30 research outputs found
RS-invariant all-orders renormalon resummations for some QCD observables
We propose a renormalon-inspired resummation of QCD perturbation theory based
on approximating the renormalization scheme (RS) invariant effective charge
beta-function coefficients by the portion containing the highest power of
=--, for SU() QCD with quark flavours.
This can be accomplished using exact large- all-orders results. The
resulting resummation is RS-invariant and the exact next-to-leading order (NLO)
and next-to-NLO (NNLO) coefficients in any RS are included. This improves on a
previously employed naive resummation of the leading- piece of the
perturbative coefficients which is RS-dependent, making its comparison with
fixed-order perturbative results ambiguous. The RS-invariant resummation is
used to assess the reliability of fixed-order perturbation theory for the
-ratio, the analogous -lepton decay ratio , and
Deep Inelastic Scattering (DIS) sum rules, by comparing it with the exact NNLO
results in the effective charge RS. For the -ratio and , where
large-order perturbative behaviour is dominated by a leading ultra-violet
renormalon singularity, the comparison indicates fixed-order perturbation
theory to be very reliable. For DIS sum rules, which have a leading infra-red
renormalon singularity, the performance is rather poor. In this way we estimate
that at LEP/SLD energies ideal data on the -ratio could determine
to three-significant figures, and for the we
estimate a theoretical uncertainty
corresponding to . This encouragingly small
uncertainty is much less than has recently been deduced from comparison with
the ambiguous naive resummation.Comment: 25 pages, uses LaTeX, 12 Postscript figures, epsfig.sty 'elsart.sty'
and 'elsart12.sty' are available via anonymous-ftp at
ftp://ftp.tex.ac.uk/tex-archive/macros/latex/contrib/supported/elsevie
On the Behavior of the Effective QCD Coupling alpha_tau(s) at Low Scales
The hadronic decays of the tau lepton can be used to determine the effective
charge alpha_tau(m^2_tau') for a hypothetical tau-lepton with mass in the range
0 < m_tau' < m_tau. This definition provides a fundamental definition of the
QCD coupling at low mass scales. We study the behavior of alpha_tau at low mass
scales directly from first principles and without any renormalization-scheme
dependence by looking at the experimental data from the OPAL Collaboration. The
results are consistent with the freezing of the physical coupling at mass
scales s = m^2_tau' of order 1 GeV^2 with a magnitude alpha_tau ~ 0.9 +/- 0.1.Comment: 15 pages, 4 figures, submitted to Physical Review D, added
references, some text added, no results nor figures change
A Constraint on Planck-scale Modifications to Electrodynamics with CMB polarization data
We show that the Cosmic Microwave Background (CMB) polarization data gathered
by the BOOMERanG 2003 flight and WMAP provide an opportunity to investigate
{\it in-vacuo} birefringence, of a type expected in some quantum pictures of
space-time, with a sensitivity that extends even beyond the desired
Planck-scale energy. In order to render this constraint more transparent we
rely on a well studied phenomenological model of quantum-gravity-induced
birefringence, in which one easily establishes that effects introduced at the
Planck scale would amount to values of a dimensionless parameter, denoted by
, with respect to the Planck energy which are roughly of order 1. By
combining BOOMERanG and WMAP data we estimate at
the 68% c.l. Moreover, we forecast on the sensitivity to achievable by
future CMB polarization experiments (PLANCK, Spider, EPIC), which, in the
absence of systematics, will be at the 1- confidence of (PLANCK), (Spider), and
(EPIC) respectively. The cosmic variance-limited sensitivity from CMB is
.Comment: 16 page
Studies on mosquitoes (Diptera: Culicidae) and anthropic environment: 6 - Breeding in empty conditions of rice fields in South-Eastern Brazil Estudos sobre mosquitos (Diptera: Culicidae) e ambiente antrópico: 6 - Observações em campos de arroz não cultivados, na região sudeste do Brasil
Studies on culicid breeding in empty rice fields were carried out during the cultivation cycle from May to November 1993. This period corresponded to stages 1 and 2, when empty conditions prevailed. Breeding occurred in stage 1 and the first part of stage 2, corresponding respectively to fallow uncultivated and ploughing situations. No breeding was found to take place during the second part of stage 2 when transient floods and harrowing occurred. The predominant species were Aedes scapularis, Culex nigripalpus and Cx. mollis. The Pilosus Group of Culex (Melanoconion) was found at lower densities. Some epidemiological considerations are presented.<br>Apresentam-se os resultados obtidos com a pesquisa sistemática de criadouros nas fases não cultivadas dos campos de arroz, correspondendo ao período de maio a novembro de 1993. Esse lapso de tempo foi o referente aos estádios de repouso, de aradura do terreno e de gradeamento. Este foi realizado mediante inundações transitórias e a homogeneização mecânica da terra. O primeiro correspondeu ao estádio 1, enquanto os outros dois, ao estádio 2. Foram encontrados criadouros nas etapas concernentes aos estados de repouso e de aradura. Foram negativas as pesquisas referentes às fases de gradeamento. As espécies predominantes foram Aedes scapularis, Culex nigripalpus e Cx. mollis. Observou-se a existência de associações com o ritmo local das precipitações atmosféricas. São apresentadas considerações sobre o potencial epidemiológico desses encontros
All-orders infrared freezing of Re+e− in perturbative QCD.
We consider the behaviour of the perturbative QCD corrections to the Re+e− ratio, in the limit that the c.m. energy vanishes. Writing , with Qf denoting the electric charge of quark flavour f, we find that for Nf<9 flavours of massless quarks, the perturbative correction to the parton model result smoothly approaches from below the infrared limit , as s→0. Here b=(33−2Nf)/6 is the first QCD beta-function coefficient. This freezing holds to all-orders in perturbation theory. The s-dependence can be written analytically in closed form in terms of the Lambert W function