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Jun 30, 2018
06/18

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J. A. Aguilar-Saavedra

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New colour octets stand out among the new physics proposals to explain the anomalous forward-backward asymmetry measured in $t \bar t$ production by the CDF experiment at the Tevatron. We perform a fit to $t \bar t$ observables at the Tevatron and the LHC, including total cross sections, various asymmetries and the top polarisation and spin correlations, to find the most likely parameters of a light colour octet to be consistent with data. In particular, an octet coupling only to right-handed...

Topics: High Energy Physics - Phenomenology, High Energy Physics - Experiment

Source: http://arxiv.org/abs/1405.5826

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Sep 21, 2013
09/13

by
J. A. Aguilar-Saavedra

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If new leptons exist close to the electroweak scale, they can be produced in pairs at LHC through standard or new interactions. We study the production of heavy lepton pairs in SM extensions with: (i) a Majorana or Dirac lepton triplet, as those appearing in type-III seesaw; (ii) a lepton isodoublet (N,E)_{L,R}; (iii) a charged isosinglet E_{L,R}; (iv) a Majorana or Dirac neutrino singlet N and an additional Z' gauge boson. It is shown that the trilepton final state l+-l+-l-+, which has a small...

Source: http://arxiv.org/abs/0905.2221v2

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Sep 20, 2013
09/13

by
J. A. Aguilar-Saavedra

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We examine the LHC discovery potential for new Q=2/3 quark singlets T in the process gg,qq -> T Tbar -> W+ b W- bbar, with one W boson decaying hadronically and the other one leptonically. A particle-level simulation of this signal and its main backgrounds is performed, showing that heavy quarks with masses of 500 GeV or lighter can be discovered at the 5 sigma level after a few months of running, when an integrated luminosity of 3 fb^-1 is collected. With a luminosity of 100 fb^-1, this...

Source: http://arxiv.org/abs/hep-ph/0506187v2

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Sep 18, 2013
09/13

by
J. A. Aguilar-Saavedra

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We investigate single top production in the presence of anomalous Wtb couplings. We explicitly show that, if these couplings arise from gauge invariant effective operators, the only relevant couplings for single top production and decay are the usual gamma^mu and sigma^mu nu q_nu terms, where q is the W boson momentum. This happens even in the single top production processes where the Wtb interaction involves off-shell top and/or bottom quarks. With this parameterisation for the Wtb vertex, we...

Source: http://arxiv.org/abs/0803.3810v3

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Sep 23, 2013
09/13

by
J. A. Aguilar-Saavedra

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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.

Source: http://arxiv.org/abs/1202.2382v1

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra

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Top flavour-changing neutral interactions with a light quark q=u,c and a gauge or Higgs boson are very suppressed within the Standard Model (SM), but can reach observable levels in many of its extensions. We review the possible size of the effective vertices Ztq, gamma tq, gtq and Htq in several SM extensions, and discuss the processes in which these interactions might show up at LHC and at a high energy e+ e- linear collider.

Source: http://arxiv.org/abs/hep-ph/0409342v4

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Sep 21, 2013
09/13

by
J. A. Aguilar-Saavedra

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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...

Source: http://arxiv.org/abs/1008.3562v3

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Sep 19, 2013
09/13

by
J. A. Aguilar-Saavedra

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In this letter we introduce a possible measure of the size of CP violation in the Standard Model and its extensions, based on quantities invariant under the change of weak quark basis. We also introduce a measure of the ``average size'' of CP violation in a model, which can be used to compare the size of CP violation in models involving extra sequential or vector-like quarks, or left-right symmetry.

Source: http://arxiv.org/abs/hep-ph/9703461v1

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra

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In this talk we review some of the most relevant effects that the mixing with a Q=2/3 quark singlet might give at high energy colliders and K and B factories.

Source: http://arxiv.org/abs/hep-ph/0307176v2

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra

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We simplify the general form of the fermion-fermion-gauge boson interactions generated by dimension-six gauge-invariant effective operators by using the equations of motion to remove redundant operators. It is found that the most general vertex for off-shell fermions fi, fj and an off-shell boson V=W,Z,gamma,g only involves gamma^mu and sigma^{mu nu} q_nu terms, with q=p_i-p_j. Examples are given for the Wtb, Ztt, gamma tt and gtt interactions, whose general expression is greatly simplified...

Source: http://arxiv.org/abs/0811.3842v2

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Sep 20, 2013
09/13

by
J. A. Aguilar-Saavedra

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The decays ~nu_e -> e- ~X_1^+, ~nu_e* -> e+ ~X_1^-, when kinematically allowed, constitute a source of 100% polarised charginos. We study the process e+ e- -> ~nu_e* ~nu_e -> e+ ~X_1^- e- ~X_1^+, with subsequent chargino decays \~X_1^- -> nu_mu mu- ~X_1^0, X_1^+ -> q qbar' ~X_1^0 or their charge conjugate \~X_1^- -> qbar q' ~X_1^0, ~X_1^+ -> nu_mu mu+ ~X_1^0. The kinematics of this process allows the reconstruction of the sneutrino and chargino rest frames, and thus a...

Source: http://arxiv.org/abs/hep-ph/0410068v2

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Sep 19, 2013
09/13

by
J. A. Aguilar-Saavedra

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We present an algorithm which allows a fast numerical computation of Feldman-Cousins confidence intervals for Poisson processes, even when the number of background events is relatively large. This algorithm incorporates an appropriate treatment of the singularities that arise as a consequence of the discreteness of the variable.

Source: http://arxiv.org/abs/hep-ex/9911024v1

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra

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New quark singlets T can be produced in pairs at LHC through standard QCD interactions, with a large cross section for masses of several hundreds of GeV. Their charged current decays T Tbar T -> W+ b W- bbar, with semileptonic decay of the W pair, give a final state l nu bb jj$, as in top pair production. The mixed decay modes T Tbar -> W+ b H tbar, H t W- bbar -> W+ b W- bbar H, T Tbar -> W+ b Z tbar, Z t W- bbar -> W+ b W- bbar Z, with H,Z -> jj, yield two more jets, l nu bb...

Source: http://arxiv.org/abs/hep-ph/0603199v1

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Sep 23, 2013
09/13

by
J. A. Aguilar-Saavedra

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We use the equations of motion to simplify the general form of fermion-fermion-Higgs interactions generated by dimension-six gauge-invariant effective operators. After removing redundant operators it is found that the most general H f_i f_j vertex for off-shell fermions f_i, f_j and an off-shell Higgs boson only involves scalar and pseudo-scalar terms, without derivatives. Examples are presented for the Htt and Htq interactions, where q=u,c, giving the explicit expressions of the vertices in...

Source: http://arxiv.org/abs/0904.2387v2

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Jun 28, 2018
06/18

by
J. A. Aguilar-Saavedra

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The ATLAS excess in fat jet pair production is kinematically compatible with the decay of a heavy resonance into two gauge bosons plus an extra particle. This hypothesis would explain the absence of such a large excess in the analogous CMS analysis of fat dijet final states, as well as the negative results of diboson resonance searches in the semi-leptonic decay modes. If the extra particle is the Higgs boson, this hypothesis might also explain -statistical fluctuations aside- why the CMS...

Topics: High Energy Physics - Phenomenology, High Energy Physics - Experiment

Source: http://arxiv.org/abs/1506.06739

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Jun 30, 2018
06/18

by
J. A. Aguilar-Saavedra

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We discuss top polarisation observables at the Tevatron and the LHC with special attention to some that have not been measured and provide new, independent information about the top polarisation.

Topics: High Energy Physics - Phenomenology, High Energy Physics - Experiment

Source: http://arxiv.org/abs/1411.5697

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Jun 30, 2018
06/18

by
J. A. Aguilar-Saavedra

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We revisit the relation between the asymmetries $A_{FB}$ and $A_{FB}^\ell$ in $t \bar t$ production at the Tevatron, using as new physics benchmark a colour octet. We find that $A_{FB}^\ell$ receives large contributions from the interference between $\lambda = \pm 1/2$ top helicity states, which has been ignored in some of the previous literature on the subject. The omission of these contributions results in a severe underestimation of the asymmetry, around $1/2$ and $1/50$ of the true value...

Topics: High Energy Physics - Phenomenology, High Energy Physics - Experiment

Source: http://arxiv.org/abs/1405.1412

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Sep 18, 2013
09/13

by
J. A. Aguilar-Saavedra

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The mixing of the known quarks with new heavy singlets can modify significantly some observables with respect to the Standard Model predictions. We analyse the range of deviations permitted by the constraints from precision electroweak data and flavour-changing neutral processes at low energies. We study top charged current and neutral current couplings, which will be directly tested at top factories, like LHC and TESLA. We discuss some examples of observables from K and B physics, as the...

Source: http://arxiv.org/abs/hep-ph/0210112v4

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra

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We evaluate the LHC discovery potential for a light Higgs boson in t tbar H (-> l nu b bbar b bbar jj) production, within the Standard Model and if a new Q=2/3 quark singlet T with a moderate mass exists. In the latter case, T pair production with decays T Tbar -> W+ b H tbar / H t W- bbar -> W+ b W- bbar H provides an important additional source of Higgs bosons giving the same experimental signature, and other decay modes T Tbar -> H t H tbar -> W+ b W- bbar H H, T Tbar -> Z...

Source: http://arxiv.org/abs/hep-ph/0603200v2

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Sep 18, 2013
09/13

by
J. A. Aguilar-Saavedra

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We introduce a new parameterisation of the effect of unknown corrections from new physics on quark and lepton mass matrices. This parameterisation is used in order to study how the hierarchies of quark masses and mixing angles are modified by random perturbations of the Yukawa matrices. We discuss several examples of flavour relations predicted by different textures, analysing how these relations are influenced by the random perturbations. We also comment on the unlikely possibility that...

Source: http://arxiv.org/abs/hep-ph/0212057v1

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Sep 17, 2013
09/13

by
J. A. Aguilar-Saavedra

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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...

Source: http://arxiv.org/abs/1003.3173v2

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra

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Selectron decays constitute a source of 100% polarised neutralinos, whose helicity is fixed by the charge and "chirality" of the decaying selectron. In SUSY scenarios where the second neutralino X_2^0 has three-body decays, the cascade decay e_L -> e X_2^0 -> e X_1^0 mu+ mu- provides a clean place to study CP violation in the neutralino sector, through the analysis of CP-violating asymmetries involving the X_2^0 spin s and the momenta of the muons. We show that a CP-violating...

Source: http://arxiv.org/abs/hep-ph/0403243v2

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Sep 21, 2013
09/13

by
J. A. Aguilar-Saavedra

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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.

Source: http://arxiv.org/abs/1008.3225v1

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Sep 23, 2013
09/13

by
J. A. Aguilar-Saavedra

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The diagonalization of general mass matrices is a more delicate problem when eigenvalue degeneracies exist. In this case, often overlooked in the literature, some difficulties arise related to the freedom in the choice of basis in degenerate subspaces. Here two simple algorithms are developed to deal with quark and neutrino mass matrices with arbitrary degeneracies.

Source: http://arxiv.org/abs/hep-ph/9607313v1

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Sep 20, 2013
09/13

by
J. A. Aguilar-Saavedra

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We discuss the observability of CP-violating asymmetries in the process e+ e- -> X_1^0 X_2^0 -> X_1^0 X_1^0 l+ l-, with l = e,mu. We consider two examples of supersymmetric scenarios: (i) with decays X_2^0 -> l_R l -> X_1^0 l+ l-; (ii) with X_2^0 three-body decays. The asymmetries can be of order 0.1 but they are partially washed out by the large backgrounds from W+ W- and slepton pair production, being the observed asymmetries one order of magnitude smaller. However, with...

Source: http://arxiv.org/abs/hep-ph/0404104v2

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Sep 23, 2013
09/13

by
J. A. Aguilar-Saavedra

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We discuss selectron pair production in e- e- scattering, in the processes e- e- -> e~_L e~_L, e~_R e~_R -> e- X_1^0 e- X_2^0 -> e- X_1^0 e- X_1^0 f f. This decay channel has in general a smaller branching ratio than the e~ e~ -> e- X_1^0 e- X_1^0 mode, but has the advantage that the momenta of all final state particles can be determined without using the selectron masses as input. The reconstruction of the momenta allows the simultaneous study of: (i) selectron mass distributions;...

Source: http://arxiv.org/abs/hep-ph/0312140v2

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Sep 18, 2013
09/13

by
J. A. Aguilar-Saavedra

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We present an analysis of the sensitivity of the TESLA e+ e- collider to top flavour-changing neutral couplings to the Z boson and photon. We consider the cases without beam polarization, with only e- polarization and with e- and e+ polarization, showing that the use of the latter substantially enhances the sensitivity to discover or bound these vertices. For some of the couplings the expected LHC limits could be improved up to an order of magnitude for equal running times.

Source: http://arxiv.org/abs/hep-ph/0012305v2

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra; M. Perez-Victoria

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We review the model-independent description of the couplings of the top quark to the Higgs and gauge bosons in theories beyond the Standard Model. Then we examine these couplings in the case of arbitrary heavy vector-like quarks mixing with the third family. We also discuss the couplings of these top partners, and comment on implications for LHC searches.

Source: http://arxiv.org/abs/1302.5634v1

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Sep 20, 2013
09/13

by
J. A. Aguilar-Saavedra; A. Onofre

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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...

Source: http://arxiv.org/abs/1002.4718v2

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Jun 28, 2018
06/18

by
J. A. Aguilar-Saavedra; F. R. Joaquim

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In models with an extra $\text{SU}(2)_R$ gauge group and an extended scalar sector, the cascade decays of the $W'$ boson can provide various multiboson signals. In particular, diboson decays $W' \to WZ$ can be suppressed while $W' \to WZX$, with $X$ one of the scalars present in the model, can reach branching ratios around 4%. We discuss these multiboson signals focusing on possible interpretations of the ATLAS excess in fat jet pair production.

Topics: High Energy Physics - Experiment, High Energy Physics - Phenomenology

Source: http://arxiv.org/abs/1512.00396

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Sep 21, 2013
09/13

by
F. del Aguila; J. A. Aguilar-Saavedra

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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...

Source: http://arxiv.org/abs/0808.2468v2

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Jun 30, 2018
06/18

by
J. A. Aguilar-Saavedra; F. R. Joaquim

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The CMS collaboration has recently reported a 2.8$\sigma$ excess of $eejj$ events with an invariant mass around 2 TeV. This observation can be explained in the context of standard model extensions with new gauge bosons $W'$, $Z'$ and heavy neutrinos coupling (mainly) to the electron. We discuss additional signals that allow to confirm or discard the $W'$ and $Z'$ hypotheses.

Topics: High Energy Physics - Phenomenology, High Energy Physics - Experiment

Source: http://arxiv.org/abs/1408.2456

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Sep 20, 2013
09/13

by
J. A. Aguilar-Saavedra; F. R. Joaquim

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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...

Source: http://arxiv.org/abs/1207.4193v2

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Sep 24, 2013
09/13

by
J. A. Aguilar-Saavedra; Jose Santiago

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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...

Source: http://arxiv.org/abs/1112.3778v2

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Sep 18, 2013
09/13

by
F. del Aguila; J. A. Aguilar-Saavedra

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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...

Source: http://arxiv.org/abs/0809.2096v3

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Sep 18, 2013
09/13

by
J. A. Aguilar-Saavedra; B. M. Nobre

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Top flavour-changing neutral decays are extremely suppressed within the Standard Model (SM) by the GIM mechanism, but can reach observable rates in some of its extensions. We compute the branching ratios for t -> c gamma and t -> c g in minimal SM extensions where the addition of a vector-like up or down quark singlet breaks the unitarity of the 3 x 3 Cabibbo-Kobayashi-Maskawa (CKM) matrix. The maximum rates obtained indicate to what extent present experimental data allow 3 x 3 CKM...

Source: http://arxiv.org/abs/hep-ph/0210360v2

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra; A. M. Teixeira

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We discuss selectron pair production in e- e- scattering. These processes can only occur via t-channel neutralino exchange, provided that the neutralinos are Majorana fermions. We concentrate on the processes e- e- -> e^~_L e^~_L,e^~_R e^~_R -> e- X^~_1^0 e- X^~_2^0 -> e- X^~_1^0 e- X^~_1^0 f f, in which a complete determination of the final state momenta is possible without using the selectron masses as input. The experimental reconstruction of the selectron masses in this decay...

Source: http://arxiv.org/abs/hep-ph/0307001v2

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Jul 19, 2013
07/13

by
J. A. Aguilar-Saavedra; J. Bernabeu

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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...

Source: http://arxiv.org/abs/1005.5382v2

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra; M. Perez-Victoria

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We present the current status of the Tevatron charge asymmetry and its sister asymmetry at the LHC. The relation between both is elucidated, using as framework the collider-independent asymmetries they originate from. Other related observables, such as the t tbar differential distribution and top polarisation, are also discussed.

Source: http://arxiv.org/abs/1302.6618v1

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Sep 23, 2013
09/13

by
F. del Aguila; J. A. Aguilar-Saavedra

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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.

Source: http://arxiv.org/abs/hep-ph/9909222v2

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Sep 23, 2013
09/13

by
J. A. Aguilar-Saavedra; M. Perez-Victoria

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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.

Source: http://arxiv.org/abs/1105.4606v4

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Sep 19, 2013
09/13

by
J. A. Aguilar-Saavedra; T. Riemann

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We present a comprehensive analysis of the sensitivity of the TESLA e+ e- collider to top flavour-changing neutral couplings to the Z boson and photon. We study single top production and top decay processes, and we consider the cases without beam polarization, with only e- polarization and with e- and e+ polarization. We show that the use of the latter substantially enhances the sensitivity to discover or bound these vertices, and for some of the couplings the expected LHC limits could be...

Source: http://arxiv.org/abs/hep-ph/0102197v2

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Sep 19, 2013
09/13

by
F. del Aguila; J. A. Aguilar-Saavedra

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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...

Source: http://arxiv.org/abs/hep-ph/0208171v3

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Sep 22, 2013
09/13

by
J. A. Aguilar-Saavedra; G. C. Branco

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Top decays into a light Higgs boson and an up or charm quark can reach detectable levels in Standard Model extensions with two Higgs doublets or with new exotic quarks, and in the Minimal Supersymmetric Standard Model. Using both a standard and a neural network analysis we show that the CERN Large Hadron Collider will give 3 sigma evidence of decays with Br(t -> Hc) >= 6.5 10^-5 or set a limit Br(t -> Hc)

Source: http://arxiv.org/abs/hep-ph/0004190v3

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Sep 20, 2013
09/13

by
J. A. Aguilar-Saavedra; A. Juste

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We introduce the forward-backward asymmetries A_u, A_d corresponding to u ubar, d dbar -> t tbar production, respectively, at hadron colliders. These are collider- and center-of-mass-independent observables, directly related to the forward-backward and charge asymmetries measured at the Tevatron and the LHC, respectively. We discuss how to extract these asymmetries from data. Because these asymmetries are collider-independent, their measurement at these two colliders could elucidate the...

Source: http://arxiv.org/abs/1205.1898v2

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2.0

Jun 28, 2018
06/18

by
J. A. Aguilar-Saavedra; J. Bernabeu

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We discuss the eight independent spin observables for the $W$ boson in terms of its vector and tensor polarizations and identify the angular distributions and asymmetries able to separate them in collider experiments. The results are applied to the study of polarized top quark decays and diboson resonances. These novel observables are of great value for disentangling new physics mechanisms in $W$ boson production.

Topics: High Energy Physics - Phenomenology, High Energy Physics - Experiment

Source: http://arxiv.org/abs/1508.04592

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Jul 20, 2013
07/13

by
J. A. Aguilar-Saavedra; M. Perez-Victoria

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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...

Source: http://arxiv.org/abs/1107.0841v2

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33

Sep 18, 2013
09/13

by
F. del Aguila; J. A. Aguilar-Saavedra

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We find a minimal set of constraints which are independent of the choice of weak quark basis and necessary and sufficient for CP conservation for four quark families, including also the case of degenerate quark masses. These invariant conditions are written in the mass eigenstate basis as a function of the fermion masses and charged current mixings. CP violation is then related to the areas of three unitarity quadrangles and the CP violating effects of the fourth family are discussed in the...

Source: http://arxiv.org/abs/hep-ph/9605418v2

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Sep 18, 2013
09/13

by
F. del Aguila; J. A. Aguilar-Saavedra

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A new leptophobic neutral gauge boson Z' with small mixing to the Z can have a mass as light as M_Z' ~ 350 GeV, and still have escaped detection at LEP and Tevatron. Such a Z' boson can be derived from E6 and, if the new heavy neutrino singlets in the 27 representation are lighter than M_Z'/2, the process p p(pbar) -> Z' -> NN -> l+- l+- X is observable. Indeed, this new signal could explain the small excess of like-sign dileptons found at Tevatron. Implications for LHC are also...

Source: http://arxiv.org/abs/0705.4117v2

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Sep 17, 2013
09/13

by
J. A. Aguilar-Saavedra; G. C. Branco

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We generalize the geometrical description of CP violation in the Standard Model in terms of a unitarity triangle. For three left-handed Majorana neutrinos CP violation in the lepton sector is determined by three unitarity triangles. With three additional right-handed neutrinos 15 quadrangles are required to characterize CP violation. We show the relation of the unitarity polygons with physical observables.

Source: http://arxiv.org/abs/hep-ph/0007025v2