Quantum Corrections to Q-Balls

We extend calculational techniques for static solitons to the case of field configurations with simple time dependence in order to consider quantum effects on the stability of Q-balls. These nontopological solitons exist classically for any fixed value of an unbroken global charge Q. We show that one-loop quantum effects can destabilize very small Q-balls. We show how the properties of the soliton are reflected in the associated scattering problem, and find that a good approximation to the full one-loop quantum energy of a Q-ball is given by $\omega - E_0$, where $\omega$ is the frequency of the classical soliton's time dependence, and $E_0$ is the energy of the lowest bound state in the associated scattering problem.Comment: 6 pages, 2 figures, uses RevTex4; v2: replaced figure

Upper bounds for the order of an additive basis obtained by removing a finite subset of a given basis

Let $A$ be an additive basis of order $h$ and $X$ be a finite nonempty subset of $A$ such that the set $A \setminus X$ is still a basis. In this article, we give several upper bounds for the order of $A \setminus X$ in function of the order $h$ of $A$ and some parameters related to $X$ and $A$. If the parameter in question is the cardinality of $X$, Nathanson and Nash already obtained some of such upper bounds, which can be seen as polynomials in $h$ with degree $(|X| + 1)$. Here, by taking instead of the cardinality of $X$ the parameter defined by d := \frac{\diam(X)}{\gcd\{x - y | x, y \in X\}}, we show that the order of $A \setminus X$ is bounded above by $(\frac{h (h + 3)}{2} + d \frac{h (h - 1) (h + 4)}{6})$. As a consequence, we deduce that if $X$ is an arithmetic progression of length $\geq 3$, then the upper bounds of Nathanson and Nash are considerably improved. Further, by considering more complex parameters related to both $X$ and $A$, we get upper bounds which are polynomials in $h$ with degree only 2.Comment: 17 page

A Quantum Monte Carlo Method at Fixed Energy

In this paper we explore new ways to study the zero temperature limit of quantum statistical mechanics using Quantum Monte Carlo simulations. We develop a Quantum Monte Carlo method in which one fixes the ground state energy as a parameter. The Hamiltonians we consider are of the form $H=H_{0}+\lambda V$ with ground state energy E. For fixed $H_{0}$ and V, one can view E as a function of $\lambda$ whereas we view $\lambda$ as a function of E. We fix E and define a path integral Quantum Monte Carlo method in which a path makes no reference to the times (discrete or continuous) at which transitions occur between states. For fixed E we can determine $\lambda(E)$ and other ground state properties of H