LHC Constraints on Scalar Diquarks

A number of years ago, low-energy constraints on scalar diquarks, particles that couple to two quarks, were examined. It was found that the two most weakly-constrained diquarks are $D^u$ and $D^d$, colour antitriplets that couple to $u_R^i{u}_R^j$ and $d_R^i{d}_R^j$, respectively. These diquarks have not been observed at the LHC. In this paper, we add the LHC measurements to the low-energy analysis, and find that the constraints are significantly improved. As an example, denoting $x^u$ as the $D^u$ coupling to the first and second generations, for $M_{D^u}=600$ GeV, the low-energy constraint is $\mathrm{\mid }{x}^u\mathrm{\mid }\le 14.4$, while the addition of the LHC dijet measurement leads to $\mathrm{\mid }{x}^u\mathrm{\mid }\le 0.13$–$0.15$. Further improvements are obtained by adding the measurement of single top production with a $p_T$ cut. These new constraints must be taken into account in making predictions for other low-energy indirect effects of diquarks.