CMS-PAS-HIN-19-005

CMS-PAS-HIN-19-005
Evidence for $\chi_{c1}\text{(3872)}$ in PbPb collisions and studies of its prompt production at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV
CMS Collaboration
November 2019

Abstract: The first evidence for $\chi_{c1}\text{(3872)}$ in relativistic heavy ion collisions is reported. Prompt $\chi_{c1}\text{(3872)}$ production is studied in 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 $\chi_{c1}\text{(3872)} \rightarrow {\rm J}/\psi\pi^+\pi^-\rightarrow \mu^+\mu^-\pi^+\pi^-$ . The data were recorded by the CMS detector in 2018 and correspond to an integrated luminosity of 1.7 $\pm$ 0.1 nb $^{-1}$ . The measurement is performed in a kinematic range in which the $\chi_{c1}\text{(3872)}$ candidates have a transverse momentum 15 $< p_{\mathrm{T}} <$ 50 GeV/ $c$ and rapidity $\|y\| <$ 1.6. The statistical significance of the $\chi_{c1}\text{(3872)}$ signal is in excess of 3 standard deviations. The production and survival of the prompt $\chi_{c1}\text{(3872)}$ in the quark-gluon plasma is expected to depend upon its internal structure. The ratio of the $\chi_{c1}\text{(3872)}$ and $\psi\text{(2S)}$ yields is found to be $R =$ 1.10 $\pm$ 0.51(stat) $\pm$ 0.53 (syst). This results is compared to the similar ratio measured in pp collisions at $\sqrt{s} =$ 7 and 8 TeV. An indication of larger $R$ in PbPb collisions with respect to the pp reference is reported.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of $m_{\mu \mu \pi \pi}$ in PbPb collisions. The mass of the dimuon pair is forced to be equal to the nominal ${\mathrm{J}/\psi}$ mass [35]. The red line presents the unbinned maximum-likelihood fit result. The signal components of $\psi\text{(2S)}$ and $\chi_{c1}\text{(3872)}$ are shown by the blue and the green line respectively. The pull distribution is presented by the red boxes.
png pdf	Figure 2: The yield ratio $R$ of prompt $\chi_{c1}\text{(3872)}$ over $\psi\text{(2S)}$ in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The yield ratio $R$ of prompt and nonprompt $\chi_{c1}\text{(3872)}$ over $\psi\text{(2S)}$ in pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8 TeV from the ATLAS experiment [22] and $R$ of inclusive $\chi_{c1}\text{(3872)}$ over $\psi\text{(2S)}$ in pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 7 TeV from the CMS experiment [7] is also shown.

Summary

In summary, the first evidence for

$\chi_{c1}\text{(3872)}$ production in heavy ion collisions is presented, using PbPb collisions at a center-of-mass energy of

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV per nucleon pair. The

$\chi_{c1}\text{(3872)}$ state is reconstructed using the decay chain

$\chi_{c1}\text{(3872)}\rightarrow {\rm J}/\psi\pi^+\pi^-\rightarrow \mu^+\mu^-\pi^+\pi^-$ . The measurement is performed in a kinematic range in which the

$\chi_{c1}\text{(3872)}$ candidates have a transverse momentum15

$< p_{\mathrm{T}} <$ 50 GeV/

$c$ and rapidity

$|y| <$ 1.6. The ratio

$R$ of the prompt

$\chi_{c1}\text{(3872)}$ and

$\psi\text{(2S)}$ yields times their branching fractions into

${\rm J}/\psi\pi^+\pi^-$ is found to be 1.10

$\pm$ 0.51 (stat)

$\pm$ 0.48 (syst). This result provides a unique experimental input to the theory community, towards elucidating the production mechanism and the nature of the

$\chi_{c1}\text{(3872)}$ state.

Additional Figures
png pdf	Additional Figure 1: Pseudo-proper decay length ${l_{xy}}$ distributions of prompt and nonprompt signal X(3872) and $\psi$ (2S) in PYTHIA+HYDJET Monte Carlo samples.
png pdf	Additional Figure 2: Invariant mass distribution of $m_{\mu \mu \pi \pi}$ in PbPb collisions with a nonprompt X(3872) enriched selection ${l_{xy}} >$ 1 mm. The mass of dimuon pair is forced to be equal to the nominal ${{\mathrm {J} / \psi}}$ mass. The red line presents the unbinned maximum-likelihood fit result. The signal components of $\psi$ (2S) and X(3872) are shown by the blue and the green line respectively. The pull distribution is presented by the red boxes.
png pdf	Additional Figure 3: The variation of the likelihood as a function of the number of X(3872) signals. The red lines indicate the values in the profile likelihood at the confidence level intervals of 68% and 95%.

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