arXiv subjects

Simon Catterall

Publications and source records attributed to Simon Catterall.

At least 19 recordsLinked to original sources

arXiv2026

Lorentzian K\"{a}hler-Dirac fermions

We examine the formulation of K\"{a}hler fermions on spacetimes with Lorentz signature. In practice we focus on Minkowski spacetime since most of the difficulties that are encountered are visible even when the spacetime is flat. We show that the theory, when interpreted as a Lorentz invariant theory of forms or antisymmetric tensor fields, is non-unitary. We show that unitarity can be restored provided one adopts a modified inner product on the Hilbert space. This modified inner product requires the insertion of an operator J that anti-commutes with certain modes of the K\"{a}hler field in such a way as to guarantee all states have positive norm. We give an explicit (and local) form for J and show that while it commutes with the Hamiltonian, it is incompatible with the Lorentz transformation properties of the tensor fields. In flat space, the unitarized formulation is equivalent to 4 Dirac fermions.

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arXiv2026

Searching for symmetric mass generation with staggered fermions in four dimensions

We conduct numerical simulations to map out the phase diagram and critical behavior of a lattice Higgs model composed of two massless staggered fermion fields forming a doublet under a global $SU(2)$ and coupled to a scalar field in the adjoint representation of the group. The scalar action consists of a potential comprising quadratic and quartic terms and a scalar kinetic term. At fixed quartic coupling we explore a two-dimensional parameter space finding a massless symmetric phase at weak coupling and a massive symmetric phase (SMG phase) at strong coupling. An intermediate anti-ferromagnetic phase separates these two regimes. These results are consistent with leading order weak and strong coupling expansions. We find that the critical lines bounding the intermediate phase merge at a unique point where all fermion bilinear condensates vanish but fermion susceptibilities diverge as non-trivial powers of the lattice size. We conjecture that this merged point corresponds to a multicritical point and may describe a phase consisting of a condensate of certain topological defects.

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arXiv2025

Quantum Ising Model on $(2+1)-$Dimensional Anti$-$de Sitter Space using Tensor Networks

We study the quantum Ising model on (2+1)-dimensional anti-de Sitter space using Matrix Product States (MPS) and Matrix Product Operators (MPOs). We explore the bulk phase diagram of the theory on regular tessellations of hyperbolic space with coordination number seven and find disordered and ordered phases separated by a phase transition. We find that the boundary-boundary spin correlation function exhibits power law scaling deep in the disordered phase of the Ising model consistent with holography. At the critical point, we find the boundary entanglement entropy scales logarithmically with subsystem size but away from this, we see a linear scaling. In comparison, the full system exhibits a volume law scaling, which is expected in chaotic and/or highly connected systems. We also measure Out of time Ordered Correlators (OTOCs) to explore the scrambling behavior of the theory.

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arXiv2025

Supersymmetric lattice theories on curved space

We show how to construct Hamiltonian lattice theories with one exact supersymmetry on arbitrary triangulations of curved space in any number of dimensions. Both bosons and fermions satisfy discrete K\"{a}hler-Dirac equations. The quantization of the fermions proceeds by imposing conventional anti-commutation relations while the bosons require a modification of the usual canonical commutator. On regular lattices we construct parity, time reversal and translation-by-one (shift) symmetries. We argue that the latter are generically non-invertible symmetries. We also show how to couple these degrees of freedom to background gauge fields which leads to a theory with enhanced supersymmetry.

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arXiv2025

Symmetries and Anomalies of Hamiltonian Staggered Fermions

We review the shift (translation) and time reversal symmetries of Hamiltonian staggered fermions and their connection to continuum symmetries concentrating in particular on the case of massless fermions and (3+1) dimensions. We construct operators using the staggered fields that implement these symmetries on finite lattices. We show that shifts composed of an odd multiple of the elementary shift anti-commute with time reversal and are related to continuum axial transformations. We argue that the presence of these non-trivial commutation relations implies the existence of lattice 't Hooft anomalies. From the shifts we also construct a set of conserved, quantized charges that generate continuous symmetries of the lattice theory. In general these do not commute with the vector charge signaling further 't Hooft anomalies.

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arXiv2024

Symmetric Mass Generation with four SU(2) doublet fermions

We study a single exactly massless staggered fermion in the fundamental representation of an $SU(2)$ gauge group. We utilize an nHYP-smeared fermion action supplemented with additional heavy Pauli-Villars fields which serve to decrease lattice artifacts. The phase diagram exhibits a clear two-phase structure with a conformal phase at weak coupling and a novel new phase, the Symmetric Mass Generation (SMG) phase, appearing at strong coupling. The SMG phase is confining with all states gapped and chiral symmetry unbroken. Our finite size scaling analysis provides strong evidence that the phase transition between these two phases is continuous, which would allow for the existence of a continuum SMG phase. Furthermore, the RG flows are consistent with a $\beta$-function that vanishes quadratically at the new fixed point suggesting that the $N_f=4$ flavor SU(2) gauge theory lies at the opening of the conformal window.

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arXiv2024

Staggered bosons and Kahler-Dirac bosons

We describe a novel way to think about bosonic lattice theories in Hamiltonian form where each lattice site has only a half boson degree of freedom. The construction requires a non-trivial Poisson bracket between neighboring sites and leads to gapless theories with non-invertible symmetries. We also describe a bosonic version of Kahler-Dirac fermions, dubbed Kahler-Dirac bosons that can be performed on any triangulation of a manifold. This also leads to a straightforward implementation of supersymmetry on the lattice and one immediately deduces the Dirac equation of the corresponding Kahler-Dirac fermions.

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arXiv2024

Gauging staggered fermion shift symmetries

Staggered fermion shift symmetries correspond to translations of the fermion field within the unit cell of a hypercubic lattice. They satisfy an algebra and in four Euclidean dimensions can be related to a discrete subgroup of an $SU(4)$ flavor symmetry which plays a crucial role in showing that staggered fermions lead to a theory of four degenerate Dirac fermions in the continuum limit. They are associated with the appearance of certain $Z_2$ valued global parameters. We propose a strategy to try to partially gauge these translation symmetries by allowing these parameters to vary locally in the lattice. To maintain invariance of the action requires the addition of $Z_2$ valued higher form lattice gauge fields. An analogous procedure can be carried out for reduced staggered fermions where the shifts correspond to a discrete subgroup of an $SO(4)$ flavor symmetry.

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arXiv2024

Lattice Regularization of Reduced Kähler-Dirac Fermions and Connections to Chiral Fermions

We show how a path integral for reduced Kähler-Dirac fermions suffers from a phase ambiguity associated with the fermion measure that is an analog of the measure problem seen for chiral fermions. However, unlike the case of chiral symmetry, a doubler free lattice action exists which is invariant under the corresponding onsite symmetry. This allows for a clear diagnosis and solution to the problem using mirror fermions resulting in a unique gauge invariant measure. By introducing an appropriate set of Yukawa interactions which are consistent with 't Hooft anomaly cancellation we conjecture the mirrors can be decoupled from low energy physics. Moreover, the minimal such Kähler-Dirac mirror model yields a light sector which corresponds, in the flat space continuum limit, to the Pati-Salam GUT model.

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arXiv2024

Fermions, quantum gravity and holography in two dimensions

We study a model comprising $N$ flavors of K\"ahler Dirac fermion propagating on a triangulated two dimensional disk which is constrained to have a negative average bulk curvature. Dirichlet boundary conditions are chosen for the fermions. Quantum fluctuations of the geometry are included by summing over all possible triangulations consistent with these constraints. We show in the limit $N\to \infty$ that the partition function is dominated by a regular triangulation of two dimensional hyperbolic space. We use strong coupling expansions and Monte Carlo simulation to show that in this limit boundary correlators of the fermions have a power law dependence on boundary separation as one expects from holography. However we argue that this behavior breaks down for any finite number of massive fields in the thermodynamic limit and quantum fluctuations of the bulk geometry drive the theory into a non-holographic phase. In contrast, for massless fermions we find evidence that the boundary is conformal even for finite $N$. This is consistent with theoretical results in quantum Liouville theory.

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arXiv2024

Simulating Field Theories with Quantum Computers

In this study, we investigate Trotter evolution in the Gross-Neveu and hyperbolic Ising models in two spacetime dimensions, using quantum computers. We identify different sources of errors prevalent in various quantum processing units and discuss challenges to scale up the size of the computation. We present benchmark results obtained on a variety of platforms and employ a range of error mitigation techniques to address coherent and incoherent noise. By comparing these mitigated outcomes with exact diagonalization results and density matrix renormalization group calculations, we assess the effectiveness of our approaches. Moreover, we demonstrate the implementation of an out-of-time-ordered correlator (OTOC) protocol using IBM's quantum computers.

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arXiv2023

Tensor network representation of non-abelian gauge theory coupled to reduced staggered fermions

We show how to construct a tensor network representation of the path integral for reduced staggered fermions coupled to a non-abelian gauge field in two dimensions. The resulting formulation is both memory and computation efficient because reduced staggered fermions can be represented in terms of a minimal number of tensor indices while the gauge sector can be approximated using Gaussian quadrature with a truncation. Numerical results obtained using the Grassmann TRG algorithm are shown for the case of $SU(2)$ lattice gauge theory and compared to Monte Carlo results.

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arXiv2023

Quantum Ising model on two dimensional anti-de Sitter space

This paper investigates the transverse Ising model on a discretization of two-dimensional anti-de Sitter space. We use classical and quantum algorithms to simulate real-time evolution and measure out-of-time-ordered correlators (OTOC). The latter can probe thermalization and scrambling of quantum information under time evolution. We compared tensor network-based methods both with simulation on gated-based superconducting quantum devices and analog quantum simulation using Rydberg arrays. While studying this system's thermalization properties, we observed different regimes depending on the radius of curvature of the space. In particular, we find a region of parameter space where the thermalization time depends only logarithmically on the number of degrees of freedom.

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arXiv2023

Holography from lattice $N=4$ super Yang-Mills

In this paper we use lattice simulation to study four dimensional $N=4$ super Yang-Mills (SYM) theory. We have focused on the three color theory on lattices of size $12^4$ and for 't Hooft couplings up to $λ=40.0$. Our lattice action is based on a discretization of the Marcus or GL twist of $N=4$ SYM and retains one exact supersymmetry for non-zero lattice spacing. We show that lattice theory exists in a single non-Abelian Coulomb phase for all 't Hooft couplings. Furthermore the static potential we obtain from correlators of Polyakov lines is in good agreement with that obtained from holography - specifically the potential has a Coulombic form with a coefficent that varies as the square root of the 't Hooft coupling.

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arXiv2023

't Hooft anomalies for staggered fermions

We show that the phase structure of certain staggered fermion theories can be understood on the basis of exact anomalies. These anomalies arise when staggered fermions are coupled to gravity which can be accomplished by replacing them by discrete Kähler-Dirac fermions. We first show the existence of a perturbative anomaly in even dimensions which breaks an exact $U(1)$ symmetry of the massless theory down to $Z_4$. If we attempt to gauge this $Z_4$ symmetry we find a 't Hooft anomaly which can only be cancelled for multiples of two Kähler-Dirac fields. This result is consistent with the cancellation of a further mixed non-perturbative 't Hooft anomaly between the global $Z_4$ and a reflection symmetry. In four dimensional flat space, theories of two staggered fields yield eight Dirac or sixteen Majorana fermions in the continuum limit and this critical number of fermions agrees with results in condensed matter theory literature on the fermion content required to gap boundary fermions in $4+1$ dimensional topological superconductors. It is also consistent with constraints stemming from the cancellation of spin-$Z_4$ anomalies of Weyl fermions. Indeed, cancellation of 't Hooft anomalies is a necessary requirement for symmetric mass generation and this result gives a theoretical explanation of recent numerical work on the phase diagram of interacting staggered fermions. As an application of these ideas we construct a lattice model whose low energy continuum limit is conjectured to yield the Pati-Salam GUT theory.

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arXiv2022

Quantum Simulation of the N flavor Gross-Neveu Model

We discuss the use of quantum simulation to study an $N$ flavor theory of interacting relativistic fermions in(1+1) dimensions on NISQ era machines. The case of two flavors is particularly interesting as it can be mapped to the Hubbard model. We derive the appropriate qubit Hamiltonians and associated quantum circuits. We compare classical simulation and DMRG/TEBD calculations with the results of quantum simulation on various platforms for $N$=2 and 4. We demonstrate that the four steps of the calculations of real-time scattering can actually be implemented using current NISQ devices.

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arXiv2022

Improved coarse-graining methods on two dimensional tensor networks including fermions

We show how to apply renormalization group algorithms incorporating entanglement filtering methods and a loop optimization to a tensor network which includes Grassmann variables which represent fermions in an underlying lattice field theory. As a numerical test a variety of quantities are calculated for two dimensional Wilson--Majorana fermions and for the two flavor Gross--Neveu model. The improved algorithms show much better accuracy for quantities such as the free energy and the determination of Fisher's zeros.

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arXiv2022

Lattice QCD and Particle Physics

Contribution from the USQCD Collaboration to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021).

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