The Lightest Higgs Boson Mass in the Minimal Supersymmetric Standard Model

We compute the upper bound on the mass of the lightest Higgs boson in the Minimal Supersymmetric Standard Model in a model-independent way, including leading (one-loop) and next-to-leading order (two-loop) radiative corrections. We find that (contrary to some recent claims) the two-loop corrections are negative with respect to the one-loop result and relatively small (\simlt 3\%). After defining physical (pole) top quark mass $M_t$, by including QCD self-energies, and physical Higgs mass $M_H$, by including the electroweak self-energies $\Pi\left(M_H^2\right)-\Pi(0)$, we obtain the upper limit on $M_H$ as a function of supersymmetric parameters. We include as supersymmetric parameters the scale of supersymmetry breaking $M_S$, the value of $\tan \beta$ and the mixing between stops $X_t= A_t + \mu \cot\beta$ (which is responsible for the threshold correction on the Higgs quartic coupling). Our results do not depend on further details of the supersymmetric model. In particular, for $M_S\leq 1$ TeV, maximal threshold effect $X_t^2=6M_S^2$ and any value of $\tan\beta$, we find $M_H\leq 140$ GeV for $M_t\leq 190$ GeV. In the particular scenario where the top is in its infrared fixed point we find $M_H\leq 86$ GeV for $M_t = 170$ GeV.Comment: 24 pages + 15 figures in one compressed uuencoded tarred postscript file (The figures can be obtained by e-mail from [email protected]; also, the whole postscript file of the text including the figures can be obtained by ANONYMOUS FTP from ROCA.CSIC.ES (161.111.20.20) at the directory HEP the file being HIGGS.PS: just type GET HEP/HIGGS.PS), Latex, CERN-TH.7334/9

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

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

Conflict and Uncertainty: A Dynamic Approach

Most of the conflict theory papers have used a one-shot game set-up. This does not correspond to reality and is certainly incapable of modeling real conflict situations. We propose a dynamic model with N-agents in an infinite time frame which allow us to adequately analyze conflicts. The dynamic aspects of the conflict come at least from two sources: first, the preferences on the good in dispute are not static; second, agentsin conflict can influence the future of the conflict by making investment in conflict's technology. We use a simple deterministic rule that defines the evolution of the subjective valuation for the good in dispute according to the results obtained by the agents in the recent past. During each period the realization of stochastic variables of the nature's states induces uncertainty in the game. The model is a theoretical approach that can be applied to evaluate the role of uncertainty and valuations' evolution on the optimal choices of forward-looking economic agents that seek to appropriate a share of a divisible resource.Conflict Theory, Dynamic Economic Model, Uncertainty

Strategic Behaviour, Resource Valuation and Competition in Electricity Markets

By means of a suitable Bayesian game we study spot electricity markets from a structural point of view. We address the problem of individual and aggregate eficciency and we show how to value water from market observables. We compare the former to engineering methods and apply our methodology to Colombian spot electricity market. Our results show that big gas and small hydro plants overbid, resources are undervalued by engineering costs and aggregate costs would have been considerably smaller if agents had played optimally. Revealed costs show a substantial gain in eficciency in the Vickrey auction compared to the actual uniform auction.Multi-unit auctions, Oligopoly, electricity markets

On business cycles and countercyclical policies

Since the third quarter of 2000, the U.S. economy began to experience a slowdown in its rate of growth. This slowdown serves as a reminder that the business cycle is still alive and raises the following questions: What do we know about the driving forces behind the business cycle? What should policymakers do in the face of economic fluctuations? ; The authors examine two explanations for business cycles that are well-known in academic circles: the animal spirits theory and the real business cycle theory. The former is closely connected with the Keynesian economic tradition and identifies market participants' mood swings as the key source of economic fluctuations. The second explanation is rooted in the classical economic tradition and views productivity shocks as the driving force behind economic fluctuations. The article then looks at what these theories suggest about countercyclical policies, which try to eliminate business cycle fluctuations or insulate market participants from their effects. The authors conclude that neither theory makes an unambiguous case supporting countercyclical policies. ; This conclusion may come as a surprise to government and business economists who have an ingrained belief in the benefits of such policies. It is important to remember, however, that attempts to understand business cycles and the effects and desirability of policies that may (or may not) moderate them are still at a very early stage.Business cycles ; Monetary policy ; Keynesian economics