These particles could arise, for instance, from $SU(5)$ GUT [@Becirevic:2018afm] or composite Higgs models [@Marzocca:2018wcf].

   Field   Spin             Quantum Numbers              $g_{V_L}$      $g_{S_R}$      $g_{S_L}$        $g_T$           $R_{D^{(\ast)}}$
  ------- ------ ------------------------------------- -------------- -------------- -------------- -------------- --------------------------
   $R_2$   $0$       $(\mathbf{3},\mathbf{2},7/6)$           –              –         $\checkmark$   $\checkmark$   $\color{blue}\checkmark$
   $S_1$   $0$    $(\mathbf{\bar{3}},\mathbf{1},1/3)$   $\checkmark$        –         $\checkmark$   $\checkmark$   $\color{blue}\checkmark$
   $S_3$   $0$       $(\mathbf{3},\mathbf{2},7/6)$      $\checkmark$        –              –              –        
   $V_2$   $1$    $(\mathbf{\bar{3}},\mathbf{2},5/6)$        –         $\checkmark$        –              –        
   $U_1$   $1$       $(\mathbf{3},\mathbf{1},2/3)$      $\checkmark$   $\checkmark$        –              –         $\color{blue}\checkmark$
   $U_3$   $1$       $(\mathbf{3},\mathbf{3},2/3)$      $\checkmark$        –              –              –        

  :  *List of LQ states contributing to the transition $b\to c\ell \bar{\nu}$ in terms of the SM quantum numbers, $(SU(3)_c,SU(2)_L,Y)$, and corresponding Wilson coefficients generated in Eq. . The last column summarizes the models which can accommodate $R_{D^{(\ast)}}$ while complying with other existing constraints [@lqpaper]. See text for details.*[]{data-label="tab:states"}

Phenomenological implications {#sec:pheno}
-----------------------------

We conclude this Section by discussing the phenomenological implications of $R_{D^{(\ast)}}$ and the strategies to disentangle the viable NP explanations, if the anomalies persist. A first possibility is to study LFUV ratios of other decay modes based on the transition $b\to c \ell\bar{\nu}$, such as $B_s\to D_s^{(\ast)} \tau \bar{\nu}$ and $B_c\to J/\psi \tau \bar{\nu}$. In particular, LHCb performed a first measurement of the ratio [@Aaij:2017tyk] $$R_{J/\psi}=\dfrac{\mathcal{B}(B_c\to J/\psi \tau \bar{\nu})}{\mathcal{B}(B_c\to J/\psi 