‘ Per Aspera Ad Astra ’ 1 – A Short Essay on the Long Quest for CP Violation

After briefly explaining the special role played by violations of CP and T invariance and their connection with the baryon number of the Universe, I sketch the history of CP violation studies since its totally unexpected discovery in 1964. For about 30 years CP violation could be described by a single number; this has changed dramatically in the years around the turn of the millenium: (i) The existence of direct CP violation was unequivocally established in the decays of long lived kaons. (ii) For the first time CP violation was observed in a system other than that of neutral kaons, namely in B → ψKS. The findings are in impressive agreement with the prediction of the CKM ansatz, which thus has been promoted to the status of a tested theory. These new insights were made possible by close feedback between theory and experiment as well as advances in detector design and a novel machine concept, namely that of an asymmetric collider. We also have direct experimental evidence that the observed CP violation in KL and B decays is matched by a violation of microscopic time reversal violation, as required by CPT symmetry. More recently CP violation has been observed also in B → ππ and B → Kπ. A few comments are added on subtle aspects of direct CP violation. While we know that the CKM dynamics are irrelevant for generating the baryon number of the Universe – i.e. hitherto unknown forces have to be driving it – we have also learnt that such ‘New Physics’ is likely to contain CP violation of sufficient strength. Prologue The conference ‘Time and Matter’ has as subtitle ‘An International Colloquium on the Science of Time’. The tale of the physicists’ quest for CP violation fits naturally into this frame, since it presents us with several variations on the theme of time: the time it took to perform the experimental studies, the uneven rate of progress in our understanding, the time that had to be measured to reveal the sought-after CP asymmetry and the preference Nature shows on the microscopic level for the flow of time. It involves glorious applications of fundamental quantum mechanics, its superposition principle and of EPR correlations [1] with their effects building up over macroscopic distances of centimeters, ”Through the roughs to the stars” To appear in the proceedings of ‘Time and Matter – An International Colloquium on the Science of Time’, Venice, Italy, August 11-17, 2002 meters and even hundreds of meters. Finally it connects the ”heavens” to the ”earth” in that it provides us with a scenario where the seeds for the preponderance of matter over antimatter observed today can be generated dynamically in the very early Universe. 1 On the Special Role of CP Violation There are three discrete transformations of general interest, namely parity P, microscopic time reversal T (operationally amounting to reversal of motion ~p → −~ p) and charge conjugation C, which replaces particles by their antiparticles. Originally it had been assumed without much reflection that all three represent symmetries of nature, since they were known to be conserved by the strong and electromagnetic forces. The first to fall from this pedestal were P and C. The 1957 discovery of P (and subsequently also of C) being violated by the weak forces did cause a paradigm shift. It was, however, realized that even maximal parity violation – meaning there are left-, but no right-handed neutrinos – does not necessarily imply that nature exhibits a genuine preference for left over right. For while the decay π → μνL produces only left-handed neutrinos, the antiparticle decay π → μν̄R yields right-handed neutrinos. They are referred to as antineutrinos, but at this point what is called particle and antiparticle is pure convention. For CP transformations relate the two processes; as long as CP invariance holds, they exhibit identical rates, and ”left” and ”right” is defined in terms of what one calls ”positive” or ”negative”. This is reminiscent of the definition ”the thumb is left on your right hand” – which is as correct as it is circular and thus useless. The observation of CP violation in 1964 by the Fitch-Cronin experiment [2] then came as another shock. 3 For Γ(KL → μ νπ) Γ(KL → μ−ν̄π+) ≃ 1.006 6= 1 , (1) which is related to KL → ππ [3], allows distinguishing a positive charge from a negative one through observation rather than convention. The discovery of CP violation thus changes our picture of Nature’s structure even more profoundly than that of parity violation. At the same time it is quite ‘frustrating’ that a CP invariant – i.e. matter-antimatter symmetric – world is such a ‘near-miss’ with the difference on the 10 level, Eq.(1), in contrast to ‘maximal’ parity violation: Γ(π → μνR)/Γ(π + → μνL) = 0. There are more features singling out CP violation as particularly special and more fundamental than parity violation: (i) Almost any Lorentz invariant local quantum field theory has to possess CPT invariance. CP violation thus has to be matched by a commensurate T violation. I.e., nature distinguishing ‘left’ and ‘right’ implies her to do likewise between a ‘forward’ and ‘backward’ flow of time already on the microscopic level beyond the macroscopic statistical consideration expressed through thermodynamics’ second law of entropy increase. To my knowledge only Okun had stated explicitly before the Fitch-Cronin experiment that the question of CP invariance is one to be decided by experiment rather than theory.