Simplification of Flavour Combinatorics in the Evaluation of Hadronic Processes

A serious computational problem in the evaluation of hadronic collision processes is connected with the large number of partonic subprocesses included in the calculation. These are from the quark and gluon content of the initial hadrons, and from CKM quark mixing. For example, there are 180 subprocesses which contribute to the $W$+2jets process, and 292 subprocesses in $W$+3jets production at the LHC, even when quarks from only the first two generations are taken into account. We propose a simple modification of the rules for evaluation of cross sections and distributions, which avoids multiplication of channels from the mixture of quark states. The method is based on a unitary rotation of down quarks, thus, transporting the mixing matrix elements from vertices of Feynman diagrams to the parton distribution functions (PDF). As a result, one can calculate cross sections with significantly fewer subprocesses. For the example mentioned above, with the new rules, one need evaluate only 21 and 33 subprocesses respectively. The matrix elements of the subprocesses are calculated without quark mixing but with a modified PDF convolution which depends on the quark mixing angle, and on the topologies of gauge invariant classes of diagrams. The proposed method has been incorporated into the CompHEP program and checked with various examples.Comment: 10 pages (standard LaTeX code), 3 figures, 2 table

Looking for anomalous gamma-gamma-H and Z-gamma-H couplings at future linear collider

We consider the possibility of studying anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices through the process e-gamma--> e-H at future e-gamma linear colliders, with Sqrt(S)=500-1500 GeV. We make a model independent analysis based on SU(2)xU(1) invariant effective operators of dim=6 added to the standard model lagrangian. We consider a light Higgs boson (mostly decaying in bar(b)-b pairs), and include all the relevant backgrounds. Initial e-beam polarization effects are also analyzed. We find that the process e-gamma--> e-H provides an excellent opportunity to strongly constrain both the CP-even and the CP-odd anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices.Comment: LaTeX, 33 pages, 16 eps figures, extended section

Optimization of symbolic evaluation of helicity amplitudes

We present a method for symbolic evaluation of Feynman amplitudes. We construct special polarization basis for spinor particles which produces compact expressions for tensor products of basis spinors.Comment: Standard LaTeX, 6 pages. To appear in the proceedings of the Seventh International Workshop on Advanced Computing and Analysis Technics in Physics Research (ACAT2000, Fermilab, October 16-20, 2000

Branching Fraction Measurements of the SM Higgs with a Mass of 160 GeV at Future Linear \ee Colliders

Assuming an integrated luminosity of 500 fb$^{-1}$ and a center-of-mass energy of 350 GeV, we examine the prospects for measuring branching fractions of a Standard Model-like Higgs boson with a mass of 160 GeV at the future linear \ee collider TESLA when the Higgs is produced via the Higgsstrahlung mechanism, \ee \pfr HZ. We study in detail the precisions achievable for the branching fractions of the Higgs into WW$^*$, ZZ$^*$ and \bb. However, the measurement of BF(H \pfr \gaga) remains a great challence. Combined with the expected error for the inclusive Higgsstrahlung production rate the uncertainty for the total width of the Higgs is estimated.Comment: 17 pages Latex, including 7 figure

Testing the ZγHZ\gamma H vertex at future linear colliders for intermediate Higgs masses

Higgs production in $e\gamma$ collisions, through the one-loop reaction $e\gamma\to eH$ at large $p_T$, can provide a precise determination of the $Z\gamma H$ vertex.Comment: 2 pages (standard LaTeX code); Contribution to the Proceedings of the Int. Europhysics Conference on High Energy Physics (Jerusalem, 19-26 August 1997

Bounds on anomalous gamma-gamma-H and Z-gamma-H couplings at future e-gamma linear colliders

We study the bounds on the anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices that can be obtained via the process e-gamma-->e H. We consider the representative case of an intermediate Higgs mass production of MH=120 GeV and for a center of mass energy of Sqrt{S}=500 GeV and Sqrt{S}=1500 GeV. We use a model independent analysis based on SU(2)xU(1) invariant operators of dimension 6 added to the Standard Model lagrangian. We find that this process provides an excelent way to put strong constraints both in the sector of CP-even and CP-odd anomalous couplings contribution to the gamma-gamma-H and Z-gamma-H vertices.Comment: LaTeX, 6 pages, To appear in the proceedings of the International Workshop on Linear Colliders, Sitges, Barcellona, Spain, April 28 - May 5, (1999

Anomalous gamma-gamma-H and Z-gamma-H couplings in the process e,gamma->e,H

The bounds on the anomalous contributions to the ggh and zgh vertices that can be obtained via the process eg->eh are discussed through a model independent analysis based on SU(2) X U(1) invariant operators of dim=6. A light Higgs boson with m_H ~ 120 GeV and center-of-mass energies of \sqrt{S}=500 GeV and \sqrt{S}=1500 GeV are assumed. The process eg->eh is shown to provide an excellent way to strongly constraint both the CP-even and CP-odd anomalous contributions in the ggH and ZgH couplings.Comment: 8 pages. To appear in the DESY report on "Physics Studies for a Future Linear Collider", DESY 123