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Searches for low-mass dimuon resonances

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Abstract

Searches are performed for a low-mass dimuon resonance, $X$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using a data sample corresponding to an integrated luminosity of 5.1 fb$^{-1}$ and collected with the LHCb detector. The $X$ bosons can either decay promptly or displaced from the proton-proton collision, where in both cases the requirements placed on the event and the assumptions made about the production mechanisms are kept as minimal as possible. The searches for promptly decaying $X$ bosons explore the mass range from near the dimuon threshold up to 60 GeV, with nonnegligible $X$ widths considered above 20 GeV. The searches for displaced $X \to \mu^+\mu^-$ decays consider masses up to 3 GeV. None of the searches finds evidence for a signal and 90 confidence-level exclusion limits are placed on the $X \to \mu^+\mu^-$ cross sections, each with minimal model dependence. In addition, these results are used to place world-leading constraints on GeV-scale bosons in the two-Higgs-doublet and hidden-valley scenarios.

Figures and captions

Prompt dimuon mass spectra showing the (black) inclusive and (red) $X+b$ candidates with all fiducial and selection requirements applied. The grey boxes show the regions vetoed due to large contributions from QCD resonances.

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Displaced dimuon mass spectra showing the (black) inclusive and (red) promptly produced candidates with all fiducial and selection requirements applied. The grey box shows the region vetoed due to the large doubly misidentified $ K ^0_{\mathrm{S}}$ background, whose low-mass tail extends into the search region. A dedicated study of this region is presented in a search for $ K ^0_{\mathrm{S}} \rightarrow \mu^+\mu^-$ decays [44].

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Signed local significances in the $ m( X ) < 20\text{ Ge V} $ region for the (top) inclusive and (bottom) associated beauty searches for prompt $ X \rightarrow \mu^+\mu^-$ decays. If the best-fit signal-yield estimator is negative, the signed significance is negative and { vice versa}. The grey regions are excluded either due to a nearby large QCD resonance contribution, or because the overlap of the bin with the fiducial region in Table 1 is small.

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Signed local significances in the $ m( X ) > 20\text{ Ge V} $ region for the (top) inclusive and (bottom) associated beauty searches for prompt $ X \rightarrow \mu^+\mu^-$ decays. The lower limit on the vertical axis of $\log_{10}[\Gamma( X ) / \text{ Me V} ] = -\infty$ corresponds to $\Gamma( X ) = 0$.

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Fit to the $ m(\mu^+\mu^-)$ spectrum in events with at least two $b$-tagged jets. The ${27 < m( X ) < 30\text{ Ge V} }$ search region is marked by the vertical dashed lines.

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Signed local significances for the (top) promptly produced and (bottom) inclusive searches for displaced $ X \rightarrow \mu^+\mu^-$ decays. The black points show the individual candidates.

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Upper limits at 90% confidence level on the cross section $\sigma( X \rightarrow \mu^+\mu^- )$ in the $ m( X ) < 20\text{ Ge V} $ region for the (top) inclusive and (bottom) associated beauty searches for prompt $ X \rightarrow \mu^+\mu^-$ decays.

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Upper limits at 90% confidence level on the cross section $\sigma( X \rightarrow \mu^+\mu^- )$ in the $ m( X ) > 20\text{ Ge V} $ region for the (top) inclusive and (bottom) associated beauty searches for prompt $ X \rightarrow \mu^+\mu^-$ decays.

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Upper limits at 90% confidence level on the cross section $\sigma( X \rightarrow \mu^+\mu^- )$ for the (top) promptly produced and (bottom) inclusive searches for displaced $ X \rightarrow \mu^+\mu^-$ decays.

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Upper limits at 90% confidence level on the $X$--$H$ mixing angle, $\theta_H$, for the 2HDM scenario discussed in the text (blue) from this analysis compared with existing limits from (red) {\sc BaBar} [58], (green) CMS Run 1 [59], (magenta) CMS Run 2 [60] and (yellow) LHCb Run 1 [61].

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Upper limits at 90% confidence level on the $\gamma$--$Z_{\rm HV}$ kinetic mixing strength for the HV scenario discussed in the text. Here, $X$ denotes a composite HV vector boson.

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Animated gif made out of all figures.

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Tables and captions

Fiducial regions of the searches for prompt and displaced $ X \rightarrow \mu^+\mu^-$ decays.

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Summary of systematic uncertainties. The luminosity and efficiency uncertainties are highly correlated, which is accounted for when obtaining the total uncertainties.

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Created on 20 February 2021.