arXiv subjects

J. A. Aguilar-Saavedra

Publications and source records attributed to J. A. Aguilar-Saavedra.

At least 91 records · Page 5Linked to original sources

arXiv2012

Measuring heavy neutrino couplings at the LHC

The existence of heavy neutrinos mediating neutrino masses via a type-I seesaw can be directly probed at the LHC, or indirectly in rare lepton flavor-violating processes. The synergy between these two approaches requires a direct measurement of the heavy neutrino couplings. We discuss a strategy to perform such measurements at the LHC in the context of left-right symmetric models, which is also applicable to other models implementing a type-I or type-III seesaw. We demonstrate that the ambiguities in the determination of the heavy neutrino mixing parameters can be resolved by performing an exclusive analysis of dilepton final states, discriminated by flavor and missing energy.

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arXiv2012

Top effective operators at the ILC

We investigate the effect of top trilinear operators in t tbar production at the ILC. We find that the sensitivity to these operators largely surpasses the one achievable by the LHC either in neutral or charged current processes, allowing to probe new physics scales up to 4.5 TeV for a centre of mass energy of 500 GeV. We show how the use of beam polarisation and an eventual energy upgrade to 1 TeV allow to disentangle all effective operator contributions to the Ztt and gamma tt vertices.

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arXiv2012

Flavour in heavy neutrino searches at the LHC

Heavy neutrinos at the TeV scale have been searched for at the LHC in the context of left-right models, under the assumption that they couple to the electron, the muon, or both. We show that current searches are also sensitive to heavy neutrinos coupling predominantly to the tau lepton, and present limits can significantly constrain the parameter space of general flavour mixing.

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arXiv2012

Overview of models for the t tbar asymmetry

We review the most popular models proposed to explain the anomalous forward-backward asymmetry in t tbar production at the Tevatron. We discuss their viability in view of recent LHC data. We summarise their predictions for charge asymmetries at the LHC.

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arXiv2012

Four tops and the t tbar forward-backward asymmetry

New colour octet vectors below the TeV scale could explain the anomalous t tbar forward-backward asymmetry observed at the Tevatron experiments, while being consistent with the current LHC data. These models generally lead to four-top final states at the LHC at observable levels. We compute the four-top production cross section at the LHC in a model with a massive colour octet vector as a function its mass, its width and its coupling to the top quark. Octet masses in the vicinity of the t tbar threshold are generally excluded by present limits on the production of same-sign dileptons and trileptons. Masses above 650 GeV are allowed, quite independently of the couplings, but they can be probed with the luminosity of 5 fb^-1 already collected at the LHC, up to around 800 GeV. The four-top production cross section is increased by a factor ~ 2 with sqrt{s}=8 TeV and by up to almost two orders of magnitude with sqrt{s}=14 TeV, thus greatly increasing the reach for massive gluons after the LHC energy upgrade.

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arXiv2011

Boosting the t tbar charge asymmetry

We propose a kinematical enhancement of the t tbar charge asymmetry at the LHC by selecting events with the t tbar centre of mass frame highly boosted along the beam axis. This kinematical selection increases the asymmetries and their significance up to a factor of two in a rather model-independent fashion. Hence, it can be a perfect complement to enhance model discrimination at the LHC.

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arXiv2011

Asymmetries in t tbar production: LHC versus Tevatron

The measurement of a charge asymmetry in t tbar production at LHC constitutes more than an independent confirmation of the forward-backward asymmetry found at Tevatron. Indeed, both measurements together can be used to identify the source of the asymmetry. This is demonstrated for the case of new Z', W' vector bosons and colour-sextet and triplet scalars, exchanged in t, u channels respectively, and a very heavy axigluon in the s channel.

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arXiv2011

Shaping the top asymmetry

We study different profiles of the distribution of the top forward-backward asymmetry, depending on the invariant mass of the t tbar pair. We show that they can be reproduced by one or more light colour octets, while keeping moderate departures of the t tbar cross section and invariant mass distributions with respect to the Standard Model predictions at Tevatron and LHC.

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arXiv2011

Simple models for the top asymmetry: constraints and predictions

We perform a comprehensive study of the allowed range for the Tevatron t tbar forward-backward asymmetries in six representative new physics models: a flavour-changing Z' boson, a scalar isodoublet, a W' boson, a heavy axigluon, a colour-triplet and a colour-sextet scalar. We devote special attention to the constraints from the t tbar tail at LHC on the parameter space, which will be dramatic if the measurements agree with the SM prediction, specially for Z' and W' bosons. We also study the predictions for the charge asymmetries at LHC and compare several proposed definitions.

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arXiv2011

No like-sign tops at Tevatron: Constraints on extended models and implications for the t tbar asymmetry

We use a recent upper limit from CDF on like-sign top pair production to place constraints on general new vector bosons and scalars mediating u u -> t t. The possible vector bosons comprise neutral colour singlets or octets, and charge 4/3 colour triplets or sextets, whereas the new scalars can be neutral colour singlets or octets and charge 4/3 colour sextets. We also estimate the expected bounds from like-sign top pair production at LHC in the near future. Then, we address the implications of these limits for the forward-backward asymmetry in t tbar production measured at Tevatron. In particular, we find that models explaining the observed asymmetry by the exchange of a single t-channel heavy Z' boson are already excluded. On the other hand, light Z' bosons with a mass M_Z' ~ 150 GeV, which could also account for a recent CDF dijet excess in W+jet production, are barely allowed.

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arXiv2011

Effective four-fermion operators in top physics: a roadmap

We write down a minimal basis for dimension-six gauge-invariant four-fermion operators, with some operator replacements with respect to previous ones which make it simpler for calculations. Using this basis we classify all four-fermion operator contributions involving one or two top quarks. Taking into account the different fermion chiralities, possible colour contractions and independent flavour combinations, a total number of 572 gauge-invariant operators are involved. We apply this to calculate all three-body top decay widths t -> d_k u_i dbar_j, t -> d_k e_i+ nu_j, t -> u_k u_i ubar_j, t -> u_k e_j+ e_i-, t -> u_k nu_j nu_i (with i,j,k generation indices) mediated by dimension-six four-fermion operators, including the interference with the Standard Model amplitudes when present. All single top production cross sections in pp, p pbar and e+ e- collisions are calculated as well, namely u_i d_k -> d_j t, dbar_j d_k -> ubar_i t, u_i dbar_j -> dbar_k t, u_i u_k -> u_j t, u_i ubar_j -> ubar_k t, e+ e- -> ubar_k t and the charge conjugate processes. We also compute all top pair production cross sections, ubar_i u_j -> t tbar, dbar_i d_j -> t tbar, u_i u_j -> t t and e+ e- -> t tbar. Our results are completely general, without assuming any particular relation among effective operator coefficients.

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arXiv2011

Constraints on the Wtb vertex from early LHC data

We use the recent measurements of top quark decay asymmetries in ATLAS and the t-channel single top cross section in CMS to set the first combined LHC limits on the Wtb vertex. This combination allows to obtain much better limits than the separate measurements. The resulting constraints are comparable, although still weaker, than the ones obtained using Tevatron data with much more statistics.

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arXiv2011

Probing the Tevatron t tbar asymmetry at LHC

We use an effective operator framework to study the contributions to the Tevatron t tbar asymmetry from arbitrary vector bosons and scalars, and compare with their effect on the t tbar tail at LHC. Our study shows, for example, that models reproducing the t tbar asymmetry by exchange of Z' and W' bosons or colour-triplet scalars lead to a large enhancement in the t tbar tail at LHC. This fact can be used to exclude these models as the sole explanation for the asymmetry, using the data already collected by CMS and ATLAS. Our analysis is model independent in the sense that we scan over all possible extra particles contributing to the asymmetry, and allow for general couplings. We also explore a class of Standard Model extensions which can accommodate the Tevatron asymmetry without contributing to the total t tbar cross section at first order, so that the enhancement of the tail at Tevatron and LHC is moderate.

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arXiv2011

Using single top rapidity to measure V_td, V_ts, V_tb at hadron colliders

Single top production processes are usually regarded as the ones in which V_tb can be directly measured at hadron colliders. We show that the analysis of the single top rapidity distribution in t-channel and tW production can also set direct limits on V_td. At LHC with 10 fb^-1 at 14 TeV the combined limits on V_td may be reduced by almost a factor of two when the top rapidity distribution is used. This also implies that the limits on V_tb can also be reduced by 15%, since both parameters as well as V_ts must be simultaneously obtained from a global fit to data. At Tevatron the explotation of this distribution would require very high statistics.

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arXiv2010

Effective operators in top physics

Top interactions beyond the Standard Model can be conveniently described in the framework of gauge-invariant effective operators. We briefly review the general form of the fermion-fermion-gauge boson and fermion-fermion-Higgs interactions arising from dimension-6 operators, focusing on the top quark. We discuss in detail an expansion in powers of 1/Lambda^2, where Lambda is the new physics scale, to consistently calculate observables in terms of top anomalous couplings.

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arXiv2010

W polarisation beyond helicity fractions in top quark decays

We calculate the density matrix for the decay of a polarised top quark into a polarised W boson and a massive b quark, for the most general Wtb vertex arising from dimension-six gauge-invariant effective operators. We show that, in addition to the well-known W helicity fractions, for polarised top decays it is worth to define and study the transverse and normal W polarisation fractions, that is, the W polarisation along two directions orthogonal to its momentum. In particular, a rather simple forward-backward asymmetry in the normal direction is found to be very sensitive to complex phases in one of the Wtb anomalous couplings. This asymmetry, which indicates a normal W polarisation, can be generated for example by a P-odd, T-odd transition electric dipole moment. We also investigate the angular distribution of decay products in the top quark rest frame, calculating the spin analysing powers for a general Wtb vertex. Finally we show that, using a combined fit to top decay observables and the tW cross section, at LHC it will be possible to obtain model-independent measurements of all the (complex) Wtb couplings as well as the single top polarisation. Implications for spin correlations in top pair production are also discussed.

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arXiv2010

Zt, gamma t and t production at hadron colliders via strong flavour-changing neutral couplings

We consider gq -> Zt, gq -> gamma t and gq -> t production (q=u,c) mediated by strong flavour-changing neutral interactions within an effective operator framework. We provide total cross sections for Tevatron and LHC, showing explicitly that the six processes can be described in full generality in terms of only two parameters (anomalous couplings) for q=u plus two for q=c. In our work we take into account and study in detail the effects of top quark decay. For gamma t, the inclusion of the top quark decay in the matrix element reveals an striking result: the largest contribution to the final state, e.g. gamma l nu b, with l = e, mu, tau, does not result from gq -> gamma t -> gamma l nu b but from on-shell gq -> t production with t -> gamma l nu b, being the photon radiated off the top decay products. This contribution, missed in previous literature, increases the signal cross sections by factors ranging between 3 and 6.5.

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arXiv2010

The Hunt for New Physics at the Large Hadron Collider

The Large Hadron Collider presents an unprecedented opportunity to probe the realm of new physics in the TeV region and shed light on some of the core unresolved issues of particle physics. These include the nature of electroweak symmetry breaking, the origin of mass, the possible constituent of cold dark matter, new sources of CP violation needed to explain the baryon excess in the universe, the possible existence of extra gauge groups and extra matter, and importantly the path Nature chooses to resolve the hierarchy problem - is it supersymmetry or extra dimensions. Many models of new physics beyond the standard model contain a hidden sector which can be probed at the LHC. Additionally, the LHC will be a top factory and accurate measurements of the properties of the top and its rare decays will provide a window to new physics. Further, the LHC could shed light on the origin of neutralino masses if the new physics associated with their generation lies in the TeV region. Finally, the LHC is also a laboratory to test the hypothesis of TeV scale strings and D-brane models. An overview of these possibilities is presented in the spirit that it will serve as a companion to the Technical Design Reports (TDRs) by the particle detector groups ATLAS and CMS to facilitate the test of the new theoretical ideas at the LHC. Which of these ideas stands the test of the LHC data will govern the course of particle physics in the subsequent decades.

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