Supersymmetric virtual effects in heavy quark pair production at LHC

We consider the production of heavy ($b,t$) quark pairs at proton colliders in the theoretical framework of the MSSM. Under the assumption of a "moderately" light SUSY scenario, we first compute the leading logarithmic MSSM contributions at one loop for the elementary processes of production from a quark and from a gluon pair in the 1 TeV c.m. energy region. We show that in the initial gluon pair case (dominant in the chosen situation at LHC energies) the electroweak and the strong SUSY contributions concur to produce an enhanced effect whose relative value in the cross sections could reach the twenty percent size for large $\tan\beta$ values in the realistic proton-proton LHC process.Comment: 19 pages and 8 figures; version to appear in Phys. Rev.

The two-dimensional Wess-Zumino model in the Hamiltonian lattice formulation

We investigate a Hamiltonian lattice version of the two-dimensional Wess-Zumino model, with special emphasis to the pattern of supersymmetry breaking. Results are obtained by Quantum Monte Carlo simulations and Density Matrix Renormalization group techniques.Comment: 3 pages, Lattice2003(theory

Reliability of a high energy one-loop expansion of e+e- --> W+W- in the SM and in the MSSM

We compare the logarithmic Sudakov expansions of the process e+e- --> W+W- in the one-loop approximation and in the resummed version, to subleading order accuracy, in the SM case and in a light SUSY scenario for the MSSM. We show that the two expansions are essentially identical below 1 TeV, but differ drastically at higher (2,3 TeV) center of mass energies. Starting from these conclusions, we argue that a complete one-loop calculation in the energy region below 1 TeV does not seem to need extra two-loop corrections, in spite of the relatively large size of the one-loop effects.Comment: 15 pages, 1 Encapsulated PostScript figur

Two-loop electroweak corrections at high energies

We discuss two-loop leading and angular-dependent next-to-leading logarithmic electroweak virtual corrections to arbitrary processes at energies above the electroweak scale. The relevant Feynman diagrams involving soft-collinear gauge bosons gamma, Z, W, have been evaluated in eikonal approximation. We present results obtained from the analytic evaluation of massive loop integrals. To isolate mass singularities we used the Sudakov method and alternatively the sector decomposition method in the Feynman-parameter representation.Comment: 5 pages. Talk presented by S.P. at the International Symposium on Radiative Corrections RADCOR 2002, September 8-13, Kloster Banz, Germany. To appear in the proceeding

Double scaling limit of N=2 chiral correlators with Maldacena-Wilson loop

We consider $\mathcal N=2$ conformal QCD in four dimensions and the one-point correlator of a class of chiral primaries with the circular $\frac{1}{2}$-BPS Maldacena-Wilson loop. We analyze a recently introduced double scaling limit where the gauge coupling is weak while the R-charge of the chiral primary $\Phi$ is large. In particular, we consider the case $\Phi=(\text{tr}\varphi^{2})^{n}$ , where $\varphi$ is the complex scalar in the vector multiplet. The correlator defines a non-trivial scaling function at fixed $\kappa = n\,g_{\rm YM}^{2}$ and large $n$ that may be studied by localization. For any gauge group $SU(N)$ we provide the analytic expression of the first correction $\sim \zeta(3)\,\kappa^{2}$ and prove its universality. In the $SU(2)$ and $SU(3)$ theories we compute the scaling functions at order $\mathcal O(\kappa^{6})$. Remarkably, in the $SU(2)$ case the scaling function is equal to an analogous quantity describing the chiral 2-point functions $\langle\Phi\overline\Phi\rangle$ in the same large R-charge limit. We conjecture that this $SU(2)$ scaling function is computed at all-orders by a $\mathcal N=4$ SYM expectation value of a matrix model object characterizing the one-loop contribution to the 4-sphere partition function. The conjecture provides an explicit series expansion for the scaling function and is checked at order $\mathcal O(\kappa^{10})$ by showing agreement with the available data in the sector of chiral 2-point functions.Comment: 21 page

Maximal Stability Regions for Superconducting Ground States of Generalized Hubbard Models

For a class of generalized Hubbard models, we determine the maximal stability region for the superconducting eta-pairing ground state. We exploit the Optimized Ground State (OGS) approach and the Lanczos diagonalization procedure to derive a sequence of improved bounds. We show that some pieces of the stability boundary are asymptotic, namely independent on the OGS cluster size. In this way, necessary and sufficient conditions are obtained to realize superconductivity in terms of an eta-pairing ground state. The phenomenon is explained by studying the properties of certain exact eigenstates of the OGS hamiltonians.Comment: 9 pages, 5 PostScript figures, submitted to Phys. Rev. Let

Monte Carlo study of exact S-matrix duality in non simply laced affine Toda theories

The $(g_2^{(1)}, d_4^{(3)})$\ pair of non simply laced affine Toda theories is studied from the point of view of non perturbative duality. The classical spectrum of each member is composed of two massive scalar particles. The exact S-matrix prediction for the dual behaviour of the coupling dependent mass ratio is found to be in strong agreement with Monte Carlo data.Comment: 15 pages, 2 Postscript figures. Packed by uufiles. Two references adde

On the large R-charge N=2\mathcal N=2 chiral correlators and the Toda equation

We consider $\mathcal N=2$ $SU(N)$ SQCD in four dimensions and a weak-coupling regime with large R-charge recently discussed in arXiv:1803.00580. If $\varphi$ denotes the adjoint scalar in the $\mathcal N=2$ vector multiplet, it has been shown that the 2-point functions in the sector of chiral primaries $(\text{Tr} \varphi^2)^n$ admit a finite limit when $g_\text{YM}\to 0$ with large R-charge growing like $\sim 1/g^2_\text{YM}$. The correction with respect to $\mathcal N=4$ correlators is a non-trivial function $F(\lambda; N)$ of the fixed coupling $\lambda=n\,g^2_\text{YM}$ and the gauge algebra rank $N$. We show how to exploit the Toda equation following from the $tt^*$ equations in order to control the R-charge dependence. This allows to determine $F(\lambda; N)$ at order $O(\lambda^{10})$ for generic $N$, greatly extending previous results and placing on a firmer ground a conjecture proposed for the $SU(2)$ case. We show that a similar Toda equation, discussed in the past, may indeed be used for the additional sector $(\text{Tr}\varphi^2)^n\,\text{Tr}\varphi^3$ due to the special mixing properties of these composite operators on the 4-sphere. We discuss the large R-limit in this second case and compute the associated scaling function $F$ at order $O(\lambda^7)$ and generic $N$. Large $N$ factorization is also illustrated as a check of the computation.Comment: 27 pages. v2: minor clarifications adde

Sudakov Expansions and Top Quark Physics at LHC

We review some peculiar features of Sudakov expansions in the calculation of electroweak radiative corrections in the MSSM at high energy. We give specific examples and consider in particular the process b g -> t W of single top quark production relevant for the top quark physics programme at LHC.Comment: 4 pages, PostScript fil

Two-loop electroweak angular-dependent logarithms at high energies

We present results on the two-loop leading and angular-dependent next-to-leading logarithmic virtual corrections to arbitrary processes at energies above the electroweak scale. In the `t Hooft-Feynman gauge the relevant Feynman diagrams involving soft and collinear gauge bosons \gamma, Z, W^\pm coupling to external legs are evaluated in the eikonal approximation in the region where all kinematical invariants are much larger than the electroweak scale. The logarithmic mass singularities are extracted from massive multi-scale loop integrals using the Sudakov method and alternatively the sector-decomposition method in the Feynman-parameter representation. The derivations are performed within the spontaneously broken phase of the electroweak theory, and the two-loop results are in agreement with the exponentiation prescriptions that have been proposed in the literature based on a symmetric SU(2) x U(1) theory matched with QED at the electroweak scale.Comment: 31 pages, LaTe