Direct CP symmetry violation

Press release Direct CP symmetry violation and matter-antimatter asymmetry: new findings from an experiment at CERN
Paris, June 28, 1999

At a seminar held on June 18 at CERN, the NA48 experiment presented its first findings on the measurement of the "direct CP symmetry violation" parameter. This experiment, in which French physicists from CEA and from IN2P3/CNRS are taking part, has confirmed a characteristic effect of our Universe, in which matter predominates over antimatter. CP symmetry violation is one of the three conditions necessary to create the matter-antimatter asymmetry that can be observed in the Universe. In 1964, J. Christenson, J. Cronin, V. Fitch and R.Turlay brought to light CP symmetry violation by studying the properties of neutral kaons (1), a system in which particles mix with their antiparticles. Two symmetries are involved in this process: the relationship between a particle and its mirror image (P symmetry), and the exchange between particles and antiparticles (C symmetry). That discovery, made in Brookhaven, USA, was based on observation of a specific neutral kaon particle disintegration that is impossible if CP symmetry is conserved. That observation made it possible to conclude that the exchange processes do not occur with strictly the same probability, as CP conservation would require (2). The Standard Model, a theory that is firmly established in particle physics, explains this CP symmetry violation using a mechanism based on the existence of three families of quarks. This model also predicts another CP symmetry violation mechanism, then referred to as "direct violation" and which is even more rare. In 1993, teams performing two experiments, NA31 at CERN and E731 at FermiLab in the United States, published their respective findings on direct CP violation. The measurement by NA31 indicated that the existence of this direct violation was highly probable. A new generation of experiments followed in order to achieve even higher measurement accuracy. Among these experiments, KTeV at Fermilab confirmed the effect in February 1999 and the new findings from NA48 confirm those of NA31 and of KTeV, thus firmly establishing direct CP violation in the neutral kaon system. The NA48 team is constituted by collaboration between about one hundred physicists from European laboratories and universities from CERN, Italy, Great Britain, Germany, Russia, and France. The French teams from CEA-Saclay (DAPNIA) and from IN2P3/CNRS (LAL) have been particularly involved in the construction of detectors and in analysis. Additional data is in the process of being acquired and analyzed, which will enable the physicists from NA48 to perform even more accurate measurements of this fundamental asymmetry of the Universe. 1 - A kaon is a particle made up of a quark and of an antiquark, one of which is "down" or "up", and the other of which is "strange." 2 - CP symmetry violation is measured using the parameter , equal to 0.002. Direct CP symmetry violation is quantified by the parameter '. The two recent experiments have measured the ratio '/. KTeV published preliminary findings with '/ = 28 ± 4.110-4, the findings of NA48 are '/ = 18.5 ± 7.310-4 Scientific contacts: CNRS, Lydia Iconomicou-Fayard Tel. +33 1 64 46 84 47 CEA, Michel Spiro Tel. +33 1 69 08 23 86 Press contact: IN2P3 / CNRS, Geneviève Edelheit Tel. +33 1 44 96 47 60