Precision tests with Kl3 and Kl2 decays

The analysis made in 2000 indicated that the unitarity relation Vud^2 + Vus^2 + Vub^2 = 1 might be broken at the 2.3 sigma level. At that time, however, Vus was inferred from old experimental data. Since then, a great experimental and theoretical effort has been invested to understand the source of that discrepancy. Thanks to the new and improved measurements by BNL-E865, KLOE, KTeV, ISTRA+ and NA48, the old Kl3 decay rate got shifted so that the new Vus is now consistent with unitarity. On the theory side, much progress in the lattice QCD has been made in order to tame the systematic uncertainties related to the computation of the Kl3 form factors. This joint progress allowed to assess the validity of the CKM unitarity relation at the level of less than 1%. The key challenge of the future lattice studies will be to simulate lighter pions in the region in which ChPT predictions apply. Also interesting is the recent progress in accurately computing the kaon and pion decay constants on the lattice, which then give us access to Vus and Vud from the corresponding leptonic decays. In addition, we discuss that the Kl3 and Kl2 decays offer the possibility to test various scenarios of physics beyond Standard Model.Comment: Contributed to the International Europhysics Conference on High Energy Physics (EPS-HEP2007), Manchester, England, 19-25 Jul 200

Rare Kaon Decays K->pi n n and KL->pi l+l-

Over the next years, the Flavour Physics community will be looking for inconsistencies of the Standard Model (SM) by exploiting new and precise measurements. In these ``indirect new physics'' searches, the key strategy is to concentrate on observables, which are theoretically clean and preferably suppressed in the SM. In this respect, the four golden modes K+->pi+ n n, KL->pi n n and KL->pi0 l+l- are very promising. The pollution from hadronic uncertainties in these decays is at present under control with good accuracy, thanks to the interplay between theory information (Chiral Perturbation Theory and OPE) and experimental inputs (KTeV, NA48). Their measurements might thus give rise to some unexpected scenario. Here, we briefly review the present situation for these four exclusive rare decays.Comment: Talk given at EPS International Europhysics Conference on High Energy Physics (HEPP-EPS 2005), Lisbon, Portugal, 21-27 Jul 200

Unitarity and causality constraints in composite Higgs models

We study the scattering of longitudinally polarized W bosons in extensions of the Standard Model where anomalous Higgs couplings to gauge sector and higher order O(p^4) operators are considered. These new couplings with respect to the Standard Model should be thought as the low energy remnants of some new dynamics involving the electroweak symmetry breaking sector. By imposing unitarity and causality constraints on the WW scattering amplitudes we find relevant restrictions on the possible values of the new couplings and the presence of new resonances above 300 GeV. We investigate the properties of these new resonances and their experimental detectability. Custodial symmetry is assumed to be exact throughout and the calculation avoids using the Equivalence Theorem as much as possible.Comment: added references, better plot resolutio

B -> D tau nu Branching Ratios: Opportunity for Lattice QCD and Hadron Colliders

In the Standard Model, scalar contributions to leptonic and semileptonic decays are helicity suppressed. The hypothesis of additional physical neutral/charged Higgses can enhance such scalar contributions and give detectable effects especially in B physics. For the charged Higgs, experimental information on both Br(B -> D tau nu) and Br(B -> tau nu) has already become available and in particular the B -> D tau nu branching ratio measurements will be further improved in the coming years. Hadronic uncertainties of scalar contributions in semileptonic decays are already in much better shape than the ones plaguing the helicity suppressed leptonic decays B -> tau nu. Combining existing experimental information form the B factories, we explore which existing and future lattice estimates will be useful to directly address new physics effects from measurements of Br(B_{u,d,s} -> D_{u,d,s} tau nu), which can be performed also at hadron colliders.Comment: 8 pages, 5 figure

Light-quark Loops in K->pi nu nu

We present a comprehensive analysis of the contributions to K->pi nu nu decays not described by the leading dimension-six effective Hamiltonian. These include both dimension-eight four-fermion operators generated at the charm scale, and genuine long-distance contributions which can be described within the framework of chiral perturbation theory. We show that a consistent treatment of the latter contributions, which turn out to be the dominant effect, requires the introduction of new chiral operators already at O(GF^2 p^2). Using this new chiral Lagrangian, we analyze the long-distance structure of K->pi nu nu amplitudes at the one-loop level, and discuss the role of the dimension-eight operators in the matching between short- and long-distance components. From the numerical point of view, we find that these O(GF^2 LambdaQCD^2) corrections enhance the SM prediction of Br(K+->pi+ nu nu) by about 6

Axion-Higgs interplay in the two Higgs-doublet model

We study the Dine-Fischler-Srednicki (DFS) model in the light of the recent Higgs LHC results and electroweak precision data. The DFS model is a natural extension of the two-Higgs doublet model endowed with a Peccei-Quinn symmetry and leading to a physically acceptable axion. For generic couplings, the model reproduces the minimal Standard Model showing only tiny deviations (extreme decoupling scenario) whereas all additional degrees of freedom (with the exception of the axion) are very heavy. Recently, new corners of this model have been highlighted where it may exhibit enlarged global symmetries making the corresponding models technically natural (naturalness scenario). In some cases an additional Higgs could be present at the weak scale. In this case, the new light $0^+$ state would be accompanied by relatively light charged and neutral pseudoscalar Higgses. We will use the oblique corrections, particularly $\Delta\rho$, to constrain the mass spectrum in this case. As a final result, we also work out the non-linear parametrization of the DFS model in the generic case where all scalars except the lightest Higgs and the axion have masses at or beyond the TeV scale.Comment: comments and references added. results unchage

Vus from Kl3 Decays

Important progress made this year, both in theory and in experiment, helped solving the problem of 2 sigma-deviation from the unitarity of the first row elements in the CKM matrix. Today we have, |Vus|^2 + |Vud|^2 + |Vub|^2-1 = -0.0008(13), -0.0010(13), or -0.0005(13), depending on whether the q^2-dependence of the relevant Kl3 form factor is considered as pole-like, linear or quadratic function, and on the Leutwyler-Roos value of f+(0)=0.961(8), whose validity has recently been reinforced by lattice studies. In this talk we summarize the recent developments.Comment: Contributed to 32nd International Conference on High-Energy Physics (ICHEP 04), Beijing, China, 16-22 Aug 2004; minor typos correcte

Semileptonic D-decays and Lattice QCD

We explore four different strategies to extract the D-meson semileptonic decay form factors from the Green functions computed in QCD numerically on the lattice. From our numerical tests we find that two such strategies, based on the use of double ratios of 3-point correlation functions, lead to an appreciable reduction of systematic uncertainties. This is an important step in reducing the overall uncertainty in the lattice QCD results for the D-decay form factors which are needed to determine the CKM entries |Vcd| and |Vcs| experimentally, that are nowadays known by imposing the unitarity of the CKM matrix.Comment: 7pages, proceedings write-up for Lattice 200

Interpreting a 2 TeV resonance in WW scattering

A diboson excess has been observed ---albeit with very limited statistical significance--- in $WW$, $WZ$ and $ZZ$ final states at the LHC experiments using the accumulated 8 TeV data. Assuming that these signals are due to resonances resulting from an extended symmetry breaking sector in the standard model and exact custodial symmetry we determine using unitarization methods the values of the relevant low-energy constants in the corresponding effective Lagrangian. Unitarity arguments also predict the widths of these resonances. We introduce unitarized form factors to allow for a proper treatment of the resonances in Monte Carlo generators and a more precise comparison with experiment.Comment: 17 pages, 7 figures. The new version contains an updated and revised list of references as well as a slightly extended conclusion sectio

Radiative corrections to WL WL scattering in composite Higgs models

The scattering of longitudinally polarized electroweak bosons is likely to play an important role in the elucidation of the fundamental nature of the Electroweak Symmetry Breaking sector and in determining the Higgs interactions with this sector. In this paper, by making use of the Equivalence Theorem, we determine the renormalization properties of the electroweak effective theory parameters in a model with generic Higgs couplings to the W and Z bosons. When the couplings between the Higgs and the electroweak gauge bosons deviate from their Standard Model values, additional counterterms of O(p^4) in the usual chiral counting are required. We also determine in the same approximation the full radiative corrections to the WL WL->ZL ZL process in this type of models. Assuming custodial invariance, all the related processes can be easily derived from this amplitude.Comment: minor changes and typos correcte