2,306 research outputs found
On the NLO Power Correction to Photon-Pion Transition Form Factor
We propose a perturbative evaluation for the next-to-leading-order (NLO)
power correction to the photon-pion transition form factor. The
effects of the NLO power correction are analyzed.Comment: 4 pages, 3 figures, Revtex, revised versio
Second moment of the pion's distribution amplitude
We present preliminary results from the QCDSF/UKQCD collaborations for the
second moment of the pion's distribution amplitude with two flavours of
dynamical fermions. We use nonperturbatively determined renormalisation
coefficients to convert our results to the MSbar scheme at 5 GeV^2. Employing a
linear chiral extrapolation from our large pion masses >550 MeV, we find
=0.281(28), leading to a value of a_2=0.236(82) for the second Gegenbauer
moment.Comment: 6 pages, 5 figures, 1 table. To appear in: Procs. of the Workshop on
Light-Cone QCD and Nonperturbative Hadron Physics 2005 (LC 2005), Cairns,
Australia, 200
QED radiative corrections to the decay pi^0 to e^+e^-
We reconsider QED radiative corrections (RC) to the
decay width. One kind of RC investigated earlier has a renormalization group
origin and can be associated with the final state interaction of electron and
positron. It determines the distribution of lepton pair invariant masses in the
whole kinematic region. The other type of RC has a double-logarithmic character
and is related to almost on-mass-shell behavior of the lepton form factors. The
total effect of RC for the decay is estimated to be
3.2% and for the decay is 4.3%.Comment: 12 pages, 3 figure
Quark-hadron duality, axial anomaly and mixing
Interplay between axial anomaly and quark-hadron duality in the presence of
strong mixing is considered. The anomaly sum rule for meson transition form
factors based on the dispersive representation of axial anomaly and
quark-hadron duality in octet channel is analyzed. The comparison of this sum
rule to the experimental data on and mesons transition form
factors shows that the interval of duality in this channel is rather small,
contradicting the usual understanding of quark-hadron duality. The same values
of interval of duality are supported by considering the two-point correlator in
the local duality limit. This contradiction may be resolved by introducing of
some nonperturbative non-OPE correction to the relevant spectral density. The
form and value of this correction are discussed.Comment: 9 pages, 1 figure, reference adde
Instanton Contribution to the Pion Electro-Magnetic Formfactor at Q^2 > 1 GeV^2
We study the effects of instantons on the charged pion electro-magnetic
formfactor at intermediate momenta. In the Single Instanton Approximation
(SIA), we predict the pion formfactor in the kinematic region Q^2=2-15 GeV^2.
By developing the calculation in a mixed time-momentum representation, it is
possible to maximally reduce the model dependence and to calculate the
formfactor directly. We find the intriguing result that the SIA calculation
coincides with the vector dominance monopole form, up to surprisingly high
momentum transfer Q^2~10 GeV^2. This suggests that vector dominance for the
pion holds beyond low energy nuclear physics.Comment: 8 pages, 5 figures, minor revision
Detecting and Studying Higgs Bosons at a Photon-Photon Collider
We examine the potential for detecting and studying Higgs bosons at a
photon-photon collider facility associated with a future linear collider. Our
study incorporates realistic \gam\gam luminosity spectra based on the most
probable available laser technology. Results include detector simulations. We
study the cases of: a) a SM-like Higgs boson; b) the heavy MSSM Higgs bosons;
c) a Higgs boson with no couplings from a general two Higgs doublet
model.Comment: 52 pages, 26 figures, revised version with new appendi
Eddy current studies from the undulator-based positron source target wheel prototype
The efficiency of future positron sources for the next generation of high-energy particle colliders (e.g. ILC, CLIC, LHeC) can be improved if the positron-production target is immersed in the magnetic field of adjacent capture optics. If the target is also rotating due to heat deposition considerations then eddy currents may be induced and lead to additional heating and stresses. In this paper we present data from a rotating target wheel prototype for the baseline ILC positron source. The wheel has been operated at revolution rates up to 1800rpm in fields of the order of 1 Tesla. Comparisons are made between torque data obtained from a transducer on the target drive shaft and the results of finite-element simulations. Rotordynamics issues are presented and future experiments on other aspects of the positron source target station are considered
Application of Jain and Munczek's bound-state approach to gamma gamma-processes of pi0, eta_c and eta_b
We point out the problems affecting most quark--antiquark bound state
approaches when they are faced with the electromagnetic processes dominated by
Abelian axial anomaly. However, these problems are resolved in the consistently
coupled Schwinger-Dyson and Bethe-Salpeter approach. Using one of the most
successful variants of this approach, we find the dynamically dressed
propagators of the light u and d quarks, as well as the heavy c and b quarks,
and find the Bethe-Salpeter amplitudes for their bound states pi0, eta_c and
\eta_b. Thanks to incorporating the dynamical chiral symmetry breaking, the
pion simultaneously appears as the (pseudo)Goldstone boson. We give the
theoretical predictions for the gamma-gamma decay widths of pi0, eta_c and
eta_b, and for the pi0 gamma* -> gamma transition form factor, and compare them
with experiment. In the chiral limit, the axial-anomaly result for
pi0->gamma-gamma is reproduced analytically in the consistently coupled
Schwinger-Dyson and Bethe-Salpeter approach, provided that the quark-photon
vertex is dressed consistently with the quark propagator, so that the vector
Ward-Takahashi identity of QED is obeyed. On the other hand, the present
approach is also capable of quantitatively describing systems of heavy quarks,
concretely eta_c and possibly eta_b, and their gamma-gamma decays. We discuss
the reasons for the broad phenomenological success of the bound-state approach
of Jain and Munczek.Comment: RevTeX, 37 pages, 7 eps figures, submitted to Int. J. Mod. Phys.
An analysis of two-body non-leptonic B decays involving light mesons in the standard model
We report a theoretical analysis of the exclusive non-leptonic decays of B
mesons into two light mesons,some of which have been measured recently by the
CLEO collaboration. Our analysis is carried out in the context of an effective
Hamiltonian based on the Standard Model using next-to-leading order
perturbative QCD calculations. Using a factorization ansatz for the hadronic
matrix elements, we show that existing data are accounted for in this approach.
Thus, theoretical scenarios with a substantially enhanced Wilson coefficient of
the chromomagnetic dipole operator (as compared to the SM) and/or those with a
substantial color-singlet component in the wave function of
are not required by these data. Implications of some of these
measurements for the parameters of the CKM matrix are presented.Comment: 42 pages including 21 postscript figures; uses epsfig, some ref.
added; improved mixing scheme for (eta,eta') system implemented; error
corrected in (3.61) as explained below eq. (3.64) in the present version.
Figs. 17-20 update
The vertex with arbitrary gluon virtualities in the perturbative QCD hard scattering approach
We study the vertex for arbitrary gluon virtualities in
the time-like and space-like regions, using the perturbative QCD hard
scattering approach and an input wave-function of the -meson
consistent with the measured transition form
factor. The contribution of the gluonic content of the -meson is
taken into account, enhancing the form factor over the entire virtuality
considered. However, data on the electromagnetic transition form factor of the
-meson is not sufficient to quantify the gluonic enhancement. We
also study the effect of the transverse momenta of the partons in the
-meson on the vertex, using the modified
hard scattering approach based on Sudakov formalism. Analytic expressions for
the vertex are presented in limiting kinematic regions
and parametrizations are given satisfying the QCD anomaly, for real gluons, and
perturbative QCD behavior for large gluon virtualities, in both the time-like
and space-like regions. Our results have implications for the inclusive decay
and exclusive decays, such as , and in hadronic production processes .Comment: 23 pages, 19 figures (requires revtex4, amssymb, epsf); several typos
corrected, this version now identical to the one accepted for publication in
Phys. Rev.
- …