arXiv subjects
Publications and source records attributed to Laura Reina.
We illustrate the possibility of observing signals from Flavor Changing Neutral Currents, originating from the scalar sector of a Two Higgs Doublet Model. In particular, we focus on the tree level process mu^+ mu^- --> tbar c + cbar t, via scalar exchange in the s-channel, as a distinctive process for mu^+ mu^- colliders.
We present a complete calculation of the Leading Order QCD corrections to the quark level decay amplitude for b -> s gamma gamma and study their relevance for both the inclusive branching ratio BR(B -> X_s gamma gamma) and for the exclusive decay channel B_s -> gamma gamma. In addition to the uncertainties in the short distance calculation, due to the choice of the renormalization scale, an appreciable uncertainty in both B_s -> gamma gamma and B -> X_s gamma gamma is introduced by the matrix element calculation. We also briefly discuss some long distance effects, especially those due to the eta_c resonance for the inclusive rate. Finally, the analysis of the IR singularities of the two photon spectrum in the inclusive case is given.
We present a general phenomenological analysis of a class of Two Higgs Doublet Models with Flavor Changing Neutral Currents arising at the tree level. The existing constraints mainly affect the couplings of the first two generations of quarks, leaving the possibility for non negligible Flavor Changing couplings of the top quark open. The next generation of lepton and hadron colliders will offer the right environment to study the physics of the top quark and to unravel the presence of new physics beyond the Standar Model. In this context we discuss some interesting signals from Flavor Changing Scalar Neutral Currents.
The rare decay B -> X_s gamma gamma is studied in the Standard Model (SM) and in two different versions (Model I and Model II) of the Two Higgs Doublet Model (2HDM). In the SM the branching ratio into ``hard photons'' is about 10^(-7) and can be appreciably different in the 2HDM. We also introduce a forward-backward asymmetry which gives an additional handle to discriminate different models.
We investigate the possibility of having sizable effects from flavor changing neutral currents at the tree level in a particular class of Two Higgs Doublet Models. Constraints from existing experimental information are discussed. Some interesting signals at the future generation of lepton colliders are illustrated.
A comprehensive phenomenological analysis of a two Higgs doublet model, with flavor changing scalar currents at the tree level, called model III, is presented. Constraints from existing experimental information especially on (Delta F)=2 processes are systematically incorporated. Constraints emerging from rare B-decays, Z-->b\bar b, and the ρparameter are also examined. Experimental implications for e^+e^- (μ^+μ^-)-->t\bar c+\bar t c, t-->cγ(Z,g), D^0-\bar D^0, and B^0_s-\bar B^0_s oscillations, and for e^+e^-(Z)-->b\bar s+s\bar b are investigated and experimental effort towards these is stressed. We also emphasize the importance of clarifying the experimental issues pertaining to Z-->b\bar b.
A study of $R_b$ and $R_c$ is presented in the context of a Two Higgs Doublet Model (2HDM) with flavor changing scalar currents (FCSC). Implications of the model for the $ρ$-parameter and for $b\to sγ$ are also considered. The experimental data on $R_b$ places stringent constraints on the model parameters. The configuration of the model needed to account for $R_b$ is found to be irreconcilable with constraints from $b\to sγ$ and $ρ$. In particular, if $R^{\rm exp}_b>R^{\sss{\rm SM}}_b$ persists then this version of 2HDM will be ruled out or require significant modifications. Noting that aspects of the experimental analysis for $R_b$ and $R_c$ may be of some concern, we also disregard $R^{\rm exp}_b$ and $R^{\rm exp}_c$ and give predictions for these using constraints from $b\to sγ$ and $ρ$ parameter only. We emphasize the theoretical and experimental advantages of the observable $R_{b+c}\equiv Γ(Z\to b\bar b\mbox{ or } c\bar c)/Γ(Z\to\mbox{hadrons})$. We also stress the role of $R_\ell\equiv Γ(Z\to\mbox{hadrons})/Γ(Z\to \ell^+\ell^-)$ in testing the Standard Model (SM) despite its dependence on QCD corrections. Noting that in models with FCNC the amplitude for $Z\to c\bar c$ receives a contribution which grows with $m^2_t$, the importance and uniqueness of precision $Z\to c\bar c$ measurements for constraining flavor changing $t\bar c$ currents is underscored.
In the contest of a Two Higgs Doublet Model without flavor conservation, the presence of Flavor Changing Neutral Scalar Currents may affect in particular the couplings of the top quark. Some ideas on how to look for new signals from Flavor Changing Interactions at the next generation of lepton colliders are presented.
The prospect of observing the flavor changing decay $\fH\to t\bar c$ of a neutral Higgs boson produced via $s$-channel and its subsequent decay into $t\bar c$ is considered at a $μ^+μ^-$ collider. Numerical estimates are given in the context of a two Higgs doublet model with flavor changing couplings. It is found that for many values of the model parameters such {\it tree-level} flavor changing decays will be produced at an observable level. In addition studies of the helicity of the top will allow the determination of the relative strengths of the flavor changing Higgs couplings and these may be measured with about $10^3$ events.
Due to the very large mass of the top-quark, probing the flavor changing top-charm-scalar vertex is clearly very important. Fortunately the largeness of $m_t$ endows a unique signature to the resulting reaction i.e., $e^+e^-\to t\bar c (\bar t c)$ that should be helpful in identification of such events. A two Higgs doublet model, without natural flavor conservation, is used to give an illustrative estimate for the rate for these reactions.