RENORMALONS FROM EIGHT LOOP EXPANSION OF THE GLUON CONDENSATE IN LATTICE GAUGE THEORY,

We use a numerical method to obtain the weak coupling perturbative coefficients of local operators with lattice regularization. Such a method allows us to extend the perturbative expansions obtained so far by analytical Feynman diagrams calculations. In SU(3) lattice gauge theory in four dimensions we compute the first eight coefficients of the expectation value of the Wilson loop on the elementary plaquette which is related to the gluon condensate. The computed eight coefficients grow with the order much faster than predicted by the presence of the infrared renormalon associated to the dimension of the gluon condensate. However the renormalon behaviour for large order is quite well reproduced if one considers the expansion coefficients in a new coupling related to the lattice coupling by large perturbative corrections. This is expected since the lattice and continuum Lambda scales differ by almost two orders of magnitude.Comment: 11 pages, LaTe

Phosphoenolpyruvate carboxykinase and the critical role of cataplerosis in the control of hepatic metabolism

BACKGROUND: The metabolic function of PEPCK-C is not fully understood; deletion of the gene for the enzyme in mice provides an opportunity to fully assess its function. METHODS: The gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK-C) was deleted in mice by homologous recombination (PEPCK-C(-/- )mice) and the metabolic consequences assessed. RESULTS: PEPCK-C(-/-) mice became severely hypoglycemic by day two after birth and then died with profound hypoglycemia (12 mg/dl). The mice had milk in their stomachs at day two after birth and the administration of glucose raised the concentration of blood glucose in the mice but did not result in an increased survival. PEPCK-C(-/- )mice have two to three times the hepatic triglyceride content as control littermates on the second day after birth. These mice also had an elevation of lactate (2.5 times), β-hydroxybutyrate (3 times) and triglyceride (50%) in their blood, as compared to control animals. On day two after birth, alanine, glycine, glutamine, glutamate, aspartate and asparagine were elevated in the blood of the PEPCK-C(-/- )mice and the blood urea nitrogen concentration was increased by 2-fold. The rate of oxidation of [2-(14)C]-acetate, and [5-(14)C]-glutamate to (14)CO(2 )by liver slices from PEPCK-C(-/- )mice at two days of age was greatly reduced, as was the rate of fatty acid synthesis from acetate and glucose. As predicted by the lack of PEPCK-C, the concentration of malate in the livers of the PEPCK-C(-/- )mice was 10 times that of controls. CONCLUSION: We conclude that PEPCK-C is required not only for gluconeogenesis and glyceroneogenesis but also for cataplerosis (i.e. the removal of citric acid cycle anions) and that the failure of this process in the livers of PEPCK-C(-/- )mice results in a marked reduction in citric acid cycle flux and the shunting of hepatic lipid into triglyceride, resulting in a fatty liver

Residual Cajal bodies in coilin knockout mice fail to recruit Sm snRNPs and SMN, the spinal muscular atrophy gene product

Cajal bodies (CBs) are nuclear suborganelles involved in the biogenesis of small nuclear ribonucleoproteins (snRNPs). In addition to snRNPs, they are highly enriched in basal transcription and cell cycle factors, the nucleolar proteins fibrillarin (Fb) and Nopp140 (Nopp), the survival motor neuron (SMN) protein complex, and the CB marker protein, p80 coilin. We report the generation of knockout mice lacking the COOH-terminal 487 amino acids of coilin. Northern and Western blot analyses demonstrate that we have successfully removed the full-length coilin protein from the knockout animals. Some homozygous mutant animals are viable, but their numbers are reduced significantly when crossed to inbred backgrounds. Analysis of tissues and cell lines from mutant animals reveals the presence of extranucleolar foci that contain Fb and Nopp but not other typical nucleolar markers. These so-called “residual” CBs neither condense Sm proteins nor recruit members of the SMN protein complex. Transient expression of wild-type mouse coilin in knockout cells results in formation of CBs and restores these missing epitopes. Our data demonstrate that full-length coilin is essential for proper formation and/or maintenance of CBs and that recruitment of snRNP and SMN complex proteins to these nuclear subdomains requires sequences within the coilin COOH terminus

Scale Setting in QCD and the Momentum Flow in Feynman Diagrams

We present a formalism to evaluate QCD diagrams with a single virtual gluon using a running coupling constant at the vertices. This method, which corresponds to an all-order resummation of certain terms in a perturbative series, provides a description of the momentum flow through the gluon propagator. It can be viewed as a generalization of the scale-setting prescription of Brodsky, Lepage and Mackenzie to all orders in perturbation theory. In particular, the approach can be used to investigate why in some cases the ``typical'' momenta in a loop diagram are different from the ``natural'' scale of the process. It offers an intuitive understanding of the appearance of infrared renormalons in perturbation theory and their connection to the rate of convergence of a perturbative series. Moreover, it allows one to separate short- and long-distance contributions by introducing a hard factorization scale. Several applications to one- and two-scale problems are discussed in detail.Comment: eqs.(51) and (83) corrected, minor typographic changes mad

Optimal Renormalization Scale and Scheme for Exclusive Processes

We use the BLM method to fix the renormalization scale of the QCD coupling in exclusive hadronic amplitudes such as the pion form factor and the photon-to-pion transition form factor at large momentum transfer. Renormalization-scheme-independent commensurate scale relations are established which connect the hard scattering subprocess amplitudes that control exclusive processes to other QCD observables such as the heavy quark potential and the electron-positron annihilation cross section. The commensurate scale relation connecting the heavy quark potential, as determined from lattice gauge theory, to the photon-to-pion transition form factor is in excellent agreement with $\gamma e \to \pi^0 e$ data assuming that the pion distribution amplitude is close to its asymptotic form $\sqrt{3}f_\pi x(1-x)$. We also reproduce the scaling and normalization of the $\gamma \gamma \to \pi^+ \pi^-$ data at large momentum transfer. Because the renormalization scale is small, we argue that the effective coupling is nearly constant, thus accounting for the nominal scaling behavior of the data. However, the normalization of the space-like pion form factor $F_\pi(Q^2)$ obtained from electroproduction experiments is somewhat higher than that predicted by the corresponding commensurate scale relation. This discrepancy may be due to systematic errors introduced by the extrapolation of the $\gamma^* p \to \pi^+ n$ electroproduction data to the pion pole.Comment: 22 pages, Latex, 7 Latex figures. Several references added, discussion of scale fixing revised for clarity. Final version to appear in Phys. Rev.

Standard Model Matrix Elements for Neutral B-Meson Mixing and Associated Decay Constants

We present results of quenched lattice calculations of the matrix elements relevant for B_d-\bar B_d and B_s-\bar B_s mixing in the Standard Model. Results for the corresponding SU(3)-breaking ratios, which can be used to constrain or determine |V_{td}|, are also given. The calculations are performed at two values of the lattice spacing, corresponding to \beta = 6.0 and \beta = 6.2, with quarks described by a mean-field-improved Sheikholeslami-Wohlert action. As a by-product, we obtain the leptonic decay constants of B and D mesons. We also present matrix elements relevant for D^0-\bar D^0 mixing. Our results are summarized in the Introduction.Comment: 27 pages (RevTeX), 26 figures, version published in Phys. Rev. D: improved estimate of the systematic error associated with the uncertainty on the strange quark mass and other small improvements to analysis (results change only slightly); correction of typos and minor changes to text; RevTeX formattin

Model-Independent Global Constraints on New Physics

Using effective-lagrangian techniques we perform a systematic survey of the lowest-dimension effective interactions through which heavy physics might manifest itself in present experiments. We do not restrict ourselves to special classes of effective interactions (such as `oblique' corrections). We compute the effects of these operators on all currently well-measured electroweak observables, both at low energies and at the $Z$ resonance, and perform a global fit to their coefficients. Despite the fact that a great many operators arise in our survey, we find that most are quite strongly bounded by the current data. We use our survey to systematically identify those effective interactions which are {\it not} well-bounded by the data -- these could very well include large new-physics contributions. Our results may also be used to efficiently confront specific models for new physics with the data, as we illustrate with an example.Comment: plain TeX, 68 pages, 2 figures (postscript files appended), McGill-93/12, NEIPH-93-008, OCIP/C-93-6, UQAM-PHE-93/08, UdeM-LPN-TH-93-15

High-p_T pion and kaon production in relativistic nuclear collisions

High-p_T pion and kaon production is studied in relativistic proton-proton, proton-nucleus, and nucleus-nucleus collisions in a wide energy range. Cross sections are calculated based on perturbative QCD, augmented by a phenomenological transverse momentum distribution of partons (``intrinsic k_T''). An energy dependent width of the transverse momentum distribution is extracted from pion and charged hadron production data in proton-proton/proton-antiproton collisions. Effects of multiscattering and shadowing in the strongly interacting medium are taken into account. Enhancement of the transverse momentum width is introduced and parameterized to explain the Cronin effect. In collisions between heavy nuclei, the model over-predicts central pion production cross sections (more significantly at higher energies), hinting at the presence of jet quenching. Predictions are made for proton-nucleus and nucleus-nucleus collisions at RHIC energies.Comment: 26 pages in Latex, 19 EPS figure

Renormalon Ambiguities in NRQCD Operator Matrix Elements

We analyze the renormalon ambiguities that appear in factorization formulas in QCD. Our analysis contains a simple argument that the ambiguities in the short-distance coefficients and operator matrix elements are artifacts of dimensional-regularization factorization schemes and are absent in cutoff schemes. We also present a method for computing the renormalon ambiguities in operator matrix elements and apply it to a computation of the ambiguities in the matrix elements that appear in the NRQCD factorization formulas for the annihilation decays of S-wave quarkonia. Our results, combined with those of Braaten and Chen for the short-distance coefficients, provide an explicit demonstration that the ambiguities cancel in the physical decay rates. In addition, we analyze the renormalon ambiguities in the Gremm-Kapustin relation and in various definitions of the heavy-quark mass.Comment: 29 pages, REVTEX; revised Abstract, Introduction, Summary, corrected some typo