Scientific progress is driven by important, infrequent discoveries that cannot be readily identified and quantified, which makes research assessment very difficult. Bibliometric indicators of important discoveries have been formulated using an empirical approach, based on the mathematical properties of the high-citation tail of the citation distribution. To investigate the theoretical basis of such formulations this study compares the US/European research performance ratios expressed in terms of highly cited papers and Nobel Prize-winning discoveries. The research performance ratio in terms of papers was studied from the citation distributions in the fields of chemistry, physics, and biochemistry and molecular biology. It varied as a function of the citation level. Selecting an appropriate high citation level, the ratios in terms of highly cited papers were compared with the corresponding ratios for Nobel Prize-winning discoveries in Chemistry, Physics, and Physiology or Medicine. Research performance ratios expressed in terms of highly cited papers and Nobel Prize-winning discoveries are reasonably similar, and suggest that the research success of the United States is almost 3 times that of Europe. A similar conclusion was obtained using articles published in Nature and Science.