Implications for New Physics from Fine-Tuning Arguments: II. Little Higgs Models

We examine the fine-tuning associated to electroweak breaking in Little Higgs scenarios and find it to be always substantial and, generically, much higher than suggested by the rough estimates usually made. This is due to implicit tunings between parameters that can be overlooked at first glance but show up in a more systematic analysis. Focusing on four popular and representative Little Higgs scenarios, we find that the fine-tuning is essentially comparable to that of the Little Hierarchy problem of the Standard Model (which these scenarios attempt to solve) and higher than in supersymmetric models. This does not demonstrate that all Little Higgs models are fine-tuned, but stresses the need of a careful analysis of this issue in model-building before claiming that a particular model is not fine-tuned. In this respect we identify the main sources of potential fine-tuning that should be watched out for, in order to construct a successful Little Higgs model, which seems to be a non-trivial goal.Comment: 39 pages, 26 ps figures, JHEP forma

Large mixing angles for neutrinos from infrared fixed points

Radiative amplification of neutrino mixing angles may explain the large values required by solar and atmospheric neutrino oscillations. Implementation of such mechanism in the Standard Model and many of its extensions (including the Minimal Supersymmetric Standard Model) to amplify the solar angle, the atmospheric or both requires (at least two) quasi-degenerate neutrino masses, but is not always possible. When it is, it involves a fine-tuning between initial conditions and radiative corrections. In supersymmetric models with neutrino masses generated through the Kahler potential, neutrino mixing angles can easily be driven to large values at low energy as they approach infrared pseudo-fixed points at large mixing (in stark contrast with conventional scenarios, that have infrared pseudo-fixed points at zero mixing). In addition, quasi-degeneracy of neutrino masses is not always required.Comment: 36 pages, 7 ps figure

Theoretical Constraints on the Vacuum Oscillation Solution to the Solar Neutrino Problem

The vacuum oscillation (VO) solution to the solar anomaly requires an extremely small neutrino mass splitting, Delta m^2_{sol}\leq 10^{-10} eV^2. We study under which circumstances this small splitting (whatever its origin) is or is not spoiled by radiative corrections. The results depend dramatically on the type of neutrino spectrum. If m_1^2 \sim m_2^2 \geq m_3^2, radiative corrections always induce too large mass splittings. Moreover, if m_1 and m_2 have equal signs, the solar mixing angle is driven by the renormalization group evolution to very small values, incompatible with the VO scenario (however, the results could be consistent with the small-angle MSW scenario). If m_1 and m_2 have opposite signs, the results are analogous, except for some small (though interesting) windows in which the VO solution may be natural with moderate fine-tuning. Finally, for a hierarchical spectrum of neutrinos, m_1^2 << m_2^2 << m_3^2, radiative corrections are not dangerous, and therefore this scenario is the only plausible one for the VO solution.Comment: 13 pages, LaTeX, 3 ps figures (psfig.sty

Bounds on the Higgs-Boson Mass in the Presence of Non-Standard Interactions

The triviality and vacuum stability bounds on the Higgs-boson mass are revisited in the presence of new interactions parameterized in a model-independent way by an effective lagrangian. When the scale of new physics is below 50 TeV the triviality bound is unchanged but the stability lower bound is increased by 40-60 GeV. Should the Higgs-boson mass be close to its current lower experimental limit, this leads to the possibility of new physics at the scale of a few TeV, even for modest values of the effective lagrangian parameters.Comment: 5 pages, 2 figures, RevTex, submitted to PR

General RG Equations for Physical Neutrino Parameters and their Phenomenological Implications

The neutral leptonic sector of the Standard Model presumably consists of three neutrinos with non-zero Majorana masses with properties further determined by three mixing angles and three CP-violating phases. We derive the general renormalization group equations for these physical parameters and apply them to study the impact of radiative effects on neutrino physics. In particular, we examine the existing solutions to the solar and atmospheric neutrino problems, derive conclusions on their theoretical naturalness, and show how some of the measured neutrino parameters could be determined by purely radiative effects. For example, the mass splitting and mixing angle suggested by solar neutrino data could be entirely explained as a radiative effect if the small angle MSW solution is realized. On the other hand, the mass splitting required by atmospheric neutrino data is probably determined by unknown physics at a high energy scale. We also discuss the effect of non-zero CP-violating phases on radiative corrections