arXiv subjects

S. F. Novaes

Publications and source records attributed to S. F. Novaes.

At least 37 records · Page 2Linked to original sources

arXiv1997

Search for Non-standard Higgs Boson in Diphoton Events at p pbar Collisions

We estimate the attainable limits on the coupling of a non-standard Higgs boson to two photons taking into account the data collected by the Fermilab collaborations on diphoton events. We based our analysis on a general set of dimension--6 effective operators that give rise to anomalous couplings in the bosonic sector of the Standard Model. If the coefficients of all ``blind'' operators are of the same order of magnitude, bounds on the anomalous triple vector-boson couplings can also be obtained.

hep-ph↗

arXiv1997

Limits on Anomalous Top Couplings from Z Pole Physics

We obtain constraints on possible anomalous interactions of the top quark with the electroweak vector bosons arising from the precision measurements at the Z pole. In the framework of $SU(2)_L \otimes U(1)_Y$ chiral Lagrangians, we examine all effective CP-conserving operators of dimension five which induce fermionic currents involving the top quark. We constrain the magnitudes of these anomalous interactions by evaluating their one-loop contributions to the Z pole physics. Our analysis shows that the operators that contribute to the LEP observables get bounds close to the theoretical expectation for their anomalous couplings. We also show that those which break the $SU(2)_C$ custodial symmetry are more strongly bounded.

hep-ph↗

arXiv1997

Anomalous Higgs Couplings in Triple Gauge Boson Production at the NLC

We present the sensitivity limits on the coefficients of a dimension-6 effective operators that parametrizes the possible effects of new physics beyond the Standard Model. Our results are based on the study of the processes $e^+ e^- \to W^+ W^- γ$, $Z Zγ$, and $Zγγ$ at NLC energies. In our calculations, we include all the anomalous interactions involving vector and Higgs bosons, and take into account the Standard Model irreducible background. We analyse the impact of these new interactions on the total cross section and on some kinematical distributions of the final state particles.

hep-ph↗

arXiv1997

New Higgs Interactions in ZZγand ZγγProduction

The effect of new operators that only modify the bosonic couplings of the Higgs boson, without altering the WWγor WWZ three-point functions, are examined in the e+e- -> ZZγand Zγγprocesses. We analyse the constraints on these interactions that can be imposed by the LEP II collider at CERN and at the Next Linear Collider.

hep-ph↗

arXiv1997

Anomalous Couplings in e^+ e^- --> W^+ W^- gamma at LEP2 and NLC

We present sensitivity limits on the coefficients of a dimension-6 effective Lagrangian that parametrizes the possible effects of new physics beyond the Standard Model. Our results are based on the study of the process $e^+ e^- \to W^+ W^- γ$ at LEP2 and NLC energies. In our calculations, we include all the new anomalous interactions, involving vector and Higgs bosons, and take into account the Standard Model irreducible background. We analyse the impact of these new interactions on the total cross section, including the effects of the initial electron and final W polarizations. We then focus on the operators that will not be constrained by the $e^+ e^- \to W^+ W^-$ process, obtaining limits based on the photon energy distribution.

hep-ph↗

arXiv1997

Discriminating New Physics Scenarios at NLC: The Role of Polarization

We explore the potential of the Next Linear Collider (NLC), operating in the $eγ$ mode, to disentangle new physics scenarios on single $W$ production. We study the effects related with the exchange of composite fermion in the reaction $eγ\to W ν_e$, and compare with those arising from trilinear gauge boson anomalous couplings. We stress the role played by the initial state polarization to increase the reach of this machine and to discriminate the possible origin of the new phenomena.

hep-ph↗

arXiv1997

Bounds on Contact Interactions from LEP1 Data and the High-Q^2 HERA Events

A contact four-fermion interaction between light quarks and electrons has been evoked as a possible explanation for the excess of events observed by HERA at high-$Q^2$. We explore the 1-loop effects of such interaction in $Γ(Z^0 \to e^+ e^-)$ measured at LEP and impose strong bounds on the lower limit of the effective scale. Our results are able to discard some of the contact interactions as possible explanation for the HERA events.

hep-ph↗

arXiv1996

Compositeness Effects in the Anomalous Weak-Magnetic Moment of Leptons

We investigate the effects induced by excited leptons, at the one-loop level, in the anomalous magnetic and weak-magnetic form factors of the leptons. Using a general effective Lagrangian approach to describe the couplings of the excited leptons, we compute their contributions to the weak-magnetic moment of the $τ$ lepton, which can be measured on the $Z$ peak, and we compare it with the contributions to $g_μ- 2$, measured at low energies.

hep-ph↗

arXiv1996

Excited Fermion Contribution to Z Physics at One Loop

We investigate the effects induced by excited leptons at the one-loop level in the observables measured on the $Z$ peak at LEP. Using a general effective Lagrangian approach to describe the couplings of the excited leptons, we compute their contributions to both oblique parameters and $Z$ partial widths. Our results show that the new effects are comparable to the present experimental sensitivity, but they do not lead to a significant improvement on the available constraints on the couplings and masses of these states.

hep-ph↗

arXiv1996

Probing Higgs Couplings in e^+ e^- \to γγγ

We investigate the existence of anomalous Higgs boson couplings, $Hγγ$ and $HZγ$, through the analysis of the process $e^+ e^- \to γγγ$ at LEP2 energies. We suggest some kinematical cuts to improve the signal to background ratio and determine the capability of LEP2 to impose bounds on those couplings by looking for a Higgs boson signal in this reaction.

hep-ph↗

arXiv1996

Anomalous Higgs Boson Contribution to e^+ e^- \to b \bbar γat LEP2

We study the effect of anomalous $Hγγ$ and $HZγ$ couplings, described by a general effective Lagrangian, on the process $e^+ e^- \to b \bar{b} γ$ at LEP2 energies. We include the relevant irreducible standard model background to this process, and from the photon energy spectrum, we determine the reach of LEP2 to unravel the anomalous couplings by analyzing the significance of the signal for Higgs boson with mass up to $150$ GeV.

hep-ph↗

arXiv1995

Excited Leptonic States in Polarized e^-γand e^+ e^- Collisions

We analyze the capability of the next generation of linear electron--positron colliders to unravel the spin and couplings of excited leptons predicted by composite models. Assuming that these machines will be able to operate both in the $e^+e^-$ and $e^-γ$ modes, we study the effects of the excited electrons of spin $\frac{1}{2}$ and $\frac{3}{2}$ in the reactions $e^-γ\rightarrow e^-γ$ and $e^+e^- \rightarrow γγ$. We show how the use of polarized beams is able not only to increase the reach of these machines, but also to determine the spin and couplings of the excited states.

hep-ph↗

arXiv1995

Quartic Anomalous Couplings in $γγ$ Colliders

We study the constraints on the vertices $W^+W^- Zγ$, $W^+W^-γγ$, and $ZZγγ$ that can be obtained from triple-gauge-boson production at the next generation of linear $e^+e^-$ colliders operating in the $γγ$ mode. We analyze the processes $γγ\to W^+W^-V$ ($V=Z$, or $γ$) and show that these reactions increase the potential of $e^+e^-$ machines to search for anomalous four-gauge-boson interactions.

hep-ph↗

arXiv1994

$ε_b$ Constraints on Self--Couplings of Vector Bosons

We analyze the constraints on possible anomalous contributions to the $W^+W^-Z$ vertex coming from non-universal radiative corrections to the $Z \rightarrow b \bar{b}$ width. We parametrize these corrections in terms of $ε_b$ and use the LEP data to establish the allowed values for the anomalous triple couplings. We examine all CP conserving effective operators that exhibit $SU(2)_L\times U(1)_Y$ gauge invariance and do not give any tree level contribution to the present experimental observables. For some of these operators our constraints are comparable with the bounds coming from a global fit of the oblique parameters, which evidences the increasing relevance of the precise measurement of the $b$--quark parameters at LEP for the search of new physics.

hep-ph↗

arXiv1994

Fermion Helicity Flip in Weak Gravitational Fields

The helicity flip of a spin-${\textstyle \frac{1}{2}}$ Dirac particle interacting gravitationally with a scalar field is analyzed in the context of linearized quantum gravity. It is shown that massive fermions may have their helicity flipped by gravity, in opposition to massless fermions which preserve their helicity.

gr-qc↗

arXiv1993

Searching for an Invisibly Decaying Higgs Boson in E^+E^-, E-Gamma, and Gamma-Gamma Collisions

Higgs bosons can have a substantial ``invisible'' branching ratio in many extensions of the Standard Model, such as models where the Higgs bosons decay predominantly into light or massless weakly interacting Goldstone bosons. In this work, we examine the production mechanisms and backgrounds for invisibly decaying Higgs bosons at the Next Linear $e^+e^-$ Collider operating in the modes $e^+e^-$, $eγ$, and $γγ$. We demonstrate that such machine is much more efficient to survey for invisibly decaying Higgs bosons than the Large Hadron Collider at CERN.

hep-ph↗

arXiv1993

Triple Vector Boson Processes in $γγ$ Colliders

We study the production of three gauge bosons at the next generation of linear $e^+e^-$ colliders operating in the $γγ$ mode. The processes $γγ\rightarrow W^+W^-V$ ($V=Z^0$, or $γ$) can provide direct information about the quartic gauge-boson couplings. We analyze the total cross section as well as several dynamical distributions of the final state particles including the effect of kinematical cuts. We find out that a linear $e^+e^-$ machine operating in the $γγ$ mode will produce 5--10 times more three-gauge-boson states compared to the standard $e^+e^-$ mode at high energies.

hep-ph↗

arXiv1993

Quartic Anomalous Couplings in $eγ$ Colliders

We study the production of gauge boson pairs at the next generation of linear $e^+e^-$ colliders operating in the $eγ$ mode. The processes $eγ\rightarrow VV^\prime F$ ($V,V^\prime =W$, $Z$, or $γ$ and $F=e$ or $ν$) can give valuable information on possible deviations of the quartic vector boson couplings from the Standard Model predictions. We establish the range of the new couplings that can be explored in these colliders based on a $3σ$ effect in the total cross section. We also present several kinematical distributions of the final state particles that could manifest the underlying new dynamics. Our results show that an $eγ$ collider can extend considerably the bounds on anomalous interactions coming from oblique radiative corrections and from direct searches in $e^+e^-$ colliders.

hep-ph↗