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

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

Electroweak constraints on new physics

We briefly review the limits on new interactions implied by electroweak precision data. Special attention is payed to the bounds on the Higgs boson mass. We also comment on the required cancellation among the new contributions to precisely measured electroweak observables in any Standard Model extension, if the new particles have to evade the indirect constraints on their couplings and masses but still remain at the LHC reach.Comment: Latex 4 pages, 1 table, 2 figures. To appear in the proceedings of 10th Hellenic School and Workshops on Elementary Particle Physics and Gravity (Corfu 2010), Corfu, Greece, Aug 29 - Sep 19, 201

Physical Parameters and Renormalization of U(1)_a x U(1)_b Models

We analize the structure of models with unbroken and spontaneously broken U(1)_a x U(1)_b gauge symmetry. We show that the quantum corrections to the 2N gauge charges, with N = #fermions + #scalars, can be absorbed in the redefinition of three independent gauge couplings (g_a,g_b and g_ab). We establish the (one-loop) conditions on the matter cotent for g_ab= = 0 (a value usually assumed in the literature) and we show that in the minimal extensions of the Standard Model with an extra U(1) symmetry the choice g_ab = 0 is not stable under radiative corrections induced by the standard Higgs fields. Moreover, g_ab = 0 to all orders seems to require an exact symmetry. The spontaneous breaking of the gauge symmetry induces further mixing between the two gauge bosons and introduces a fourth independent physical parameter. A consequence of our analysis is that the usual tree-level description with only three physical parameters (i.e., two gauge couplings and one gauge boson mixing angle) is not in general a justified zero order limit of the treatment including radiative corrections.Comment: 24 pages, tex, 1 figur

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.

Quark mixing: determination of top couplings

The top flavour-changing neutral couplings can be large in extended models with vector-like quarks. In the next decade(s) the CERN Large Hadron Collider will allow to measure (bound) them with a precision of few per cent.Comment: LaTeX 15 pages,19 PS figures, uses epsfig.sty. To be published in Acta Physica Polonica

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

Neutrino physics beyond neutrino masses

We briefly summarise the current status of neutrino masses and mixing, paying special attention to the prospects for observing new leptonic interactions.Comment: Latex 6 pages. To appear in the proceedings of 9th Hellenic School and Workshops on Elementary Particle Physics and Gravity (CORFU2009), Corfu, Greece, August 30-September 20, 2009

Combined analysis of Z' -> t tbar and Z' -> t tbar j production for vector resonance searches at LHC

We have implemented a code for Z' + n jets production in ALPGEN, with Z' decays into several final states, including l+ l- and t tbar. The MLM prescription is used for matching the matrix element with the parton shower, including in this way the leading soft and collinear corrections. In order to demonstrate its capabilities, we perform a combined analysis of Z' -> t tbar and Z' -> t tbar j production for a heavy leptophobic gauge boson. It is found that the effect of the extra jet cannot only be accounted for by a K factor multiplying the leading-order cross section. In fact, the combined analysis for Z' -> t tbar and Z' -> t tbar j presented improves the statistical significance of the signal by 25% (8.55 sigma versus 6.77 sigma for a Z' mass of 1 TeV), compared with the results of an inclusive analysis carried out on the same sample of t tbar + t tbar j events.Comment: LaTeX 17 pages, 13 PS figure

Light neutrino propagation in matter without heavy neutrino decoupling

We review the propagation of light neutrinos in matter assuming that their mixing with heavy neutrinos is close to present experimental limits. The phenomenological implications of the non-unitarity of the light neutrino mixing matrix for neutrino oscillations are discussed. In particular we show that the resonance effect in neutrino propagation in matter persists, but for slightly modified values of the parameters and with the maximum reduced by a small amount proportional to the mixing between light and heavy neutrinos squared.Comment: 11 pages, 1 figure, contribution to Stefan Pokorski 60th birthda