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J. A. Aguilar-Saavedra

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

At least 127 records · Page 7Linked to original sources
arXiv2006

New signals in pair production of heavy Q=2/3 singlets at LHC

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 jjjj, or the same state in the latter case for Z -> nu nubar. We extend previous work examining in detail the extra H and Z contributions and stressing one of the salient features of the T Tbar -> W+ b W- bbar signal: the presence of a very energetic charged lepton. We finally compare with the discovery potential in l nu bbbb jj final states, with four b tags.

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arXiv2005

Supersymmetry Parameter Analysis: SPA Convention and Project

High-precision analyses of supersymmetry parameters aim at reconstructing the fundamental supersymmetric theory and its breaking mechanism. A well defined theoretical framework is needed when higher-order corrections are included. We propose such a scheme, Supersymmetry Parameter Analysis SPA, based on a consistent set of conventions and input parameters. A repository for computer programs is provided which connect parameters in different schemes and relate the Lagrangian parameters to physical observables at LHC and high energy e+e- linear collider experiments, i.e., masses, mixings, decay widths and production cross sections for supersymmetric particles. In addition, programs for calculating high-precision low energy observables, the density of cold dark matter (CDM) in the universe as well as the cross sections for CDM search experiments are included. The SPA scheme still requires extended efforts on both the theoretical and experimental side before data can be evaluated in the future at the level of the desired precision. We take here an initial step of testing the SPA scheme by applying the techniques involved to a specific supersymmetry reference point.

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arXiv2005

Pair production of heavy Q=2/3 singlets at LHC

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 process can signal the presence of heavy quarks with masses up to approximately 1 TeV. Finally, we discuss the complementarity among T Tbar, Tj production and indirect constraints from precise electroweak data in order to discover a new quark or set bounds on its mass.

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arXiv2005

Top flavour-changing neutral interactions: theoretical expectations and experimental detection

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.

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arXiv2005

Sneutrino cascade decays ~nu_e -> e- ~X_1^+ -> e- f f' ~X_1^0 as a probe of chargino spin properties and CP violation

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 clean analysis of the angular distributions of the visible final state products in these reference systems. Furthermore, a triple product CP asymmetry can be built in ~X_1^+- hadronic decays, involving the chargino spins and the momenta of the quark and antiquark. With a good c tagging efficiency, this CP asymmetry is quite sensitive to the phase of the bino mass term M_1 and can test CP violation in the neutralino sector.

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arXiv2005

l W nu production at CLIC: a window to TeV scale non-decoupled neutrinos

We discuss single heavy neutrino production e+ e- -> N nu -> l W nu, l = e, mu, tau, at a future high energy collider like CLIC, with a centre of mass energy of 3 TeV. This process could allow to detect heavy neutrinos with masses of 1-2 TeV if their coupling to the electron V_eN is in the range 0.004-0.01. We study the dependence of the limits on the heavy neutrino mass and emphasise the crucial role of lepton flavour in the discovery of a positive signal at CLIC energy. We present strategies to determine heavy neutrino properties once they are discovered, namely their Dirac or Majorana character and the size and chirality of their charged current couplings. Conversely, if no signal is found, the bound V_eN < 0.002-0.006 would be set for masses of 1-2 TeV, improving the present limit up to a factor of 30. We also extend previous work examining in detail the flavour and mass dependence of the corresponding limits at ILC, as well as the determination of heavy neutrino properties if they are discovered at this collider.

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arXiv2005

Flavour and polarisation in heavy neutrino production at e+ e- colliders

We analyse l W nu production at ILC, paying special attention to the role of the final lepton flavour and beam polarisation in the search for a new heavy neutrino N. We show that a sizeable coupling to the electron V_eN ~ 10^-2 is necessary to have an observable signal in any of the channels, despite the fact that the signal may be more visible in muon or tau final states. The non-observation of a heavy neutrino at ILC will improve the present upper bound on its mixing with the electron by more than one order of magnitude, V_eN < 0.007 for m_N between 200 and 400 GeV.

hep-ph↗
arXiv2004

The size of chi = arg (-Vts Vtb* Vcs* Vcb) and physics beyond the Standard Model

We analyse the allowed range of values of chi, both in the Standard Model and in models with New Physics, pointing out that a relatively large value of chi, e.g. of order lambda, is only possible in models where the unitarity of the 3x3 Cabibbo-Kobayashi-Maskawa matrix is violated through the introduction of extra Q=2/3 quarks. We study the interesting case where the extra quark is an isosinglet, determining the allowed range for chi and the effect of a large chi on various low-energy observables, such as CP asymmetries in B meson decays. We also discuss the correlated effects which would be observable at high energy colliders, like decays t -> cZ, modifications of the cross section and forward-backward asymmetry in e+ e- -> ttbar and the direct production of a new quark.

hep-ph↗
arXiv2004

Study of selectron properties in the e~ e~ -> e- X_1^0 e- X_2^0 decay channel

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; (ii) selectron spins, via the angular distributions of the e- in the selectron rest frames; (iii) selectron masses andspins, using the e- energy distributions in the CM frame; (iv) the selectron "chirality", with the analysis of the spin of the produced X_2^0.

hep-ph↗
arXiv2004

CP violation in X_1^0 X_2^0 production at a linear collider

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 appropriate kinematical cuts they can be observed at an e+ e- collider with a centre of mass energy of 500 GeV and high luminosity.

hep-ph↗
arXiv2004

CP violation in selectron cascade decays e_L -> e X_2^0 -> e X_1^0 mu+ mu-

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 asymmetry in the triple product s . (p_mu- x p_mu+) could be observable at a 800 GeV linear collider provided the gaugino mass M_1 has a large phase at the electroweak scale. We compare our results with X_1^0 X_2^0 production, where a CP asymmetry is also observable within these scenarios.

hep-ph↗
arXiv2004

Effects of mixing with quark singlets

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 branching ratio of K_L -> pi^0 nu nu, the B^0_s mass difference or the time-dependent CP asymmetry in B_s^0 -> D_s^+ D_s^-, which can also show large new effects.

hep-ph↗
arXiv2003

Neutrino Masses, Mixing and New Physics Effects

We introduce a parametrization of the effects of radiative corrections from new physics on the charged lepton and neutrino mass matrices, studying how several relevant quantities describing the pattern of neutrino masses and mixing are affected by these corrections. We find that the ratio omega = sin theta / tan theta_atm is remarkably stable, even when relatively large corrections are added to the original mass matrices. It is also found that if the lightest neutrino has a mass around 0.3 eV, the pattern of masses and mixings is considerably more stable under perturbations than for a lighter or heavier spectrum. We explore the consequences of perturbations on some flavor relations given in the literature. In addition, for a quasi-degenerate neutrino spectrum it is shown that: (i) starting from a bi-maximal mixing scenario, the corrections to the mass matrices keep tan theta_atm very close to unity while they can lower tan theta_sol to its measured value; (ii) beginning from a scenario with a vanishing Dirac phase, corrections can induce a Dirac phase large enough to yield CP violation observable in neutrino oscillations.

hep-ph↗
arXiv2003

Testing the Majorana nature of neutralinos in supersymmetric theories

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 channel provides clear evidence of the Majorana character of the neutralinos, which is confirmed by the analysis of the electron energy spectrum.

hep-ph↗
arXiv2002

Rare top decays t -> c gamma, t -> c g and CKM unitarity

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 unitarity to be broken in these models, and are too small to be observed in the near future. As a by-product, we reproduce the calculation of these branching ratios in the SM, and with an improved set of parameters we obtain values one order of magnitude smaller than the ones usually quoted in the literature. We study the CP asymmetries between the decay rates of the top quark and antiquark, which can be much larger than in the SM, also as a consequence of the partial breaking of 3 x 3 CKM unitarity.

hep-ph↗
arXiv2002

Hierarchy plus anarchy in quark masses and mixings

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 unknown corrections contribute significantly to the hierarchy of masses and mixings.

hep-ph↗
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.

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