Power corrections in models with extra dimensions

We critically revisit the issue of power-law running in models with extra dimensions. The general conclusion is that, in the absence of any additional physical principle, the power-corrections tend to depend strongly on the details of the underlying theory.Comment: Talk given at EPS2003 - Aachen, Germany, July 2003, 3 pages, 1 figur

On the connection between the pinch technique and the background field method

The connection between the pinch technique and the background field method is further explored. We show by explicit calculations that the application of the pinch technique in the framework of the background field method gives rise to exactly the same results as in the linear renormalizable gauges. The general method for extending the pinch technique to the case of Green's functions with off-shell fermions as incoming particles is presented. As an example, the one-loop gauge independent quark self-energy is constructed. We briefly discuss the possibility that the gluonic Green's functions, obtained by either method, correspond to physical quantities.Comment: 13 pages and 3 figures, all included in a uuencoded file, to appear in Physical Review

Strangeness contribution to the vector and axial form factors of the nucleon

The strangeness contribution to the vector and axial form factors of the nucleon is presented for momentum transfers in the range $0.45<Q^2<1.0$ GeV$^2$. The results are obtained via a combined analysis of forward-scattering parity-violating elastic $\vec{e}p$ asymmetry data from the $G^0$ and HAPPEx experiments at Jefferson Lab, and elastic $\nu p$ and $\bar{\nu} p$ scattering data from Experiment 734 at Brookhaven National Laboratory. The parity-violating asymmetries measured in elastic $\vec{e}p$ scattering at forward angles establish a relationship between the strange vector form factors $G_E^s$ and $G_M^s$, with little sensitivity to the strange axial form factor $G_A^s$. On the other hand, elastic neutrino scattering at low $Q^2$ is dominated by the axial form factor, with still some significant sensitivity to the vector form factors as well. The combination of the two data sets allows the simultaneous extraction of $G_E^s$, $G_M^s$, and $G_A^s$ over a significant range of $Q^2$ for the very first time.Comment: 3 pages, 1 figure, will appear in AIP Conference Proceedings for PANIC 200

Electroweak pinch technique to all orders

The generalization of the pinch technique to all orders in the electroweak sector of the Standard Model within the class of the renormalizable 't Hooft gauges, is presented. In particular, both the all-order PT gauge-boson-- and scalar--fermions vertices, as well as the diagonal and mixed gauge-boson and scalar self-energies are explicitly constructed. This is achieved through the generalization to the Standard Model of the procedure recently applied to the QCD case, which consist of two steps: (i) the identification of special Green's functions, which serve as a common kernel to all self-energy and vertex diagrams, and (ii) the study of the (on-shell) Slavnov-Taylor identities they satisfy. It is then shown that the ghost, scalar and scalar--gauge-boson Green's functions appearing in these identities capture precisely the result of the pinching action at arbitrary order. It turns out that the aforementioned Green's functions play a crucial role, their net effect being the non-trivial modification of the ghost, scalar and scalar--gauge-boson diagrams of the gauge-boson-- or scalar--fermions vertex we have started from, in such a way as to dynamically generate the characteristic ghost and scalar sector of the background field method. The pinch technique gauge-boson and scalar self-energies are also explicitly constructed by resorting to the method of the background-quantum identities.Comment: 48 pages, 8 figures; v2: typos correcte

Magnetic field-dependent interplay between incoherent and Fermi liquid transport mechanisms in low-dimensional tau phase organic conductors

We present an electrical transport study of the 2-dimensional (2D) organic conductor tau-(P-(S,S)-DMEDT-TTF)_2(AuBr)_2(AuBr_2)_y (y = 0.75) at low temperatures and high magnetic fields. The inter-plane resistivity rho_zz increases with decreasing temperature, with the exception of a slight anomaly at 12 K. Under a magnetic field B, both rho_zz and the in-plane resistivity plane rho_xx show a pronounced negative and hysteretic magnetoresistance with Shubnikov de Haas (SdH)oscillations being observed in some (high quality)samples above 15 T. Contrary to the predicted single, star-shaped, closed orbit Fermi surface from band structure calculations (with an expected approximate area of 12.5% of A_FBZ), two fundamental frequencies F_l and F_h are detected in the SdH signal. These orbits correspond to 2.4% and 6.8% of the area of the first Brillouin zone(A_FBZ), with effective masses F_l = 4.0 +/- 0.5 and F_h = 7.3 +/- 0.1. The angular dependence, in tilted magnetic fields of F_l and F_h, reveals the 2D character of the FS and Angular dependent magnetoresistance (AMRO) further suggests a FS which is strictly 2-D where the inter-plane hopping t_c is virtually absent or incoherent. The Hall constant R_xy is field independent, and the Hall mobility increases by a factor of 3 under moderate magnetic fields. Our observations suggest a unique physical situation where a stable 2D Fermi liquid state in the molecular layers are incoherently coupled along the least conducting direction. The magnetic field not only reduces the inelastic scattering between the 2D metallic layers, but it also reveals the incoherent nature of interplane transport in the AMRO spectrum. The apparent ferromagnetism of the hysteretic magnetoresistance remains an unsolved problem.Comment: 33 pages, 11 figure

The pinch technique at two-loops: The case of mass-less Yang-Mills theories

The generalization of the pinch technique beyond one loop is presented. It is shown that the crucial physical principles of gauge-invariance, unitarity, and gauge-fixing-parameter independence single out at two loops exactly the same algorithm which has been used to define the pinch technique at one loop, without any additional assumptions. The two-loop construction of the pinch technique gluon self-energy, and quark-gluon vertex are carried out in detail for the case of mass-less Yang-Mills theories, such as perturbative QCD. We present two different but complementary derivations. First we carry out the construction by directly rearranging two-loop diagrams. The analysis reveals that, quite interestingly, the well-known one-loop correspondence between the pinch technique and the background field method in the Feynman gauge persists also at two-loops. The renormalization is discussed in detail, and is shown to respect the aforementioned correspondence. Second, we present an absorptive derivation, exploiting the unitarity of the $S$-matrix and the underlying BRS symmetry; at this stage we deal only with tree-level and one-loop physical amplitudes. The gauge-invariant sub-amplitudes defined by means of this absorptive construction correspond precisely to the imaginary parts of the $n$-point functions defined in the full two-loop derivation, thus furnishing a highly non-trivial self-consistency check for the entire method. Various future applications are briefly discussed.Comment: 29 pages, uses Revtex, 22 Figures in a separate ps fil

On the zero crossing of the three-gluon vertex

We report on new results on the infrared behaviour of the three-gluon vertex in quenched Quantum Chormodynamics, obtained from large-volume lattice simulations. The main focus of our study is the appearance of the characteristic infrared feature known as 'zero crossing', the origin of which is intimately connected with the nonperturbative masslessness of the Faddeev-Popov ghost. The appearance of this effect is clearly visible in one of the two kinematic configurations analyzed, and its theoretical origin is discussed in the framework of Schwinger-Dyson equations. The effective coupling in the momentum subtraction scheme that corresponds to the three-gluon vertex is constructed, revealing the vanishing of the effective interaction at the exact location of the zero crossing.Comment: 6 pages, 4 figure

Strange Quark Contribution to the Vector and Axial Form Factors of the Nucleon: Combined Analysis of G0, HAPPEx, and Brookhaven E734 Data

The strange quark contribution to the vector and axial form factors of the nucleon has been determined for momentum transfers in the range $0.45<Q^2<1.0$ GeV$^2$. The results are obtained via a combined analysis of forward-scattering, parity-violating elastic $\vec{e}p$ asymmetry data from the G0 and HAPPEx experiments at Jefferson Lab, and elastic $\nu p$ and $\bar{\nu} p$ scattering data from Experiment 734 at Brookhaven National Laboratory. The parity-violating asymmetries measured in elastic $\vec{e}p$ scattering at forward angles establish a relationship between the strange vector form factors $G_E^s$ and $G_M^s$, with little sensitivity to the strange axial form factor $G_A^s$. On the other hand, elastic neutrino scattering at low $Q^2$ is dominated by the axial form factor, with some significant sensitivity to the vector form factors as well. Combination of the two data sets allows the simultaneous extraction of $G_E^s$, $G_M^s$, and $G_A^s$ over a significant range of $Q^2$ for the very first time. The $Q^2$-dependence of the strange axial form factor suggests that the strange quark contribution to the proton spin, $\Delta s$, is negative.Comment: 21 pages, 2 figures, 6 tables, 63 references; revised to fix minor typos and to add a missing reference; to be submitted to Physical Review

Low-Energy Constraints on New Physics Revisited

It is possible to place constraints on non-Standard-Model gauge-boson self-couplings and other new physics by studying their one-loop contributions to precisely measured observables. We extend previous analyses which constrain such nonstandard couplings, and we present the results in a compact and transparent form. Particular attention is given to comparing results for the light-Higgs scenario, where nonstandard effects are parameterized by an effective Lagrangian with a linear realization of the electroweak symmetry breaking sector, and the heavy-Higgs/strongly interacting scenario, described by the electroweak chiral Lagrangian. The constraints on nonstandard gauge-boson self-couplings which are obtained from a global analysis of low-energy data and LEP/SLC measurements on the Z pole are updated and improved from previous studies. Replaced version: tables and figures of Section VIb recalculated. There were roundoff problems, especially in Fig. 8. Text unchanged.Comment: \documentstyle[preprint,aps,floats,psfig]{revtex}, 10 figures, postscript version available from ftp://ftp.kek.jp/kek/preprints/TH/TH-51

Two-loop electroweak corrections to the ρ\rho parameter beyond the leading approximation

We show that in the framework of the pinch technique the universal part of the $\rho$ parameter can be meaningfully defined, beyond one loop. The universal part so obtained satisfies the crucial requirements of gauge-independence, finiteness, and process-independence, even when subleading contributions of the top quark are included. The mechanism which enforces the aforementioned properties is explained in detail, and several subtle field theoretical issues are discussed. Explicit calculations of the sub-leading two-loop corrections of order $O(G_{\mu}^{2}m^{2}_{t}M_{Z}^{2})$ are carried out in the context of an $SU(2)$ model, with $M_{W}=M_{Z}$, and various intermediate and final results are reported.Comment: 40 pages, TeXsis, uu encoded ps files, 14 figures include