arXiv subjects

Jose Santiago

Publications and source records attributed to Jose Santiago.

At least 37 records · Page 2Linked to original sources

arXiv2015

Observable Effects of General New Scalar Particles

We classify all possible new scalar particles that can have renormalizable linear couplings to Standard Model fields and therefore be singly produced at colliders. We show that this classification exhausts the list of heavy scalar particles that contribute at the tree level to the Standard Model effective Lagrangian to dimension six. We compute this effective Lagrangian for a general scenario with an arbitrary number of new scalar particles and obtain flavor-preserving constraints on their couplings and masses. This completes the tree-level matching of the coefficients of dimension five and six operators in the effective Lagrangian to arbitrary extensions of the Standard Model.

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arXiv2015

Four and two-lepton signals of leptophilic gauge interactions at large colliders

Many Standard Model extensions can contribute to four-lepton signals at large colliders. We review the particular case of leptophilic interactions eventually observable at the LHC and the ILC, paying special attention to the addition of a new vector boson coupled to muon minus tau lepton number, $Z'_{μ- τ}$, and emphasizing the prospects at a very large hadron collider with $\sqrt s =$ 100 TeV. We also discuss in this case the new contribution to two-lepton (Drell-Yan) production when the new leptophilic interaction has a non-vanishing kinetic mixing with the SM.

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arXiv2015

Collider limits on leptophilic interactions

Leptophilic interactions can only be observed at the LHC in four-lepton final states. If these interactions are mediated by a resonance in the di-leptonic channel with renormalizable couplings, the mediator must have spin 1. We study the LHC reach for such a vector boson allowing for arbitrary couplings. We find that only couplings to muons can be probed at the LHC because lepton flavor violating couplings are constrained by rare processes, couplings to electrons by LEP and the LHC is not sensitive to final states involving taus in this case. The ILC becomes then complementary to the LHC as it will provide the best limits on $Z'$ couplings to tau leptons. A prominent example is the case of the anomaly-free $Z'$ coupling to the muon minus tau lepton number ${\rm L}_μ- {\rm L}_τ$. If no departure from the Standard Model is observed at the LHC, the most stringent bounds on this vector boson are provided from events with only three charged leptons plus missing energy. Masses of the order of 1 TeV can be probed at the high-luminosity phase of the LHC for $Z^\prime$ couplings of order one. Generic four-lepton operators parametrizing leptophilic interactions can be also constrained using three and four (or two at the ILC) charged-lepton samples, but the corresponding limits are marginal, if meaningful, because the resonant behavior appears to be essential for the signal to be significant.

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arXiv2015

The Elusive Gluon

We study the phenomenology of vector resonances in the context of natural composite Higgs models. A mild hierarchy between the fermionic partners and the vector resonances can be expected in these models based on the following arguments. Both direct and indirect (electroweak and flavor precision) constraints on fermionic partners are milder than the ones on spin one resonances. Also the naturalness pressure coming from the top partners is stronger than that induced by the gauge partners. This observation implies that the search strategy for vector resonances at the LHC needs to be modified. In particular, we point out the importance of heavy gluon decays (or other vector resonances) to top partner pairs that were overlooked in previous experimental searches at the LHC. These searches focused on simplified benchmark models in which the only new particle beyond the Standard Model was the heavy gluon. It turns out that, when kinematically allowed, such heavy-heavy decays make the heavy gluon elusive, and the bounds on its mass can be up to 2 TeV milder than in the simpler models considered so far for the LHC14. We discuss the origin of this difference and prospects for dedicated searches.

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arXiv2014

From Tevatron's top and lepton-based asymmetries to the LHC

We define a lepton-based asymmetry in semi-leptonic ttbar production at the LHC. We show that the ratio of this lepton-based asymmetry and the ttbar charge asymmetry, measured as a function of the lepton transverse momentum or the ttbar invariant mass is a robust observable in the Standard Model. It is stable against higher order corrections and mis-modeling effects. We show that this ratio can also be a powerful discriminant among different new physics models and between them and the Standard Model. Finally, we show that a related ratio defined at the Tevatron is also robust as a function of the ttbar invariant mass.

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arXiv2013

Global Constraints on Lepton-Quark Contact Interactions

The Large Hadron Collider can do precision physics at a level that is competitive with electroweak precision constraints when probing physics beyond the Standard Model. We present a simple yet general parameterization of the effect of an arbitrary number of lepton-quark contact interactions on any di-lepton observable at hadron colliders. This parameterization can be easily adopted by the experimental collaborations to put bounds on arbitrary combinations of lepton-quark contact interactions. We compute the corresponding bounds from current di-lepton resonance searches at the LHC and find that they are competitive with and often complementary to indirect constraints from electroweak precision data. We combine all current constraints in a global analysis to obtain the most stringent bounds on lepton-quark contact interactions. We also show that the high-energy phase of the LHC has a unique potential in terms of discovery and discrimination power among different types of lepton-quark contact interactions.

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arXiv2013

Searches for New Vector Like Quarks: Higgs Channels

New vector-like quarks can mix sizably with first generation Standard Model quarks without conflicting with current experimental constraints. Searches for such new quarks have been performed in pair production and electroweak single production channels with subsequent decays into electroweak gauge bosons. To fully explore the underlying structure of the theory the channels with heavy quark decays into Higgs bosons are crucial and in this article we consider for the first time the LHC reach for such channels. The two main production mechanisms involve single production of new quarks through the fusion of a vector boson and the Higgs and single production in association with a Higgs boson. We show that both channels have promising reach at the LHC and that they complement the current direct searches involving decays into electroweak gauge bosons.

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arXiv2013

$Hb\bar{b}$ production in Composite Higgs Models

New vector-like quarks with electric charge 2/3 and -1/3 can be singly produced at hadron colliders through the exchange of a color octet vector resonance in models of strong electroweak symmetry breaking. We show that electroweak symmetry breaking effects can have a significant impact on the decay pattern of these new quarks. In particular, single production of charge -1/3 fermion resonances, mediated by a color octet vector resonance, typically results in an $H b\bar{b}$ final state with a sizeable cross section and very distinctive kinematics. We consider the leading $H\to b\bar{b}$ decay and show that the $4b$ signal can be very efficiently disentangled from the background: heavy octet masses of up to 3 TeV can be tested with the data already collected at the LHC and up to 5 TeV with an integrated luminosity of 100 fb$^{-1}$ at $\sqrt{s}=14$ TeV. We also discuss the kinematical differences between the $Hb\bar{b}$ production in models of strong electroweak symmetry breaking and supersymmetric models and the implications on the phenomenology of non-minimal composite Higgs models.

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arXiv2013

Physics of the Interplay Between the Top Quark and the Higgs Boson

We discuss some aspects of the interplay between the top quark and the Higgs boson at the LHC. First we describe what indirect information on the top Yukawa coupling can be extracted from measurements in the Higgs sector. We then show that the study of processes involving $Ht\bar{t}$ and $Hb\bar{b}$ final states can give us information on the spectrum in models of strong electroweak symmetry breaking. In particular, we introduce a novel analysis of the $Hb\bar{b}\to bb\bar{b}\bar{b}$ channel with an excellent discovery potential at the LHC.

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arXiv2012

New Higgs Production Mechanism in Composite Higgs Models

Composite Higgs models are only now starting to be probed at the Large Hadron Collider by Higgs searches. We point out that new resonances, abundant in these models, can mediate new production mechanisms for the composite Higgs. The new channels involve the exchange of a massive color octet and single production of new fermion resonances with subsequent decays into the Higgs and a Standard Model quark. The sizable cross section and very distinctive kinematics allow for a very clean extraction of the signal over the background with high statistical significance. Heavy gluon masses up to 2.8 TeV can be probed with data collected during 2012 and up to 5 TeV after the energy upgrade to $\sqrt{s}=14$ TeV.

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arXiv2012

The Effective Lagrangian for Bulk Fermions in Models with Extra Dimensions

We compute the dimension 6 effective Lagrangian arising from the tree level integration of an arbitrary number of bulk fermions in models with warped extra dimensions. The coefficients of the effective operators are written in terms of simple integrals of the metric and are valid for arbitrary warp factors, with or without an infrared brane, and for a general Higgs profile. All relevant tree level fermion effects in electroweak and flavor observables can be computed using this effective Lagrangian.

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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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arXiv2012

Single Vectorlike Quark Production at the LHC

A gluon resonance G of mass below 1 TeV could be the origin of the t\bar{t} forward-backward asymmetry observed at the Tevatron provided that new decay modes G->\bar{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->Wq' or Q->Zq. 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\bar{t} asymmetry. Similar analyses can be applied to more general models with new massive gluons and vectorlike quarks.

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arXiv2011

Phenomenology of Non-Custodial Warped Models

We study the effect of bulk fermions on electroweak precision observables in a recently proposed model with warped extra dimensions and no custodial symmetry. We find that the top-quark mass, together with the corrections to the Zbb vertex and the one-loop contribution to the T parameter, which is finite, impose important constraints that single out a well defined region of parameter space. New massive vector bosons can be as light as 1.5 TeV and have large couplings to the t_R quark, and suppressed couplings to the t_L, b_L and lighter quarks. We discuss the implications for searches of models with warped extra dimensions at the LHC.

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arXiv2011

Stealth gluons at hadron colliders

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 \bar{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->qQ, 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\bar{t} mass invariant distribution and discuss its implications at the Tevatron and the LHC.

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arXiv2011

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}.

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arXiv2011

Warped Views on the Large Hadron Collider

Models with warped extra dimensions, and their strongly coupled duals, offer a nice solution to the hierarchy problem and a very appealing realisation of flavour. Compatibility with the very stringent electroweak and flavour tests have made a generic picture emerge, with a composite Higgs, partial compositeness and custodial symmetry as the main ingredients. We review the main features of this picture and discuss how -and when- models with warped extra dimensions could be discovered at the Large Hadron Collider.

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arXiv2010

Tau Custodian searches at the LHC

The tau lepton can be more composite than naively expected in models of strong electroweak symmetry breaking with tri-bimaximal lepton mixing. New leptonic resonances required by custodial symmetry, the tau custodians, can then be the first signal of this lepton flavor realization. Tau custodians can be very light, decaying almost exclusively into taus. The LHC reach for these new leptons is up to masses of 240, 480 and 720 GeV for sqrt{s}=14 TeV and an integrated luminosity of 30, 300 and 3000 fb^{-1}, respectively. Our analysis can be extended to any pair produced particles decaying mostly into taus and Standard Model bosons.

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