New physics searches at kaon and hyperon factories.

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

Asymmetric Higgsino dark matter.

In the supersymmetric framework, prior to the electroweak phase transition, the existence of a baryon asymmetry implies the existence of a Higgsino asymmetry. We investigate whether the Higgsino could be a viable asymmetric dark matter candidate. We find that this is indeed possible. Thus, supersymmetry can provide the observed dark matter abundance and, furthermore, relate it with the baryon asymmetry, in which case the puzzle of why the baryonic and dark matter mass densities are similar would be explained. To accomplish this task, two conditions are required. First, the gauginos, squarks, and sleptons must all be very heavy, such that the only electroweak-scale superpartners are the Higgsinos. With this spectrum, supersymmetry does not solve the fine-tuning problem. Second, the temperature of the electroweak phase transition must be low, in the (1-10) GeV range. This condition requires an extension of the minimal supersymmetric standard model.

Testing new indirect CP violation.

If new CP violating physics contributes to neutral meson mixing, but its contribution to CP violation in decay amplitudes is negligible, then there is a model independent relation between four (generally independent) observables related to the mixing: the mass splitting (x), the width splitting (y), the CP violation in mixing (1-|q/p|), and the CP violation in the interference of decays with and without mixing (phi). For the four neutral meson systems, this relation can be written in a simple approximate form: y tan phi approximately x(1-|q/p|). In the K system, all four observables have been measured and obey the relation to excellent accuracy. For the B(s) and D systems, new predictions are provided. The success or failure of these relations will probe the physics that is responsible for the CP violation.

Combining K0-K0 mixing and D0-D0 mixing to constrain the flavor structure of new physics.

New physics at high energy scale often contributes to K0-K0 and D0-D0 mixings in an approximately SU(2)L invariant way. In such a case, the combination of measurements in these two systems is particularly powerful. The resulting constraints can be expressed in terms of misalignments and flavor splittings.

Importance of the heavier singlet neutrinos in leptogenesis.

We argue that fast interactions of the lightest singlet neutrino N1 would project part of a preexisting lepton asymmetry L{p} onto a direction that is protected from N1 washout effects, thus preventing it from being erased. In particular, we consider an asymmetry generated in N2 decays, assuming that N1 interactions are fast enough to bring N1 into full thermal equilibrium. If N1 decays occur at T > or = 10{9} GeV, that is, before the muon Yukawa interactions enter into thermal equilibrium, then generically part of L{p} survives. In this case some of the constraints implied by the standard N1 leptogenesis scenario hold only if L{p} approximately 0. For T < or = 10{9} GeV, L{p} is generally erased, unless special alignment or orthogonality conditions in flavor space are realized.

Constraining the phase of Bs-Bs mixing.

New physics contributions to Bs-Bs mixing can be parametrized by the size (rs2) and the phase (2thetas) of the total mixing amplitude relative to the standard model amplitude. The phase has so far been unconstrained. We first use the D0 measurement of the semileptonic CP asymmetry ASL to obtain the first constraint on the semileptonic CP asymmetry in Bs decays, ASLs=-0.008+/-0.011. Then we combine recent measurements by the CDF and D0 Collaborations--the mass difference (DeltaMs), the width difference (DeltaGammas), and ASL;s--to constrain 2thetas. The errors on DeltaGammas and ASL;s should still be reduced to have a sensitive probe of the phase, yet the central values are such that the regions around 2thetas approximately 3pi/2 and, in particular, 2thetas approximately pi/2, are disfavored.

Leptogenesis from supersymmetry breaking.

We show that soft supersymmetry breaking terms involving the heavy sneutrinos can lead to sneutrino-antisneutrino mixing and to new sources of CP violation, which are present even if a single generation is considered. These terms are naturally present in supersymmetric versions of leptogenesis scenarios, and they induce indirect CP violation in the decays of the heavy sneutrinos, eventually generating a baryon asymmetry. This new contribution can be comparable to or even dominate over the asymmetry produced in traditional leptogenesis scenarios.

Measuring the photon polarization in B -->Kpipigamma.

We propose a way of measuring the photon polarization in radiative B decays into K resonance states decaying to Kpipi, which can test the standard model and probe new physics. The photon polarization is shown to be measured by the up-down asymmetry of the photon direction relative to the Kpipi decay plane in the K resonance rest frame. The integrated asymmetry in K1(1400)-->Kpipi, calculated to be 0.34 plus/minus 0.05 in the standard model, is measurable at currently operating B factories.