Search for narrow resonances in the $b$-tagged dijet mass spectrum in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

A search is performed for narrow resonances decaying to final states of two jets, with at least one jet originating from a $b$ quark, in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$. The data set corresponds to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ collected with the CMS detector at the LHC. Jets originating from energetic $b$ hadrons are identified through a $b$-tagging algorithm that utilizes a deep neural network or the presence of a muon inside a jet. The invariant mass spectrum of jet pairs is well described by a smooth parametrization and no evidence for the production of new particles is observed. Upper limits on the production cross section are set for excited $b$ quarks and other resonances decaying to dijet final states containing $b$ quarks. These limits exclude at 95% confidence level models of $Z^{\prime }$ bosons with masses from 1.8 TeV to 2.4 TeV and of excited $b$ quarks with masses from 1.8 TeV to 4.0 TeV. This is the most stringent exclusion of excited $b$ quarks to date.

Figure 1

Simulated signal shapes of $b^{*}$ from the process $bg\to b^{*}\to bg$. Shown are the wide jets (see text) used to reconstruct the dijet mass spectra.

Figure 2

The product of acceptance and efficiency of the event selection for a $Z^{\prime }\to b\overline{b}$ resonance as a function of the resonance mass.

Figure 3

The observed differential cross sections as a function of the dijet mass, shown as fit with the background functions, for the four tagging categories (rows) and the three data-taking periods (columns). The number of parameters in the fit, and the goodness of fit “${\chi }^2/\mathrm{ndf}$”, are listed where “ndf” is the number of degrees of freedom. The lower panel within each row shows the pulls, (data-fit)/uncertainty, in units of the statistical uncertainty in data. The upper three rows are used to search for $Z^{\prime }$ models, the bottom row is used to search for the $b^{*}$ model, and example shapes of these signal models are shown with the same arbitrary normalization for three choices of resonance mass.

Figure 4

The observed 95% C.L. upper limits (solid curve) on the product of the cross section and branching fraction (left), and multiplied by signal acceptance (right), for a resonance decaying to $b\overline{b}$. The corresponding expected limits (dashed curve) and their variations at the 1- and 2-standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for $Z^{\prime }$ bosons from the sequential SM (SSM) and the heavy vector triplet (HVT) models A and B. The latter two models follow the parameter choices of $g_{\mathrm{V}}=1$ and $g_{\mathrm{V}}=3$, respectively.

Figure 5

The coupling strengths to SM bosons ($g_{\mathrm{H}}$) and fermions ($g_{\mathrm{F}}$) of a $Z^{\prime }$ boson with mass 2.0 TeV (blue) and 2.5 TeV (red) that are excluded at 95% C.L. for the HVT model. The shading indicates the excluded side of the contour. The benchmark scenarios corresponding to HVT models A and B are represented by a purple cross and a red point, respectively. The gray shaded area corresponds to the region where the resonance natural width is predicted to be larger than the typical experimental resolution, and thus the narrow-width approximation is not fulfilled.

Figure 6

The observed 95% C.L. upper limits on the product of the cross section, branching fraction, and acceptance for dijet resonances decaying to a $b$ quark and a gluon (points). The corresponding expected limits (short dashed) and their variations at the 1- and 2-standard deviation levels (shaded bands) are also shown. Limits are compared to predictions for single $b^{*}$ production (blue, dot dashed), the resonant component of the $b^{*}$ production via contact interactions (magenta, long dashed), and the total $b^{*}$ signal from the sum of these two production modes (red, solid).