arXiv subjects

Y. A. Coutinho

Publications and source records attributed to Y. A. Coutinho.

At least 19 recordsLinked to original sources

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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arXiv2013

Bounds on $Z^\prime$ from 3-3-1 model at the LHC energies

The Large Hadron Collider will restart with higher energy and luminosity in 2015. This achievement opens the possibility of discovering new phenomena hardly described by the Standard Model, that is based on two neutral gauge bosons: the photon and the $Z$. This perspective imposes a deep and systematic study of models that predicts the existence of new neutral gauge bosons. One of such models is based on the gauge group $SU(3)_C \times SU(3)_L \times U(1)_N$ called 3-3-1 model for short. In this paper we perform a study with $Z^\prime$ predicted in two versions of the 3-3-1 model and compare the signature of this resonance in each model version. By considering the present and future LHC energy regimes, we obtain some distributions and the total cross section for the process $p + p \longrightarrow \ell^{+} + \ell^{-} + X$. Additionally, we derive lower bounds on $Z^\prime$ mass from the latest LHC results. Finally we analyze the LHC potential for discovering this neutral gauge boson at 14 TeV center-of-mass energy.

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arXiv2011

Mirror matter, inverse seesaw neutrino masses and the Higgs mass spectrum

In this work we study a mirror model with inverse seesaw neutrino masses in which symmetry breaking scales are fixed from bounds in the neutrino sector. The Higgs sector of the model has two doublets and neutral singlets. The mirror model can be tested at the LHC energies in several aspects. Two very distinctive signatures of the mirror model are a new neutral gauge boson $Z^{\prime}$, with a high invisible branching ratio, and a heavy Majorana neutrino production through the decay $Z^{\prime} \rightarrow N +\bar ν$. This result is compared with heavy Majorana production through heavy pair production and the consequent same-sign dilepton production. The other important consequence of the mirror model is the prediction of the Higgs mass. A particular solution leads to a Higgs in the same region as in the standard model. There is, however, another natural solution where the Higgs mass is above 400 GeV.

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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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arXiv2011

$Z^{\prime}_{B-L}$ phenomenology at LHC

We study the $Z^\prime$ phenomenology for two extensions of the Electroweak Standard Model (SM) which have an extra $U(1)_{B-L}$ gauge factor. We show the capabilities of the LHC in distinguishing the signals coming from these two extensions and both of them from the Standard Model background. In order to compare the behavior of these $B-L$ models we consider the reaction $p + p\longrightarrow μ^+ + μ^- + X$ and compute some observables as the total cross sections, number of events, forward-backward asymmetry, final particle distributions like rapidity, transverse momentum, and dimuon invariant mass, for two LHC regimes: $\sqrt{s}\,({\cal L})=7$ TeV ($1\, \textrm{fb}^{-1}$) and $14 $ TeV ($100\, \textrm{fb}^{-1}$) for $M_{Z^{\prime}}$ = 1000 GeV and 1500 GeV. We show that by using appropriate kinematic cuts some of the observables considered here are able to extract different properties of the $Z^\prime$ boson, and hence providing information about to which $B-L$ model it belongs to.

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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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arXiv2010

Double seesaw mechanism in a left-right symmetric model with TeV neutrinos

A left-right symmetric model is discussed with new mirror fermions and a Higgs sector with two doublets and neutral scalar singlets. The seesaw mechanism is generalized, including not only neutrino masses but also charged fermion masses. The spectrum of heavy neutrinos presents a second seesaw mass matrix and has neutrinos masses naturally in the TeV region. The model has very clear signatures for the new neutral vector gauge bosons. Two classes of models are discussed. New mirror neutrinos can be very light and a new $Z^{\prime}$ can be discriminated from other models by a very high invisible branching fraction. The other possibility is that mirror neutrinos can have masses naturally in the TeV region and can be produced through $Z^{\prime}$ decays into heavy neutrino pairs. Signatures and production processes for the model at the LHC energy are also presented.

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arXiv2009

Testing a possible W and Z structure at the LHC/CERN

One of the first channels to be experimentally analyzed at the LHC is $ p + p \longrightarrow l^+ + l ^- + X $. A resonance in this channel would be a clear indication of a new gauge neutral boson, as proposed in many extended models. In this paper we call attention to the possibility that the new resonance in this channel could have spin zero. A new high mass spin zero state could be a strong indication of the composite nature of the standard model particles. We have made a comparison between spin zero and spin one for the new hypothetical heavy gauge particle production and decays and we show some distributions that can easily identify their spins.

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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

Right-handed heavy neutrinos in the littlest Higgs model

In this paper we discuss the consequences of including a new heavy right-handed neutrino singlet $N_R$ in the littlest Higgs model. This new state is not connected with the light neutrinos {\it via} the seesaw mechanism. A very interesting property of this extended model is the full coupling of the new neutral gauge boson $A_H$ to $N_R$, giving large total cross sections and suggesting a wide range of experimental search for the $N_R$ at the p p collider CERN-LHC and future electron-positron collider ILC.

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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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arXiv2007

Discriminating among the theoretical origins of new heavy Majorana neutrinos at the CERN LHC

A study on the possibility of distinguishing new heavy Majorana neutrino models at LHC energies is presented. The experimental confirmation of standard neutrinos with non-zero mass and the theoretical possibility of lepton number violation find a natural explanation when new heavy Majorana neutrinos exist. These new neutrinos appear in models with new right-handed singlets, in new doublets of some grand unified theories and left-right symmetrical models. It is expected that signals of new particles can be found at the CERN high-energy hadron collider (LHC). We present signatures and distributions that can indicate the theoretical origin of these new particles. The single and pair production of heavy Majorana neutrinos are calculated and the model dependence is discussed. Same-sign dileptons in the final state provide a clear signal for the Majorana nature of heavy neutrinos, since there is lepton number violation. Mass bounds on heavy Majorana neutrinos allowing model discrimination are estimated for three different LHC luminosities.

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arXiv2007

Distinguishing Z' signatures and the Littlest Higgs model in e+e- Colliders at sqrt{s} \ne M_{Z'}

There is a recent proposal identifying the Higgs particle of the Standard Model as a pseudo Nambu-Goldstone boson. This new broken symmetry introduces new particles and new interactions. Among these new interactions a central role to get a new physics is played by two new heavy neutral gauge bosons. We have studied the two new neutral currents in the Littlest Higgs model and compared with other extended models. For high energy e+e- colliders we present a clear signature for these two new neutral gauge bosons that can indicate the theoretical origin of these particles. Previous analysis by other authors were done at collider energies equal to the new gauge boson mass M_{A_H}. In this paper we show that asymmetries in fermion antifermion production can display model differences in the case M_{A_H} > sqrt{s}. For M_{A_H} < sqrt{s} we show that the hard photon energy distribution in e+e- --> gamma + f + anti-f can present a model dependence. For higher energies, the hard photon energy distribution can be a clear signature for both new neutral gauge bosons. New bounds for the new neutral gauge boson masses are also presented.

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arXiv2006

On signatures for the Littlest Higgs model in electron-positron colliders

There is a recent proposal of identifying the Higgs particle of the Standard Model as a pseudo Nambu-Goldstone boson. This new broken symmetry introduces new particles and new interactions. Among these new interactions a central role to get a new physics is played by the new neutral gauge boson. We have studied the new neutral currents in the Littlest Higgs model and compared with other extended models. For high energy $e^+ + e^-$ colliders we present a clear signature for new neutral gauge bosons that can indicate the theoretical origin of these particles. Previous analysis by other authors were done at collider energies equal to the new gauge boson mass $M_{A_H}$. In this paper we show that asymmetries in fermion anti-fermion production can display model differences in the case $M_{A_H} > \sqrt{s}$. For $M_{A_H} < \sqrt{s}$ we show that the hard photon energy distribution in $e^+ + e^- \lra γ+ f + \bar f$ can present a model dependence. New bounds for the new neutral gauge boson masses are also presented.

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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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