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J. C. Romão

Publications and source records attributed to J. C. Romão.

12 recordsLinked to original sources
arXiv2017

Multi-Higgs doublet models: physical parametrization, sum rules and unitarity bounds

If the scalar sector of the Standard Model is non-minimal, one might expect multiple generations of the hypercharge-1/2 scalar doublet analogous to the generational structure of the fermions. In this work, we examine the structure of a Higgs sector consisting of N Higgs doublets (where N \geq 2). It is particularly convenient to work in the so-called charged Higgs basis, in which the neutral Higgs vacuum expectation value resides entirely in the first Higgs doublet, and the charged components of remaining N-1 Higgs doublets are mass-eigenstate fields. We elucidate the interactions of the gauge bosons with the physical Higgs scalars and the Goldstone bosons and show that they are determined by an Nx2N matrix. This matrix depends on (N-1)(2N-1) real parameters that are associated with the mixing of the neutral Higgs fields in the charged Higgs basis. Among these parameters, N-1 are unphysical (and can be removed by rephasing the physical charged Higgs fields), and the remaining 2(N-1)^2 parameters are physical. We also demonstrate a particularly simple form for the cubic interaction and some of the quartic interactions of the Goldstone bosons with the physical Higgs scalars. These results are applied in the derivation of Higgs coupling sum rules and tree-level unitarity bounds that restrict the size of the quartic scalar couplings. In particular, new applications to three Higgs doublet models with an order-4 CP symmetry and with a Z_3 symmetry, respectively, are presented.

hep-ph↗
arXiv2014

$h \rightarrow Z γ$ in the complex two Higgs doublet model

The latest LHC data confirmed the existence of a Higgs-like particle and made interesting measurements on its decays into $γγ$, $Z Z^\ast$, $W W^\ast$, $τ^+ τ^-$, and $b \bar{b}$. It is expected that a decay into $Z γ$ might be measured at the next LHC round, for which there already exists an upper bound. The Higgs-like particle could be a mixture of scalar with a relatively large component of pseudoscalar. We compute the decay of such a mixed state into $Z γ$, and we study its properties in the context of the complex two Higgs doublet model, analysing the effect of the current measurements on the four versions of this model. We show that a measurement of the $h \rightarrow Z γ$ rate at a level consistent with the SM can be used to place interesting constraints on the pseudoscalar component. We also comment on the issue of a wrong sign Yukawa coupling for the bottom in Type II models.

hep-ph↗
arXiv2014

A reappraisal of the wrong-sign $hb\overline{b}$ coupling and the study of $h \rightarrow Z γ$

It has been pointed out recently that current experiments still allow for a two Higgs doublet model where the $h b \bar{b}$ coupling ($k_D m_b/v$) is negative; a sign opposite to that of the Standard Model. Due to the importance of delayed decoupling in the $h H^+ H^-$ coupling, $h \rightarrow γγ$ improved measurements will have a strong impact on this issue. For the same reason, measurements or even bounds on $h \rightarrow Z γ$ are potentially interesting. In this article, we revisit this problem, highlighting the crucial importance of $h \rightarrow VV$, which can be understood with simple arguments. We show that the impacts on $k_D<0$ models of both $h \rightarrow b \bar{b}$ and $h \rightarrow τ^+ τ^-$ are very sensitive to input values for the gluon fusion production mechanism; in contrast, $h \rightarrow γγ$ and $h \rightarrow Z γ$ are not. We also inquire if the search for $h \rightarrow Z γ$ and its interplay with $h \rightarrow γγ$ will impact the sign of the $h b \bar{b}$ coupling. Finally, we study these issues in the context of the Flipped two Higgs doublet model.

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arXiv2008

Fermion masses, Leptogenesis and Supersymmetric SO(10) Unification

Current neutrino oscillation data indicate the existence of two large lepton mixing angles, while Kobayashi-Maskawa matrix elements are all small. Here we show how supersymmetric SO(10) with extra chiral singlets can easily reconcile large lepton mixing angles with small quark mixing angles within the framework of the successful Fritzsch ansatz. Moreover we show how this is fully consistent with the thermal leptogenesis scenario, avoiding the so-called gravitino problem. A sizeable asymmetry can be generated at relatively low scales. We present our results in terms of the leptonic CP violation parameter that characterizes neutrino oscillations.

hep-ph↗
arXiv2003

CP Phases, LFV, RpV & all that

Most phenomenological analyses of searches for supersymmetric particles have been performed within the MSSM with real SUSY parameters and conserved R-parity and lepton flavour. Here we summarize recent results obtained in the (s)lepton sector when one of the above assumptions is relaxed.

hep-ph↗
arXiv2001

Spontaneous CP Violation in the Next-to-Minimal Supersymmetric Standard Model

We re-examine spontaneous $\cp$ violation (SCPV) at the tree level in the context of the next-to-minimal supersymmetric standard model (NMSSM) with two Higgs doublets and a gauge singlet field. We analyse the most general Higgs potential without a discrete $Z_3$ symmetry, and derive an upper bound on the mass of the lightest neutral Higgs boson. We estimate $\epsi_K$ by applying the mass insertion approximation, finding that in order to account for the for the observed $\cp$ violation in the neutral kaon sector a non-trivial flavour structure in the soft-breaking $A$ terms is required and that the upper bound on the lightest Higgs-boson mass becomes stronger. We also discuss the implications of electric dipole moments of the electron and the neutron in SUSY models with SCPV.

hep-ph↗
arXiv2001

Spontaneous CP Violation in the Next-to-Minimal Supersymmetric Standard Model Revisited

We re-examine spontaneous CP violation at the tree level in the context of the next-to-minimal supersymmetric standard model (NMSSM) with two Higgs doublets and a gauge singlet field. We analyse the most general Higgs potential without a discrete Z_3 symmetry, and derive an upper bound on the mass of the lightest neutral Higgs boson consistent with present experimental data. We investigate, in particular, its dependence on the admixture and CP-violating phase of the gauge singlet field, as well as on tan(beta). To assess the viability of the spontaneous CP violation scenario, we estimate epsilon_K by applying the mass insertion approximation. We find that a non-trivial flavour structure in the soft-breaking A terms is required to account for the observed CP violation in the neutral kaon sector. Furthermore, combining the minimisation conditions for spontaneous CP violation with the constraints coming from K0-K0bar mixing, we find that the upper bound on the lightest Higgs-boson mass becomes stronger. We also point out that the electric dipole moments of electron and neutron are a serious challenge for SUSY models with spontaneous CP violation.

hep-ph↗
arXiv2000

Flavour-Conserving CP Phases in Supersymmetry and Implications for Exclusive B Decays

We study rare exclusive B decays based on the quark-level transition b->s(d)l^+l^-, where l=e or mu, in the context of supersymmetric theories with minimal flavour violation. We present analytic expressions for various mixing matrices in the presence of new CP-violating phases, and examine their impact on observables involving B and \bar{B} decays. An estimate is obtained for CP-violating asymmetries in B->K^(*)l^+l^- and B->rho(pi)l^+l^- decays for the dilepton invariant mass region 1.2 GeV < M_{l^+l^-}< M_{J/psi}. As a typical result, we find a CP-violating partial width asymmetry of about -6% (-5%) in the case of B->pi (B->rho) in effective supersymmetry with phases of O(1), taking into account the measurement of the inclusive b->s gamma branching fraction. On the other hand, CP asymmetries of less than 1% are predicted in the case of B->K^(*). We argue that it is not sufficient to have additional CP phases of O(1) to observe large CP-violating effects in exclusive b->s(d)l^+l^- decays.

hep-ph↗
arXiv1995

SINGLE PHOTON DECAYS OF THE $Z^0$ AND SUSY WITH SPONTANEOUSLY BROKEN R-PARITY

Spontaneous violation of R parity can induce rare single photon decays of the $Z^0$ involving the emission of (nearly) massless pseudoscalar Goldstone bosons, majorons, as well as massive CP even or CP odd spin zero bosons that arise in the electroweak breaking sector of these models. We show that the majoron emitting decays can have a sizeable branching ratio of $10^{-5}$ or so, without conflicting any experimental observation from neutrino physics or particle searches. These decays may lead to interesting structures for the single photon spectrum involving either mono chromatic photons as well as continuous spectra that grow with energy. They would easily account for an excess of single photon events at high energies recently hinted at by the OPAL collaboration.

hep-ph↗
arXiv1993

Model Independent Higgs Boson Mass Limits at LEP

We derive model-independent constraints on Higgs mass and couplings from associated signals for higher masses, accessible at LEP2. This work is motivated by the fact that, in many extensions of the standard model, the Higgs boson can have substantial "invisible" decay modes, for example, into light or massless weakly interacting Goldstone bosons associated to the spontaneous violation of lepton number below the weak scale.

hep-ph↗
arXiv1992

Detection of Intermediate Mass Higgs Bosons from Spontaneously Broken R Parity Supersymmetry

The Higgs sector in spontaneously broken R Parity supersymmetry (RPSUSY) shows interesting features that require new search techniques. Both the mass spectrum and production rates may differ from the standard model and minimal supersymmetric model (MSSM) expectations. For some parameter choices, the dominant Higgs boson decay mode can even be invisible, leading to events with large missing transverse momentum carried by superweakly interacting majorons. We study the reaction $pp \ra Z + H + X$, and find that it canlead to detectable events at LHC/SSC for a large region of parameter space.

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