Search for Low Mass Vector Resonances Decaying to Quark-Antiquark Pairs in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

A search is reported for a narrow vector resonance decaying to quark-antiquark pairs in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$, collected with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of $2.7{\mathrm{fb}}^{-1}$. The vector resonance is produced at large transverse momenta, with its decay products merged into a single jet. The resulting signature is a peak over background in the distribution of the invariant mass of the jet. The results are interpreted in the framework of a leptophobic vector resonance and no evidence is found for such particles in the mass range of 100–300 GeV. Upper limits at 95% confidence level on the production cross section are presented in a region of mass-coupling phase space previously unexplored at the LHC. The region below 140 GeV has not been explored by any previous experiments.

Figure 1

The average of ${\tau }_{21}^{\mathrm{DDT}}$ versus ${\rho }^{\mathrm{DDT}}$ for different bins in jet $p_T$ (upper). A schematic of the method used to estimate the background (lower). After a ${\tau }_{21}^{\mathrm{DDT}}$ selection is applied, the multijet event distributions in the passing region are predicted from their distributions in the failing region with the use of a pass-to-fail ratio extracted through a two-dimensional fit in $({\rho }^{\mathrm{DDT}},p_T)$ space. Events in a sliding window around the desired $Z^{\prime }$ mass point are vetoed when determining the pass-to-fail ratio.

Figure 2

Soft-drop jet mass distribution after final selection. Data are shown as black points. The multijet background, including uncertainties, is shown as gray boxes, while the sum of the SM processes is given by the blue line. Contributions from the SM $W/Z$ boson and a hypothetical $Z^{\prime }$ signal at a mass of 135 GeV are also indicated. In the bottom panel, we show the ratio of the data to the background prediction, including uncertainties.

Figure 3

Upper limits on $g_q$ as a function of the $Z^{\prime }$ mass at 95% C.L. [78]. Limits from other relevant searches are shown. An indirect constraint from the width of the SM $Z$ boson is also given.