QCD corrections to e+ e- --> 4 jets

We report on the next-to-leading order QCD calculation for e+ e- --> 4 jets. We explain some modern techniques which have been used to calculate the one-loop amplitudes efficiently. We further report on the general purpose numerical program ``Mercutio'', which can be used to calculate any infrared safe four-jet quantity in electron-positron annihilation at next-to-leading order.Comment: 4 pages, talk given at the UK Phenomenology Workshop on Collider Physics, Durham, 19-24 September 199

Prospects for New Physics at the LHC

High-energy collisions at the LHC are now starting. The new physics agenda of the LHC is reviewed, with emphasis on the hunt for the Higgs boson (or whatever replaces it) and supersymmetry. In particular, the prospects for discovering new physics in the 2010-2011 run are discussed.Comment: 13 pages, 9 figures, Invited Talk at Conference in Honor of Murray Gell-Mann's 80th Birthday, on Quantum Mechanics, Elementary Particles, Quantum Cosmology and Complexity, Nanyang Executive Centre, NTU, Singapore, 24th-26th February 201

Non-perturbative effective model for the Higgs sector of the Standard Model

A non-perturbative effective model is derived for the Higgs sector of the standard model, described by a simple scalar theory. The renormalized couplings are determined by the derivatives of the Gaussian Effective Potential that are known to be the sum of infinite bubble graphs contributing to the vertex functions. A good agreement has been found with strong coupling lattice simulations when a comparison can be made

Tau and Charm physics highlights

In tau physics, we are at the frontier between the completion of the LEP program and the start of analyses from b-factories, which are expected to produce results in the coming years. Nice results from CLEO are steadily delivered in the meantime. For charm, impressive progress have been achieved by fixed target experiments in the search for CP violation and D^0 - \bar D^0 oscillations. First results from b-factories demonstrate the power of these facilities in such areas. The novel measurement of the D* width by CLEO happens to be rather different from current expectations. The absence of a charm factory explains the lack or the very slow progress in the absolute scale determinations for charm decays.Comment: "Typos corrected and references added

Flipped SU(5), see-saw scale physics and degenerate vacua

We investigate the requirement of the existence of two degenerate vacua of the effective potential as a function of the Weinberg-Salam Higgs scalar field norm, as suggested by the multiple point principle, in an extension of the Standard Model including see-saw scale physics. Results are presented from an investigation of an extension of the Standard Model to the gauge symmetry group SU(3)_C\times SU(2)_L\times U(1)'\times \tilde U(1), where two groups U(1)' and \tilde U(1) originate at the see-saw scale M_{SS}, when heavy (right-handed) neutrinos appear. The consequent unification of the group SU(3)_C\times SU(2)_L\times U(1)' into the flipped SU(5) at the GUT scale leads to the group SU(5)\times \tilde U(1). We assume the position of the second minimum of the effective potential coincides with the fundamental scale, here taken to be the GUT scale. We solve the renormalization group equations in the one-loop approximation and obtain a top-quark mass of 171\pm 3 GeV and a Higgs mass of 129\pm 4 GeV, in the case when the Yukawa couplings of the neutrinos are less than half that of the top quark at the GUT scale.Comment: 12 pages and 3 Figures; Presented at 'Planck 05' conference, ICT

Probing the Higgs Field Using Massive Particles as Sources and Detectors

In the Standard Model, all massive elementary particles acquire their masses by coupling to a background Higgs field with a non-zero vacuum expectation value. What is often overlooked is that each massive particle is also a source of the Higgs field. A given particle can in principle shift the mass of a neighboring particle. The mass shift effect goes beyond the usual perturbative Feynman diagram calculations which implicitly assume that the mass of each particle is rigidly fixed. Local mass shifts offer a unique handle on Higgs physics since they do not require the production of on-shell Higgs bosons. We provide theoretical estimates showing that the mass shift effect can be large and measurable, especially near pair threshold, at both the Tevatron and the LHC.Comment: 6 pages, no figures; Version 2 corrects some typographical errors of factors of 2 in equations 14, 17, 18 and 19 (all of the same origin) and mentions a linear collider as an interesting place to test the results of this pape

Energy dependence of ratios of multiplicities and their slopes for gluon and quark jets

The difference between the ratio of multiplicities and the ratio of their derivatives on energy for gluon and quark jets is calculated up to next-to-next-to leading order of perturbative QCD. Its non-zero value is uniquely defined by the running property of the QCD coupling constant. It is shown that this difference is rather small compared to values which can be obtained from experimental data. This disagreement can be ascribed either to strong non-perturbative terms or to experimental problems with a scale choice, jets separation and inadequate assignement of soft particles to jets.Comment: 5 pages, LATEX, no Figs; submitted to JETP Let

Finite-width effects in the near-threshold ZZZ and ZWW production at ILC

We calculate the cross-section of the near-threshold off-shell $ZZZ$ and $ZW^+W^-$ production at the International Linear Collider taking into account their instability and the principal part of NLO corrections. The calculations are performed in the framework of the model of unstable particles with smeared mass-shell. We show that the contribution of the finite $Z/W$ and $H$ widths (their instability) is large in the Higgs resonance range and should be taken into account in the Higgs boson searches at future colliders.Comment: 5 pages, 6 figure