Publications

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Fermionic defects of topological phases and logical gates

Ryohei Kobayashi

SciPost Phys. 15, 028 (2023) · published 25 July 2023 | · pdf

We discuss the codimension-1 defects of (2+1)D bosonic topological phases, where the defects can support fermionic degrees of freedom. We refer to such defects as fermionic defects, and introduce a certain subclass of invertible fermionic defects called "gauged Gu-Wen SPT defects" that can shift self-statistics of anyons. We derive a canonical form of a general fermionic invertible defect, in terms of the fusion of a gauged Gu-Wen SPT defect and a bosonic invertible defect decoupled from fermions on the defect. We then derive the fusion rule of generic invertible fermionic defects. The gauged Gu-Wen SPT defects give rise to interesting logical gates of stabilizer codes in the presence of additional ancilla fermions. For example, we find a realization of the CZ logical gate on the (2+1)D $\mathbb{Z}_2$ toric code stacked with a (2+1)D ancilla trivial atomic insulator. We also investigate a gapped fermionic interface between (2+1)D bosonic topological phases realized on the boundary of the (3+1)D Walker-Wang model. In that case, the gapped interface can shift the chiral central charge of the (2+1)D phase. Among these fermionic interfaces, we study an interesting example where the (3+1)D phase has a spatial reflection symmetry, and the fermionic interface is supported on a reflection plane that interpolates a (2+1)D surface topological order and its orientation-reversal. We construct a (3+1)D exactly solvable Hamiltonian realizing this setup, and find that the model generates the $\mathbb{Z}_8$ classification of the (3+1)D invertible phase with spatial reflection symmetry and fermion parity on the reflection plane. We make contact with an effective field theory, known in literature as the exotic invertible phase with spacetime higher-group symmetry.
AdvNF: Reducing mode collapse in conditional normalising flows using adversarial learning

Vikas Kanaujia, Mathias S. Scheurer, Vipul Arora

SciPost Phys. 16, 132 (2024) · published 24 May 2024 | · pdf

Deep generative models complement Markov-chain-Monte-Carlo methods for efficiently sampling from high-dimensional distributions. Among these methods, explicit generators, such as Normalising Flows (NFs), in combination with the Metropolis Hastings algorithm have been extensively applied to get unbiased samples from target distributions. We systematically study central problems in conditional NFs, such as high variance, mode collapse and data efficiency. We propose adversarial training for NFs to ameliorate these problems. Experiments are conducted with low-dimensional synthetic datasets and XY spin models in two spatial dimensions.
Comparison of $pp$ and $p \bar{p}$ differential elastic cross sections and observation of the exchange of a colorless $C$-odd gluonic compound

C. Royon

SciPost Phys. Proc. 8, 138 (2022) · published 14 July 2022 | · pdf

We describe the discovery of the colorless $C$-odd gluonic compound, the odderon, by the D0 and TOTEM Collaborations by comparing elastic differential cross sections in $pp$ and $p \bar{p}$ interactions at high energies.
Measurements of $W$ and $Z/γ^*$ cross sections and their ratios in $pp$ collisions at STAR

Jae Nam

SciPost Phys. Proc. 8, 140 (2022) · published 14 July 2022 | · pdf

While the unpolarized parton distribution functions of the valence quarks ($d$ and $u$) are well determined from DIS experiments, the sea quark counterparts, $\bar{d}$ and $\bar{u}$, are much less constrained, in particular, near the valence region. Measurements of $W^+/W^-$ production ratio in $pp$ collider experiments, such as the STAR experiment at RHIC, can probe the $\bar{d}/\bar{u}$ ratio at a large $Q^2$ set by the $W$ mass. These proceedings will discuss recent results of $W$ and $Z$ cross-section measurements using the STAR $pp$ collision data at a center-of-mass energy of $\sqrt{s} = 500/510\;\mathrm{GeV}$ collected in 2011, 2012, 2013 and 2017.
Update on the angular resolution of GRAPES-3 experiment based on Moon shadow analysis

D. Pattanaik, S. Ahmad, M. Chakraborty, S. R. Dugad, U. D. Goswami, S. K. Gupta, B. Hariharan, Y. Hayashi, P. Jagadeesan, A. Jain, P. Jain, S. Kawakami, H. Kojima, S. Mahapatra, P. K. Mohanty, R. Moharana, Y. Muraki, P. K. Nayak, T. Nonaka, A. Oshima, B. P. Pant, M. Rameez, K. Ramesh, L. V. Reddy, S. Shibata, F. Varsi, M. Zuberi

SciPost Phys. Proc. 13, 033 (2023) · published 29 September 2023 | · pdf

GRAPES-3 is an extensive air shower array located at Ooty (11.6$^\circ$N, 76.7$^\circ$E, 2200 m a.s.l.) in southern India. Recently, the angular resolution of the GRAPES-3 array has been improved by correcting the shower front curvature based on the shower size and age. Here, we present the results of the angular resolution and pointing accuracy of the array through observation of the Moon shadow. We have analyzed the data for the period of 2014-2016 containing $\sim$ 3$×$10$^9$ air shower events with a median energy of 15 TeV. A significant improvement in the angular resolution has been observed compared to the earlier analysis by the group through the Moon shadow method and is comparable to the arrays located at a 2 km higher altitude than the GRAPES-3 experiment.
SUSY-QCD corrections to squark annihilation into gluons and light quarks

Luca Paolo Wiggering

SciPost Phys. Proc. 12, 047 (2023) · published 4 July 2023 | · pdf

We calculate the full one-loop SUSY-QCD corrections to stop annihilation into gluons and light quarks including the Sommerfeld enhancement effect and discuss their impact on the neutralino relic density in the MSSM.
Photoproduction of diffractive dijets in Pythia 8

Ilkka Helenius

SciPost Phys. Proc. 8, 145 (2022) · published 14 July 2022 | · pdf

The new framework for the simulations of hard diffractive events in photoproduction within Pythia 8 is presented. The model, originally introduced for proton-proton collisions, applies the dynamical rapidity gap survival probability based on the multiparton interaction model in Pythia. These additional interactions provide a natural explanation for the observed factorization-breaking effects in hard diffraction by filling up the rapidity gaps used to classify the events of diffractive origin. The generated cross sections are well in line with the existing data from HERA experiments and predictions for the future electron-ion collider are presented. In addition, also predictions for ultra-peripheral collisions at the LHC have been calculated where the factorization-breaking effects are found to be more pronounced.
Downfolding the Su-Schrieffer-Heeger model

Arne Schobert, Jan Berges, Tim Wehling, Erik van Loon

SciPost Phys. 11, 079 (2021) · published 20 October 2021 | · pdf

Charge-density waves are responsible for symmetry-breaking displacements of atoms and concomitant changes in the electronic structure. Linear response theories, in particular density-functional perturbation theory, provide a way to study the effect of displacements on both the total energy and the electronic structure based on a single ab initio calculation. In downfolding approaches, the electronic system is reduced to a smaller number of bands, allowing for the incorporation of additional correlation and environmental effects on these bands. However, the physical contents of this downfolded model and its potential limitations are not always obvious. Here, we study the potential-energy landscape and electronic structure of the Su-Schrieffer-Heeger (SSH) model, where all relevant quantities can be evaluated analytically. We compare the exact results at arbitrary displacement with diagrammatic perturbation theory both in the full model and in a downfolded effective single-band model, which gives an instructive insight into the properties of downfolding. An exact reconstruction of the potential-energy landscape is possible in a downfolded model, which requires a dynamical electron-biphonon interaction. The dispersion of the bands upon atomic displacement is also found correctly, where the downfolded model by construction only captures spectral weight in the target space. In the SSH model, the electron-phonon coupling mechanism involves exclusively hybridization between the low- and high-energy bands and this limits the computational efficiency gain of downfolded models.
Exclusive electro-weak production of a charmed meson at high energy

B. Pire, L. Szymanowski, J. Wagner

SciPost Phys. Proc. 8, 150 (2022) · published 14 July 2022 | · pdf

Exclusive electro-weak processes have peculiar features which make them complementary to usually discussed deeply virtual electroproduction processes such as deep virtual Compton scattering or meson production (and the corresponding crossed reactions). They allow in particular single charmed meson production, which we study in two different contexts : electroproduction at an electron ion collider and neutrino-production at a medium energy neutrino facility. We rely on the QCD collinear factorization framework where generalized parton distributions allow physicists to perform a nucleon tomography.
Lattice phase shifts and mixing angles for an arbitrary number of coupled channels

Lukas Bovermann, Evgeny Epelbaum, Hermann Krebs, Dean Lee

SciPost Phys. Proc. 3, 052 (2020) · published 27 February 2020 | · pdf

We present a lattice method for determining scattering phase shifts and mixing angles for the case of an arbitrary number of coupled channels. The proposed method combines a spherical wall boundary condition and a channel-mixing auxiliary potential to extract the full-rank S-matrix from the radial wave functions. We consider the scattering problem of two spin-1 bosons interacting with a test potential involving up to four coupled channels. For this benchmark system, the phase shifts and mixing angles are shown to agree on the lattice and in the continuum. Our method should allow to extend previous two-channel nuclear lattice EFT simulations to mixing of more than two partial waves.

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