Discrete regularisation of localised kinetic terms

We investigate the behaviour of 5d models with general brane kinetic terms by discretising the extra dimension. We show that in the continuum limit the Kaluza-Klein masses and wave functions are in general nonanalytic in the coefficients of brane terms.Comment: Presented at the 7th DESY Workshop on Elementary Particle Theory ``Loops and Legs in Quantum Field Theory'', Zinnowitz, April 25-30, 200

Impact of extra particles on indirect Z' limits

We study the possibility of relaxing the indirect limits on extra neutral vector bosons by their interplay with additional new particles. They can be systematically weakened, even below present direct bounds at colliders, by the addition of more vector bosons and/or scalars designed for this purpose. Otherwise, they appear to be robust.Comment: Latex 23 pages, 8 eps figures. Minor changes, version published in Phys. Rev.

Reconstruction of the Extended Gauge Structure from Z′Z' Observables at Future Colliders

The discovery of a new neutral gauge boson $Z'$ with a mass in the TeV region would allow for determination of gauge couplings of the $Z'$ to ordinary quarks and leptons in a model independent way. We show that these couplings in turn would allow us to determine the nature of the extended gauge structure. As a prime example we study the $E_6$ group. In this case two discrete constraints on experimentally determined couplings have to be satisfied. If so, the couplings would then uniquely determine the two parameters, $\tan \beta$ and $\delta$, which fully specify the nature of the $Z'$ within $E_6$. If the $Z'$ is part of the $E_6$ gauge structure, then for $M_{Z'}=1$ TeV $\tan \beta$ and $\delta$ could be determined to around $10\%$ at the future colliders. The NLC provides a unique determination of the two constraints as well as of $\tan \beta$ and $\delta$, though with slightly larger error bars than at the LHC. On the other hand, since the LHC primarily determines three out of four normalized couplings, it provides weaker constraints for the underlying gauge structure.Comment: 14 pages LaTeX using RevTeX and psfig.sty. TeX source and 3 PS figures, tarred, compressed and uuencoded; also available via anonymous ftp to ftp://dept.physics.upenn.edu/pub/Cvetic/UPR-636-T

Low energy constraints on orbifold models

We review the low energy limits on Kaluza-Klein excitations in orbifold models. New vector-like quarks, as well as new Z' gauge bosons, can be accommodated with masses observable at large colliders.Comment: Presented at RADCOR 2002, to appear in the proceeding

Top mixing in effective theories

We review how top mixing with light quarks constrains new physics beyond the Standard Model using the effective Lagrangian approach.Comment: 4 pages, contribution to Workshop "Loops and Legs in Quantum Field Theory", April 200

The Minimal Extension of the SM and the Neutrino Oscillation Data

We study the simplest Standard Model estension with only one extra right-handed neutrino. In this case there are two massless $m_{1,2}$ and two massive $m_{3,4}$ neutrinos, and in principle both solar and atmospheric anomalies can be described in two different scenarios, $m_3 << m_4$ (scheme I) and $m_3 \simeq m_4$ (scheme II). However, neither bi-maximal mixing nor the dark matter problem are explained in this minimal extension. Only scheme II can accommodate simultaneously maximal mixing for atmospheric neutrinos and the small mixing angle MSW solution for the solar anomaly. This scenario can be tested in the BOREXINO experiment.Comment: 9 pages, Presented by J. Gluza at the XXIII School of Theoretical Physics, Ustron'99, Poland, September 15-22, 199

Multilepton production via top flavour-changing neutral couplings at the CERN LHC

Zt and gamma t production with Z -> l+ l- and t -> Wb -> l nu b provides the best determination of top flavour-changing neutral couplings at the LHC. The bounds on tc couplings eventually derived from these processes are similar to those expected from top decays, while the limits on tu couplings are better by a factor of two. The other significant Z and W decay modes are also investigated.Comment: 30 pages, 23 PS figures. Uses epsfig.sty and elsart.sty. Added some references and corrected some typos. Added more comments about statistics. Using elsart.sty reduces the size to 30 pages. Published in Nucl. Phys.

Evidence for right-handed neutrinos at a neutrino factory

We emphasize that a muon based neutrino factory could show the existence of light right-handed neutrinos, if a deficit in the number of detected events is observed at a near detector. This could be as large as ~10% if the size of the new interactions saturates the present limits from electroweak precision data, what is not excluded by the oscillation experiments performed up to now. A simple model realizing such a scenario can be obtained adding right-handed neutrinos to the minimal Standard Model, together with an extra scalar doublet and a triplet of hypercharge 1. In this case, however, the possible deficit is reduced by a factor of ~3, and the Yukawa couplings must be adequately chosen. This is also generically required if lepton flavour violation must be below present bounds.Comment: Latex 18 pages, 2 eps figure

Neutrino oscillations beyond the Standard Model

We address the possible impact of New Physics on neutrino oscillation experiments. This can modify the neutrino production, propagation and/or detection, making the full cross section non-factorizable in general. Thus, for example, the neutrino flux may not be properly described assuming an unitary MNS matrix and/or neutrinos may propagate differently depending of their Dirac or Majorana character. Interestingly enough, present limits on New Physics still allow for observable effects at future neutrino experiments.Comment: 2 pages, 2 figures, Presented at the Neutrino 08 Conference, Christchurch, New Zealand, May 25-31, 200