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Search for beautiful tetraquarks in the $\Upsilon(1S)\mu^+\mu^-$ invariant-mass spectrum

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Abstract

The $\Upsilon(1S)\mu^+\mu^-$ invariant-mass distribution is investigated for a possible exotic meson state composed of two $b$ quarks and two $\overline{b}$ quarks, $X_{b\overline{b}b\overline{b}}$. The analysis is based on a data sample of $pp$ collisions recorded with the LHCb detector at centre-of-mass energies $\sqrt{s} =$ 7, 8 and 13 TeV, corresponding to an integrated luminosity of 6.3 fb$^{-1}$. No significant excess is found, and upper limits are set on the product of the production cross-section and the branching fraction as functions of the mass of the $X_{b\overline{b}b\overline{b}}$ state. The limits are set in the fiducial volume where all muons have pseudorapidity in the range $[2.0,5.0]$, and the $X_{b\overline{b}b\overline{b}}$ state has rapidity in the range $[2.0,4.5]$ and transverse momentum less than 15 GeV/$c$.

Figures and captions

Linear fit to the ratio of the $X$ and ${\Upsilon{(1S)} }$ widths as a function of the $X$ mass as determined from fits to simulated data samples. The error bars represent the statistical uncertainty arising from the finite size of the simulated samples.

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Distributions of $m(\mu ^+\mu ^- )$ for the normalisation datasets at $ p p $ centre-of-mass energies of (a) 7 $\mathrm{\,TeV}$ , (b) 8 $\mathrm{\,TeV}$ , (c) 13 $\mathrm{\,TeV}$ and (d) all combined. The total fit function (solid blue line), the combinatorial background (dashed red line) and the ${\Upsilon{(1S)} }$, ${\Upsilon{(2S)} }$ and ${\Upsilon{(3S)} }$ components (hatched magenta area) are shown overlaid.

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Distributions of $m(2\mu^+2\mu^-)$ for the signal datasets at $ p p $ centre-of-mass energies of (a) 7 $\mathrm{\,TeV}$ , (b) 8 $\mathrm{\,TeV}$ , (c) 13 $\mathrm{\,TeV}$ and (d) all combined, using a bin size comparable to the expected $X$ mass resolution. In each case the region around the corresponding ${\Upsilon{(1S)} }$ peak has been selected. The background-only fit function (solid blue line) is shown overlaid. The dotted black lines indicate the range in which limits are set on the product of the $X$ production cross-section and branching fractions. The dash-dotted red and long-dashed green lines show the positions of the $\eta_b\eta_b$ and ${\Upsilon{(1S)} }{\Upsilon{(1S)} }$ thresholds, respectively.

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The 95 % CL upper limits on $S\equiv \sigma( p p \rightarrow X)\times\mathcal{B}(X\rightarrow {\Upsilon{(1S)} }\mu ^+\mu ^- ) \times \mathcal{B}({\Upsilon{(1S)} }\rightarrow \mu^+\mu^-)$ as functions of the $X$ mass hypothesis at $ p p $ centre-of-mass energies of (a) 7 $\mathrm{\,TeV}$ , (b) 8 $\mathrm{\,TeV}$ and (c) 13 $\mathrm{\,TeV}$ and (d) all combined.

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The kinematic distribution of (black) simulated $X$ particles in (a) $ p_{\mathrm{ T}} $ and (b) rapidity, and (c) the 2D distribution. For comparison, the kinematic distribution of (red) simulated ${\Upsilon{(4S)} }$ particles is also shown.

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Animated gif made out of all figures.

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Tables and captions

Upper limits on $\sigma( p p \rightarrow X) \times \mathcal{B}(X\rightarrow {\Upsilon{(1S)} }\mu ^+\mu ^- ) \times \mathcal{B}({\Upsilon{(1S)} }\rightarrow \mu ^+\mu ^- )$ for different $X$ mass hypotheses in the range $[17.5,18.4] {\mathrm{\,GeV\!/}c^2} $.

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Upper limits on $\sigma( p p \rightarrow X) \times \mathcal{B}(X\rightarrow {\Upsilon{(1S)} }\mu ^+\mu ^- ) \times \mathcal{B}({\Upsilon{(1S)} }\rightarrow \mu ^+\mu ^- )$ for different $X$ mass hypotheses in the range $[18.4,19.3] {\mathrm{\,GeV\!/}c^2} $.

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Upper limits on $\sigma( p p \rightarrow X) \times \mathcal{B}(X\rightarrow {\Upsilon{(1S)} }\mu ^+\mu ^- ) \times \mathcal{B}({\Upsilon{(1S)} }\rightarrow \mu ^+\mu ^- )$ for different $X$ mass hypotheses in the range $[19.3,20.0] {\mathrm{\,GeV\!/}c^2} $.

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.