Search for heavy resonances in the all-hadronic vector-boson pair final state with a multi-dimensional fit

We present a novel fit method in the search for new resonances decaying to WW, WZ, or ZZ boson pairs in the all-hadronic final state using data corresponding to an integrated luminosity of $77.3~\mathrm{fb}^{-1}$ taken with the CMS experiment at the LHC at a centre-of-mass energy of $\sqrt{s}=13~\mathrm{TeV}$. The search is focussed on resonances with masses above $1.2~\mathrm{TeV}$, where the decay products of each W or Z boson are expected to be collimated into one single large-radius jet. The signal extraction method is based on a three-dimensional maximum likelihood fit of the dijet invariant mass and the two jet masses, which allows systematic uncertainties that affect all three dimensions to be incorporated simultaneously. The new method yields an improvement in sensitivity of up to $30\%$ with respect to previous search methods used in CMS. No excess is observed above the estimated standard model background. In a heavy vector triplet model, spin-1 $\mathrm{W}^\prime$ and $\mathrm{Z}^\prime$ resonances with masses below $3.8$ and $3.5~\mathrm{TeV}$, respectively, are excluded at $95\%$ confidence level. In a narrow-width bulk graviton model, upper limits on cross sections are set between $27$ and $0.2~\mathrm{fb}$ for resonance masses between $1.2$ and $5.2~\mathrm{TeV}$, respectively.