Searcharxiv⌕ Search

arXiv subjects

F. del Aguila

Publications and source records attributed to F. del Aguila.

At least 55 records · Page 3Linked to original sources
arXiv2002

Precise determination of the Wtb couplings at LHC

Top pair production at LHC is the ideal place to search for nonstandard Wtb couplings in t -> W b -> l nu b decays. The lb forward-backward asymmetry in the W rest frame is very sensitive to sigma_{mu nu} couplings, and can spot one-loop QCD corrections to the decay vertex with more than 5 sigma statistical significance. We discuss the potential of this asymmetry to signal nonstandard gamma_mu and sigma_{mu nu} couplings and compare with top-antitop spin correlation asymmetries, which have a lower sensitivity. We also briefly summarise the results for Tevatron.

hep-ph↗
arXiv2002

Forward-backward asymmetries of lepton pairs in events with a large transverse momentum jet at hadron colliders

We discuss forward-backward charge asymmetries for lepton pair production in association with a large transverse momentum jet at hadron colliders. The measurement of the lepton charge asymmetry relative to the jet direction A_{FB}^j gives a new determination of the effective weak mixing angle sin^2{θ_{eff}^{lept}(M_Z^2)} with in principle a statistical precision after cuts of \sim 10^{-3} (8 x 10^{-3}) at LHC (Tevatron), due to the large cross section of the process with initial gluons. The identification of b jets also allows for the measurement of the bottom quark Z asymmetry A_{FB}^b at hadron colliders although with a lower precision than at LEP, the resulting statistical precision for sin^2{θ_{eff}^{lept}(M_Z^2)} being \sim 9 x 10^{-4} (2 x 10^{-2} at Tevatron).

hep-ph↗
arXiv2002

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.

hep-ph↗
arXiv2002

Signals from extra dimensions decoupled from the compactification scale

Multilocalization provides a simple way of decoupling the mass scale of new physics from the compactification scale of extra dimensions. It naturally appears, for example, when localization of fermion zero modes is used to explain the observed fermion spectrum, leaving low energy remnants of the geometrical origin of the fermion mass hierarchy. We study the phenomenology of the simplest five dimensional model with order one Yukawa couplings reproducing the standard fermion masses and mixing angles and with a light Kaluza-Klein quark Q_{2/3} saturating experimental limits on V_{tb} and m_Q, and then with observable new effects at Tevatron.

hep-ph↗
arXiv2000

Large top mixing from extra dimensions

Fermion mixing is conveniently described using the effective Lagrangian formalism. We apply this approach to study top mixing in models with an infinite tower of Kaluza-Klein fermion excitations. In the Randall-Sundrum background with a boundary Higgs and phenomenologically viable values of the model parameters, the only effect eventually observable is the loss of universality of the top couplings. Their deviation from the SM predictions can be up to 4 % if the five-dimensional Yukawa couplings times the AdS_5 curvature scale are < 10.

hep-ph↗
arXiv2000

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.

hep-ph↗
arXiv2000

Universality limits on bulk fermions

The Kaluza-Klein fermion excitations induce mixing between the Standard Model fermions and loss of universality. The flavour mixing not present in the Standard Model can be made to vanish aligning the Yukawa couplings and the Dirac masses of the heavy modes, but universality is only recovered when these masses go to infinity. This implies a bound on the lightest new heavy quark, M1 \~ 3-5 TeV, which together with the electroweak precision data limits will allow the Large Hadron Collider to provide a crucial test of the Randall-Sundrum ansatz for solving the gauge hierarchy.

hep-ph↗
arXiv2000

Observable contributions of new exotic quarks to quark mixing

Models with new vector-like quarks can produce observable quark mixing effects which are forbidden in the Standard Model. We classify all such models and write down the effective Lagrangian that results from integrating out the new quarks. We study the relations between neutral and charged currents and discuss how to distinguish among the different possibilities.

hep-ph↗
arXiv2000

Effective description of quark mixing

We use the effective Lagrangian formalism to describe quark mixing. The new W, Z and H couplings generalizing the CKM matrix and the GIM mechanism fulfil relations and inequalities which allow to discriminate among different SM extensions. As a by-product we give a useful parametrization of the generalized CKM matrix. We also show that the largest possible departures from the SM predictions result from heavy exotic fermions, which can induce, for example, top FCNC large enough to be observable at future colliders.

hep-ph↗
arXiv2000

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.

hep-ph↗
arXiv1999

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.

hep-ph↗
arXiv1999

Zt and γt production via top flavour-changing neutral couplings at the Fermilab Tevatron

Associated single top production with a Z boson or a photon at large hadron colliders provides a precise determination of top flavour-changing neutral couplings. The best way to measure these couplings with the up quark at Tevatron is to search for events with three jets and missing energy or events with a photon, a charged lepton, a jet and missing energy. Other decay channels are also discussed.

hep-ph↗
arXiv1999

Top mixing

We discuss the existing limits on top flavour-changing neutral couplings in models with new vector-like quarks. Large hadron and e+ e- colliders can improve these bounds by more than one order of magnitude.

hep-ph↗
arXiv1999

CP violation from new quarks in the chiral limit

We characterize CP violation in the SU(2)xU(1) model due to an extra vector-like quark or sequential family, giving special emphasis to the chiral limit m_{u,d,s}=0. In this limit, CP is conserved in the three generation Standard Model (SM), thus implying that all CP violation is due to the two new CP violating phases whose effects may manifest either at high energy in processes involving the new quark or as deviations from SM unitarity equalities among imaginary parts of invariant quartets (or, equivalently, areas of unitarity triangles). In our analysis we use an invariant formulation, independent of the choice of weak quark basis or the phase convention in the generalized Cabibbo-Kobayashi-Maskawa matrix. We identify the three weak-basis invariants, as well as the three imaginary parts of quartets B_{1-3} which, in the chiral limit, give the strength of CP violation beyond the SM. We find that for an extra vector-like quark |B_i| <= 10^{-4}, whereas for an extra sequential family |B_i| <= 10^{-2}.

hep-ph↗
arXiv1999

Constraints on top couplings in models with exotic quarks

The extension of the Standard Model with exotic quark singlets and doublets introduces large flavor changing neutral couplings between ordinary fermions. We derive inequalities which translate the precise determination of the diagonal Z couplings, in particular at LEP, into stringent bounds on the off-diagonal ones. The resulting limits can be saturated in minimal extensions with one vector doublet or singlet. In this case, 23 and 6 single top events, respectively, are predicted at LEP2 for an integrated luminosity of 500 1/pb per experiment.

hep-ph↗
arXiv1999

Differential Renormalization and Dimensional Reduction

We describe the equivalence at one loop between constrained differential renormalization and regularization by dimensional reduction in the MS scheme. To illustrate it, we reexamine the calculation of supergravity corrections to (g-2)_l.

hep-ph↗
arXiv1998

Differential Renormalization of Gauge Theories

The scope of constrained differential renormalization is to provide renormalized expressions for Feynman graphs, preserving at the same time the Ward identities of the theory. It has been shown recently that this can be done consistently at least to one loop for abelian and non-abelian gauge theories. We briefly review these results, evaluate as an example the gluon selfenergy in both coordinate and momentum space, and comment on anomalies.

hep-ph↗