485 research outputs found
pi-pi and pi-K scatterings in three-flavour resummed chiral perturbation theory
The (light but not-so-light) strange quark may play a special role in the
low-energy dynamics of QCD. The presence of strange quark pairs in the sea may
have a significant impact of the pattern of chiral symmetry breaking : in
particular large differences can occur between the chiral limits of two and
three massless flavours (i.e., whether m_s is kept at its physical value or
sent to zero). This may induce problems of convergence in three-flavour chiral
expansions. To cope with such difficulties, we introduce a new framework,
called Resummed Chiral Perturbation Theory. We exploit it to analyse pi-pi and
pi-K scatterings and match them with dispersive results in a frequentist
framework. Constraints on three-flavour chiral order parameters are derived.Comment: Proceedings of the EPS-HEP 2007 Conference, Manchester (UK). 3 pages,
1 figur
Chiral dynamics with strange quarks in the light of recent lattice simulations
Several lattice collaborations performing simulations with 2+1 light
dynamical quarks have experienced difficulties in fitting their data with
standard Nf=3 chiral expansions at next-to-leading order, yielding low values
of the quark condensate and/or the decay constant in the Nf=3 chiral limit. A
reordering of these expansions seems required to analyse these data in a
consistent way. We discuss such a reordering, known as Resummed Chiral
Perturbation Theory, in the case of pseudoscalar masses and decay constants,
pion and kaon electromagnetic form factors and Kl3} form factors. We show that
it provides a good fit of the recent results of two lattice collaborations
(PACS-CS and RBC/UKQCD). We describe the emerging picture for the pattern of
chiral symmetry breaking, marked by a strong dependence of the observables on
the strange quark mass and thus a significant difference between chiral
symmetry breaking in the Nf=2 and Nf=3 chiral limits. We discuss the
consequences for the ratio of decay constants F_K/F_pi and the Kl3 form factor
at vanishing momentum transfer.Comment: 31 pages. Published versio
Determining the chiral condensate from the distribution of the winding number beyond topological susceptibility
The first two non-trivial moments of the distribution of the topological
charge (or gluonic winding number), i.e., the topological susceptibility and
the fourth cumulant, can be computed in lattice QCD simulations and exploited
to constrain the pattern of chiral symmetry breaking. We compute these two
topological observables at next-to-leading order in three-flavour Chiral
Perturbation Theory, and we discuss the role played by the eta propagation in
these expressions. For hierarchies of light-quark masses close to the physical
situation, we show that the fourth cumulant has a much better sensitivity than
the topological susceptibility to the three-flavour quark condensate, and thus
constitutes a relevant tool to determine the pattern of chiral symmetry
breaking in the limit of three massless flavours. We provide the complete
formulae for the two topological observables in the isospin limit, and predict
their values in the particular setting of the recent analysis of the RBC/UKQCD
collaboration. We show that a combination of the topological susceptibility and
the fourth cumulant is able to pin down the three-flavour condensate in a
particularly clean way in the case of three degenerate quarks.Comment: 31 pages, 7 figures. Accepted for publication in EPJ
Three-particle contributions to the renormalisation of B-meson light-cone distribution amplitudes
We study light-cone distribution amplitudes of heavy-light systems, such as a
B-meson. By an explicit computation, we determine how two-parton distribution
amplitudes mix with three-parton ones at one loop: \phi_+ is shown to mix only
into itself, whereas \phi_- mixes with the difference of three-parton
distribution amplitudes \Psi_A-\Psi_V. We determine the corresponding anomalous
dimension and we check the gauge independence of our result by considering a
general covariant gauge. Finally, we comment on some implications of our result
for phenomenological models of these distribution amplitudes.Comment: 21 pages, 5 figures, some comments and 2 references added, except for
typesetting matches version published in JHE
Extracting CP violation and strong phase in D decays by using quantum correlations in psi(3770)-> D0\bar{D}0 -> (V1V2)(V3V4) and psi(3770)->D0\bar{D}0 -> (V1V2)(K pi)
The charm quark offers interesting opportunities to cross-check the mechanism
of CP violation precisely tested in the strange and beauty sectors. In this
paper, we exploit the angular and quantum correlations in the D\bar{D} pairs
produced through the decay of the psi(3770) resonance in a charm factory to
investigate CP-violation in two different ways. We build CP-violating
observables in psi(3770) -> D\bar{D} -> (V_1V_2)(V_3 V_4) to isolate specific
New Physics effects in the charm sector. We also consider the case of psi(3770)
-> D\bar{D} -> (V_1V_2)(K\pi) decays, which provide a new way to measure the
strong phase difference delta between Cabibbo-favored and doubly-Cabibbo
suppressed D decays required in the determination of the CKM angle gamma.
Neglecting the systematics, we give a first rough estimate of the sensitivities
of these measurements at BES-III with an integrated luminosity of 20 fb^-1 at
psi(3770) peak and at a future Super tau-charm factory with a luminosity of
10^35 cm^-2.s^-1.Comment: 13 pages
Nonminimal 331 model for lepton flavor universality violation in b → sll decays
The 331 models constitute an extension of the Standard Model (SM) obtained by enlarging the SM gauge group SU(3)C×SU(2)L×U(1)Y to the group SU(3)C×SU(3)L×U(1)X. We investigate how a nonminimal 331 model may embed lepton flavor universality violating contributions to b→sℓℓ processes without introducing lepton flavor violation, as suggested by the recent LHCb measurements of the ratios RK and RK∗. We discuss the model-independent scenarios of new physics in b→sℓℓ currently favored by the data that could be accommodated by this model and consider a few phenomenological constraints on this model
The S-wave \Lambda\pi phase shift is not large
We study the strong interaction S-wave \Lambda\pi phase shift in the region
of the \Xi mass in the framework of a relativistic chiral unitary approach
based on coupled channels. All parameters have been previously determined in a
fit to strangeness S= -1 S-wave kaon-nucleon data. We find 0^\circ \le \delta_0
\le 1.1^\circ in agreement with previous chiral perturbation theory
calculations (or extensions thereof). We also discuss why a recent coupled
channel K-matrix calculation gives a result for \delta_0 that is negative and
much bigger in magnitude. We argue why that value should not be trusted.Comment: 3 pages, REVTe
Two-loop representations of low-energy pion form factors and pi-pi scattering phases in the presence of isospin breaking
Dispersive representations of the pi-pi scattering amplitudes and pion form
factors, valid at two-loop accuracy in the low-energy expansion, are
constructed in the presence of isospin-breaking effects induced by the
difference between the charged and neutral pion masses. Analytical expressions
for the corresponding phases of the scalar and vector pion form factors are
computed. It is shown that each of these phases consists of the sum of a
"universal" part and a form-factor dependent contribution. The first one is
entirely determined in terms of the pi-pi scattering amplitudes alone, and
reduces to the phase satisfying Watson's theorem in the isospin limit. The
second one can be sizeable, although it vanishes in the same limit. The
dependence of these isospin corrections with respect to the parameters of the
subthreshold expansion of the pi-pi amplitude is studied, and an equivalent
representation in terms of the S-wave scattering lengths is also briefly
presented and discussed. In addition, partially analytical expressions for the
two-loop form factors and pi-pi scattering amplitudes in the presence of
isospin breaking are provided.Comment: 57 pages, 12 figure
Chiral Extrapolation of the Strangeness Changing K pi Form Factor
We perform a chiral extrapolation of lattice data on the scalar K pi form
factor and the ratio of the kaon and pion decay constants within Chiral
Perturbation Theory to two loops. We determine the value of the scalar form
factor at zero momentum transfer, at the Callan-Treiman point and at its soft
kaon analog as well as its slope. Results are in good agreement with their
determination from experiment using the standard couplings of quarks to the W
boson. The slope is however rather large. A study of the convergence of the
chiral expansion is also performed.Comment: few minor change
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