Numerical Study of the Simplest String Bit Model

TL;DR

This paper numerically investigates a simplified string bit model with one bosonic and one fermionic bit, revealing string-like behavior, supersymmetry effects, and conditions for ground state stability at finite N.

Contribution

It introduces a minimal string bit model with supersymmetry, analyzes its spectrum numerically at finite N, and explores how additional Hamiltonian terms affect ground state existence.

Findings

Supersymmetry reduces Hamiltonian parameters from 7 to 3.

At N=∞, the model exhibits a continuous energy spectrum indicating a spatial dimension.

Ground states disappear at specific N depending on M, with patterns affected by Hamiltonian modifications.

Abstract

String bit models provide a possible method to formulate a string as a discrete chain of pointlike string bits. When the bit number $M$ is large, a chain behaves as a continuous string. We study the simplest case that has only one bosonic bit and one fermionic bit. The creation and annihilation operators are adjoint representations of the $U\left(N\right)$ color group. We show that the supersymmetry reduces the parameter number of a Hamiltonian from 7 to 3 and, at $N=\infty$, ensures a continuous energy spectrum, which implies the emergence of one spatial dimension. The Hamiltonian $H_{0}$ is constructed so that in the large $N$ limit it produces a world sheet spectrum with one Grassmann world sheet field. We concentrate on numerical study of the model in finite $N$. For the Hamiltonian $H_{0}$, we find that the would-be ground energy states disappear at $N=\left(M-1\right)/2$ for odd $M\leq11$. Such a simple pattern is spoiled if $H$ has an additional term $\xi\Delta H$ which does not affect the result of $N=\infty$. The disappearance point moves to higher (lower) $N$ when $\xi$ increases (decreases). Particularly, the $\pm\left(H_{0}-\Delta H\right)$ cases suggest a possibility that the ground state could survive at large $M$ and $M\gg N$. Our study reveals that the model has stringy behavior: when $N$ is fixed and large enough, the ground energy decreases linearly with respect to $M$, and the excitation energy is roughly of order $M^{-1}$. We also verify that a stable system of Hamiltonian $\pm H_{0}+\xi\Delta H$ requires $\xi\geq\mp1$.