arXiv subjects

E. Ramirez Barreto

Publications and source records attributed to E. Ramirez Barreto.

15 recordsLinked to original sources

arXiv2014

Is There a Hidden Principle in the Higgs Boson Decay to Photons?

It is remarkable that the measured Higgs boson mass is so close to the value which maximizes the Higgs decay rate to photons as predicted by the Standard Model. In this work we explore the consequences to assume that an $\sim 125$ GeV Higgs boson mass is not accidental, but fixed by some fundamental principle that enforces it to maximize its decay rate into photons. The principle is motivated by the evidence that only a very small volume of the parameters space of the Standard Model, which contains their measured values, could lead to a maximal Higgs boson with that mass. If the principle actually holds, several Standard Model features get fixed, as the number of fermion families, quark colors, and the CP nature of the new boson, for example. We also illustrate how such principle can place strong bounds on new physics scenarios as a Higgs dark portal model and a Two Higgs Doublet Model.

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arXiv2013

A Model with Chiral Quarks of Electric Charges -4/3 and 5/3

We present a new model based on the SU(3)xSU(2)xU(1) symmetry, in which there is a new consistent set of chiral fermion fields that renders the model free from anomalies. The new fermions do not share the usual family structure of the Standard Model and some of them have exotic electric charges, as the quarks X and Y with electric charge 5/3 and -4/3, respectively. Interestingly, the model contains a new heavy neutral lepton which may be a dark matter candidate. Two Higgs doublets are present in our construction, so that two CP even scalars are present in the model particle spectrum. One of them is similar to Standard Model Higgs boson, while the other one couples mainly with the new exotic fermions. We performed a discovery analysis showing that the 8 TeV LHC can find the Y quark from single and pair production with masses from 300 GeV up to ~ 750 GeV. We also show that the new spectrum does not contribute significantly to the oblique EW parameters, and that dangerous flavor changing neutral currents are suppressed. Characteristic signatures from the other new fermions in the model are also commented.

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arXiv2013

Vector-bilepton Contribution to Four Lepton Production at the LHC

Some extensions of the standard model predict the existence of particles having two units of leptonic charge, known as bileptons. One of such models is based on the $SU(3)_c\times SU(3)_L\times U(1)_X$ symmetry group (3-3-1 model, for short). Our search uses the minimal version of the 3-3-1 model, having exotic charges for the quarks and new gauge bosons. This model predicts the existence of bileptons as vector particles having one ($V^{\pm}$) and two ($Y^{\pm\pm}$) units of electric charge. We study the signatures for the production of four leptons by considering the contribution of a pair of bileptons in $p p $ collisions for three energy and luminosity regimes at the Large Hadron Collider (LHC). We present invariant mass and transverse momentum distributions, the total cross section and we determine the expected number of events for each bilepton type. Finally we analyze the LHC potential for discovering singly and doubly charged vector bileptons at 95$%$ C.L.. We conclude that the LHC collider can show a clear signature for the existence of bileptons as a signal of new physics.

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arXiv2012

Jets Plus Same-Sign Dileptons Signatures from Fermionic Leptoquarks at the LHC

Within the 3-3-1 model framework, we consider the production and decay of fermionic leptoquarks into a striking experimental signature at the LHC: a narrow resonance at the $b$ jet plus same-sign dileptons channel. The data already accumulated by the LHC collaborations may hide a large number of events associated to the production and decay of such exotic quarks, allowing one to investigate large portions of the parameters space of the model. The observation or not of such events would shed light on the construction of models presenting heavy exotic quarks.

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arXiv2012

Explaining the Higgs Decays at the LHC with an Extended Electroweak Model

We show that the recent discovery of a new boson at the LHC, which we assume to be a Higgs boson, and the observed enhancement in its diphoton decays compared to the SM prediction, can be explained by a new doublet of charged vector bosons from an extended electroweak gauge sector model with SU(3)$_C\otimesSU(3)_L\otimesU(1)_X$ symmetry. Our results show a good agreement between our theoretical expected sensitivity to a 126--125 GeV Higgs boson and the experimental significance observed in the diphoton channel at the 8 TeV LHC. Effects of an invisible decay channel for the Higgs boson are also taken into account, in order to anticipate a possible confirmation of deficits in the branching ratios into $ZZ^*$, $WW^*$, bottom quarks, and tau leptons.

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arXiv2011

Probing 3-3-1 Models in Diphoton Higgs Boson Decay

We investigate the Higgs boson production through gluon fusion and its decay into two photons at the LHC in the context of the minimal 3-3-1 model and its alternative version with exotic leptons. The diphoton Higgs decay channel presents an enhanced signal in this model compared to the Standard Model due to the presence of an extra singly charged vector boson and a doubly charged one. Prospects for the Higgs boson detection at 7 TeV center-of-mass energy with up to 10 fb$^{-1}$ are presented. Our results suggest that a Higgs boson from these 3-3-1 models can potentially explain the small excess for $m_H\leq 145$ GeV observed at the LHC. Otherwise, if this excess reveals to be only a statistical fluctuation of the Standard Model backgrounds severe constraints can be put on these models.

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arXiv2011

Production of Charged Higgs Bosons in a 3-3-1 Model at the CERN LHC

We perform a study of the charged Higgs production from an $SU(3)_{C}\otimes SU(3)_{L}\otimes U(1)_{X}$ model with right-handed neutrinos, postulating a custodial symmetry which reduces the number of free parameters in the scalar potential. We compute the cross sections for charged scalars for typical and new production modes. One of the new $SU(3)_{L}$ neutral gauge bosons, $Z^{\prime}$, affects some production cross sections distinguishing the model from other Standard Model extensions like, for example, the Minimal Supersymmetric Standard Model and general two Higgs doublets models. The interplay between the Higgs sector of the model and that $Z^{\prime}$ gauge boson enhances substantialy all the production rates of the lightest charged Higgs boson, $H_{1}^{\pm}$, at hadron colliders compared to the MSSM. We found that a large portion of the parameters space can be probed at the LHC running at 14 TeV center-of-mass energy in the associated $pp\rightarrow W^{\pm}H_{1}^{\mp}+X$ production channel in the low luminosity run stage of the experiment.

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arXiv2011

Top quark forward-backward asymmetry from the $3-3-1$ model

The forward-backward asymmetry $A_{FB}$ in top quark pair production, measured at the Tevatron, is probably related to the contribution of new particles. The Tevatron result is more than a $2σ$ deviation from the standard model prediction and motivates the application of alternative models introducing new states. However, as the standard model predictions for the total cross section $σ_{tt}$ and invariant mass distribution $M_{tt}$ for this process are in good agreement with experiments, any alternative model must reproduce these predictions. These models can be placed into two categories: One introduces the s-channel exchange of new vector bosons with chiral couplings to the light quarks and to the top quark and another relies on the t-channel exchange of particles with large flavor-violating couplings in the quark sector. In this work we employ a model which introduces both s- and t-channel nonstandard contributions for the top quark pair production in proton antiproton collisions. We use the minimal version of the $SU(3)_C \otimes SU(3)_L \otimes U (1)_X$ model (3-3-1 model) that predicts the existence of a new neutral gauge boson, called $Z^\prime$. This gauge boson has both flavor-changing couplings to up and top quarks and chiral coupling to the light quarks and to the top quark. This very peculiar model coupling can correct the $A_{FB}$ for top quark pair production for two ranges of $Z^\prime$ mass while leading to cross section and invariant mass distribution quite similar to the standard model ones. This result reinforces the role of the 3-3-1 model for any new physics effect.

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arXiv2011

Vector- and Scalar-Bilepton Pair Production in Hadron Colliders

We study the double-charged vector-bilepton pair production and double-charged scalar-bilepton pair production {\it via} $p + p \longrightarrow Y^{++} + Y^{--} + X$ and $p + p \longrightarrow S_1^{++} + S_1^{--} + X$, where $Y$ and $S_1$ are vector and scalar bileptons respectively, in the framework of the minimal version of the 3-3-1 model. We compute the photon, $Z$, and $Z^\prime$ s-channel contributions for the elementary process of bilepton scalar pair production, and to keep the correct unitarity behavior for the elementary $q \bar q$ interaction, we include the exotic quark t-channel contribution in the vector-bilepton pair production calculation. We explore a mass range for $Z^\prime$ and we fix the exotic quark mass within the experimental bounds. In this model, the vector-bilepton mass is directly related to $M_{Z^\prime}$ and we consider scalar mass values around the vector-bilepton mass. We show that the total cross section for vector-bilepton production is 3 orders of magnitude larger than for scalar pair production for $\sqrt s= 7$ TeV and 14 TeV and we obtain the number of events for the proposed LHC luminosities as a function of the bilepton mass. In addition we present some invariant mass and transverse momentum distributions. When comparing these distributions we observe quite different behavior providing the determination of the bilepton nature. We conclude that one can disentangle the production rates and that the LHC can be capable of detecting these predicted particles as a signal for new physics.

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arXiv2010

Searching for an Extra Neutral Gauge Boson from Muon Pair Production at LHC

We search for signatures of the extra neutral gauge boson $ Z^\prime$, predicted in some extensions of the Standard Model, from the analysis of some distributions for $p + p \longrightarrow μ^+ + μ^- + X$, where the only exotic particle involved is $ Z^\prime$. In addition to the invariant mass and charge asymmetry distributions, we propose in our search to use the transverse momentum distribution ($p_T$) as an observable. We do our calculation for two values of the LHC center of mass energy (7 and 14 TeV), corresponding to 1 and 100 fb$^{-1}$ of luminosity, in order to compare our findings from some models with the distributions following from the Standard Model. By applying convenient cuts in the invariant mass, we show that the final particles $p_T$ distributions can reveal the presence of an extra neutral gauge boson contribution. We also claim that it is possible to disentangle the models considered here and we emphasize that the minimal version of the model, based on ${SU (3)_C \times SU (3)_L \times U (1)_X}$ symmetry, presents the more clear signatures for $ Z^\prime$ existence.

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arXiv2008

Charged Bilepton Pair Production at LHC Including Exotic Quark Contribution

The production of $W^+ W^-$ pair in hadron colliders was calculated up to loop corrections by some authors in the Electroweak standard model (SM) framework. This production was also calculated, at the tree level, in some extensions of the SM such as the vector singlet, the fermion mirror fermion and the vector doublet models by considering the contributions of new neutral gauge bosons and exotic fermions. The obtained results for $e^+ e^-$ and $pp$ collisions pointed out that the new physics contributions are quite important. This motivates us to calculate the production of a more massive charged gauge boson predicted by the ${SU (3)_C \times SU (3)_L \times U (1)_X}$ model (3-3-1 model). Thus, the aim of the present paper is to analyze the role played by of the extra gauge boson ${Z^\prime}$ and of the exotic quarks, predicted in the minimal version of the 3-3-1 model, by considering the inclusive production of a pair of bileptons ($V^\pm$) in the reaction $p + p \longrightarrow V^+ + V^- + X$, at the Large Hadron Collider (LHC) energies. Our results show that the correct energy behavior of the elementary cross section follows from the balance between the contributions of the extra neutral gauge boson with those from the exotic quarks. The extra neutral gauge boson induces flavor-changing neutral currents (FCNC) at tree level, and we have introduced the ordinary quark mixing matrices for the model when the first family transforms differently to the other two with respect to $SU(3)_L$. We obtain a huge number of heavy bilepton pairs produced for two different values of the center of mass energy of the LHC.

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arXiv2008

The Production of Neutral Bilepton Bosons in Proton-Proton Collisions

We establish some signatures of the extra bilepton boson ${X^0}$ predicted in the ${SU (3)_C \times SU (3)_L \times U (1)_X}$ model with right-handed neutrinos. We analyze the process $p + p \longrightarrow X^0 +X^{0*} + {\hbox {anything}}$, for center of mass energy regime of the Large Hadron Collider. The main contributions for the neutral bilepton production in $q \bar q$ process come from the $s-$channel ($Z$ and $Z^\prime$ exchanges), when the initial quarks have charge 2/3, and from an additional $t-$channel (heavy quark exchange) when they have -1/3 of the positron electric charge. We calculate some distributions of the final bileptons and from these results we conclude that LHC can show a clear signature for the existence of the $X^0$ predicted in the 3-3-1 model.

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arXiv2007

Extra Neutral Gauge Boson from Two Versions of the 3-3-1 Model in Future Linear Colliders

Our aim is to establish some signatures of the extra gauge boson ${Z^\prime}$, predicted in two versions of ${SU (3)_C \times SU (3)_L \times U (1)_X}$ model and to show possible differences between these signatures. First, by considering the process $e^+ + e^- \longrightarrow μ^+ +μ^- $, we obtain some observables and next, by considering additional hadronic final states, we obtain lower bounds for $M_{Z^\prime}$, within 95% C.L.. We also include our preliminary results concerning $p \bar p$ and $p p$ collisions in order to support our conclusions about the possibility to discriminate the various versions of 3-3-1 model. From our analysis we conclude that linear colliders can show a clear signature for the existence of $Z^\prime$ predicted in the 3-3-1 model and also that it can discriminate the versions.

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arXiv2006

Bounds for $Z^\prime$ Mass in 3-3-1 Models from $e^+$ $e^-$ Collisions at ILC and CLIC Energies

We obtain bounds for the mass of the extra neutral gauge boson, ${Z^\prime}$, predicted by two versions of ${SU (3)_C \times SU (3)_L \times U (1)_X}$ model, namely one corresponding to its minimal version and another where the neutrinos are allowed to have right-handed projection. We explore $\sqrt s$ from 0.5 TeV to 5 TeV region, that will be accessible in next linear colliders (ILC and CLIC). We used the process $e^+ + e^- \longrightarrow f^+ + f^-$, with $f=μ$, $c$ and $b$ to obtain the energy dependent lower bounds for $M_{Z^\prime}$, within 95% C.L., by performing a $χ^{2}$ fit of the difference between the final lepton angular distribution calculated from the SM and that predicted by the 3-3-1 models, for a zero mixing angle. In addition we show that, as the angular distributions depends on $Z^\prime$ couplings, the obtained bounds allow one for disentangle the studied models. Finally, using the model with right-handed neutrinos, we calculate the total cross section for $e^+ + e^- \longrightarrow e^+ + e^- + e^+ + e^-$. We concluded that $Z^\prime$ contribution is very small when compared with the bilepton contribution of the minimal version and with the SM cross section.

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arXiv2005

Four Leptons Production in $e^-$ $e^+$ Collisions from 3-3-1 Model

We study the process $e^- + e^+ \to e^+ + e^+ + e^- + e^-$ for ILC and CLIC energy regimes, in the minimal 3-3-1 model framework. The main contributions, for the production of two bileptons each one decaying into two same-sign leptons, come from s-channel annihilation ({\it via} $γ, Z$ and ${Z^\prime}$) and t-channel electron exchange. We evaluate the number of events for an extra neutral gauge boson mass $M_{Z^\prime}$ in the range 1 TeV to 3 TeV. We compare some distributions from 3-3-1 model with the Standard Model background in order to extract a possible signature of the contribution of bilepton and ${Z^\prime}$ for the process. From our analysis we conclude that our results give clear signals for physics beyond the Standard Model in e$^-$ e$^+$ colliders.

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