Publications

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• Many-body perturbation theory for strongly correlated effective Hamiltonians using effective field theory methods

  Raphaël Photopoulos, Antoine Boulet

  SciPost Phys. 18, 003 (2025) · published 6 January 2025 | Toggle abstract · pdf

  Introducing low-energy effective Hamiltonians is usual to grasp most correlations in quantum many-body problems. For instance, such effective Hamiltonians can be treated at the mean-field level to reproduce some physical properties of interest. Employing effective Hamiltonians that contain many-body correlations renders the use of perturbative many-body techniques difficult because of the overcounting of correlations. In this work, we develop a strategy to apply an extension of the many-body perturbation theory starting from an effective interaction that contains correlations beyond the mean-field level. The goal is to re-organize the many-body calculation to avoid the overcounting of correlations originating from the introduction of correlated effective Hamiltonians in the description. For this purpose, we generalize the formulation of the Rayleigh-Schrödinger perturbation theory by including free parameters adjusted to reproduce the appropriate limits. In particular, the expansion in the bare weak-coupling regime and the strong-coupling limit serves as a valuable input to fix the value of the free parameters appearing in the resulting expression. This method avoids double counting of correlations using beyond-mean-field strategies for the description of many-body systems. The ground state energy of various systems relevant for ultracold atomic, nuclear, and condensed matter physics is reproduced qualitatively beyond the domain of validity of the standard many-body perturbation theory. Finally, our method suggests interpreting the formal results obtained as an effective field theory using the proposed reorganization of the many-body calculation. The results, like ground state energies, are improved systematically by considering higher orders in the extended many-body perturbation theory while maintaining a straightforward polynomial expansion.

• Momentum transfer dependence of heavy quarkonium electroproduction

  Michal Krelina, Jan Nemchik

  SciPost Phys. Proc. 8, 114 (2022) · published 13 July 2022 | Toggle abstract · pdf

  We investigate the momentum transfer dependence of differential cross sections $d\sigma/dt$ in diffractive electroproduction of heavy quarkonia on proton targets. Model predictions for $d\sigma/dt$ within the light-front QCD dipole formalism are based on a realistic model for a proper correlation between the impact parameter $\vec b$ of a collision and color dipole orientation $\vec r$. We demonstrate a significance of $\vec b-\vec r$ correlation by comparing with a standard simplification $\vec{b}\parallel\vec{r}$, frequently used in the literature.

• THz higher-order topological photonics in Ge-on-Si heterostructures

  Ian Colombo, Pietro Minazzi, Emiliano Bonera, Fabio Pezzoli, Jacopo Pedrini

  SciPost Phys. Core 7, 039 (2024) · published 4 July 2024 | Toggle abstract · pdf

  We design germanium-based higher-order topological cavities for terahertz applications by breaking the symmetry of a two-dimensional photonic crystal following the Su-Schrieffer-Heeger model. Calculations demonstrate the parity inversion of the electric field in differently deformed unit cells. The interface between domains of opposite topology presents edge and corner modes. The former are chiral, locking light propagation to its helicity. The latter prove that Ge-based structures can be used as high-order topological photonic crystals. These findings can accelerate the development of Si-photonic components working in a spectral range of high technological interest.

• Measurements of $t\overline{t}$ production cross-sections with the ATLAS experiment at the LHC

  Margherita Spalla

  SciPost Phys. Proc. 8, 112 (2022) · published 13 July 2022 | Toggle abstract · pdf

  Comprehensive measurements of differential cross-sections of top-quark-antiquark pair-production are presented. They are performed in the dilepton, lepton+jets and the all-hadronic channels, using $\sqrt{s}=13$ TeV data from the ATLAS detector at the LHC. Several setups of next-to-leading order and next-to-next-to-leading order generators are quantitatively compared to the measurements. In addition, total cross-section measurements using up to the full ATLAS Run 2 dataset are presented.

• Phase transitions in quantum dot-Majorana zero mode coupling systems

  Yue Mao, Qing-Feng Sun

  SciPost Phys. Core 8, 031 (2025) · published 11 March 2025 | Toggle abstract · pdf

  The magnetic doublet ground state (GS) of a quantum dot (QD) could be changed to a spin-singlet GS by coupling to a superconductor. In analogy, here we study the GS phase transitions in QD-Majorana zero mode (MZM) coupling systems: GS behaves phase transition versus intra-dot energy level and QD-MZM coupling strength. The phase diagrams of GS are obtained, for cases with and without Zeeman term. Along with the phase transition, we also study the change of spin feature and density of states. The properties of the phase transition are understood via a mean-field picture. Our study not only serves as an analogue to QD-superconductor phase transitions, but also gives alternative explanations on MZM-relevant experiments.

• WG2 Summary: Small-x, Diffraction and Vector Mesons

  Tolga Altinoluk, Marta Luszczak, Daniel Tapia Takaki

  SciPost Phys. Proc. 8, 110 (2022) · published 13 July 2022 | Toggle abstract · pdf

  Here we briefly summarize the contributions to Working Group 2: Small-x, Diffraction and Vector Mesons, at the DIS2021 Workshop.

• Latest ALICE results on J/$ψ$ photoproduction in ultra-peripheral collisions at the LHC

  Tomas Herman

  SciPost Phys. Proc. 8, 106 (2022) · published 13 July 2022 | Toggle abstract · pdf

  Ultra-peripheral collisions (UPCs), studied using the ALICE detector, allow us to investigate the low-$x$ behavior of the gluon distribution of the colliding particles. Two new measurements of coherent J/$\psi$ photoproduction cross section from Pb-Pb UPCs at $\sqrt{s_{\rm NN}}=5.02$ TeV are presented: the first measurement of the $|t|$-dependence of the cross section providing a new tool to investigate the transverse gluonic structure at low Bjorken-$x$ and a rapidity-differential measurement at midrapidity allowing us to provide stringent constraints on nuclear gluon shadowing and saturation models. In addition, prospects for heavy vector meson photoproduction measurements in LHC Run 3 and 4 are presented.

• Three-loop soft anomalous dimensions for top-quark processes

  Nikolaos Kidonakis

  SciPost Phys. Proc. 8, 104 (2022) · published 13 July 2022 | Toggle abstract · pdf

  I present results for soft anomalous dimensions through three loops for several processes involving the production of top quarks. In particular, I discuss single-top and top-pair production. I also present some numerical results for double-differential distributions in $t{\bar t}$ production through approximate N$^3$LO.

• Josephson effect in strongly disordered metallic wires

  Mustafa E. Ismagambetov, Aleksey V. Lunkin, Pavel M. Ostrovsky

  SciPost Phys. 19, 021 (2025) · published 17 July 2025 | Toggle abstract · pdf

  We study localization effects in Josephson junctions with two superconductors connected by a strongly disordered metallic wire of length $L$. The conventional description of the Josephson effect in such systems, based on the quasiclassical Usadel equation, neglects electron interference and is only applicable when $L$ is shorter than the localization length $\xi$ in the wire. We develop a more general theory for the Josephson effect using the non-linear sigma model that fully accounts for electron interference, and hence localization. We show that for $L \gg \xi$, three qualitatively different regimes of the Josephson current arise depending on the ratio of the superconducting order parameter $\Delta$ and the mean level spacing in the localization volume $\Delta_\xi$. We derive the average supercurrent as a function of the phase difference for all three regimes. Quite unexpectedly, we observe that the Ambegaokar-Baratoff relation between the average critical current and the normal-state conductance still holds in the strongly localized state when $\Delta_\xi \gg \Delta$ and $\xi \ll L \ll (\xi/\pi^2) \ln^2(\Delta_\xi/\Delta)$.

• Experimental input from $e^+e^-$ for $a_{\mu}$ light-by-light

  Yuping Guo on behalf of BESIII Collaboration

  SciPost Phys. Proc. 1, 054 (2019) · published 9 May 2019 | Toggle abstract · pdf

  The anomalous magnetic momentum of the muon, $a_{\mu}$, has been measured and calculated with a precision up to $~0.5$ ppm, but there is a 3 to 4 standard deviations between these two values. The uncertainty in the calculation is dominated by the hadronic part, including the hadronic vacuum polarization and the hadronic light-by-light. The meson transition form factors and the helicity amplitudes can be used as input or constraint to the calculation of the hadronic light-by-light contribution. Latest experimental studies of the transition form factors of $\pi^0$, $\eta$, and $\eta^{*}$ and the cross-section of $\gamma\gamma^*\to\pi^+\pi-$from $e^+e^-$ collider are presented.