Karl Schilcher

Bio: Karl Schilcher is an academic researcher from University of California, San Diego. The author has contributed to research in topics: Branching fraction & Quantum chromodynamics. The author has an hindex of 4, co-authored 5 publications receiving 85 citations.

Universality-breaking effects in leptonic z-decays

Abstract: We analyze the possibility of universality violation in diagonal leptonic decays of the [ital Z] boson, in the context of interfamily seesaw'' models. In a minimal extension of the standard model with right-handed neutrino fields, we find that universality-breaking effects increase quadratically with the heavy Majorana neutrino mass and may be observed in the current experiments at the CERN [ital e][sup +][ital e[minus]] collider LEP.

Gauge-Independent Analysis of K_L --> e \mu in Left-Right Models

Abstract: The lepton-flavour-violating decay K_L --> e \mu is studied in detail within the context of SU(2)_R x SU(2)_L x U(1)_(B-L) models, which include heavy Majorana neutrinos. Particular attention is paid to the gauge independence of this decay process to one loop. In analogy with earlier studies on the K^0\bar{K}^0 mixing, it is explicitly shown how restoration of gauge invariance occurs in the decay amplitude containing the box diagrams, when the relevant Higgs-dependent self-energy and vertex graphs are taken into account in the on-shell skeleton renormalization scheme. Based on the analytic expressions so derived, we find that the branching ratio B(K_L --> e \mu) can be considerably enhanced due to the presence of left- and right-handed currents in the loop, and can reach values close to or even larger than the present experimental limit 3.3 x 10^{-11} in the manifest left-right symmetric model. Constraints on the parameter space of typical left-right models are derived from the possible decay K_L --> e \mu and a global analysis of other low-energy data.

Heavy-Neutrino Chirality Enhancement of the Decay $K_L -> e\mu $ in Left-Right Symmetric Models

Abstract: We study the decay $K_L -> e\mu$ in minimal extensions of the Standard Model based on the gauge groups $SU(2)_L x U(1)_Y$ and $SU(2)_R x SU(2)_L x U(1)_{B-L}$, in which heavy Majorana neutrinos are present. In $SU(2)_L x U(1)_Y$ models with chiral neutral singlets, $B(K_L -> e\mu )$ cannot be much larger than $5 x 10^{-15}$ without violating other low-energy constraints. In $SU(2)_L x SU(2)_R x U(1)_{B-L}$ models, we find that heavy-neutrino-chirality enhancements due to the presence of left-handed and right- handed currents can give rise to a branching ratio close to the present experimental limit $B(K_L -> e \mu)< 3.3 x 10^{-11}$.

Electroweak-scale resonant leptogenesis

Abstract: We study minimal scenarios of resonant leptogenesis near the electroweak phase transition. These models offer a number of testable phenomenological signatures for low-energy experiments and future high-energy colliders. Our study extends previous analyses of the relevant network of Boltzmann equations, consistently taking into account effects from out of equilibrium sphalerons and single lepton flavors. We show that the effects from single lepton flavors become very important in variants of resonant leptogenesis, where the observed baryon asymmetry in the Universe is created by lepton-to-baryon conversion of an individual lepton number, for example, that of the {tau}-lepton. The predictions of such resonant {tau}-leptogenesis models for the final baryon asymmetry are almost independent of the initial lepton-number and heavy neutrino abundances. These models accommodate the current neutrino data and have a number of testable phenomenological implications. They contain electroweak-scale heavy Majorana neutrinos with appreciable couplings to electrons and muons, which can be probed at future e{sup +}e{sup -} and {mu}{sup +}{mu}{sup -} high-energy colliders. In particular, resonant {tau}-leptogenesis models predict sizable 0{nu}{beta}{beta} decay, as well as e- and {mu}-number-violating processes, such as {mu}{yields}e{gamma} and {mu}{yields}e conversion in nuclei, with rates that are within reach of the experiments proposed by the MEG and MECOmore » collaborations.« less

Heavy Majorana Neutrinos and Baryogenesis

Abstract: The scenario of baryogenesis through leptogenesis is reviewed in models involving heavy Majorana neutrinos. The various mechanisms of CP violation occurring in the out-of-equilibrium lepton-number-violating decays of heavy Majorana neutrinos are studied within a resummation approach to unstable-particle mixing. It is explicitly demonstrated how the resummation approach preserves crucial field-theoretic properties such as unitarity and CPT invariance. Predictions of representative scenarios are presented after solving numerically the Boltzmann equations describing the thermodynamic evolution of the Universe. The phenomenological consequences of loop effects of heavy Majorana neutrinos on low-energy observables, such as lepton-flavor and/or lepton-number nonconservation in τ and Z-boson decays and electron electric dipole moment, are discussed.

Rare Charm Decays in the Standard Model and Beyond

Abstract: We perform a comprehensive study of a number of rare charm decays, incorporating the first evaluation of the QCD corrections to the short distance contributions, as well as examining the long range effects. For processes mediated by the c → ul + l − transitions, we show that sensitivity to short distance physics exists in kinematic regions away from the vector meson resonances that dominate the total rate. In particular, we find that D → πl + l − and D → ρl + l − are sensitive to non-universal soft-breaking effects in the Minimal Supersymmetric Standard Model with R-parity conservation. We separately study the sensitivity of these modes to R-parity violating effects and derive new bounds on R-parity violating couplings. We also obtain predictions for these decays within extensions of the Standard Model, including extensions of the Higgs, gauge and fermion sectors, as well as models of dynamical electroweak symmetry breaking.