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

A search for narrow, low-mass, scalar, and pseudoscalar resonances decaying to bottom quark-antiquark pairs is presented. The search is based on events recorded in $\sqrt{s}=13\mathrm{TeV}$ proton-proton collisions with the CMS detector at the LHC, collected in 2016, and corresponding to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The search selects events in which the resonance would be produced with high transverse momentum because of the presence of initial- or final-state radiation. In such events, the decay products of the resonance would be reconstructed as a single large-radius jet with high mass and two-prong substructure. A potential signal would be identified as a narrow excess in the jet invariant mass spectrum. No evidence for such a resonance is observed within the mass range from 50 to 350 GeV, and upper limits at 95% confidence level are set on the product of the cross section and branching fraction to a bottom quark-antiquark pair. These constitute the first constraints from the LHC on exotic bottom quark-antiquark resonances with masses below 325 GeV.

Figure 1

One-loop Feynman diagrams of processes exchanging a scalar $\mathrm{\Phi }$ (top) or pseudoscalar $A$ (bottom) mediator, leading to a boosted $\mathrm{double}\text{−}b$ jet signature.

Figure 2

The fitted pass-fail ratio $R_{\mathrm{p}/\mathrm{f}}$ as a function of $p_{\mathrm{T}}$ and $\rho$ for the AK8 selection (left) and the CA15 selection (right).

Figure 3

The observed and fitted background $m_{\mathrm{SD}}$ distributions for the AK8 selection for the failing (left) and passing (right) regions, combining all the $p_{\mathrm{T}}$ categories. The background fit is performed under the background-only hypothesis. A hypothetical $\mathrm{\Phi }(b\overline{b})$ signal at a mass of 140 GeV is also indicated. The features at 166, 180, 201, 215, and 250 GeV in the $m_{\mathrm{SD}}$ distribution are due to the $\rho$ boundaries, which define different $m_{\mathrm{SD}}$ ranges for each $p_{\mathrm{T}}$ category. The shaded blue band shows the systematic uncertainty in the total background prediction. The bottom panel shows the difference between the data and the nonresonant background prediction, divided by the statistical uncertainty in the data.

Figure 4

The observed and fitted background $m_{\mathrm{SD}}$ distributions for the CA15 selection for the failing (top) and passing (bottom) regions, combining all the $p_{\mathrm{T}}$ categories. The background fit is performed under the background-only hypothesis. A hypothetical $A(b\overline{b})$ signal at a mass of 260 GeV is also indicated. The features at 285, 313, 341, 376, and 432 GeV in the $m_{\mathrm{SD}}$ distribution are due to the $\rho$ boundaries, which define different $m_{\mathrm{SD}}$ ranges for each $p_{\mathrm{T}}$ category. The shaded blue band shows the systematic uncertainty in the total background prediction. The bottom panel shows the difference between the data and the nonresonant background prediction, divided by the statistical uncertainty in the data.

Figure 5

The observed and fitted background $m_{\mathrm{SD}}$ distributions in each $p_{\mathrm{T}}$ category for the AK8 selection in the passing regions. The fit is performed under the background-only hypothesis. A hypothetical $\mathrm{\Phi }(b\overline{b})$ signal at a mass of 140 GeV is also indicated. The shaded blue band shows the systematic uncertainty in the total background prediction. The bottom panel shows the difference between the data and the nonresonant background prediction, divided by the statistical uncertainty in the data.

Figure 6

The observed and fitted background $m_{\mathrm{SD}}$ distributions in each $p_{\mathrm{T}}$ category for the CA15 selection in the passing regions. The fit is performed under the background-only hypothesis. A hypothetical $A(b\overline{b})$ signal at a mass of 260 GeV is also indicated. The shaded blue band shows the systematic uncertainty in the total background prediction. The bottom panel shows the difference between the data and the nonresonant background prediction, divided by the statistical uncertainty in the data.

Figure 7

Upper limits at 95% CL on the product of the $\mathrm{\Phi }$ production cross section and the branching fraction to $b\overline{b}$ (top) and on $g_{q\mathrm{\Phi }}$ (bottom), as a function of the resonance mass $m_{\mathrm{\Phi }}$. The blue dash-dotted line indicates the theoretical scalar production cross section assuming $g_{q\mathrm{\Phi }}=1$ as a chosen benchmark [5]. The vertical line at 175 GeV corresponds to the transition between the AK8 and CA15 jet selections.

Figure 8

Upper limits at 95% CL on the product of the $A$ production cross section and the branching fraction to $b\overline{b}$ (top) and on $g_{qA}$ (bottom), as a function of the resonance mass $m_A$. The blue dash-dotted line indicates the theoretical pseudoscalar production cross section assuming $g_{qA}=1$ as a chosen benchmark [5]. The vertical line at 175 GeV corresponds to the transition between the AK8 and CA15 jet selections.