Perturbation theory vs. simulation for tadpole improvement factors in pure gauge theories

We calculate the mean link in Landau gauge for Wilson and improved SU(3) anisotropic gauge actions, using two loop perturbation theory and Monte Carlo simulation employing an accelerated Langevin algorithm. Twisted boundary conditions are employed, with a twist in all four lattice directions considerably improving the (Fourier accelerated) convergence to an improved lattice Landau gauge. Two loop perturbation theory is seen to predict the mean link extremely well even into the region of commonly simulated gauge couplings and so can be used remove the need for numerical tuning of self-consistent tadpole improvement factors. A three loop perturbative coefficient is inferred from the simulations and is found to be small. We show that finite size effects are small and argue likewise for (lattice) Gribov copies and double Dirac sheets.Comment: 13 pages of revtex

Single Top Production at the Next Generation Linear e+e- Colliders

Present limits on the top mass from LEP1 and Tevatron point to a top quark that is considerably heavier than the $W$ vector boson in the standard model. Hence, e+e- colliders with \sqrt{s} \simeq 300 GeV (the c.m. energy foreseen at the first phase of the Next Linear e+e- Collider) could be well below the energy threshold for real top-pair production. We argue that, if this is the case, single top production through the process e+e- --> t\bar{b}W- (\bar{t}bW+), where t\bar{b} (\bar{t}b) are produced mainly by means of a virtual W, becomes the dominant top production mechanism. Total cross sections and kinematical distributions are evaluated and numerical results are given in ranges of m_t and \sqrts{s} where single top production can be of relevance. The relative importance of virtual-W and virtual-t contributions to the process is discussed.Comment: 20 pages, LaTeX + feynman.tex, 10 compressed (tar.Z) postscript figures included in a separate uuencoded file, revised version of Rome1 Preprint n.979 (1993), Dec 29, 1993. (In this revised version -- accepted for publication on Zeit. fur Phys.C in Jan 24, 1994 -- some sentences and 3 new refs. have been added with respect to the first one

Synchonizing U.S. States\u27 Strategic Plans to Increase Exports To Emerging Markets

The United States seeks to increase and expand exports to emerging markets. The majority of U.S. firms (98 percent) are small to medium sized enterprises (SMEs) with only 1 percent engaging in exporting. The Federal and State Governments encourage SMEs to expand exporting through the National Export Initiative (NEI) and NEI/NEXT initiatives. This paper examines NEI’s progress and strategies by leading U.S. exporting states, export activity to BRICSA, states’ alignment with NEI and the extent to which NEI has facilitated federal and state collaboration. Further, it reviews NEI/NEXT objectives and strategies to internationalize U.S. business practices

Morphologic examination of the temporal bone by cone beam computed tomography: Comparison with multislice helical computed tomography

SummaryIntroductionHigh-resolution CT imaging is essential to diagnosis and follow-up of temporal bone pathology. Morphologically, CT is the reference examination. The requirement of long-term follow-up thus exposes patients to cumulative radiation doses. Limiting exposure to ionizing radiation is an increasing concern of public health authorities. The principal advantage of Cone Beam CT (CBCT) lies in a significant reduction in radiation dose. The main objective of the present study was to assess the morphologic concordance between CBCT and Multislice Helical Computed Tomography (MSCT) on 20 anatomic landmarks corresponding to regions of interest in clinical practice. The secondary objectives were to compare the two techniques qualitatively in stapes and footplate assessment and measurement of footplate thickness, and quantitatively in terms of dosimetry.Material and methodsAn experimental anatomical study was performed on 12 temporal bones from fresh human cadavers of unknown clinical history. Each underwent CBCT and MSCT.ResultsThere was no significant difference in morphologic assessment of the temporal bones on the two techniques. Exploration of the stapes, incudostapedial joint, anterior stapediovestibular joint and footplate was qualitatively more precise on CBCT, and footplate thickness showed less overestimation than on MSCT. CBCT delivered 22 times less radiation than MSCT under the present experimental conditions.ConclusionCBCT provides reliable morphologic assessment of temporal bone, thanks to higher spatial resolution than on MSCT, with significantly reduced radiation dose

Soft pion theorem for hard near threshold pion production

We prove new soft pion theorem for the near threshold pion production by a hard electromagnetic probe. This theorem relates various near threshold pion production amplitudes to the nucleon distribution amplitudes. The new soft pion theorem is in a good agreement with the SLAC data for F_2^p(W,Q^2) for W^2 < 1.4 GeV^2 and 7 < Q^2 < 30.7 GeV^2.Comment: 9 pages, revised version, more general analysi

Probing partonic structure in gamma* gamma -> pi pi near threshold

Hadron pair production gamma* gamma -> h hbar in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new non-perturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where h is a pion is emphasized.Comment: 8 pages, 1 figure, LaTeX2

String breaking by dynamical fermions in three-dimensional lattice QCD

The first observation is made of hadronic string breaking due to dynamical fermions in zero temperature lattice QCD. The simulations are done for SU(2) color in three dimensions, with two flavors of staggered fermions. The results have clear implications for the large scale simulations that are being done to search (so far, without success) for string breaking in four-dimensional QCD. In particular, string breaking is readily observed using only Wilson loops to excite a static quark-antiquark pair. Improved actions on coarse lattices are used, providing an extremely efficient means to access the quark separations and propagation times at which string breaking occurs.Comment: Revised version to appear in Physical Review D, has additional discussion of the results, additional references, modified title, larger figure

One loop renormalisation of Lattice NRQCD currents for semileptonic B→D(∗)B\to D^{(\ast)} decays to order p⃗M\frac{\vec{p}}{M}

We present the results of a perturbative calculation to match the axial and vector currents for semileptonic $B\to D^{(\ast)}$ decays in lattice NRQCD to the continuum \MSb scheme. The matching is performed to $O(\alpha_s\frac{\vec{p}}{M})$ in Feynman gauge and in the on-shell renormalisation scheme. The spatial and temporal components of the currents renormalise differently; to this order the matching involves a straightforward renormalisation for the $V_0$ and $A_k$ currents, and a rank two and four mixing matrix for the $A_0$ and $V_k$ currents respectively. The resultant one loop corrections are of $O(5$, boding well for the accuracy of forthcoming simulations.Comment: 24 Pages, 18 Figure

External Operators and Anomalous Dimensions in Soft-Collinear Effective Theory

It has recently been argued that soft-collinear effective theory for processes involving both soft and collinear partons contains a new soft-collinear mode, which can communicate between the soft and collinear sectors of the theory. The formalism incorporating the corresponding fields into the effective Lagrangian is extended to include external current and four-quark operators relevant to weak interactions. An explicit calculation of the anomalous dimensions of these operators reveals that soft-collinear modes are needed for correctly describing the ultraviolet behavior of the effective theory.Comment: 15 pages, 2 figure

Infrared Behavior of the Gluon Propagator on a Large Volume Lattice

The first calculation of the gluon propagator using an order a^2 improved action with the corresponding order a^2 improved Landau gauge fixing condition is presented. The gluon propagator obtained from the improved action and improved Landau gauge condition is compared with earlier unimproved results on similar physical lattice volumes of 3.2^3 \times 6.4 fm^4. We find agreement between the improved propagator calculated on a coarse lattice with lattice spacing a = 0.35 fm and the unimproved propagator calculated on a fine lattice with spacing a = 0.10 fm. This motivates us to calculate the gluon propagator on a coarse large-volume lattice 5.6^3 \times 11.2 fm^4. The infrared behavior of previous studies is confirmed in this work. The gluon propagator is enhanced at intermediate momenta and suppressed at infrared momenta. Therefore the observed infrared suppression of the Landau gauge gluon propagator is not a finite volume effect.Comment: 8 pages, 4 figures, minor typos corrected and repsonse to referees comment