arXiv subjects

F. del Aguila

Publications and source records attributed to F. del Aguila.

At least 19 recordsLinked to original sources

arXiv2025

Electroweak Limits on General New Vector Bosons

We study extensions of the Standard Model with general new vector bosons. The full Standard Model gauge symmetry is used to classify the extra vectors and constrain their couplings. We derive the corresponding effective Lagrangian, valid at energies lower than the mass of the extra vectors, and use it to extract limits from electroweak precision observables, including LEP 2 data. We consider both universal and nonuniversal couplings to fermions. We study the interplay of several extra vectors, which can have the effect of opening new regions in parameter space. In particular, it allows to explain the anomaly in the bottom forward-backward asymmetry with perturbative couplings. Finally, we analyze quantitatively the implications for the Higgs mass.

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arXiv2019

The Full Lepton Flavor of the Littlest Higgs Model with T-parity

We re-examine lepton flavor violation (LFV) in the Littlest Higgs model with T--parity (LHT) including the full T--odd (non-singlet) lepton and Goldstone sectors. The heavy leptons induce two independent sources of LFV associated with the couplings necessary to give masses to the T--odd mirror fermions and to their partners in right-handed $SO(5)$ multiplets, respectively. The latter, which have been neglected in the past, can be decoupled from gauge mediated processes but not from Higgs mediated ones and must therefore also be included in a general analysis of LFV in the LHT. We also further extend previous analyses by considering on-shell $Z$ and Higgs LFV decays together with the LFV processes at low momentum transfer. We show that current experimental limits can probe the LHT parameter space up to global symmetry breaking scales $f \sim 10$ TeV. For lower $f$ values $\gtrsim 1$ TeV, $μ-e$ transitions require the misalignment between the heavy and the Standard Model charged leptons to be $\lesssim 1 \, \%$. Future LFV experiments using intense muon beams should be sensitive to misalignments below the per mille level. For $τ$ LFV transitions, which could potentially be observed at Belle II and the LHC as well as future lepton colliders, we find that generically they can not discriminate between the LHT and supersymmetric models though in some regions of parameter space this may be possible.

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arXiv2019

Lepton Flavor Changing Higgs decays in the Littlest Higgs Model with T-parity

We calculate loop induced lepton flavor violating Higgs decays in the Littlest Higgs model with T-parity. We find that a finite amplitude is obtained only when all contributions from the T-odd lepton sector are included. This is in contrast to lepton flavor violating processes mediated by gauge bosons where the partners of the right-handed mirror leptons can be decoupled from the spectrum. These partners are necessary to cancel the divergence in the Higgs mass introduced by the mirror leptons but are otherwise unnecessary and assumed to be decoupled in previous phenomenological studies. Furthermore, as we emphasize, including the partner leptons in the spectrum also introduces a new source of lepton flavor violation via their couplings to the physical pseudo-Goldstone electroweak triplet scalar. Although this extra source also affects lepton flavor changing gauge transitions, it decouples from these amplitudes in the limit of heavy mass for the partner leptons. We find that the corresponding Higgs branching ratio into taus and muons can be as large as $\sim 0.2 \times 10^{-6}$ for T-odd masses of the order a few TeV, a demanding challenge even for the high luminosity LHC.

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arXiv2011

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.

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arXiv2011

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.

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arXiv2011

Lepton flavor violation in the Simplest Little Higgs model

The flavor sector of Little Higgs models based on product groups, notably the Littlest Higgs with T parity (LHT), has been extensively studied and some amount of fine tuning was found to be required to meet the experimental constraints. However, no such attention has been paid to other classes of models. Here we analyze the phenomenology of flavor mixing in the lepton sector of a simple group model, the Simplest Little Higgs (SLH). We obtain the Feynman rules of the SLH in the 't Hooft-Feynman gauge up to the necessary order and calculate the leading contributions to the rare processes mu -> e gamma, mu -> eee and mu-e conversion in nuclei. We find results comparable to those of the LHT model, because in both cases they arise at the one-loop level. These require the flavor alignment of the Yukawa couplings of light and heavy leptons at the per cent level or an effective scale of around 10 TeV.

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arXiv2010

Muon to electron conversion in the Littlest Higgs model with T-parity

Little Higgs models provide a natural explanation of the little hierarchy between the electroweak scale and a few TeV scale, where new physics is expected. Under the same inspiring naturalness arguments, this work completes a previous study on lepton flavor-changing processes in the Littlest Higgs model with T-parity exploring the channel that will eventually turn out to be the most sensitive, μ-e conversion in nuclei. All one-loop contributions are carefully taken into account, results for the most relevant nuclei are provided and a discussion of the influence of the quark mixing is included. The results for the Ti nucleus are in good agreement with earlier work by Blanke et al., where a degenerate mirror quark sector was assumed. The conclusion is that, although this particular model reduces the tension with electroweak precision tests, if the restrictions on the parameter space derived from lepton flavor violation are taken seriously, the degree of fine tuning necessary to meet these constraints also disfavors this model.

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arXiv2010

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.

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arXiv2009

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.

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arXiv2009

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.

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arXiv2009

Trilepton signals: the golden channel for seesaw searches at LHC

The comparison of samples with different number of charged leptons shows that trilepton signals are the most significant ones for seesaw mediators. As previously pointed out, this is indeed the case for scalar (type II) and fermion (type III) triplets at LHC, which can be discovered in this channel for masses up to 500-700 GeV and an integrated luminosity of 30 fb^-1; whereas fermion singlets (type I) are marginally observable if there are no further new physics near the TeV scale. However, if there are new gauge interactions at this scale coupling to right-handed neutrinos, as in left-right models, heavy neutrinos are observable up to masses ~ 2 TeV for new gauge boson masses up to ~ 4 TeV, as we discuss in some detail.

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arXiv2009

Precise limits from lepton flavour violating processes on the Littlest Higgs model with T-parity

We recalculate the leading one-loop contributions to mu > e gamma and mu -> eee in the Littlest Higgs model with T-parity, recovering previous results for the former. When all the Goldstone interactions are taken into account, the latter is also ultraviolet finite. The present experimental limits on these processes require a somewhat heavy effective scale ~2.5 TeV, or the flavour alignment of the Yukawa couplings of light and heavy leptons at the ~10% level, or the splitting of heavy lepton masses to a similar precision. Present limits on tau decays set no bounds on the corresponding parameters involving the tau lepton

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arXiv2009

Electroweak scale seesaw and heavy Dirac neutrino signals at LHC

Models of type I seesaw can be implemented at the electroweak scale in a natural way provided that the heavy neutrino singlets are quasi-Dirac particles. In such case, their contribution to light neutrino masses has the suppression of a small lepton number violating parameter, so that light neutrino masses can arise naturally even if the seesaw scale is low and the heavy neutrino mixing is large. We implement the same mechanism with fermionic triplets in type III seesaw, deriving the interactions of the new quasi-Dirac neutrinos and heavy charged leptons with the SM fermions. We then study the observability of heavy Dirac neutrino singlets (seesaw I) and triplets (seesaw III) at LHC. Contrarily to common wisdom, we find that heavy Dirac neutrino singlets with a mass around 100 GeV are observable at the 5 sigma level with a luminosity of 13 fb^-1. Indeed, in the final state with three charged leptons l+- l+- l-+, not previously considered, Dirac neutrino signals can be relatively large and backgrounds are small. In the triplet case, heavy neutrinos can be discovered with a luminosity of 1.5 fb^-1 for a mass of 300 GeV in the same channel.

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arXiv2008

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.

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arXiv2008

Distinguishing seesaw models at LHC with multi-lepton signals

We investigate the LHC discovery potential for electroweak scale heavy neutrino singlets (seesaw I), scalar triplets (seesaw II) and fermion triplets (seesaw III). For seesaw I we consider a heavy Majorana neutrino coupling to the electron or muon. For seesaw II we concentrate on the likely scenario where the new scalars decay to two leptons. For seesaw III we restrict ourselves to heavy Majorana fermion triplets decaying to light leptons plus gauge or Higgs bosons, which are dominant except for unnaturally small mixings. The possible signals are classified in terms of the charged lepton multiplicity, studying nine different final states ranging from one to six charged leptons. Using a fast detector simulation of signals and backgrounds, it is found that the trilepton channel l+- l+- l-+ is by far the best one for scalar triplet discovery, and for fermion triplets it is as good as the like-sign dilepton channel l+- l+-. For heavy neutrinos with a mass O(100) GeV, this trilepton channel is also better than the usually studied like-sign dilepton mode. In addition to evaluating the discovery potential, we make special emphasis on the discrimination among seesaw models if a positive signal is observed. This could be accomplished not only by searching for signals in different final states, but also by reconstructing the mass and determining the charge of the new resonances, which is possible in several cases. For high luminosities, further evidence is provided by the analysis of the production angular distributions in the cleanest channels with three or four leptons.

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arXiv2008

Electroweak constraints on see-saw messengers and their implications for LHC

We review the present electroweak precision data constraints on the mediators of the three types of see-saw mechanisms. Except in the see-saw mechanism of type I, with the heavy neutrino singlets being mainly produced through their mixing with the Standard Model leptons, LHC will be able to discover or put limits on new scalar (see-saw of type II) and lepton (see-saw of type III) triplets near the TeV. If discovered, it may be possible in the simplest models to measure the light neutrino mass and mixing properties that neutrino oscillation experiments are insensitive to.

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arXiv2008

Effects of new leptons in Electroweak Precision Data

We obtain limits on generic vector-like leptons at the TeV scale from electroweak precision tests. These limits are complementary to the ones obtained from lepton flavour violating processes. In general, the quality of the global electroweak fit is comparable to the one for the Standard Model. In the case of an extra neutrino singlet mixing with the muon or electron, the global fit allows for a relatively large Higgs mass (M_H < 260 GeV at 90 % C.L.), thus relaxing the tension between the direct LEP limit and the Standard Model fit.

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arXiv2008

Collider aspects of flavour physics at high Q

This review presents flavour related issues in the production and decays of heavy states at LHC, both from the experimental side and from the theoretical side. We review top quark physics and discuss flavour aspects of several extensions of the Standard Model, such as supersymmetry, little Higgs model or models with extra dimensions. This includes discovery aspects as well as measurement of several properties of these heavy states. We also present public available computational tools related to this topic.

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