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Authors
Alexey Guskov
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The $Z_{c}(3900)$ hadron state has been found by the BES-III and Belle experiments in the decay of the hadron state with higher mass. The first attempt to search for the direct exclusive production of the $Z_c^{pm}(3900)$ hadron by virtual photons has been performed in the reaction $mu^+ N rightarrow mu^+ N Z_c(3900)^{pm} rightarrow mu^+ N J/psi pi^{pm}$ at COMPASS cite{Adolph:2014hba}. The data cover the range from 7~GeV to 19~GeV in the centre-of-mass energy of the photon-nucleon system. The full COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio $BR(Z_c^{pm}(3900)rightarrow J/psi pi^{pm} )times (sigma_{ gamma~N rightarrow Z_c^{pm}(3900)~ N} /sigma_{ gamma~N rightarrow J/psi~ N})$ of $3.7times10^{-3}$ has been established at the confidence level of 90%.
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A search for the exclusive production of the $Z_c^{pm}(3900)$ hadron by virtual photons has been performed in the channel $Z_c^{pm}(3900)rightarrow J/psi pi^{pm}$. The data cover the range from 7 GeV to 19 GeV in the centre-of-mass energy of the photon-nucleon system. The full set of the COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio $BR(Z_c^{pm}(3900)rightarrow J/psi pi^{pm} )times sigma_{ gamma~N rightarrow Z_c^{pm}(3900)~ N} /sigma_{gamma~N rightarrow J/psi~ N}$ of $3.7times10^{-3}$ has been established at the confidence level of 90%.
The decay $Z_c(3900)^pmtoomegapi^pm$ is searched for using data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies $sqrt{s}=4.23$ and $4.26$~GeV. No significant signal for the $Z_c(3900)^pm$ is found, and upper limits at the 90% confidence level on the Born cross section for the process $e^+e^-to Z_c(3900)^pmpi^mptoomegapi^+pi^-$ are determined to be $0.26$ and $0.18$ pb at $sqrt{s}=4.23$ and 4.26 GeV, respectively.
We study e+e- --> pi+pi-h_c at center-of-mass energies from 3.90 GeV to 4.42 GeV using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections are measured at 13 energies, and are found to be of the same order of magnitude as those of e+e- --> pi+pi-J/psi but with a different line shape. In the pi^pm h_c mass spectrum, a distinct structure, referred to as Z_c(4020), is observed at 4.02 GeV/c^2. The Z_c(4020) carries an electric charge and couples to charmonium. A fit to the pi^pm h_c invariant mass spectrum, neglecting possible interferences, results in a mass of (4022.9pm 0.8pm 2.7) MeV/c^2 and a width of (7.9pm 2.7pm 2.6) MeV for the Z_c(4020), where the first errors are statistical and the second systematic. No significant Z_c(3900) signal is observed, and upper limits on the Z_c(3900) production cross sections in pi^pm h_c at center-of-mass energies of 4.23 and 4.26 GeV are set.
The spin and parity of the $Z_c(3900)^pm$ state are determined to be $J^P=1^+$ with a statistical significance larger than $7sigma$ over other quantum numbers in a partial wave analysis of the process $e^+e^-to pi^+pi^-J/psi$. We use a data sample of 1.92 fb$^{-1}$ accumulated at $sqrt{s}=4.23$ and 4.26 GeV with the BESIII experiment. When parameterizing the $Z_c(3900)^pm$ with a Flatte-like formula, we determine its pole mass $M_textrm{pole}=(3881.2pm4.2_textrm{stat}pm52.7_textrm{syst})textrm{MeV}/c^2$ and pole width $Gamma_textrm{pole}=(51.8pm4.6_textrm{stat}pm36.0_textrm{syst})textrm{MeV}$. We also measure cross sections for the process $e^+e^-to Z_c(3900)^+pi^-+c.c.to J/psipi^+pi^-$ and determine an upper limit at the 90% confidence level for the process $e^+e^-to Z_c(4020)^+pi^-+c.c.to J/psipi^+pi^-$.
Using 586 pb$^{-1}$ of $e^+e^-$ annihilation data taken with the CLEO-c detector at $sqrt{s}=4170$ MeV, the peak of the charmonium resonance $psi(4160)$, we analyze the decay $psi(4160)to pi^+pi^- J/psi$, and report observation of the charged state $Z_c^pm(3900)$ decaying into $pi^pm J/psi$ at a significance level of $>5 sigma$. We obtain $M(Z_c^pm)=3886pm4(text{stat})pm 2(text{syst})$ MeV and $Gamma(Z_c^pm)=37pm4(text{stat})pm 8(text{syst})$ MeV, which are in good agreement with the results for this resonance obtained by the BES III and Belle Collaborations in the decay of the resonance Y(4260). We also present first evidence for the production of the neutral member of this isospin triplet, $Z_c^0(3900)$ decaying into $pi^0J/psi$ at a $3.5sigma$ significance level.
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