Rachid Benbrik

Motivated by the recent unexpected large forward-backward asymmetry of the top-quark observed by D0 and CDF at the Tevatron, we investigate a possible explanation for the anomaly within the framework of diquark models. In the diquark models, the top-quark pair production is mediated by the $u$-channel diagram. It is found that the color-triplet diquark can generate the forward-backward asymmetry of 20% when the constraint from the cross section of the top-quark pair production is taken into account. DOI: http://dx.doi.org/10.1103/PhysRevD.82.034034 © 2010 The American Physical Society

Motivated by the disagreement between the experimental data and lattice calculations on the decay constant of the $D_{s}$ meson, we investigate leptoquark (LQ) contributions to the purely leptonic decays of a pseudoscalar (P). We concentrate on the LQs which only couple to the second-generation quarks before the electroweak symmetry breaking and we discuss in detail how flavor symmetry breaking effects are brought into the extension of the standard model after the spontaneous symmetry breaking. We find that the hermiticity of fermion mass matrices can not only reproduce the correct Cabibbo-Kobayashi-Maskawa and Maki-Nakagawa-Sakata matrices, but also reduce the number of independent flavor mixing matrices and lead to $V^{R}_{f}=V^{L}_{f}$ with $L(R)$ denoting the chirality of the f-type fermion. Accordingly, it is found that the decays $D_{s,d}\to \ell^{+} \nu$, $B^{+}\to \tau^{+} \nu$ and $B_c\to \ell^{+} \nu$ have a strong correlation in parameters. We predict that the decay constant of the $B_c$ meson calculated by the lattice could be less than the experimental data by 23%. Intriguingly, the resultant upper limits of branching ratios for $D\to \mu^{+} \mu^{-}$ and $\tau\to \mu (\pi^0, \eta, \eta', \rho, \omega)$ are found to be around $ 5.1\times 10^{-7}$ and $(2.6, 1.5, 0.6, 7.4, 4.8)\times 10^{-8}$, which are below and close to the current experimental upper bounds, respectively.

Motivated by the recent muon $g-2$ data, we study the lepton flavor violating (LFV) $l\to l^{\prime }\gamma$ and $Z\to l{\overline{l}}^{\prime }$ ($l$, $l^{\prime }=e$, $\mu$, $\tau$ decays with $l\ne l^{\prime }$) in a scalar leptoquark model. Leptoquarks can produce sizable LFV $l\to l^{\prime }\gamma$ decay rates that can be easily reached by present or near future experiments. Leptoquark masses and couplings are constrained by the muon $g-2$ data and the current $l\to l^{\prime }\gamma$ bounds. We predict $\mathrm{Br}(Z\to {\tau }^{\mp }{e}^{\pm })$ reaching the present limit (${10}^{-5}$) and $\mathrm{Br}(Z\to {\mu }^{\mp }{\tau }^{\pm })$ reaching $2\times {10}^{-8}$, which will be accessible by future linear colliders, whereas, the current bounds on LFV impose very strong constraints on the $\mathrm{Br}(Z\to {\mu }^{\mp }{e}^{\pm })$ and the ratio is too low to be observed in the near future.

Motivated by the recent muon $g-2$ data, we study the lepton flavor violating $l \to l' \gamma$ and $Z \to l \bar l'$ ($l, l' = e, \mu, \tau$) decays with $l \neq l'$ in a scalar leptoquark model. Leptoquarks can produce sizable LFV $l\to l'\gamma$ decay rates that can be easily reached by present or near future experiments. % Leptoquark masses and couplings are constrained by the muon $g-2$ data and the current $l \to l' \gamma$ bounds. We predict $Br(Z \to \tau^\mp e^\pm)$ reaching the present limit ($10^{-5}$) and $Br(Z \to \mu^\mp \tau^\pm)$ reaching $2\times 10^{-8}$, which will be accessible by future linear colliders, whereas, the current bounds on LFV impose very strong constraints on the $Br(Z \to \mu^\mp e^\pm)$ and the ratio is too low to be observed in the near future.

In this study, we propose new Higgs production mechanisms with multiphoton final states in the fermiophobic limit of the two Higgs doublet model. The processes are: $gg\to hh$, $gg\to Hh$ followed by $H\to hh$ and $gg\to Ah$ followed by $A\to hZ$. In the fermiophobic limit, $gg\to hh$ and $gg\to Ah\to hhZ$ would give rise to $4\gamma$ signature while $gg\to Hh\to hhh$ can give a $6\gamma$ final state. We show that both the Fermilab Tevatron and CERN’s Large Hadron Collider can probe a substantial slice of the parameter space in this fermiophobic scenario of the two Higgs doublet model. If observed the above processes can give some information on the triple Higgs couplings involved.

We study double Higgs production at the future linear collider in the framework of the two Higgs doublet models through the following channels: $e^{+}{e}^{-}\to {\Phi }_i{\Phi }_{j}Z$, ${\Phi }_i=h^0$, $H^0$, $A^0$, $H^{\pm }$. All these processes are sensitive to triple Higgs couplings. Hence observations of them provide information on the triple Higgs couplings that help reconstructing the scalar potential. We also discuss the double Higgs-Strahlung $e^{+}{e}^{-}\to h^0{h}^{0}Z$ in the decoupling limit where $h^0$ mimics the standard model Higgs boson. The processes $e^{+}{e}^{-}\to h^0{h}^{0}Z$ and $e^{+}{e}^{-}\to h^0{H}^{0}Z$ are also discussed in the fermiophobic limit where distinctive signatures such as $4\gamma +X$, $2\gamma +X$, and $6\gamma +X$ are expected in the Type-I two Higgs doublet model.

One loop mediated charged Higgs bosons decays $H^\pm\to W^\pm V$, $V= Z, \gamma$ are studied in the Minimal Supersymmetric Standard Model (MSSM) with and without CP violating phases. We evaluate the MSSM contributions to these processes taking into account $B\to X_s\gamma$ constraint as well as experimental constraints on the MSSM parameters. In the MSSM, we found that in the intermediate range of $\tan\beta \la 10$ and for large A_t and large $\mu$, where the lightest top squark becomes very light and hence non-decoupled, the branching ratio of $H^\pm \to W^{\pm} Z$ can be of the order 10^{-3} while the branching ratio of $H^\pm \to W^{\pm} \gamma$ is of the order 10^{-5}. We found also that the CP violating phases of soft SUSY parameters can modify the branching ratio by about one order of magnitude. We also show that MSSM with CP violating phases lead to CP-violating asymmetry in the decays $H^+ \to W^+V$ and $H^- \to W^-V$. Such CP asymmetry can be rather large and can reach 80% in some region of parameter space.