Luca Vecchi

On Partial Compositeness and the CP Asymmetry in Charm Decays

Recently, the LHCb and CDF collaborations reported the measure of an unexpectedly large direct CP asymmetry in D meson decays. In this paper we ask if new physics associated with Partial Compositeness could plausibly explain this result. We nd that Composite Higgs models with mass scale around 10 TeV can account for it, while marginally satisfying all other avor constraints in the quark sector. The minimal framework is however inadequate in the lepton sector due to the strong constraint from ! e . This tension can be eciently alleviated by realizing Partial Compositeness within Supersymmetry. The resulting models can saturate the CP asymmetry in D decays for superpartner masses close to the TeV scale and somewhat large A-terms. The supersymmetric realization of Partial Compositeness also oers a predictive and phenomenologically viable organizing principle for R-Parity violation, and may result in very distinctive signatures at hadron colliders. With or without Supersymmetry, the neutron EDM is expected to be around the present experimental sensitivity.

Asymmetric WIMP dark matter

In existing dark matter models with global symmetries the relic abundance of dark matter is either equal to that of anti-dark matter (thermal WIMP), or vastly larger, with essentially no remaining anti-dark matter (asymmetric dark matter). By exploring the consequences of a primordial asymmetry on the coupled dark matter and anti-dark matter Boltzmann equations we find large regions of parameter space that interpolate between these two extremes. Interestingly, this new asymmetric WIMP framework can accommodate a wide range of dark matter masses and annihilation cross sections. The present-day dark matter population is typically asymmetric, but only weakly so, such that indirect signals of dark matter annihilation are not completely suppressed. We apply our results to existing models, noting that upcoming direct detection experiments will constrain a large region of the relevant parameter space.

Phenomenology of a light scalar: The dilaton

We make use of the language of nonlinear realizations to analyze electroweak symmetry breaking scenarios in which a light dilaton emerges from the breaking of a nearly conformal strong dynamics and compare the phenomenology of the dilaton to that of the well-motivated light composite Higgs scenario. We argue that - in addition to departures in the decay/production rates into massless gauge bosons mediated by the conformal anomaly - characterizing features of the light dilaton scenario are enhancements in off-shell events at high invariant mass involving two longitudinally polarized vector bosons and a dilaton, and tree-level flavor violating processes. Accommodating both electroweak precision measurements and flavor constraints appears especially challenging in the ambiguous scenario in which the Higgs and the dilaton fields strongly mix. We show that warped higgsless models of electroweak symmetry breaking are explicit and tractable realizations of this limiting case. The relation between the naive radion profile often adopted in the study of holographic realizations of the light dilaton scenario and the actual dynamical dilaton field is clarified in the Appendix.

Probing Nonstandard Standard Model Backgrounds with LHC Monojets

Abstract Monojet events at colliders have been used to probe models of dark matter and extra dimensions. We point out that these events also probe extensions of the Standard Model modifying neutrino–quark interactions. Such nonstandard interactions (NSI) have been discussed in connection with neutrino oscillation experiments. Assuming first that NSI remain contact at LHC energies, we derive stringent bounds that approach the levels suggested by the B 8 solar data. We next explore the possibility that the mediators of the NSI can be produced at colliders. The constraints are found to be strongest for mediator masses in the 10 2 – 10 3 GeV range, with the best bounds above ∼ 200 GeV coming from ATLAS and below from CDF. For mediators with masses below 30 GeV the monojet bounds are weaker than in the contact limit. These results also directly apply to light dark matter searches. Lastly, we discuss how neutrino NSI can be distinguished from dark matter or Kaluza–Klein states with charged lepton searches.

Multitrace deformations, Gamow states, and stability of AdS/CFT

We analyze the effect of multitrace deformations in conformal field theories at leading order in a large N approximation. These theories admit a description in terms of weakly coupled gravity duals. We show how the deformations can be mapped into boundary terms of the gravity theory and how to reproduce the RG equations found in field theory. In the case of doubletrace deformations, and for bulk scalars with masses in the range − d2/4 <m2< − d2/4 + 1, the deformed theory flows between two fixed points of the renormalization group, manifesting a resonant behavior at the scale characterizing the transition between the two CFT’s. On the gravity side the resonance is mapped into an IR non-normalizable mode (Gamow state) whose overlap with the UV region increases as the dual operator approaches the free field limit. We argue that this resonant behavior is a generic property of large N theories in the conformal window, and associate it to a remnant of the Nambu-Goldstone mode of dilatation invariance. We emphasize the role of nonminimal couplings to gravity and establish a stability theorem for scalar/gravity systems with AdS boundary conditions in the presence of arbitrary boundary potentials and nonminimal coupling.

Fermions and Goldstone bosons in an asymptotically safe model

Abstract We consider a model in which Goldstone bosons, described by a SU ( N ) chiral nonlinear σ model, are coupled to an N-plet of colored fermions by means of a Yukawa interaction. We study the one-loop renormalization group flow and show that the non-Gaussian UV fixed point, which is present in the purely bosonic model, is lost because of fermion loop effects unless N is sufficiently large. We then add four-fermion contact interactions to the Lagrangian and show that in this case there exist several non-Gaussian fixed points. The strength of the contact interactions, predicted by the requirement that the theory flows towards a fixed point in the UV, is compared to the current experimental bounds. This toy model could provide an important building block of an asymptotically safe model of the weak interactions.

The conformal window of deformed conformal field theories in the planar limit

We discuss in the planar approximation the effect of double-trace deformations on conformal field theories. We show that this large class of models posses a conformal window describing a nontrivial flow between two fixed points of the renormalization group and reveal the presence of a resonance which we associate to the remnant of a dilaton pole. As the conformal window shrinks to zero measure, the theory undergoes a conformal phase transition separating a symmetric from a nonsymmetric phase. The recently conjectured strongly coupled branch of nonsupersymmetric, non-Abelian gauge theories with a large number of flavors is analyzed in light of these results, and a model for the strong branch is proposed. Some phenomenological implications in the context of unparticle physics are also emphasized.

Stability of thick brane worlds nonminimally coupled to gravity

We analyze a class of 5D models where a 3 brane is generated by a bulk scalar field nonminimally coupled to gravity. We show that perturbative stability of such branes is normally guaranteed although nonminimal couplings are not innocuous in general. After the physical states are identified, the linearized equations for the propagating modes are evaluated into a Schroedinger form. The spectrum contains a tower of spin 2 and spin 0 fields with continuous masses starting from zero ones. For regular geometries the scalar spectrum contains a massless state which is always non-normalizable. The propagating massive scalar states are repelled off the brane due to a centrifugal potential.

Technicolor at criticality

We discuss an asymptotically non-free, natural model for dynamical electroweak symmetry breaking characterized by the emergence of a weakly coupled Higgs in the IR regime. Due to the large anomalous dimension of the Higgs operator, the model is capable of solving the hierarchy problem without losing the phenomenologically appealing features typical of weakly coupled Higgs sectors.We speculate on the possibility that such a scenario be realized as a strongly coupled phase of non-supersymmetric non-abelian gauge theories.

Color & weak triplet scalars, the dimuon asymmetry in B s decay, the top forward-backward asymmetry, and the CDF dijet excess

The new physics required to explain the anomalies recently reported by the D0 and CDF collaborations, namely the top forward-backward asymmetry (FBA), the like-sign dimuon charge asymmetry in semileptonic b decay, and the CDF dijet excess, has to feature an amount of flavor symmetry in order to satisfy the severe constrains arising from flavor violation. In this paper we show that, once baryon number conservation is imposed, color & weak triplet scalars with hypercharge Y = −1/3 can feature the required flavor structure. The color & weak triplet model can simultaneously explain the top FBA and the dimuon charge asymmetry or the dimuon charge asymmetry and the CDF dijet excess. However, the CDF dijet excess appears to be incompatible with the top FBA in the minimal framework. Our model for the dimuon asymmetry predicts the observed pattern hd ≪ hs in the region of parameter space required to explain the top FBA, whereas our model for the CDF dijet anomalyis characterized by the absence of beyond the SM b-quark jets in the excess region. Compatibility of the color & weak triplet with the electroweak constraints is also discussed. We show that a Higgs boson mass exceeding the LEP bound is typically favored in this scenario, and that both Higgs production and decay can be significantly altered by the triplet. The most promising collider signature is found if the splitting among the components of the triplet is of weak scale magnitude.