Threshold Effects Beyond the Standard Model

I am very happy to contribute to the Festschrift celebrating Gabriele Veneziano on his 65th birthday. I have known Gabriele for more than 25 years, and worked with him on many projects, learning not only physics, but how to enjoy physics. “Amusing” is the word that he often uses to describe interesting ideas, and that single word characterizes best a unique style of joyful research that led to his pioneering work on string theory, particle physics and cosmology described in this book. When researching Gabriele’s original work on running coupling constants, preceding our 1988 collaboration [1] described below, I ran into a write-up of his lectures on “Topics in String Theory” delivered in 1987 in China and in India [2]. His paper concludes with: “But my moral, I hope, is a clear one for the young string theorist: If string math. is lots of fun, string phys. is no less.” Indeed, I had much fun working on string physics over the following twenty years. In this contribution, I discuss the threshold effects of extra dimensions and their applications to physics beyond the standard model, focusing on superstring theory.