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CMS-EXO-17-027 ; CERN-EP-2019-068
Search for low-mass quark-antiquark resonances produced in association with a photon at $\sqrt s = $ 13 TeV
Phys. Rev. Lett. 123 (2019) 231803
Abstract: A search for narrow low-mass resonances decaying to quark-antiquark pairs is presented. The search is based on proton-proton collisions events collected at 13 TeV by the CMS detector at the CERN LHC. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$, recorded in 2016. The search considers the case where the resonance has high transverse momentum due to initial-state radiation of a photon. To study this process, the decay products of the resonance are reconstructed as a single large-radius jet with two-pronged substructure. The signal is identified as a localized excess in the jet invariant mass spectrum. No evidence for such a resonance is observed in the mass range 10 to 125 GeV. Upper limits at the 95% confidence level are set on the coupling strength of vector resonances decaying to quark pairs. The results obtained with this novel technique provide the first direct constraints on quark-antiquark resonance masses below 50 GeV obtained at a hadron collider.
Figures & Tables Summary References CMS Publications

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Figure 1:
The soft drop [39,40] jet mass distribution of the signal region after the main background estimation fit is performed. The nonresonant background is indicated by a dashed line, while the total background composed of the sum of this nonresonant background and the resonant backgrounds is shown by the solid line. Representative signals are plotted for comparison. The bottom panel shows the difference between the data and the final background estimate, divided by the statistical uncertainty of the data in each bin. The shaded region represents the total uncertainty in the background estimate in each bin.

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Figure 2:
Upper limits at 95% CL on the coupling strength $g^\prime _ {\mathrm {q}}$ of $ \mathrm{Z}' \to {{\mathrm {q}} {\overline {\mathrm {q}}}} $. The observed limit is shown as a solid black line, while the expected limit is dashed. The green (dark) and yellow (light) bands represent 1 and 2 standard deviation intervals. Limits from other searches and the indirect constraint from measurements of the ${\Upsilon}$ and Z boson decay widths [18] are also shown.

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Table 1:
The systematic uncertainties included in the computation of the limit on the coupling strength of Z' to quarks. Parameters denoted by the $\star $ symbol affect both the shape and normalization of the affected processes; otherwise only the normalization is modified. The parameters affecting normalizations have log normal priors, and those affecting the shape have Gaussian priors, unless marked with the $\dagger $ symbol, which denotes that this parameter was floating and constrained by the final simultaneous fit of the passing and failing distributions.
In summary, a search for a low mass Z' resonance decaying to $\mathrm{q\overline{q}}$ pairs has been presented, using data from proton-proton collisions at the LHC with a center-of-mass energy of 13 TeV. Jet substructure and decorrelation techniques are implemented to search for narrow resonances over a smoothly falling background of the jet groomed mass. No significant excess is observed above the standard model expectation. Upper limits are placed on the quark coupling strength $g'_{\mathrm{q}}$ of Z' bosons with masses between 10 and 125 GeV. Below 50 GeV, the results obtained with this novel technique probe the lowest diquark resonance masses reached by a hadron collider.
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