Rare Top Decays in Minimal and Non-minimal Universal Extra Dimension
Ujjal Kumar Dey, Tapoja Jha
备注：
31 pages, 11 figures; added more discussion, one section, a few clarifications and minor changes in the text and abstract; fixed typos; updated references; matches published version
The flavour changing decays of the top quark are severely suppressed in the Standard Model by virtue of the Glashow-Iliopoulos-Maiani mechanism. Many beyond Standard Model extensions predict the decay rates at a level that is observable in the LHC. We perform a complete one-loop calculation of the flavour changing top quark decays $t\to cγ$ and $t\to ch$ in the universal extra dimensional model. Apart from considering the decay rates in the minimal version of the model, we also calculate the rates in the non-minimal scenario where the presence of boundary localised terms interestingly modify the set-up. We find that the decay rates in the minimal variant of the model do not change much from their Standard Model values. In the non-minimal version of this model, these decay rates can be higher for specific choices of the boundary localised parameters for a certain range of inverse compactification radius. But these model parameters lead to Kaluza-Klein particle masses that are in tension with various searches at the LHC.
The rare top quark decays $t\to cV$ in the topcolor-assisted technicolor model
在顶色辅助人工色（TC 2）模型框架下研究了顶夸克的稀有衰变。我们发现，由TC 2模型预测的top-pion和top-Higgs的贡献可以提高SM分支比多达6-9个数量级。也就是说，在大多数情况下，分支比的数量级为$Br(t\to c g)\sim 10^{-5}$、$Br(t\to c Z)\sim 10^{-5}$、$Br(t\to c γ)\sim 10^{-7}$。在TC 2模型参数合理取值的情况下，这种罕见的顶夸克衰变有可能在未来的实验中得到验证。因此，罕见的顶夸克衰变为我们提供了一种独特的方法来检验TC 2模型。
Double-Lepton Polarization Asymmetries and Polarized Forward Backward Asymmetries in the Rare b \to s \ell^+ \ell^- Decays in a Single Universal Extra Dimension Scenario
研究了在标准模型和Appelquist-Cheng-Dobrescu模型中稀有b \to s \ell^+ \ell^-模式的双轻子极化不对称性和单、双轻子极化前后不对称性.特别地，我们研究了观测量对紧化额外维度半径R的敏感性，这是这个新物理模型中的新参数。
Motivated by the search for flavor-changing neutral current (FCNC) top quark decays at the LHC, we calculate rare Higgs three body decay $H\rightarrow Wbc$ induced by top-Higgs FCNC coupling in the littlest Higgs model with T-parity(LHT). We find that the branching ratios of $H\rightarrow Wbc$ in the LHT model can reach $\mathcal O(10^{-7})$ in the allowed parameter space.
Top physics provides a fertile ground for new-physics searches. At present, most top observables appear to be in good agreement with the respective Standard Model predictions. However, in the case of decay modes that are suppressed in the Standard Model, new-physics contributions of comparable magnitude may exist and yet go unnoticed because their impact on the total decay width is small. Hence it is interesting to probe rare top decays. This analysis focuses on the decay $t \to b \bar b c$. Useful observables are identified and prospects for measuring new-physics parameters are examined.
Measurements at B factories have provided important constraints on new physics in several rare processes involving the B meson. New Physics, if present in the b quark sector may also affect the top sector. In an effective Lagrangian approach, we write down operators where effects in the bottom and the top sector are related. Assuming the couplings of the operators to be of the same size as the weak coupling g of the Standard Model and taking into account constraints on new physics from the bottom sector as well as top branching ratios, we make predictions for the rare top decays t -> cV where …
The study of the top quark's properties is an important part of the LHC programme. In earlier work, we have studied the rare decay $t \to b \bar b c$, using effective operators to capture the effects of physics beyond the Standard Model. However top decay is primarily sensitive to new physics in the sub-TeV energy regime. If this new physics resides at a higher energy scale, then one needs to turn to single-top production. In this paper, we use the $s$-channel and $t$-channel single-top production measurements to constrain the new physics parameter space associated with such contact interactio…
The flavor-changing rare top decays $t\to c V V$ in topcolor-assisted technicolor theory
在顶色辅助的人工色（TC 2）理论框架下，我们计算了标量（中性顶介子$π_{t}^{0}$和顶希格斯粒子$h_{t}^{0}$）对变味稀有顶衰变$t\to c V V$（V= W，g，$γ$或Z）的贡献.我们的研究结果表明，$h_{t}^{0}$可以提高标准模型$B_{r}^{SM}(t\longrightarrow cWW)$的几个数量级的参数空间。当顶希格斯粒子的质量在$2m_{W}$和$m_{t}$之间时，分支比共振峰出现。分支比$ B_{r}(t\to c W W)$可在窄范围内达到$ 10^{-3}$。
New physics can manifest itself by an appreciable increase of the decay rate of top quarks in rare flavour-changing final states. Exploiting the large top quark production rate at the LHC, we bound four-fermion operators contributing to non-resonant $t\rightarrow \ell^+\ell^- j$ using different signal regions of the latest LHC searches for $t\rightarrow Zj$. We also provide prospects for the high-luminosity LHC to test these as well as four-fermion operators contributing to $t\rightarrow b\overline{b}j$, based on improved analysis strategies of existing searches. We single out all weakly-coupl…
The recently observed mass difference of the $D^0-\overline{D^0}$ mixing is used to predict the branching ratios of the rare top quark decays $t\to uγ$ and $t\to ug$ in a model independent way using the effective Lagrangian approach. It is found that $Br(t\to uγ)<4\times 10^{-4}$ and $Br(t\to ug)<2\times 10^{-3}$, which still may be within reach of the LHC collider.
We study a version of Two Higgs Doublet Models with non-standard flavor violation in the up quark sector. We find branching ratios for the rare top decays $t\to hc$ and $t\to hu$ that are within reach of current and future colliders, while other flavor constraints from rare $B$ decays and neutral $D$ meson mixing, as well as constraints from Higgs signal strength measurements remain under control. The most prominent collider signature of the considered setup is $pp \to t H \to tt \bar c$, providing continued motivation to search for same-sign tops at the LHC as well as a simple framework to in…
Neutral top-pion and the rare top decays $t\to c l_{i} l_{j}$
我们在顶色辅助人工色（$TC2$）模型的框架下研究了稀有顶衰变$t\to c l_{i} l_{j}(l=τ,μ,or e)$。我们发现，中性的top-pion $π_{t}^{0}$可以产生显着的贡献，这些过程通过改变味道耦合$π_{t}^{0} \bar{t} c$和$π_{t}^{0} l_{i} l_{j}$。对于$π_{t}^{0}$质量$m_{π_{t}}=150 GeV$和参数$ε=0.08$，分支比$Br$（t）-$\to c ττ)$可以达到$7.1\times10^{-7}$。考虑到过程$μ\to e γ$的当前实验极限对$TC2$模型的自由参数的约束，我们发现$Br$（$t\to c τμ$）$\approx$ $Br$（$t\to c τe$）的值在1.8 $\times10^{-10}\sim1.7\times10^{-8}$的范围内.
Rare Decay of the Top t->c l lbar and Single Top Production at International Linear Collider
本文在单圈水平上，对标准模型、双Higgs双重模型（I和II）和MSSM中的变味中性流稀有顶夸克衰变t-> cl+l-和t->c nu_i bar nu_i进行了全面详细的分析。所有模型的分支比都非常小，为O（10^-14），除了无约束MSSM的情况，在无约束MSSM中，e+e-和nu_i bar nu_i的分支比可以达到O（10^-6），tau+ tau-的分支比可以达到O（10^-5）。这个分支比与t-> cV，cH的分支比相当。我们还研究了e+e-> tcbar中的单顶产生率和前后不对称性，并评论了这种信号在国际直线对撞机上的可观测性。
Within the Standard Model the Higgs couplings to fermions and gauge bosons, as function of the particle mass, are predicted to lay on a single line. However, new patterns appear in multi-Higgs models that employ the Froggart-Nielsen mechanism, where the diagonal couplings could lay on different lines and flavor-violating Higgs couplings could appear too. These aspects are studied for a specific model with 3+1 Higgs doublets and a singlet FN field. Constraints on the model are derived from the Higgs search at LHC, and some remarks are presented on their implications for the rare top and Higgs d…
We examine the rare decays $B_s\to l^+l^-$ and $B\to Kl^+l^-$ in the framework of the topcolor-assisted technicolor ($TC2$) model. The contributions of the new particles predicted by this model to these rare decay processes are evaluated. We find that the values of their branching ratios are larger than the standard model predictions by one order of magnitude in wide range of the parameter space. The longitudinal polarization asymmetry of leptons in $B_s \to l^+l^-$ can approach $\ord(10^{-2})$. The forward-backward asymmetry of leptons in $B \to Kl^+l^-$ is not large enough to be measured in …
The rare top decay t-> c l+l-, which involves flavor violation, is studied as a possible probe of new physics. This decay is analyzed with the simplest Standard Model extensions with additional gauge symmetry formalism. The considered extension is the Left-Right Symmetric Model, including a new neutral gauge boson Z' that allows to obtain the decay at tree level through Flavor Changing Neutral Currents (FCNC) couplings. The neutral gauge boson couplings are considered diagonal but family non-universal in order to induce these FCNC. We find the $BR(t-> c l+l-)~10^{-13} for a range 1 TeV < M_{Z'…
The decay t -> c V (V=γ, Z, g) process in the mirror twin Higgs models with the colorless top partners are studied in this paper. We found that the branching ratios of these decays can in some parameter spaces alter the standard model expectations greatly and may be detectable according to the currently precision electroweak measurements. Thus, the constraints on the model parameters may be obtained from the branching fraction of the decay processes, which may serve as a robust detection to this new physics model.
In the littlest Higgs model with T-parity (LHT), the mirror quarks have flavor structures and will contribute to the top quark flavor changing neutral current. In this work, we perform an extensive investigation of the top quark rare three-body decays $t\to cVV (V=γ,Z,g)$ and $t\to cf\bar{f} (f=b,τ,μ,e)$ at one-loop level. Our results show that the branching ratios of $t\to cgg$ and $t\to cb\bar{b}$ could reach $\mathcal {O}(10^{-3})$ in the favorite parameter space of the littlest Higgs model with T-parity, which implies that these decays may be detectable at the LHC or ILC, while for the oth…
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。
We investigate extremely rare top quark decays at a future proton-proton collider with centre-of-mass energy of 100 TeV. We focus on two decay modes: radiative decay with a $Z$ boson, $t \rightarrow b WZ$, and flavour-changing neutral decay with a Higgs boson, $t \rightarrow h c$, the former being kinematically suppressed with a branching ratio of $\mathcal{O}(10^{-6})$, and the latter highly loop-suppresed, with a branching ratio of $\mathcal{O}(10^{-15})$. We find that $t \rightarrow b WZ$ will be very challenging to observe in top quark pair production, even within well-motivated beyond-the…
The rare radiative flavour changing top decays $t\to cγ$ and $t\to cg$ (and the even rarer $t\to uγ$ and $t\to ug$) have been processes of interest for decades as they offer a key probe for studying top quark properties. However an explicit analytical study of the branching ratios and $CP$ asymmetries resulting from these loop level processes has thus far evaded attention. In this work, we provide the formulation for the $CP$ asymmetry resulting from the total kinetic contribution of the loop integrals and their imaginary parts, as well as an updated numerical computation of the predicted Stan…
We study the effects of models with a warped extra dimension on rare charm decays. While the new degrees of freedom in these models are bounded above a few TeV, they can leave signatures in rare charm decays with their tiny Standard Model background. We look at several channels with leptons in the final state along with several asymmetries of both kinematical and dynamical nature. CPT invariance is `usable' for analyzing the impacts of new dynamics in $D$ meson decays and in particular, rare decays of charm mesons.
Measurements of single top and rare top quark production may be sensitive to theories of physics beyond the standard model (SM), including those in which the energy scale of new physics is beyond the energies directly accessible at the Large Hadron Collider. Such models may be observable through signatures like forbidden SM interactions or deviations of the top quark's couplings from the SM predictions. An overview of recent searches from the ATLAS and CMS Collaborations is presented, with a focus on flavor-changing neutral currents.
The amount of data collected by the CMS experiment at the CERN LHC in the second data taking period provides the possibility to study absolute and differential cross sections of top quark interaction processes with high precision. Utilizing these results, fundamental standard model parameters such as the top quark mass are extracted. Moreover, given standard model measurements are interpreted in the context of Beyond the standard model theories. Recent results on top quark properties and rare or anomalous top quark interactions by CMS are presented in these proceedings. Furthermore, the implic…
The calculation of the rare $t$-quark decay $t \to b W b \bar{b}$ within the Standard Model as well as the charged Higgs contribution to this decay is presented. The role of possible background processes is discussed. It is shown that this decay provides an additional possibility for charged Higgs searches.
In this paper we study new-physics contributions to the top-quark decay $t \to b \bar b c$. We search for ways of detecting such new physics via measurements at the LHC. As top quarks are mainly produced at the LHC in $t \bar t$ production via gluon fusion, we analyze the process $gg \to t \bar t \to (b \bar b c) (\bar b \ell \bar ν)$. We find six observables that can be used to reveal the presence of new physics in $t \to b \bar b c$. Three are invariant mass-squared distributions involving two of the final-state particles in the top decay, and three are angular correlations between the final…
In this talk I present the total cross sections, accurate at the NLO in QCD, for rare Standard-Model hadroproduction processes involving multi-Higgs-boson, multi-electroweak-boson and multi-top-quark final states. The comparison between cross sections at the LHC and at a future circular hadronic collider with up to 100 TeV centre-of-mass energy is detailed. Results relevant to the hadronic production of five electroweak bosons, and of a top-antitop pair in association with an electroweak vector boson and two jets are presented here for the first time with NLO accuracy.
The calculation of the partial width of the rare t-quark decay into Upsilon-meson, W-boson and b-quark (t -> ΥW b) is presented. The branching ratio equals Br(t -> Upsilon W b) = 1.3 * 10^{-5} that make possible searches for this rare $t$-quark decay at LHC.
Anomalous couplings of the $Z$ boson to top quarks are only marginally constrained by direct searches and are still sensitive to new particle dynamics at the TeV scale. Employing an effective field theory approach we consider the dimension-six operators which generate deviations from the standard-model vector and axial-vector interactions. We show that rare $B$ and $K$ meson decays together with electroweak precision observables provide strong constraints on these couplings. We also consider constraints from t-channel single-top production.
