from the $B_c$–meson lifetime, which allow us to exclude the solutions with large values of $|g_P|$, as explained in the text [@Feruglio:2018fxo].*[]{data-label="fig:RD-RDst-1TeV"}](gSL-gSR-mb.pdf "fig:"){width="0.5\linewidth"} ![*Allowed regions by $R_D$ and $R_{D^\ast}$ in the planes $g_{S}^\tau$ vs. $g_P^\tau$ (left panel), and $g_{S_L}^\tau$ vs. $g_T^\tau$ (right panel) are shown to $1$, $2$ and $3\,\sigma$ accuracy in blue (darker to lighter). The black lines show the constraints from the $B_c$–meson lifetime, which allow us to exclude the solutions with large values of $|g_P|$, as explained in the text [@Feruglio:2018fxo].*[]{data-label="fig:RD-RDst-1TeV"}](gSL-gT-mb.pdf "fig:"){width="0.5\linewidth"}

Concrete New Physics scenarios {#sec:concrete}
------------------------------

Several concrete models of NP can generate the viable Wilson coefficients discussed above. These scenarios require new bosonic degrees of freedom at the TeV scale [@DiLuzio:2017chi] with tree-level contributions to the transition $b\to c\tau \bar{\nu}$. The main challenge is to comply with nontrivial constraints coming flavor physics observables, electroweak precision tests [@Feruglio:2016gvd; @Feruglio:2017rjo; @Cornella:2018tfd] and the direct searches at the LHC [@Faroughy:2016osc]. The proposed mediators can be (i) a charged Higgs, (ii) colorless vector bosons ($W^\prime$), and (iii) scalar or vector leptoquarks (LQ). Scenarios with charged Higgs boson cannot accommodate $R_{D^{(\ast)}}$ due to the $B_c$-lifetime constraint discussed above, cf. Fig. \[fig:RD-RDst-1TeV\]. On the other hand, $W^\prime$ models have difficulties 