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Determination of the $X(3872)$ meson quantum numbers

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Abstract

The quantum numbers of the X(3872) meson are determined to be JPC = 1++ based on angular correlations in B+ to X(3872) K+ decays, where X(3872) to pi+pi- J/psi and J/psi to \mu+\mu-. The data correspond to 1.0 fb-1 of pp collisions collected by the LHCb detector. The only alternative assignment allowed by previous measurements, JPC=2-+, is rejected with a confidence level equivalent to more than eight Gaussian standard deviations using the likelihood-ratio test in the full angular phase space. This result favors exotic explanations of the X(3872) state.

Figures and captions

Distribution of $\Delta M$ for $B^+\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^+\pi^+\pi^-$ candidates. The fits of the $\psi(2S)$ and $X(3872)$ signals are displayed. The solid blue, dashed red, and dotted green lines represent the total fit, signal component, and background component, respectively.

jppj.pdf [30 KiB]
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jppj.pdf

Distribution of the test statistic $t$ for the simulated experiments with $J^{PC}=2^{-+}$ and $\alpha=\hat\alpha$ (black circles on the left) and with $J^{PC}=1^{++}$ (red triangles on the right). A Gaussian fit to the $2^{-+}$ distribution is overlaid (blue solid line). The value of the test statistic for the data, $t_{\rm data}$, is shown by the solid vertical line.

dchi2L_log.pdf [26 KiB]
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dchi2L_log.pdf

Distribution of $-\ln[{\cal P(\Omega_i|2^{-+},\hat\alpha)}/{\cal P(\Omega_i|1^{++})}]$ for the data (points with error bars) compared to the distributions for the simulated experiments with $J^{PC}=1^{++}$ (red solid histogram) and with $J^{PC}=2^{-+}$, $\alpha=\hat\alpha$ (blue dashed histogram) after the background subtraction using sWeights. The simulated distributions are normalized to the number of signal candidates observed in the data. Bin contents and uncertainties are divided by bin width because of unequal bin sizes.

llr.pdf [15 KiB]
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llr.pdf

Background-subtracted distribution of $\cos\theta_X$ for (top) all candidates and for (bottom) candidates with $|\cos\theta_{\pi\pi}|>0.6$ for the data (points with error bars) compared to the expected distributions for the $J^{PC}=1^{++}$ (red solid histogram) and $J^{PC}=2^{-+}$ and $\alpha=\hat\alpha$ hypotheses (blue dashed histogram). The simulated distributions are normalized to the number of signal candidates observed in the data across the full phase space.

cosxco[..].pdf [15 KiB]
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cosxcosr_new.pdf

Animated gif made out of all figures.

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Created on 19 August 2019.Citation count from INSPIRE on 19 August 2019.