Extra Higgs bosons in ttbar production at the LHC

The top quark has a large Yukawa coupling with the Higgs boson. In the usual extensions of the standard model the Higgs sector includes extra scalars, which also tend to couple strongly with the top quark. Unlike the Higgs, these fields have a natural mass above 2m_t, so they could introduce anomalies in ttbar production at the LHC. We study their effect on the ttbar invariant mass distribution at sqrt{s}=7 TeV. We focus on the bosons (H,A) of the minimal SUSY model and on the scalar field (r) associated to the new scale f in Little Higgs (LH) models. We show that in all cases the interference with the standard amplitude dominates over the narrow-width contribution. As a consequence, the mass difference between H and A or the contribution of an extra T-quark loop in LH models become important effects in order to determine if these fields are observable there. We find that a 1 fb^{-1} luminosity could probe the region tan beta \le 3 of SUSY and v/(sqrt{2}f) \ge 0.3 in LH models.Comment: 18 pages, version to appear in PR

Energy conditions and their cosmological implications

The energy conditions of general relativity permit one to deduce very powerful and general theorems about the behaviour of strong gravitational fields and cosmological geometries. However, the energy conditions these theorems are based on are beginning to look a lot less secure than they once seemed: (1) there are subtle quantum effects that violate all of the energy conditions, and more tellingly (2), there are also relatively benign looking classical systems that violate all the energy conditions. This opens up a Pandora's box of rather disquieting possibilities --- everything from negative asymptotic mass, to traversable wormholes, to warp drives, up to and including time machines.Comment: Plenary talk presented at Cosmo99, Trieste, Sept/Oct 1999. 8 pages, latex 209, World Scientific style file ltwol.sty (included

Combinatorial Representation Theory

We attempt to survey the field of combinatorial representation theory, describe the main results and main questions and give an update of its current status. We give a personal viewpoint on the field, while remaining aware that there is much important and beautiful work that we have not been able to mention

From vacuum fluctuations across an event horizon to long distance correlations

We study the stress energy two-point function to show how short distance correlations across the horizon transform into correlations among asymptotic states, for the Unruh effect, and for black hole radiation. In the first case the transition is caused by the coupling to accelerated systems. In the second, the transition is more elusive and due to the change of the geometry from the near horizon region to the asymptotic one. The gradual transition is appropriately described by using affine coordinates. We relate this to the covariant regularization used to evaluate the mean value of the stress energy. We apply these considerations to analogue black holes, i.e. dispersive theories. On one hand, the preferred rest frame gives further insight about the transition, and on the other hand, the dispersion tames the singular behavior found on the horizon in relativistic theories.Comment: 21 pages, 4 figures, new section on growth of correlation

Group size effect on cooperation in one-shot social dilemmas II. Curvilinear effect

In a world in which many pressing global issues require large scale cooperation, understanding the group size effect on cooperative behavior is a topic of central importance. Yet, the nature of this effect remains largely unknown, with lab experiments insisting that it is either positive or negative or null, and field experiments suggesting that it is instead curvilinear. Here we shed light on this apparent contradiction by considering a novel class of public goods games inspired to the realistic scenario in which the natural output limits of the public good imply that the benefit of cooperation increases fast for early contributions and then decelerates. We report on a large lab experiment providing evidence that, in this case, group size has a curvilinear effect on cooperation, according to which intermediate-size groups cooperate more than smaller groups and more than larger groups. In doing so, our findings help fill the gap between lab experiments and field experiments and suggest concrete ways to promote large scale cooperation among people.Comment: Forthcoming in PLoS ON

A minimal Little Higgs model

We discuss a Little Higgs scenario that introduces below the TeV scale just the two minimal ingredients of these models, a vectorlike T quark and a singlet component (implying anomalous couplings) in the Higgs field, together with a pseudoscalar singlet \eta. In the model, which is a variation of Schmaltz's simplest Little Higgs model, all the extra vector bosons are much heavier than the T quark. In the Yukawa sector the global symmetry is approximate, implying a single large coupling per flavour, whereas in the scalar sector it is only broken at the loop level. We obtain the one-loop effective potential and show that it provides acceptable masses for the Higgs h and for the singlet \eta with no need for an extra \mu term. We find that m_\eta can be larger than m_h/2, which would forbid the (otherwise dominant) decay mode h -> \eta\eta.Comment: 16 pages. References added, fine tuning analysis included. Version to appear in PR