We analyze the semileptonic $B \to a_1 \ell^+\ell^-$, $\ell=τ, μ, e$ transitions in the frame work of the three-point QCD sum rules in the standard model. These rare decays governed by flavor-changing neutral current transition of $b \to d$. Considering the quark condensate contributions, the relevant form factors as well as the branching fractions of these transitions are calculated.
在一个扩展的额外维模型中计算了中性顶夸克衰变t -> cX的变味过程，其中X是中性标准模型粒子.对于光子，X= γ $, and a Standard Model Higgs boson, X = H, are analyzed in detail in a non-linear$ R_x规范。我们发现，分支比可以增强的动力学起源于额外的维。在1/R >> ->的极限下，我们发现当1/R = 0.5 TeV时，Br（t -> c γ）\simeq 10^{-10}。对于衰变t -> cH，我们发现在低Higgs质量值下Br（t -> cH）\simeq 10^{-10}。当1/R -> \infty时，分支比变为零。
In a covariant quark-diquark model, we investigate the rare decay of $Λ_b \rightarrow n l^+ l^-$ and $Λ_b \rightarrow n γ$ in the Bethe-Salpeter equation approach. In this model the baryons are treated as bound states of a constituent quark and a diquark interacting via a gluon exchange and the linear confinement. We find that the ratio of form factors $R$ is varies from $-0.90$ to $-0.25$ and the branching ratios $Br(Λ_b \rightarrow n l^+ l^-)\times 10^{8}$ are $6.79(l=e),~4.08 ~ (l=μ),~2.9 ~(l=τ) $ and the branching ratio $Br(Λ_b \rightarrow γ)\times 10^{7} )=3.69$ in central values of param…
Rare B meson decays mediated by flavour changing neutral current (FCNC) transition play interesting role to probe the flavour sector of the standard model (SM). Generally at the tree level, FCNC processes are not allowed in the SM but occurs at the loop levels. This gives an excellent hunting ground for new physics (NP). From various experimental studies it is found that the FCNC processes having quark level transition $b \to s$ are challenging. Here, we investigate different kinematic observables like forward-backward asymmetry, differential branching ratio and lepton polarization asymmetry f…
FCNC in leptonic and semileptonic decays of D mesons in a general two-Higgs doublet model
FCNC半轻子D衰变中大的长距离标准模型效应可以使这些过程在未来的测量中观测到。在这个部门和/或轻子族违反（LFV）D衰变的观察最终的分歧将需要超越SM框架的解释。在本文中，我们面对轻子和半轻子FCNC和LFV D介子衰变的实验数据与版本的两希格斯偶极子模型，允许这些影响发生在树水平。从D ^0-> l^+ l '^-和D -> pi l^+ l'^-衰变中得到了模型参数的严格界。模型的一致性要求D -> Vl ^+ l '^-衰变的分支分数应低于10^{-9}能级。
Scalar Dark Matter Search at the LHC through FCNC Top Decay
我们讨论了一个扩展的标准模型电弱扇区，它包含一个稳定的标量暗物质粒子，D玻色子。为了在大型强子对撞机上寻找D玻色子，我们利用了变味中性流（FCNC）顶夸克衰变，t->c D D，在一个双希格斯模型框架中，由最轻的标准模型--如希格斯h^0--介导。考虑到D玻色子残余丰度的限制，在这种情况下，t->c D D的分支比可以高达10^{-3}。在10（100）fb^{-1}的积分光度下，14 TeV LHC可以探测到这个分支比的值，它比t \bar{t}的产生小一（两）个数量级，其中t \bar{t} \to c \bar{b} \ell^-（\bar{c} b \ell^+）+缺失的E_T。当D玻色子质量<60 GeV，m_{h^0} < 2 M_Z，光度为10 fb^{-1}，分支比BR（t->CDD）为10^{-4}时，14 TeV LHC的信号事件估计数为80个量级。
With the discovery of the doubly heavy $Ξ_{cc}$ baryon, comprehensive studies of the properties of the doubly heavy baryons are started. In the present work, we examine the $Ξ_{bb} \to Σ_b \ell^+ \ell^-$ and $Ξ_{cc} \to Σ_c \ell^+ \ell^-$ decays induced by flavor-changing neutral currents (FCNC) in the framework of the light-cone sum rules. After obtaining the sum rules for the form factors induced by the tensor current, the branching ratios of the relevant transitions are estimated. We found that the branching ratio for the $c \to u$ transition is around five orders smaller than the $b \to d$…
SUSY induced top quark FCNC decay $t \to c h$ after Run I of LHC
鉴于希格斯玻色子的发现和在大型强子对撞机上未观测到超对称粒子，我们重新审视了超对称诱导的顶夸克味变衰变为希格斯玻色子。通过考虑Higgs质量测量、LHC对SUSY的搜索、真空稳定性、精确电弱观测量以及$B \to X_s γ$的约束，我们对相关的SUSY参数空间进行了扫描。我们得到了以下结果：（1）在MSSM中，$ t \to c h$的分支比只能达到$3.0\times10^{-6}$，这比LHC出现之前的结果小了一个数量级。在考虑的约束，希格斯质量和LHC搜索sparticles被发现发挥了重要作用，限制预测。(2)在MSSM的单重态扩展中，由于夸克扇区不受Higgs质量的限制，$t \to c h$的分支比在允许的参数空间中可以达到$10^{-5}$的量级. (3)在重超对称极限下，手性守恒的混合物$δ_{LL}$和$δ_{RR}$对$t \to c h$可能有抑制作用.在方子和胶子能量大于3TeV，CP奇Higgs玻色子质量约为1TeV的MSSM中，$t\to c h$的分支比仍能达到$10^{-8}$的量级.
The weak decays of doubly heavy $Ξ_{QQ}$ baryon induced by flavor changing $b \to d$ and $ c \to u$ flavor changing neutral current within the light cone sum rules are studied. Using the parallel components of the light cone distribution amplitudes for $Λ_Q$ baryon, the sum rules are constructed and analyzed for the corresponding transition form factors. Having the results for the form factors, the corresponding branching ratios for $Ξ_{QQ} \to Λ_Q l^+ l^-$ decays are estimated. While the branching ratio due to $b \to d$ transition is around $10^{-9}$, in $c \to u$ transition case, it is found…
In this work we fit the available binned data of the differential decay distribution of the exclusive $B \to K^{(*)} \ell^+ \ell^-$ and $B_s \to ϕμ^+ μ^-$ decays to the mass and mixing parameters of a light dark vector boson model. Due to an incorrect assessment of the dominant contribution of the dark $Z$ model to the FCNC B-meson decays, a previous work in literature reported that $\mathcal{O}(1)$ mixings were allowed by the data. In this talk we report the correct calculations and constraints on the mixing parameters as well as the mass of the dark vector boson using the marginalization tec…
We study the invisible scalar of $S$ in charmed meson and baryon decays with flavor-changing neutral currents (FCNCs) based on the model-independent effective Lagrangian between the quarks and invisible scalar. From the bounds of the coupling constants extracted from the recent BES III experiment on the decay of $D^{0}\toπ^{0}\barνν$, we predict that the decay branching ratios of $D^{+}\toπ^{+}S$, $D_s^+\to K^+S$, $Λ_c\to pS$, $Ξ_c^{0(+)}\to Σ^{0(+)}S$, and $Ξ_c^0\toΛS$ can be as large as $10.6,~2.53,~2.39,~0.963,~5.77$, and $2.92\times10^{-4}$ with $m_{_S}\approx 1.1~{\rm GeV}$, respectively.…
In this paper, we study the FCNC decay processes of $B$ and $B_c$ meson, in which one invisible particle is emitted. Both the spin-0 and spin-1 cases are considered. The model-independent effective Lagrangian is introduced to describe the coupling between the light invisible boson and quarks. The constraints of the coupling coefficients are extracted by experimental upper limits of the missing energy in $B$ meson decays. The bounds are used to predict the upper limits of branching fractions of corresponding $B_c$ decays, which are of the order of $10^{-6}$ or $10^{-5}$ when final meson is pseu…
Some interpretations of $R_{D^{(*)}}$ anomaly in $B$ meson decay using leptoquark (LQ) models can also generate top quark decays through Flavor Changing Neutral Current (FCNC). In this work we focus on two LQs, i.e. scalar $S_1$ and vector $U_1$ which are both singlet under the $SU(2)_L$ gauge group in the Standard Model (SM). We investigate their implications on the 3-body top FCNC decays $t\to c \ell_i \ell_j$ at tree level and the 2-body $t\to c V$ at one-loop level, with $\ell$ being the SM leptons and $V=γ, Z, g$ being the SM gauge bosons. We utilize the $2σ$ parameter fitting ranges of t…
Flavor changing neutral current (FCNC) interactions for a top quark $t$ decays into $Xq$ with $X$ represents a neutral gauge or Higgs boson, and $q$ a up- or charm-quark are highly suppressed in the Standard Model (SM) due to the Glashow-Iliopoulos-Miami mechanism. Whilst current limits on the branching ratios of these processes have been established at the order of $10^{-4}$ from the Large Hadron Collider experiments, SM predictions are at least nine orders of magnitude below. In this work, we study some of these FCNC processes in the context of an extended mirror fermion model, originally pr…
Motivated by the rencent LHC data on the lepton-flavor violating (LFV) decays $h\to \ell_1 \ell_2$ and $B_{s,d}\to \ell_1 \ell_2$, we study the Higgs-mediated flavor-changing neutral current (FCNC) interactions in the effective field theory (EFT) approach without and with the minimal flavor violation (MFV) hypothesis, and concentrate on the later. After considering the $B$ and $K$ physics data, the various LFV processes, and the LHC Higgs data, severe constraints on the Higgs FCNC couplings are derived, which are dominated by the LHC Higgs data, the $B_s - \bar B_s$ mixing, and the $μ\to e γ$ …
The observed flavor-changing neutral-current (FCNC) processes in the standard model (SM) arise from the loop diagrams involving the weak charged currents mediated by the $W$-gauge boson. Nevertheless, the top-quark FCNCs and lepton-flavor violating processes resulting from the same mechanism are highly suppressed. We investigate possible new physics effects that can enhance the suppressed FCNC processes, such as $t\to q(h,V)$ with $V=γ,Z,g$, $h\to \ell \ell'$, and $\ell\to \ell' γ$. To achieve the assumption that the induced-FCNCs are all from quantum loops, we consider the scotogenic mechanis…
We revisit rare radiative leptonic decays $B_{s,d}\to γe^+e^-$ and $B_{s,d}\to γμ^+μ^-$ in the Standard Model and provide the updated estimates for various differential distributions (the branching ratios, the forward-backward asymmetry, and $R_{μ/e}$, the ratio of the differential distribution for muons over electrons in the final state). The new ingredients of this work compared to the existing theoretical analyses are the following: (i) we calculate all $B_d\toγ$ and $B_s\toγ$ form factors induced by the vector, axial-vector, tensor and pseudotensor quark currents within the relativistic di…
Motivated by the recent anomalies in $b \to s \ell^+ \ell^-$ transitions, we explore a minimal $Z^\prime$ scenario, in which the $Z^\prime$ boson has a flavour-changing coupling to charm and top quarks and a flavour-conserving coupling to muons. It is found that such a $Z^\prime$ boson can explain the current $b \to s \ell^+ \ell^-$ anomalies, while satisfying other flavour and collider constraints simultaneously. The $Z^\prime$ boson can be as light as few hundreds GeV. In this case, the $t \to c μ^+ μ^-$ decay and the $tZ^\prime$ associated production at the LHC could provide sensitive probe…
In this work, we study the flavor-changing neutral-current process $B_{c}\to{D}_{s}^{*}(\to{D}_{s}π)\ell^{+}\ell^{-}$ ($\ell$= $e$, $μ$, $τ$). The relevant weak transition form factors are obtained by using the covariant light-front quark model, in which, the main inputs, i.e., the meson wave functions of $B_{c}$ and $D_{s}^{*}$, are adopted as the numerical wave functions from the solution of the Schrödinger equation with the modified Godfrey-Isgur model. With the obtained form factors, we further investigate the relevant branching fractions and their ratios, and some angular observables, i.e…
The discovery of doubly heavy baryon provides us with a new platform for precisely testing Standard Model and searching for new physics. As a continuation of our previous works, we investigate the FCNC processes of doubly heavy baryons. Light-front approach is adopted to extract the form factors, in which the two spectator quarks are viewed as a diquark. Results for form factors are then used to predict some phenomenological observables, such as the decay width and the forward-backward asymmetry. We find that most of the branching ratios for $b\to s$ processes are $10^{-8}\sim10^{-7}$ and thos…
本文研究了顶夸克$t\to cγ$和$t\to c g$的变味中性流衰变。根据标准模型的预测，这些衰变的分支比分别约为$\sim 5\times 10^{-14}$和$\sim 1\times 10^{-12}$。最近由ATLAS协作组提出的研究给出了在$%95$ C. L.处关于$\sim 10^{-5}$的这些分支比的灵敏度。获得了$t\to cγ$和$t\to c g$衰变分支比超过这些预测的$λ$、$Λ$和$d$的参数空间。
