115 research outputs found
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 state would be accompanied by relatively light charged and
neutral pseudoscalar Higgses. We will use the oblique corrections, particularly
, 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 , and 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
- …