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Flavor signatures of complex anomalous tcZ couplings

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Abstract

In this work, we study the effects of anomalous tcZ couplings. Such couplings would potentially affect several neutral current decays of K and B mesons via Z-penguin diagrams. Using constraints from relevant observables in K and B sectors, we find that the 2\(\sigma \) upper bound on the branching ratio of \(\mathcal{B}(t \rightarrow c Z)\) is \(1.47 \times 10^{-5}\) for real coupling and \(1.91 \times 10^{-4}\) for complex coupling. The current experimental upper bound from ATLAS and CMS collaborations on the branching ratio of \(t \rightarrow c Z\) is \(2.4 \times 10^{-4}\) and \(4.9\times 10^{-4}\) at \(95\%\) C.L., respectively. Hence the possibility of observation of \(t \rightarrow c Z\) decay at the level of \(10^{-4}\) would imply the anomalous couplings to be complex. Such complex couplings should also show up its presence in other related decays. We find that an order of magnitude enhancement is possible in the branching ratio of \(K_L \rightarrow \pi ^0 \nu {\bar{\nu }}\). Further, the complex tcZ coupling can also provide large enhancements in many CP violating angular observables in \(B \rightarrow K^* \mu ^+ \mu ^-\) decay.

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Notes

  1. Apart from introducing new anomalies in \(b \rightarrow s\, \mu ^+ \,\mu ^-\) sector, the LHC reinforced the prevailing anomalies in the decays included by \(b \rightarrow c \tau \nu \) transition. A series of experiments performed by BaBar [1, 2], Belle [3,4,5,6] indicated an excess in the values of the ratios \(R_{D^{(*)}} = {\Gamma (B\rightarrow D^{(*)}\,\tau \,{\bar{\nu }})}/{ \Gamma (B\rightarrow D^{(*)}\, e/\mu \, {\bar{\nu }})}\) over their SM predictions. This was corroborated by the LHCb measurements [7, 8]. Refs. [9,10,11,12,13,14] identified new physics operators which can account for \(R_{D^{(*)}}\) measurements.

  2. This can also be attributed to underestimation of hadronic uncertainties within the SM. For example, in [31], it was shown that the experimental data can be accommodated within the SM itself if one assumes sizable non factorizable power corrections.

  3. In Ref. [68], all possible FCNC transitions of top quark was considered. It was shown that there could be a large number of gauge-invariant terms which can lead to different top quark FCNC processes. In this work, we concentrate only on the effective tcZ anomalous couplings and obtain model-independent bounds using constraints from the current flavor data. In the presence of other anomalous top quark FCNC couplings, the allowed tcZ parameter space, in general may be relaxed. Further, in a specific new physics model, there can be additional constraints which may shrink the allowed tcZ parameter space obtained in this work.

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Acknowledgements

We thank A. K. Alok, Dinesh Kumar, Jacky Kumar and Gauhar Abbas for useful discussions. We are thankful to the referee for his/her comments and suggestions regarding the manuscript.

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  1. Indian Institute of Technology Bombay, Mumbai, 400076, India

    Suman Kumbhakar

  2. Indian Institute of Technology Jodhpur, Jodhpur, 342037, India

    Jyoti Saini

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Kumbhakar, S., Saini, J. Flavor signatures of complex anomalous tcZ couplings. Eur. Phys. J. Plus 135, 330 (2020). https://doi.org/10.1140/epjp/s13360-020-00341-8

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