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

Abstract

A search for resonances with masses from 50 to 300 GeV decaying into a quark-antiquark pair is presented. The search uses proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS experiment at the CERN LHC in 2016$-$2018, corresponding to an integrated luminosity of 13 fb$^{-1}$. Two coupling scenarios are considered, with the resonances coupled either equally to all flavors of quarks or preferentially to bottom quarks. The search targets resonances produced in association with hard initial-state radiation, resulting in a large-radius jet with a two-pronged substructure. The ParticleNet algorithm is used to distinguish resonance decays to bottom quark pairs from lighter quark pairs and to suppress background processes. The invariant jet mass spectrum is scrutinized for peaking excesses over a falling background. No evidence for such resonances is observed. Limits are set on the couplings of new scalar and vector resonances to quarks, representing the most stringent limits to date in the mass range of 50$-$250 GeV.

Figures (5)

• Figure 1: The jet $m_{\mathrm{SD}}$ distributions in the low-$p_{{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace}$ (upper) and high-$p_{{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace}$ (lower) SRs, fitted with the background-only hypothesis. The distributions are summed across all five $p_{\mathrm{T}}$ bins and the four data-taking periods. The steplike features are due to the $\rho$ requirement, which results in a different $m_{\mathrm{SD}}$ range for each $p_{\mathrm{T}}$ bin. The hatched area shows the expected $\mathup{{{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace}}{} _{ {}} ^{ {\prime}}}$ signal yields for $g_{{ \mathup{{{q}}{} _{ {}} ^{ {}}} }\xspace}\xspace=0.25$. The lower panel shows the residual difference between the data and the overall background, divided by the statistical uncertainty of the data; the gray band represents the systematic uncertainty in the background estimation.
• Figure 2: Upper limits at 95% $\text{CL}$ on the universal coupling $g_{{ \mathup{{{q}}{} _{ {}} ^{ {}}} }\xspace}$ between the $\mathup{{{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace}}{} _{ {}} ^{ {\prime}}}$ boson and quarks (upper) and the coupling parameters $g_{{ \mathup{{{q}}{} _{ {}} ^{ {}}} }\xspace\phi}$ (left vertical axis) and $g_{{ \mathup{{{q}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{A}}{} _{ {}} ^{ {}}} }\xspace}$ (right vertical axis) of the spin-0 model (lower). The $\mathup{{{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace}}{} _{ {}} ^{ {\prime}}}$ boson is assumed to decay to quark-antiquark pairs with a 100% branching fraction. The $\phi$ and $\mathup{{{A}}{} _{ {}} ^{ {}}}$ bosons couple to quarks with a coupling given by $g_{{ \mathup{{{q}}{} _{ {}} ^{ {}}} }\xspace\phi}$ or $g_{{ \mathup{{{q}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{A}}{} _{ {}} ^{ {}}} }\xspace}$ times the SM Yukawa couplings, respectively, and decay dominantly to bottom quark-antiquark pairs. The solid line represents the observed limits. The dashed line and the inner (yellow) and outer (blue) bands represent the median and the regions containing 68% and 95%, respectively, of the distribution of expected limits under the background-only hypothesis.
• Figure 3: Recast of the 95% $\text{CL}$ exclusion limits into the kinetic mixing parameter, $\epsilon^2$, of a dark-photon model (${ \mathup{{{A}}{} _{ {}} ^{ {}}} }\xspace^{\prime}$, upper) and the effective gluon coupling, $c_{{ \mathup{{{g}}{} _{ {}} ^{ {}}} }\xspace}/\Lambda$, of an axion-like particle model (ALP, lower). The solid (dashed) red lines represent the observed (expected) upper limits from this analysis. The orange and blue (upper), and magenta and gray (lower) lines represent the minimum allowed coupling compatible with the observed DM relic density for representative values of $m_{{ \mathup{{{DM}}{} _{ {}} ^{ {}}} }\xspace}\xspace/m_{{ \mathup{{{M}}{} _{ {}} ^{ {}}} }\xspace}\xspace$. The hatched lines show the direction of the excluded regions.
• Figure 4: Efficiencies of the selection criteria in the low-$p_{{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace}$ (upper) and high-$p_{{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace}$ (lower) signal regions, derived from simulated $\mathup{{{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace}}{} _{ {}} ^{ {\prime}}}$ events where the $\mathup{{{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace}}{} _{ {}} ^{ {\prime}}}$ satisfies $p_{\mathrm{T}}\xspace>500\,\text{Ge\spaceV}\xspace$ and $\lvert \eta \rvert<2.5$. The efficiencies are shown separately for different quark flavors (denoted "total"). The efficiencies for all selection criteria, except for the ParticleNet requirements (denoted "kinematic only"), are also shown to illustrate the performance of the jet tagging algorithm.
• Figure 5: Upper limits at 95% $\text{CL}$ on the visible cross section, $\sigma_{\text{vis}}$, defined as the number of signal events divided by the integrated luminosity, for the low-$p_{{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace}$ (upper) and high-$p_{{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace}$ (lower) signal regions. The solid line represents the observed limits, and the dashed line and the inner (green) and outer (yellow) bands represent the median and the regions containing 68% and 95%, respectively, of the distribution of expected limits under the background-only hypothesis.