\pi$. A relatively low mass and large coupling is found for the lightest $D_1$, suggestive of a state that will evolve into a broad resonance as the light quark mass is reduced. An earlier calculation of the scalar $D_0^\ast$ using the same light-quark mass enables comparisons to the heavy-quark limit.

hep-ph↗

arXiv2021

The lightest $D_0^\ast$ resonance from lattice QCD

We recently presented elastic $I=1/2$ $Dπ$ scattering from lattice QCD at $m_π = 239$ MeV. The amplitude features a pole corresponding to a mass $m \approx 2200$ MeV and a width $Γ\approx 400$ MeV. The results were compared to an earlier study at a higher pion mass and to a similar study in the charm-strange sector. In this contribution to LATTICE2021 I summarize these results and compare them with experiment, based on the values reported by the particle data group. Our result lies significantly below the experimental $D_0^\ast$. I also relate our findings to recent studies in chiral perturbation theory. Based on work presented in JHEP2021(7),123 for the Hadron Spectrum Collaboration.

hep-lat↗

arXiv2021

Isospin-1/2 $Dπ$ scattering and the lightest $D_0^\ast$ resonance from lattice QCD

Isospin-1/2 $Dπ$ scattering amplitudes are computed using lattice QCD, working in a single volume of approximately $(3.6\; \mathrm{fm})^3$ and with a light quark mass corresponding to $m_π\approx239$ MeV. The spectrum of the elastic $Dπ$ energy region is computed yielding 20 energy levels. Using the Lüscher finite-volume quantisation condition, these energies are translated into constraints on the infinite-volume scattering amplitudes and hence enable us to map out the energy dependence of elastic $Dπ$ scattering. By analytically continuing a range of scattering amplitudes, a $D_0^\ast$ resonance pole is consistently found strongly coupled to the $S$-wave $Dπ$ channel, with a mass $m\approx 2200$ MeV and a width $Γ\approx400$ MeV. Combined with earlier work investigating the $D_{s0}^\ast$, and $D_0^\ast$ with heavier light quarks, similar couplings between each of these scalar states and their relevant meson-meson scattering channels are determined. The mass of the $D_0^\ast$ is consistently found well below that of the $D_{s0}^\ast$, in contrast to the currently reported experimental result.

hep-lat↗