Roberto Barcelo

Stealth gluons at hadron colliders

Abstract We find that a heavy gluon G of mass 800–900 GeV with small, mostly axial-vector couplings to the light quarks and relatively large vector and axial-vector couplings to the top quark can explain the t t ¯ forward–backward asymmetry observed at the Tevatron with no conflict with other top-quark or dijet data. The key ingredient is a complete treatment of energy-dependent width effects and a new decay mode G → q Q , where q is a standard quark and Q a vector-like quark of mass 400–600 GeV. We show that this new decay channel makes the heavy gluon invisible in the t t ¯ invariant mass distribution and discuss its implications at the Tevatron and the LHC.

Single vectorlike quark production at the LHC

Abstract A gluon resonance G of mass below 1 TeV could be the origin of the t t ¯ forward–backward asymmetry observed at the Tevatron provided that new decay modes G → q Q ¯ , with q a standard quark and Q its massive excitation, make G broad enough. We consider all the different cases, with q the top, the bottom or a light quark and dominant decay modes Q → W q ′ or Q → Z q . We show that current experimental searches are unable to probe the model, but that minimal departures from these analyses can explore a large region of its parameter space for the current LHC luminosity. This includes the challenging case with the new quarks decaying mostly into light quark flavors. In some channels not only the heavy quark but also the massive gluon can be reconstructed, which would stablish the origin of the t t ¯ asymmetry. Similar analyses can be applied to more general models with new massive gluons and vectorlike quarks.

Gluon excitations in t tbar production at hadron colliders

We argue that a relatively light massive gluon with mass <= 1 TeV, small purely axial couplings to light quarks and sizable vector and axial couplings to the top quark can reproduce the large forward-backward asymmetry observed at the Tevatron without conflicting with the t tbar and the dijet invariant mass distributions measured at the Tevatron and the LHC. We show that realistic Higgsless models with warped extra dimensions naturally fulfil all the necessary ingredients to realize this scenario. While current data is unable to discover or exclude these heavy gluons with masses 850 GeV, they should be observed at the (7 TeV) LHC with a luminosity of the order of 300 pb^{-1}.

Extra Higgs bosons in t t ¯ production at the LHC

The top quark has a large Yukawa coupling with the Higgs boson. In the usual extensions of the standard model the Higgs sector includes extra scalars, which also tend to couple strongly with the top quark. Unlike the Higgs, these fields have a natural mass above 2m{sub t}, so they could introduce anomalies in tt production at the LHC. We study their effect on the tt invariant mass distribution at {radical}(s)=7 TeV. We focus on the bosons (H,A) of the minimal supersymmetric model and on the scalar field (r) associated with the new scale f in Little Higgs (LH) models. We show that in all cases the interference with the standard amplitude dominates over the narrow-width contribution. As a consequence, the mass difference between H and A or the contribution of an extra T-quark loop in LH models becomes an important effect in order to determine if these fields are observable there. We find that a 1 fb{sup -1} luminosity could probe the region tan{beta}{ =}0.3 in LH models.

Cosmic ray knee and new physics at the TeV scale

We analyze the possibility that the cosmic ray knee appears at an energy threshold where the proton-dark matter cross section becomes large due to new TeV physics. It has been shown that such interactions could break the proton and produce a diffuse gamma ray flux consistent with MILAGRO observations. We argue that this hypothesis implies knees that scale with the atomic mass for the different nuclei, as KASKADE data seem to indicate. We find that to explain the change in the spectral index in the flux from E−2.7 to E−3.1 the cross section must grow like E0.4+β above the knee, where β = 0.3–0.6 parametrizes the energy dependence of the age (τ∝E−β) of the cosmic rays reaching the Earth. The hypothesis also requires mbarn cross sections (that could be modelled with TeV gravity) and large densities of dark matter (that could be clumped around the sources of cosmic rays). We argue that neutrinos would also exhibit a threshold at E = (mχ/mp) Eknee ≈ 108 GeV where their interaction with a nucleon becomes strong. Therefore, the observation at ICECUBE or ANITA of standard neutrino events above this threshold would disprove the scenario.

Little Higgs models with a light T quark

Abstract We study little Higgs models based on a SU ( 3 ) 1 × SU ( 3 ) 2 global symmetry and with two scales (the two vacuum expectation values f 1 , 2 ) substantially different. We show that all the extra vector boson fields present in these models may be much heavier than the vectorlike T quark necessary to cancel top-quark quadratic corrections. In this case the models become an extension of the Standard Model with a light ( ≈ 500 GeV ) T quark and a scalar Higgs field with a large singlet component. We obtain that the Yukawa and the gauge couplings of the Higgs are smaller than in the standard model, a fact that may reduce significantly the Higgs production rate through glu–glu and WW fusion. The T quark decay into Higgs boson becomes then the dominant Higgs production channel in hadron colliders.

A minimal little Higgs model

We discuss a little Higgs scenario that introduces below the TeV scale just the two minimal ingredients of these models, a vectorlike

Supersymmetry with long-lived staus at the LHC

T

Gluon excitations in tt production at hadron colliders

quark and a singlet component (implying anomalous couplings) in the Higgs field, together with a pseudoscalar singlet

Extra Higgs bosons in tt production at the LHC

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