CMS-HIN-19-005

CMS-HIN-19-005 ; CERN-EP-2020-219
Evidence for X(3872) in PbPb collisions and studies of its prompt production at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV
CMS Collaboration
25 February 2021
Phys. Rev. Lett. 128 (2022) 032001
Abstract: The first evidence for X(3872) production in relativistic heavy ion collisions is reported. The X(3872) production is studied in lead-lead (PbPb) collisions at a center-of-mass energy of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV per nucleon pair, using the decay chain ${\mathrm{X}(3872)} \to \mathrm{J}/\psi\,\pi^{+}\pi^{-}\to \mu^{+}\mu^{-}\pi^{+}\pi^{-}$. The data were recorded with the CMS detector in 2018 and correspond to an integrated luminosity of 1.7 nb$^{-1}$. The measurement is performed in the rapidity and transverse momentum ranges $\|y\| < $ 1.6 and 15 $ < {p_{\mathrm{T}}} < $ 50 GeV/$c$ . The significance of the inclusive X(3872) signal is 4.2 standard deviations. The prompt X(3872) to $\psi(\text{2S})$ yield ratio is found to be $\rho^{\mathrm{PbPb}} =$ 1.08 $\pm$ 0.49 (stat) $\pm$ 0.52 (syst), to be compared with typical values of 0.1 for pp collisions. This result provides a unique experimental input to theoretical models of the X(3872) production mechanism, and of the nature of this exotic state.
Links: e-print arXiv:2102.13048 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of $m_{\mu \mu \pi \pi}$ in PbPb collisions, for the inclusive (upper) and Pb-enriched (bottom) samples. The vertical lines represent statistical uncertainties in the data. The results of the unbinned maximum-likelihood fits for the signal$+$background and background alone are also shown by the solid and dashed lines, respectively. The pull distribution is represented by the shaded bars.
png pdf	Figure 2: The yield ratio $\rho ^{{\mathrm {Pb}\mathrm {Pb}}}$ of prompt X(3872) over $\psi(\text{2S})$ production in PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The yield ratios $\rho ^{{{\mathrm{p}} {\mathrm{p}}}}$ of prompt X(3872) over $\psi(\text{2S})$ production in pp collisions at $\sqrt {s} = $ 8 TeV, measured by ATLAS [8], and at $\sqrt {s} = $ 7 TeV, measured by CMS [7] are also shown.

Summary

In summary, the first evidence for X(3872) production in heavy ion collisions is presented using lead-lead collisions at a center-of-mass energy of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV per nucleon pair, recorded with the CMS detector. The X(3872) state is reconstructed using the decay chain $\mathrm{X}(3872)\to \mathrm{J}/\psi\,\pi^{+}\pi^{-}\to \mu^+\mu^-\pi^{+}\pi^{-}$. The measurement is performed for transverse momentum values of the X(3872) of 15 $< {p_{\mathrm{T}}} < $ 50 GeV/$c$ and rapidity $|y|<$ 1.6. The significance of the inclusive X(3872) signal is 4.2 standard deviations. The ratio $\rho^{\text{PbPb}}$ between the prompt X(3872) and $\psi(\text{2S})$ yields times their branching fractions into $\mathrm{J}/\psi\,\pi^{+}\pi^{-}$ is found to be 1.08 $\pm$ 0.49 (stat) 0.52 (syst), to be compared with typical values of 0.1 for pp collisions. This result provides a unique experimental input to theoretical models of the X(3872) production mechanism, and of the nature of this exotic state.

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