2,240 research outputs found
Analytic structure in the coupling constant plane in perturbative QCD
We investigate the analytic structure of the Borel-summed perturbative QCD
amplitudes in the complex plane of the coupling constant. Using the method of
inverse Mellin transform, we show that the prescription dependent Borel-Laplace
integral can be cast, under some conditions, into the form of a dispersion
relation in the a-plane. We also discuss some recent works relating resummation
prescriptions, renormalons and nonperturbative effects, and show that a method
proposed recently for obtaining QCD nonperturbative condensates from
perturbation theory is based on special assumptions about the analytic
structure in the coupling plane that are not valid in QCD.Comment: 14 pages, revtex4, 1 eps-figur
Synchonisation of Resonances with Thresholds
The mechanism by which a resonance may be attracted to a sharp threshold is
described with several examples. It involves a threshold cusp interfering
constructively with either or both (i) a resonance produced via confinement,
(ii) attractive t- and u-channel exchanges. More generally, it is suggested
that resonances are eigenstates generated by mixing between confined states and
long-range meson and baryon exchanges.Comment: 8 pages, 4 figures. For Meson08 Proceedings. One important typo
correcte
Convergence of the expansion of the Laplace-Borel integral in perturbative QCD improved by conformal mapping
The optimal conformal mapping of the Borel plane was recently used to
accelerate the convergence of the perturbation expansions in QCD. In this work
we discuss the relevance of the method for the calculation of the Laplace-Borel
integral expressing formally the QCD Green functions. We define an optimal
expansion of the Laplace-Borel integral in the principal value prescription and
establish conditions under which the expansion is convergent.Comment: 10 pages, no figure
Gravitational wave generation from bubble collisions in first-order phase transitions: an analytic approach
Gravitational wave production from bubble collisions was calculated in the
early nineties using numerical simulations. In this paper, we present an
alternative analytic estimate, relying on a different treatment of
stochasticity. In our approach, we provide a model for the bubble velocity
power spectrum, suitable for both detonations and deflagrations. From this, we
derive the anisotropic stress and analytically solve the gravitational wave
equation. We provide analytical formulae for the peak frequency and the shape
of the spectrum which we compare with numerical estimates. In contrast to the
previous analysis, we do not work in the envelope approximation. This paper
focuses on a particular source of gravitational waves from phase transitions.
In a companion article, we will add together the different sources of
gravitational wave signals from phase transitions: bubble collisions,
turbulence and magnetic fields and discuss the prospects for probing the
electroweak phase transition at LISA.Comment: 48 pages, 14 figures. v2 (PRD version): calculation refined; plots
redone starting from Fig. 4. Factor 2 in GW energy spectrum corrected. Main
conclusions unchanged. v3: Note added at the end of paper to comment on the
new results of 0901.166
Theory of unitarity bounds and low energy form factors
We present a general formalism for deriving bounds on the shape parameters of
the weak and electromagnetic form factors using as input correlators calculated
from perturbative QCD, and exploiting analyticity and unitarity. The values
resulting from the symmetries of QCD at low energies or from lattice
calculations at special points inside the analyticity domain can beincluded in
an exact way. We write down the general solution of the corresponding Meiman
problem for an arbitrary number of interior constraints and the integral
equations that allow one to include the phase of the form factor along a part
of the unitarity cut. A formalism that includes the phase and some information
on the modulus along a part of the cut is also given. For illustration we
present constraints on the slope and curvature of the K_l3 scalar form factor
and discuss our findings in some detail. The techniques are useful for checking
the consistency of various inputs and for controlling the parameterizations of
the form factors entering precision predictions in flavor physics.Comment: 11 pages latex using EPJ style files, 5 figures; v2 is version
accepted by EPJA in Tools section; sentences and figures improve
Gravitational waves from stochastic relativistic sources: primordial turbulence and magnetic fields
The power spectrum of a homogeneous and isotropic stochastic variable,
characterized by a finite correlation length, does in general not vanish on
scales larger than the correlation scale. If the variable is a divergence free
vector field, we demonstrate that its power spectrum is blue on large scales.
Accounting for this fact, we compute the gravitational waves induced by an
incompressible turbulent fluid and by a causal magnetic field present in the
early universe. The gravitational wave power spectra show common features: they
are both blue on large scales, and peak at the correlation scale. However, the
magnetic field can be treated as a coherent source and it is active for a long
time. This results in a very effective conversion of magnetic energy in
gravitational wave energy at horizon crossing. Turbulence instead acts as a
source for gravitational waves over a time interval much shorter than a Hubble
time, and the conversion into gravitational wave energy is much less effective.
We also derive a strong constraint on the amplitude of a primordial magnetic
field when the correlation length is much smaller than the horizon.Comment: Replaced with revised version accepted for publication in Phys Rev
Detection of gravitational waves from the QCD phase transition with pulsar timing arrays
If the cosmological QCD phase transition is strongly first order and lasts
sufficiently long, it generates a background of gravitational waves which may
be detected via pulsar timing experiments. We estimate the amplitude and the
spectral shape of such a background and we discuss its detectability prospects.Comment: 7 pages, 5 figs. Version accepted by PR
Improvements to the Method of Dispersion Relations for B Nonleptonic Decays
We bring some clarifications and improvements to the method of dispersion
relations in the external masses variables, that we proposed recently for
investigating the final state interactions in the B nonleptonic decays. We
first present arguments for the existence of an additional term in the
dispersion representation, which arises from an equal-time commutator in the
LSZ formalism and can be approximated by the conventional factorized amplitude.
The reality properties of the spectral function and the Goldberger-Treiman
procedure to perform the hadronic unitarity sum are analyzed in more detail. We
also improve the treatment of the strong interaction part by including the
contributions of both t and u-channel trajectories in the Regge amplitudes.
Applications to the and decays are
presented.Comment: 16 pages, 4 new figures. modifications of the dispersion
representatio
Stringent constraints on the scalar K pi form factor from analyticity, unitarity and low-energy theorems
We investigate the scalar K pi form factor at low energies by the method of
unitarity bounds adapted so as to include information on the phase and modulus
along the elastic region of the unitarity cut. Using at input the values of the
form factor at t=0 and the Callan-Treiman point, we obtain stringent
constraints on the slope and curvature parameters of the Taylor expansion at
the origin. Also, we predict a quite narrow range for the higher order ChPT
corrections at the second Callan-Treiman point.Comment: 5 pages latex, uses EPJ style files, 3 figures, replaced with version
accepted by EPJ
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