131 research outputs found

### A novel way to probe distribution amplitudes of neutral mesons in e^+e^- annihilation

We derive the amplitude for the process $e^+e^-\to \pi^0\pi^0$ at large
invariant energy. The process goes through the two-photon exchange and its
amplitude is expressed in terms of the convolution integral which depends on
the shape of the pion distribution amplitude (DA) and the centre of mass
scattering angle. Remarkable feature of the integral is that it is very
sensitive to the end-point behaviour of the pion DA -- it starts to diverge if
pion DA nullifies at the end-point as $\sqrt x$ or slower. That makes the
$e^+e^-\to \pi^0\pi^0$ process unique probe of the shape of the meson DAs. The
estimated cross section is rather small, for $\sqrt s = 3$ GeV it ranges from a
fraction of femtobarn (for the asymptotic DA) to couple of femtobarn (for the
Chernyak-Zhitnitsky DA). The observation of the process $e^+e^-\to\pi^0\pi^0$
with the cross section higher as estimated here would imply very unusual form
of the pion DA, e.g. the flat one. The derived amplitude can be easily
generalized to other processes like $e^+e^-\to \sigma\sigma, K_SK_S, \eta\eta,
\eta^\prime\eta, \pi^0 f_2$, etc.Comment: 5 pages, 3 figure

### Coulomb dissociation of a fast pion into two jets

We calculate the electromagnetic contribution to the scattering amplitude of
pion diffractive dissociation into di-jets which is described by one photon
exchange. The result shows that the factorization procedure known for the
description of exclusive reactions holds also for this quasi-exclusive process.
We find that the longitudinal momentum distribution of di-jets does not depend
on the form of the pion distribution amplitude. We discuss the magnitude of the
cross section.Comment: 7 pages, 3 .eps figures, Late

### Operator Relations for SU(3) Breaking Contributions to K and K* Distribution Amplitudes

We derive constraints on the asymmetry a1 of the momentum fractions carried
by quark and antiquark in K and K* mesons in leading twist. These constraints
follow from exact operator identities and relate a1 to SU(3) breaking
quark-antiquark-gluon matrix elements which we determine from QCD sum rules.
Comparing our results to determinations of a1 from QCD sum rules based on
correlation functions of quark currents, we find that, for a1^\parallel(K*) the
central values agree well and come with moderate errors, whereas for a1(K) and
a1^\perp(K*) the results from operator relations are consistent with those from
quark current sum rules, but come with larger uncertainties. The consistency of
results confirms that the QCD sum rule method is indeed suitable for the
calculation of a1. We conclude that the presently most accurate predictions for
a1 come from the direct determination from QCD sum rules based on correlation
functions of quark currents and are given by: a1(K) = 0.06\pm 0.03,
a1^\parallel(K*) = 0.03\pm 0.02, a1^\perp(K*) = 0.04\pm 0.03.Comment: 21 page

### On mixed phases in gauge theories

In many gauge theories at different values of parameters entering Lagrangian,
the vacuum is dominated by coherent condensates of different mutually non-local
fields (for instance, by condensates of electric or magnetic charges, or by
various dyons). It is argued that the transition between these "dual to each
other" phases proceeds through the intermediate "mixed phase", having
qualitatively different features. The examples considered include: ordinary YM,
N=1 SYM, N=1 SQCD, and broken N=2 SYM and SQCD.Comment: Latex, 19 pages; Talk given at "Continuous Advances in
QCD-2002/Arkadyfest", honoring the 60-th birthday of Arkady Vainshtein; 17-23
May 2002, University of Minneapolis, Minnesota, USA; v.3: the extended and
improved versio

### Mass corrections in $J/\psi \to B\bar B$ decay and the role of distribution amplitudes

We consider mass correction effects on the polar angular distribution of a
baryon--antibaryon pair created in the chain decay process $e^-e^+ \to J/\psi
\to B\bar B$, generalizing a previous analysis of Carimalo. We show the
relevance of the features of the baryon distribution amplitudes and estimate
the electromagnetic corrections to the QCD results.Comment: 26 pages + 3 figures, REVTEX 3.0, figures appended as uuencoded,
tar-compressed postscript fil

### QCD Calculation of the $B \rightarrow \pi,K$ Form Factors

We calculate the form factors for the heavy-to-light transitions
$B\rightarrow \pi,K$ by means of QCD sum rules using $\pi$ and $K$ light-cone
wave functions. Higher twist contributions as well as gluonic corrections are
taken into account. The sensitivity to the shape of the leading-twist wave
functions and effects of SU(3)-breaking are discussed. The results are compared
with quark model predictions and with the results from QCD sum rules for
three-point correlators.Comment: 13 pages +5 figures available upon request , LaTeX , CERN-TH.6880/93,
MPI-Ph/93-32, LMU-07/9

### The B-Meson Distribution Amplitude in QCD

The B-meson distribution amplitude is calculated using QCD sum rules. In
particular we obtain an estimate for the integral relevant to exclusive
B-decays \lambda_B = 460 \pm 110 MeV at the scale 1 GeV. A simple QCD-motivated
parametrization of the distribution amplitude is suggested.Comment: 17 pages, 8 figures, Latex styl

### Models for Light-Cone Meson Distribution Amplitudes

Leading-twist distribution amplitudes (DAs) of light mesons like pi,rho etc.
describe the leading nonperturbative hadronic contributions to exclusive QCD
reactions at large energy transfer, for instance electromagnetic form factors.
They also enter B decay amplitudes described in QCD factorisation, in
particular nonleptonic two-body decays. Being nonperturbative quantities, DAs
cannot be calculated from first principles, but have to be described by models.
Most models for DAs rely on a fixed order conformal expansion, which is
strictly valid for large factorisation scales, but not always sufficient in
phenomenological applications. We derive models for DAs that are valid to all
orders in the conformal expansion and characterised by a small number of
parameters which are related to experimental observables.Comment: 19 pages, 10 figure

### Symmetries and Asymmetries of B -> K* mu+ mu- Decays in the Standard Model and Beyond

The rare decay B -> K* (-> K pi) mu+ mu- is regarded as one of the crucial
channels for B physics as the polarization of the K* allows a precise angular
reconstruction resulting in many observables that offer new important tests of
the Standard Model and its extensions. These angular observables can be
expressed in terms of CP-conserving and CP-violating quantities which we study
in terms of the full form factors calculated from QCD sum rules on the
light-cone, including QCD factorization corrections. We investigate all
observables in the context of the Standard Model and various New Physics
models, in particular the Littlest Higgs model with T-parity and various MSSM
scenarios, identifying those observables with small to moderate dependence on
hadronic quantities and large impact of New Physics. One important result of
our studies is that new CP-violating phases will produce clean signals in
CP-violating asymmetries. We also identify a number of correlations between
various observables which will allow a clear distinction between different New
Physics scenarios.Comment: 56 pages, 18 figures, 14 tables. v5: Missing factor in eqs. (3.31-32)
and fig. 6 corrected. Minor misprints in eq. (2.10) and table A corrected.
Conclusions unchange

### $D^*D\pi$ and $B^*B\pi$ couplings in QCD

We calculate the $D^*D\pi$ and $B^*B\pi$ couplings using QCD sum rules on the
light-cone. In this approach, the large-distance dynamics is incorporated in a
set of pion wave functions. We take into account two-particle and
three-particle wave functions of twist 2, 3 and 4. The resulting values of the
coupling constants are $g_{D^*D\pi}= 12.5\pm 1$ and $g_{B^*B\pi}= 29\pm 3$.
From this we predict the partial width \Gamma (D^{*+} \ra D^0 \pi^+ )=32 \pm
5~ keV . We also discuss the soft-pion limit of the sum rules which is
equivalent to the external axial field approach employed in earlier
calculations. Furthermore, using $g_{B^*B\pi}$ and $g_{D^*D\pi}$ the pole
dominance model for the B \ra \pi and D\ra \pi semileptonic form factors
is compared with the direct calculation of these form factors in the same
framework of light-cone sum rules.Comment: 27 pages (LATEX) +3 figures enclosed as .uu file MPI-PhT/94-62 ,
CEBAF-TH-94-22, LMU 15/9

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