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Searches for low-mass dimuon resonances

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Abstract

Searches are performed for low-mass dimuon resonances, $X$, 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 $X \to \mu^+\mu^-$ decays can be either prompt-like 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 prompt-like $X$ searches 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 two-Higgs-doublet and hidden-valley scenarios.

Figures and captions

Prompt-like dimuon mass spectra showing the (black) inclusive and (red) $X+b$ samples. 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 samples. 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.

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Signed local significances in the $ m( X ) < 20\text{ Ge V} $ region for the (top) inclusive and (bottom) associated beauty prompt-like $ X \rightarrow \mu^+\mu^-$ searches. 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 prompt-like $ X \rightarrow \mu^+\mu^-$ searches. 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 prompt-like $ X \rightarrow \mu^+\mu^-$ searches.

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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 prompt-like $ X \rightarrow \mu^+\mu^-$ searches.

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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.

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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-like 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 08 August 2020.