CP violation and the matter–antimatter asymmetry of the Universe

In our everyday environment one observes only matter. Thatʼs quite a fortunate situation! Any sizeable presence of antimatter on Earth, from the enormous energy it would release through annihilation with matter, would prevent us talking about it! For the physicist this fact, at first sight obvious, is nevertheless a kind of surprise: antimatter, which is observed in cosmic rays, in radioactive decays of nuclei, which has been copiously produced and extensively studied in accelerators and which is nowadays currently used in hospitals, turns out to have pretty much the same properties as matter. Moreover, the fact that matter dominates appears to be a general property of our Universe: no evidence of large quantities of antimatter has been observed at any distance from us. Why would matter have taken the advantage on antimatter? In this short review we explain how, through a limited number of basic elements, one can find answers to this question. Matter and antimatter have, in fact, not exactly the same properties: from laboratory experiments CP conservation is known not to be a fundamental law of nature.     

Explicit CP violation in the Dine–Seiberg–Thomas model

The possibility of explicit CP violation is studied in a supersymmetric model proposed by Dine, Seiberg, and Thomas, with two effective dimension-five operators. The explicit CP violation may be triggered by complex phases in the coefficients for the dimension-five operators in the Higgs potential, and by a complex phase in the scalar top quark masses. Although the scenario of explicit CP violation is found to be inconsistent with the experimental data at LEP2 at tree level, it may be possible at the one-loop level. For a reasonable parameter space, the masses of the neutral Higgs bosons and their couplings to a pair of Z bosons are consistent with the LEP2 data, at the one-loop level.     

The effect of the violation of Vegard's law on the optical bowing in Si1 − xGex alloys

The effect of the deviation of the lattice parameters from Vegard's rule on the optical bowing of the fundamental gap in Si_{1 − x}Ge_x semiconductor alloys is discussed. Our computations are based on the pseudopotential method. To make allowance for the chemical disorder, the virtual-crystal approximation is used, including a correction to the alloy potential. Our results show that the bowing parameter is highly improved when the lattice relaxation effect is included which indicates the importance of the lattice mismatch in the electronic band structure calculations of IV-IV semiconductor alloys.     

Search for CP violation in Z0 → τ + τ − and an upper limit on the weak dipole moment of the τ lepton

An improved test of invariance in the reaction e⁺e^? τ ⁺ τ ^? on the Z0 peak is performed using the data sample recorded between 1991 and 1995 with the OPAL detector at LEP. Optimal observables, requiring the reconstruction of the τ flight direction and spin, have been used for different final state topologies. From the non-observation of violation we derive 95% confidence level upper limits on the real and imaginary parts of the weak dipole moment of the τ lepton of |Re(d _τ ^w (m _Z ² | < 5.6 × 10^?18 e cm and |Im(d _τ ^w (m _Z ² ))| < 1.5 × 10^?17 e cm, respectively.     

Measurement of the leptonic branching ratios of the τ lepton

The leptonic branching ratios of the tau lepton have been determined from data collected by the OPAL detector in 1991 and 1992. From a sample of 27196 e⁺e^–+– candidates we find 7322 and 7941 candidates. Using efficiency and background estimates determined from a study of Monte Carlo events and control samples of data, the branching ratios =(18.14±0.20±0.28)% and =(17.48±0.18±0.23)% have been obtained. These new results have been combined with the published results for the 1990 OPAL data to yield the following branching ratios for data taken between 1990 and 1992:

Analysis of the neutralino system in three–body leptonic decays of neutralinos

Neutralinos in supersymmetric theories, the spin–1/2 Majorana–type superpartners of the U(1) and SU(2) neutral electroweak gauge bosons and SU(2) neutral Higgs bosons, are expected to be among light supersymmetric particles so that they can be produced copiously via direct pair production and/or from cascade decays of other sparticles such as sleptons at the planned Large Hadron Collider and the prospective International Linear Collider. Considering the prospects of having both highly polarized neutralinos and possibility of reconstructing their decay rest frames, we provide a systematic investigation of the three–body leptonic decays of the neutralinos in the minimal supersymmetric standard model and demonstrate alternative ways for probing the Majorana nature of the neutralinos and CP violation in the neutralino system.     

The CP violation of theB→Kϕ decay at γ(4s) in the non-standard models

The CP violating asymmetry in theB→K? decay is studied in both supersymmetric model and three Higgs doublets model. The absorptive part of the penguin amplitude leads to the CP violating asymmetry in theB _u ^+- →K ^±? decays, which is at most 3% due to the GIM mechanism in the standard model. However, the asymmetry could be relatively large if there exists a new source of the imaginary component in the decay amplitude. In the supersymmetric model, the contributions of the imaginary part of the super-penguin and the super-box processes rather suppress the magnitude of the asymmetry, whereas in the three Higgs doublets model, that of the charged Higgs-penguin process could significantly enhance it.