Search for low mass vector resonances decaying into quark-antiquark pairs in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

A search for low mass narrow vector resonances decaying into quark-antiquark pairs is presented. The analysis is based on data collected in 2017 with the CMS detector at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $41.1{\mathrm{fb}}^{-1}$. The results of this analysis are combined with those of an earlier analysis based on data collected at the same collision energy in 2016, corresponding to $35.9{\mathrm{fb}}^{-1}$. Signal candidates will be recoiling against initial state radiation and are identified as energetic, large-radius jets with two pronged substructure. The invariant jet mass spectrum is probed for a potential narrow peaking signal over a smoothly falling background. No evidence for such resonances is observed within the mass range of 50–450 GeV. Upper limits at the 95% confidence level are set on the coupling of narrow resonances to quarks, as a function of the resonance mass. For masses between 50 and 300 GeV these are the most sensitive limits to date. This analysis extends the earlier search to a mass range of 300–450 GeV, which is probed for the first time with jet substructure techniques.

Figure 1

High-level trigger efficiency as a function of the soft-drop jet mass ($m_{\mathrm{SD}}$) for AK8 jets with $p_{\mathrm{T}}>525\mathrm{GeV}$ (blue squares) and CA15 jets with $p_{\mathrm{T}}>575\mathrm{GeV}$ (red circles). The trigger selection is $>95\%$ efficient for 2017 data for both cone sizes and is applied to AK8 jets with masses between 50 and 275 GeV and CA15 jets with masses between 150 and 450 GeV. For jet masses above 200 GeV, the trigger efficiency for the larger CA15 jet decreases slightly. This is due to events for which a reconstructed jet passing the CA15 jet selection does not satisfy the AK8 jet selection at the trigger level.

Figure 2

Jet $m_{\mathrm{SD}}$ distribution for AK8 jets for each $p_{\mathrm{T}}$ category of the fit. Data are shown by the black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the $W$ and $Z$ bosons, and top quark background processes are shown as well. A hypothetical $Z^{\prime }$ boson signal with a mass of 110 GeV is also indicated. In the bottom panel, the ratio of the data to its statistical uncertainty, after subtracting the nonresonant backgrounds, is shown.

Figure 3

Jet $m_{\mathrm{SD}}$ distribution for CA15 jets for the different $p_{\mathrm{T}}$ ranges of the fit from 575 to 1500 GeV. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Smaller contributions from the $W$ and $Z$ bosons, and top quark background processes are shown as well. A hypothetical $Z^{\prime }$ boson signal with a mass of 210 GeV is also indicated. In the bottom panel, the ratio of the data to its statistical uncertainty, after subtracting the nonresonant backgrounds, is shown.

Figure 4

Upper limits at 95% CL on the coupling $g_{\mathrm{q}}^{\prime }$ as a function of the resonance mass for a leptophobic $Z^{\prime }$ boson that couples only to quarks, based on the 2017 analysis. The observed limits (solid), expected limits (dashed), and their variation at the 1 and 2 standard deviation levels (shaded bands) are shown. The vertical line at 175 GeV corresponds to the transition between the AK8 and CA15 jet selections.

Figure 5

Upper limits at 95% CL on the coupling $g_{\mathrm{q}}^{\prime }$ as a function of the resonance mass for a leptophobic $Z^{\prime }$ boson that couples only to quarks. The observed limits (solid), expected limits (dashed), and their variation at the 1 and 2 standard deviation levels (shaded bands) are shown. For masses between 50 and 220 GeV the limits correspond to a $Z^{\prime }$ boson reconstructed in AK8 jets using $77.0{\mathrm{fb}}^{-1}$ of statistically combined data from 2016 and 2017. For masses above 220 up to 450 GeV, the results correspond to a $Z^{\prime }$ resonance reconstructed in CA15 jets using $41.1{\mathrm{fb}}^{-1}$ of data collected in 2017.

Figure 6

Distributions of $X_{(5\%)}$ used to define the $N_2^{1,\mathrm{DDT}}$ variable for AK8 jets (right) and CA15 jets (left), corresponding to the 5% quantile of the $N_2^1$ distribution in simulated multijet events. The distributions are shown as a function of the jet $\rho$ and $p_{\mathrm{T}}$. The $N_2^1$ variable is mostly insensitive to the jet $\rho$ and $p_{\mathrm{T}}$ in the kinematic phase space considered for this analysis: $-5.5<\rho <-2.0$ (AK8 jets) and $-4.7<\rho <-1.0$ (CA15 jets). The distributions of $X_{(5\%)}$ are used to take into account residual correlations in simulation by applying a decorrelation procedure that yields the $N_2^{1,\mathrm{DDT}}$ variable. In order to ensure smoothness of the transformation, we simulate particle-level QCD multijet events and smear them using a parametric detector response derived for the $N_2^1$ variable as a function of $\rho$ and $p_{\mathrm{T}}$. This method overcomes the limitation from the limited event count in simulated samples by generating $10^4$ the original number of events available in the multijet simulation.

Figure 7

Pass-to-fail ratio, $R_{\mathrm{p}/\mathrm{f}}(\rho (m_{\mathrm{SD}},p_{\mathrm{T}}))$, defined from the events passing and failing the $N_2^{1,\mathrm{DDT}}$ selection. The variable $N_2^{1,\mathrm{DDT}}$ is constructed so that, for simulated multijet events, $R_{\mathrm{p}/\mathrm{f}}$ is constant at $\mathrm{p}=5\%$ and $\mathrm{f}=95\%$ (blue). To account for residual differences between data and simulation, $R_{\mathrm{p}/\mathrm{f}}$ is extracted by performing a two-dimensional fit to data in ($\rho ,p_{\mathrm{T}}$) space (orange). The $R_{\mathrm{p}/\mathrm{f}}$ shown is derived for AK8 jets using $41.1{\mathrm{fb}}^{-1}$ of data collected in 2017 and corresponds to a polynomial in the Bernstein basis of third order in $p_{\mathrm{T}}$ and fifth order in $\rho$.

Figure 8

Upper limits at 95% CL on the coupling $g_{\mathrm{q}}^{\prime }$ as a function of the resonance mass for a leptophobic $Z^{\prime }$ boson that couples only to quarks. Based on the statistical combination of the 2016 and 2017 analyses using AK8 jets. The observed limits (solid), expected limits (dashed), and their variation at the 1 and 2 standard deviation levels (shaded bands) are shown.