arXiv subjects

Henry Lamm

Publications and source records attributed to Henry Lamm.

At least 55 records · Page 3Linked to original sources

arXiv2021

Path integral contour deformations for observables in $SU(N)$ gauge theory

Path integral contour deformations have been shown to mitigate sign and signal-to-noise problems associated with phase fluctuations in lattice field theories. We define a family of contour deformations applicable to $SU(N)$ lattice gauge theory that can reduce sign and signal-to-noise problems associated with complex actions and complex observables. For observables, these contours can be used to define deformed observables with identical expectation value but different variance. As a proof-of-principle, we apply machine learning techniques to optimize the deformed observables associated with Wilson loops in two dimensional $SU(2)$ and $SU(3)$ gauge theory. We study loops consisting of up to 64 plaquettes and achieve variance reduction of up to 4 orders of magnitude.

hep-lat↗

arXiv2020

Toward Quantum Simulations of $\mathbb{Z}_2$ Gauge Theory Without State Preparation

Preparing strongly-coupled particle states on quantum computers requires large resources. In this work, we show how classical sampling coupled with projection operators can be used to compute Minkowski matrix elements without explicitly preparing these states on the quantum computer. We demonstrate this for the 2+1d $\mathbb{Z}_2$ lattice gauge theory on small lattices with a quantum simulator.

hep-lat↗

arXiv2020

Gluon Field Digitization via Group Space Decimation for Quantum Computers

Efficient digitization is required for quantum simulations of gauge theories. Schemes based on discrete subgroups use fewer qubits at the cost of systematic errors. We systematize this approach by deriving a single plaquette action for approximating general continuous gauge groups through integrating out field fluctuations. This provides insight into the effectiveness of these approximations, and how they could be improved. We accompany the scheme by simulations of pure gauge over the largest discrete subgroup of $SU(3)$ up to the third order.

hep-lat↗

arXiv2020

Suppressing Coherent Gauge Drift in Quantum Simulations

Simulations of field theories on noisy quantum computers must contend with errors introduced by that noise. For gauge theories, a large class of errors violate gauge symmetry, and thus may result in unphysical processes occurring in the simulation. We present a method, applicable to non-Abelian gauge theories, for suppressing coherent gauge drift errors through the repeated application of pseudorandom gauge transformation. In cases where the dominant errors are gauge-violating, we expect this method to be a practical way to improve the accuracy of NISQ-era simulations.

quant-ph↗

arXiv2020

Parton Physics on a Quantum Computer

Parton distribution functions and hadronic tensors may be computed on a universal quantum computer without many of the complexities that apply to Euclidean lattice calculations. We detail algorithms for computing parton distribution functions and the hadronic tensor in the Thirring model. Their generalization to QCD is discussed, with the conclusion that the parton distribution function is best obtained by fitting the hadronic tensor, rather than direct calculation. As a side effect of this method, we find that lepton-hadron cross sections may be computed relatively cheaply. Finally, we estimate the computational cost of performing such a calculation on a digital quantum computer, including the cost of state preparation, for physically relevant parameters.

hep-lat↗

arXiv2020

Quantum Simulation of Field Theories Without State Preparation

We propose an algorithm for computing real-time observables using a quantum processor while avoiding the need to prepare the full quantum state. This reduction in quantum resources is achieved by classically sampling configurations in imaginary-time using standard lattice field theory. These configurations can then be passed to quantum processor for time-evolution. This method encounters a signal-to-noise problem which we characterize, and we demonstrate the application of standard lattice QCD methods to mitigate it.

hep-lat↗

arXiv2020

Nucleon form factors from basis light front quantization

We investigate the electromagnetic form factors of the nucleon in the framework of basis light front quantization. We compute the form factors using the light front wavefunctions obtained by diagonalizing the effective Hamiltonian consisting of the holographic QCD confinement potential, the longitudinal confinement, and a one-gluon exchange interaction with fixed coupling. The electromagnetic radii of the nucleon are also computed.

hep-ph↗

arXiv2019

Precision Model-Independent Bounds from Global Analysis of $b \to c \ell ν$ Form Factors

We present a model-independent global analysis of hadronic form factors for the semileptonic decays $b\rightarrow c\ellν$ that exploits lattice-QCD data, dispersion relations, and heavy-quark symmetries. The analysis yields predictions for the relevant form factors, within quantifiable bounds. These form factors are used to compute the semileptonic ratios $R(H_c)$ and various decay-product polarizations. In particular, we find $R(D_s^*)=0.20(3)$ and $R(J/ψ)=0.25(3)$, predictions that can be compared to results of upcoming LHCb measurements. In developing this treatment, we obtain leading-order NRQCD results for the nonzero-recoil relations between the $B_c^+ \rightarrow \{J/ψ, η_c \}$ form factors.

hep-ph↗

arXiv2019

General Methods for Digital Quantum Simulation of Gauge Theories

A general scheme is presented for simulating gauge theories, with matter fields, on a digital quantum computer. A Trotterized time-evolution operator that respects gauge symmetry is constructed, and a procedure for obtaining time-separated, gauge-invariant operators is detailed. We demonstrate the procedure on small lattices, including the simulation of a 2+1D non-Abelian gauge theory.

hep-lat↗

arXiv2019

Gluon Field Digitization for Quantum Computers

Simulations of gauge theories on quantum computers require the digitization of continuous field variables. Digitization schemes that uses the minimum amount of qubits are desirable. We present a practical scheme for digitizing $SU(3)$ gauge theories via its discrete subgroup $S(1080)$. The $S(1080)$ standard Wilson action cannot be used since a phase transition occurs as the coupling is decreased, well before the scaling regime. We proposed a modified action that allows simulations in the scaling window and carry out classical Monte Carlo calculations down to lattice spacings of order $a\approx 0.08$ fm. We compute a set of observables with sub-percent precision at multiple lattice spacings and show that the continuum extrapolated value agrees with the full $SU(3)$ results. This suggests that this digitization scheme provides sufficient precision for NISQ-era QCD simulations.

hep-lat↗

arXiv2019

Sigma models on quantum computers

We formulate a discretization of sigma models suitable for simulation by quantum computers. Space is substituted by a lattice, as usually done in lattice field theory, while the target space (a sphere) is replaced by the "fuzzy sphere", a construction well known from non-commutative geometry. Contrary to more naive discretizations of the sphere, in this construction the exact $O(3)$ symmetry is maintained, which suggests that the discretized model is in the same universality class as the continuum model. That would allow for continuum results to be obtained for very rough discretizations of the target space as long as the space discretization is made fine enough. The cost of performing time-evolution, measured as the number of CNOT operations necessary, is $12 L T/Δt $, where $L$ is the number of spatial sites, $T$ the maximum time extent and $Δt$ the time spacing.

hep-lat↗

arXiv2018

Flowing Gauge Theories: Finite-Density $QED_{1+1}$

Finite-density calculations in lattice field theory are typically plagued by sign problems. A promising way to ameliorate this issue is the holomorphic flow equations that deform the manifold of integration for the path integral to manifolds in the complex space where the sign fluctuations are less dramatic. We discuss some novel features of applying the flow equations to gauge theories and present results for finite-density $QED_{1+1}$.

hep-lat↗

arXiv2018

Model-Independent Bounds on $R(J/ψ)$

We present a model-independent bound on $R(J/ψ) \! \equiv \! \mathcal{BR} (B_c^+ \rightarrow J/ψ\, τ^+ν_τ)/ \mathcal{BR} (B_c^+ \rightarrow J/ψ\, μ^+ν_μ)$. This bound is constructed by constraining the form factors through a combination of dispersive relations, heavy-quark relations at zero-recoil, and the limited existing determinations from lattice QCD. The resulting 95\% confidence-level bound, $0.20\leq R(J/ψ)\leq0.39$, agrees with the recent LHCb result at $1.3 \, σ$, and rules out some previously suggested model form factors.

hep-ph↗

arXiv2018

Scouring meson decays for true muonium

Rare meson decay experiments promise to measure branching ratios as small as $10^{-13}$. This presents an opportunity to discover the $μ^+μ^-$ bound state true muonium. We consider a set of possible channels, all with branching ratios above $\sim10^{-11}$. For the electromagnetic decays $η/η'\rightarrow(μ^+μ^-)γ$, theoretical and phenomenological form factors $F_{η/η'γγ^*}(Q^2)$ allow predictions of $\mathcal{BR}(η'\rightarrow (μ^+μ^-) γ)\sim 4.8\times10^{-10}$ and $\mathcal{BR}(η'\rightarrow (μ^+μ^-) γ)\sim 3.7\times10^{-11}$ at the 5% level. Discussion of experimental prospects and potential backgrounds are made.

hep-ph↗

arXiv2018

Model-Independent Bounds on $R(J/ψ)$ via Dispersive Relations

Model-independent bounds on $R(J/ψ) \! \equiv \!\mathcal{BR} (B_c^+ \rightarrow J/ψ\, τ^+ν_τ)/$ $\mathcal{BR} (B_c^+ \rightarrow J/ψ\, μ^+ν_μ)$ are obtained through a combination of dispersive relations, heavy-quark relations at zero-recoil, and the limited existing form factor determinations from lattice QCD. The resulting 95% confidence-level bound, $0.20\leq R(J/ψ)\leq0.39$, agrees with the recent LHCb result at $1.3 \, σ$, and removes the dominant model-dependent uncertainty from theory predictions. Using the same techniques, a prediction of $R(η_c)=0.29(5)$ is obtained.

hep-ph↗

arXiv2018

Fermions at Finite Density in (2+1)d with Sign-Optimized Manifolds

We present Monte Carlo calculations of the thermodynamics of the (2+1) dimensional Thirring model at finite density. We bypass the sign problem by deforming the domain of integration of the path integral into complex space in such a way as to maximize the average sign within a parameterized family of manifolds. We present results for lattice sizes up to $10^3$ and we find that at high densities and/or temperatures the chiral condensate is abruptly reduced.

hep-lat↗

arXiv2018

Model-Independent Prediction of $R(η_c)$

We present a model-independent prediction for $R(η_c) \! \equiv \! \mathcal{BR} (B_c \rightarrow η_c \, τ^+ν_τ)/ \mathcal{BR} (B_c \rightarrow η_c \, μ^+ν_μ)$. This prediction is obtained from the form factors through a combination of dispersive relations, heavy-quark relations at zero-recoil, and the limited existing determinations from lattice QCD. The resulting prediction, $R(η_c)=0.29(5)$, agrees with the weighted average of previous model predictions, but with reduced uncertainties.

hep-ph↗

arXiv2018

Tests of the Standard Model in $B \to D\ell ν_\ell$, $B \to D^* \ell ν_\ell$ and $B_c \to J/ψ\, \ell ν_\ell$

A number of recent experimental measurements suggest the possibility of a breakdown of lepton ($\ell$) universality in exclusive $b \to c \ell ν_\ell$ semileptonic meson decays. We analyze the full differential decay rates for several such processes, and show how to extract combinations of the underlying helicity amplitudes that are completely independent of $m_\ell$. Ratios of these combinations for different $\ell$ (as well as some combinations for a single value of $\ell$) therefore equal unity in the Standard Model and provide stringent tests of lepton universality. Furthermore, the extractions assume the form of weighted integrals over the differential decay rates and therefore are useful even in situations where data in some regions of allowed phase space may be sparse.

hep-ph↗