low energies. There are a number of challenging questions for nuclear physicists at the particle physics boundary:

  • Do neutrinos have mass? Are the leptons from different generations mixed together by the weak interaction?
  • Is the pattern of symmetry violations found experimentally at low energies consistent with the Standard Model description? Can a direct manifestation of a violation of time-reversal symmetry be discovered at low energies?
  • What is the significance of the mixing of quarks from different generations? Will more precise experiments show that mixing parameters satisfies the quantum mechanical laws of probability within the context of three generations of quarks and leptons?

The Standard Model

The first hint of the existence of the fundamental weak force was the discovery of radioactivity about a century ago. It has taken most of the last hundred years to establish the Standard Model, a surprisingly complete description of the fundamental particles and their interactions. The most important clues came from elegant experiments that studied weak-interaction phenomena. The Standard Model describes the electromagnetic, weak, and strong interactions; gravity is the only known interaction missing. While there are occasional hints of the need for revision, no confirmed experiment contradicts the Standard Model. Nevertheless, despite the successes, most physicists are convinced that a more complete theory of nature will eventually replace it. The Standard Model has a disturbingly large number of parameters whose numerical values are not explained; many aspects of the model seem unnatural.

Since the early 1970s, experimentalists have mounted a two-pronged assault on the Standard Model attempting to discover its limitations: trying to verify its quantitative predictions to the highest possible precision, and searching for new, unexpected, and inconsistent phenomena. The top priority of high-energy physics is to discover direct evidence for the Higgs boson, the particle responsible for all finite particle masses according to the Standard Model. If there is no Higgs boson, then the Standard Model must be revised. The prudent search for the Higgs is at the highest available energy. Searches for specific extensions to the Standard Model (for example, a class of attractive theories classified as grand unified supersymmetric models) are often, but not always, carried out at high energies. Nevertheless, it may turn out that testing the Standard Model precisely at low energies is the most economical way to find new physics. Low-energy experiments often have unique discovery potential.

The best-established part of the Standard Model is its description of the electromagnetic interaction. Called quantum electrodynamics (QED), it developed well before the weak and strong interaction were incorporated into the



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