2,423 research outputs found
On "dynamical mass" generation in Euclidean de Sitter space
We consider the perturbative treatment of the minimally coupled, massless,
self-interacting scalar field in Euclidean de Sitter space. Generalizing work
of Rajaraman, we obtain the dynamical mass m^2 \propto sqrt{lambda} H^2 of the
scalar for non-vanishing Lagrangian masses and the first perturbative quantum
correction in the massless case. We develop the rules of a systematic
perturbative expansion, which treats the zero-mode non-perturbatively, and goes
in powers of sqrt{lambda}. The infrared divergences are self-regulated by the
zero-mode dynamics. Thus, in Euclidean de Sitter space the interacting,
massless scalar field is just as well-defined as the massive field. We then
show that the dynamical mass can be recovered from the diagrammatic expansion
of the self-energy and a consistent solution of the Schwinger-Dyson equation,
but requires the summation of a divergent series of loop diagrams of
arbitrarily high order. Finally, we note that the value of the long-wavelength
mode two-point function in Euclidean de Sitter space agrees at leading order
with the stochastic treatment in Lorentzian de Sitter space, in any number of
dimensions.Comment: LaTeX, 15 page
Lepton flavour violation in RS models with a brane- or nearly brane-localized Higgs
We perform a comprehensive study of charged lepton flavour violation in
Randall-Sundrum (RS) models in a fully 5D quantum-field-theoretical framework.
We consider the RS model with minimal field content and a "custodially
protected" extension as well as three implementations of the IR-brane localized
Higgs field, including the non-decoupling effect of the Kaluza-Klein (KK)
excitations of a narrow bulk Higgs. Our calculation provides the first complete
result for the flavour-violating electromagnetic dipole operator in
Randall-Sundrum models. It contains three contributions with different
dependence on the magnitude of the anarchic 5D Yukawa matrix, which can all be
important in certain parameter regions. We study the typical range for the
branching fractions of mu -> e gamma, mu -> 3e, mu N -> e N as well as tau ->
mu gamma, tau -> 3 mu and the electron electric dipole moment by a numerical
scan in both the minimal and the custodial RS model. The combination of mu -> e
gamma and mu N -> e N currently provides the most stringent constraint on the
parameter space of the model. A typical lower limit on the KK scale T is around
2 TeV in the minimal model (up to 4 TeV in the bulk Higgs case with large
Yukawa couplings), and around 4 TeV in the custodially protected model, which
corresponds to a mass of about 10 TeV for the first KK excitations, far beyond
the lower limit from the non-observation of direct production at the LHC.Comment: 64 pages, 21 figures, LaTeX, v2: electron EDM analysis added, matches
published versio
Lepton fluxes from atmospheric charm revisited
We update predictions for lepton fluxes from the hadroproduction of charm
quarks in the scattering of primary cosmic rays with the Earth's atmosphere.
The calculation of charm-pair hadroproduction applies the latest results from
perturbative QCD through next-to-next-to-leading order and modern parton
distributions, together with estimates on various sources of uncertainties. Our
predictions for the lepton fluxes turn out to be compatible, within the
uncertainty band, with recent results in the literature. However, by taking
into account contributions neglected in previous works, our total uncertainties
are much larger. The predictions are crucial for the interpretation of results
from neutrino experiments like IceCube, when disentangling signals of neutrinos
of astrophysical origin from the atmospheric background.Comment: 40 pages, 24 figure
The Three-Loop Splitting Functions in QCD: The Helicity-Dependent Case
We present the next-to-next-to-leading order (NNLO) contributions to the main
splitting functions for the evolution of longitudinally polarized parton
densities of hadrons in perturbative QCD. The quark-quark and gluon-quark
splitting functions have been obtained by extending our previous all Mellin-N
calculations to the structure function g_1 in electromagnetic deep-inelastic
scattering (DIS). Their quark-gluon and gluon-gluon counterparts have been
derived using third-order fixed-N calculations of structure functions in
graviton-exchange DIS, relations to the unpolarized case and mathematical tools
for systems of Diophantine equations. The NNLO corrections to the splitting
functions are small outside the region of small momentum fractions x where they
exhibit a large double-logarithmic enhancement, yet the corrections to the
evolution of the parton densities can be unproblematic down to at least x about
10^{-4}.Comment: 52 pages, Latex, 10 figures. FORM and Fortran files of the main
results available with the sourc
Two-loop conformal generators for leading-twist operators in QCD
QCD evolution equations in minimal subtraction schemes have a hidden
symmetry: One can construct three operators that commute with the evolution
kernel and form an algebra, i.e. they satisfy (exactly) the
commutation relations. In this paper we find explicit expressions for these
operators to two-loop accuracy going over to QCD in non-integer
space-time dimensions at the intermediate stage. In this way conformal symmetry
of QCD is restored on quantum level at the specially chosen (critical) value of
the coupling, and at the same time the theory is regularized allowing one to
use the standard renormalization procedure for the relevant Feynman diagrams.
Quantum corrections to conformal generators in effectively
correspond to the conformal symmetry breaking in the physical theory in four
dimensions and the commutation relations lead to nontrivial constraints
on the renormalization group equations for composite operators. This approach
is valid to all orders in perturbation theory and the result includes
automatically all terms that can be identified as due to a nonvanishing QCD
-function (in the physical theory in four dimensions). Our result can be
used to derive three-loop evolution equations for flavor-nonsinglet
quark-antiquark operators including mixing with the operators containing total
derivatives. These equations govern, e.g., the scale dependence of generalized
hadron parton distributions and light-cone meson distribution amplitudes.Comment: 36 page
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