We explore the signatures of Majorana fermions in a nanowire based topological superconductor-quantum dot-topological superconductor hybrid device by charge transport measurements. The device is made from an epitaxially grown InSb nanowire with two superconductor Nb contacts on a Si/SiO$_2$ substrate. At low temperatures, a quantum dot is formed in the segment of the InSb nanowire between the two Nb contacts and the two Nb contacted segments of the InSb nanowire show superconductivity due to the proximity effect. At zero magnetic field, well defined Coulomb diamonds and the Kondo effect are observed in the charge stability diagram measurements in the Coulomb blockade regime of the quantum dot. Under the application of a finite, sufficiently strong magnetic field, a zero-bias conductance peak structure is observed in the same Coulomb blockade regime. It is found that the zero-bias conductance peak is present in many consecutive Coulomb diamonds, irrespective of the even-odd parity of the quasi-particle occupation number in the quantum dot. In addition, we find that the zero-bias conductance peak is in most cases accompanied by two differential conductance peaks, forming a triple-peak structure, and the separation between the two side peaks in bias voltage shows oscillations closely correlated to the background Coulomb conductance oscillations of the device. The observed zero-bias conductance peak and the associated triple-peak structure are in line with the signatures of Majorana fermion physics in a nanowire based topological superconductor-quantum dot-topological superconductor system, in which the two Majorana bound states adjacent to the quantum dot are hybridized into a pair of quasi-particle states with finite energies and the other two Majorana bound states remain as the zero-energy modes located at the two ends of the entire InSb nanowire.
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