How can we find out what causes CP violation?

Abstract

Although there have been a number of very fine experiments probing the nature of CP violation, even after 25 years we don't know whether we understand the basic theory involved. We do not know if CP violation is a profound clue to physics beyond the Standard Model and to the origin of the baryon asymmetry, or a trivial accident involving (for example) diagonalizing a mass matrix of quarks or Higgs scalars or neutralinos or squarks. The Standard Model can give a satisfactory description of CP violation in kaon decays to pions without any other contribution, if the phase angle in the CKM matrix is fairly large. The Standard model description of e and e I depends on some poorly known parameters. Some of those, such as m,, will be better known soon. Unfortunately, the hadronic matrix elements that enter are not well known, and may not be for a long time. Better measurements of e and e I will provide valuable input to eventually understanding the origin of CP violation, but unless the hadronic matrix elements can be calculated, measurements of e and e ~ can never give us real confidence that we understand CP violation. In particular, they may never be able to tell us whether a second source of CP violation, about the same size as the Standard Model one, is present. Most people do not expect the Standard Model to be the sole source of CP violation, for very general reasons. Although the Standard Model describes the world accessible to experiment so far, it leaves unanswered many questions and it is conceptually unsatisfactory. Everyone believes that new physics will appear to help explain why the weak scale is where it is, why there are families, and so on. Every form of new physics that has been thought of so far involves new sets of particles whose interactions will normally have phase factors. That all the new phase angles which enter the more complete theory should be zero would seem to be a miraculous coincidence. These particles could come in the Higgs sector, and/or be supersymmetric partners, and/or leptoquarks, and/or new gauge bosons, or new quarks, or other objects. It cannot be emphasized enough that if the Standard Model were the only origin of CP violation it would be a remarkable fact that would require explanation. Indeed, if the view of a number of people that the excess baryon number of the universe is generated at the weak scale is correct, either new sources of CP violation must exist that are larger than the CP violation in K decay, or that source must have a rapid growth with energy. Given this situation, it is extremely important to find new ways to get information about the source of CP violation. Even if very accurate experiments