A dynamical gluon mass solution in Mandelstam's approximation

We discuss the pure gauge Schwinger-Dyson equation for the gluon propagator in the Landau gauge within an approximation proposed by Mandelstam many years ago. We show that a dynamical gluon mass arises as a solution. This solution is obtained numerically in the full range of momenta that we have considered without the introduction of any ansatz or asymptotic expression in the infrared region. The vertex function that we use follows a prescription formulated by Cornwall to determine the existence of a dynamical gluon mass in the light cone gauge. The renormalization procedure differs from the one proposed by Mandelstam and allows for the possibility of a dynamical gluon mass. Some of the properties of this solution, such as its dependence on $\Lambda_{QCD}$ and its perturbative scaling behavior are also discussed.Comment: 23 pages, 4 figures. Revised version with improved discussion on the renormalization procedur

Phenomenological tests for the freezing of the QCD running coupling constant

We discuss phenomenological tests for the frozen infrared behavior of the running coupling constant and gluon propagators found in some solutions of Schwinger-Dyson equations of the gluonic sector of QCD. We verify that several observables can be used in order to select the different expressions of alpha_s found in the literature. We test the effect of the nonperturbative coupling in the tau-lepton decay rate into nonstrange hadrons, in the rho vector meson helicity density matrix that are produced in the chi_{c2} --> rho rho decay, in the photon to pion transition form factor, and compute the cross sections for elastic proton-proton scattering and exclusive rho production in deep inelastic scattering. These quantities depend on the infrared behavior of the coupling constant at different levels, we discuss the reasons for this dependence and argue that the existent and future data can be used to test the approximations performed to solve the Schwinger-Dyson equations and they already seems to select one specific infrared behavior of the coupling.Comment: 15 pages, 8 figure

Bosonic string - Kaluza Klein theory exact solutions using 5D-6D dualities

We present the explicit formulae which allow to transform the general solution of the 6D Kaluza--Klein theory on a 3--torus into the special solution of the 6D bosonic string theory on a 3--torus as well as into the general solution of the 5D bosonic string theory on a 2--torus. We construct a new family of the extremal solutions of the 3D chiral equation for the SL(4,R)/SO(4) coset matrix and interpret it in terms of the component fields of these three duality related theories.Comment: 13 pages in LaTe

Gluon mass and freezing of the QCD coupling

Infrared finite solutions for the gluon propagator of pure QCD are obtained from the gauge-invariant non-linear Schwinger-Dyson equation formulated in the Feynman gauge of the background field method. These solutions may be fitted using a massive propagator, with the special characteristic that the effective mass employed drops asymptotically as the inverse square of the momentum transfer, in agreement with general operator-product expansion arguments. Due to the presence of the dynamical gluon mass the strong effective charge extracted from these solutions freezes at a finite value, giving rise to an infrared fixed point for QCD.Comment: 3 pages, 2 figures, based on talk given at the 2007 Europhysics Conference on High Energy Physics, Manchester, 19-25 Jul

Using single top rapidity to measure V_td, V_ts, V_tb at hadron colliders

Single top production processes are usually regarded as the ones in which V_tb can be directly measured at hadron colliders. We show that the analysis of the single top rapidity distribution in t-channel and tW production can also set direct limits on V_td. At LHC with 10 fb^-1 at 14 TeV the combined limits on V_td may be reduced by almost a factor of two when the top rapidity distribution is used. This also implies that the limits on V_tb can also be reduced by 15%, since both parameters as well as V_ts must be simultaneously obtained from a global fit to data. At Tevatron the explotation of this distribution would require very high statistics.Comment: LaTeX 19 pages, 48 PS figures. A section added with estimations of systematics, LO/NLO comparison and a fast detector simulation. Final version to appear in PR

Profile of multiboson signals

We investigate the visibility of signals characterised by wide bumps over a smoothly falling background that cannot be accurately predicted by Monte Carlo calculations. Examples of such are the wide bumps that triboson and quadriboson resonance cascade decays would yield in diboson resonance searches in fully hadronic final states. We find that the sensitivity to triboson bumps is rather small: signals of a moderate size could be present in current data and yet remain unnoticed. For quadriboson cascade decays the signals can hardly be distinguished from the background in the current searches.Comment: LaTeX 17 pages. v2: fixed typos and comments added; journal versio

Ultraboosted ZtZt and γt\gamma t production at the HL-LHC and FCC-hh

Searches for anomalous $Zt$ and $\gamma t$ production provide an excellent probe of flavour-changing top interactions when the energies considered are very large. In this note we estimate the sensitivity to these interactions at the high luminosity phase of the LHC and a future 100 TeV $pp$ collider (FCC-hh). For the LHC, the expected limits on $t \to uZ / u\gamma$ branching ratios from $Zt$ and $\gamma t$ production will reach the $10^{-5}$ level, one order of magnitude better than the existing projections for $t \to uZ$ from $t \bar t$ production. For the FCC-hh, the limits on $t \to uZ / u\gamma$ could reach an impressive sensitivity at the $10^{-6}$ level, with limits on $t \to cZ / c\gamma$ at the $10^{-5}$ level.Comment: LaTeX 11 pages. Plots fixed, journal versio