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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%.
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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%.
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.
Exotic charmonium-like states have been observed by various experiments over the last 15 years, but their nature is still under discussion. Photo-(muo)production is a new promising instrument to study them. COMPASS, a fixed target experiment at CERN, analyzed the full set of the data collected with a muon beam between 2002 and 2011, covering the range from 7 GeV to 19 GeV in the centre-of-mass energy of the virtual photon-nucleon system. A signal in the mass spectrum of $J/psipi^+pi^-$ with the statistical significance of 4.1 $sigma$ was observed in the reaction $mu^+~N rightarrow mu^+(J/psipi^+pi^- )pi^{pm} N$. Its mass and width are consistent with those of the $X(3872)$. The shape of the $pi^+pi^-$ mass distribution from the observed decay into $J/psipi^+pi^-$ is different from previous observations for $X(3872)$. The observed signal may be interpreted as possible evidence of a new charmonium state $widetilde{X}(3872)$. It could be associated with a neutral partner of $X(3872)$ with $C = -1$ predicted by a tetraquark model.
Exclusive photoproduction of $rho^0(770)$ mesons is studied using the H1 detector at the $ep$ collider HERA. A sample of about 900000 events is used to measure single- and double-differential cross sections for the reaction $gamma p to pi^{+}pi^{-}Y$. Reactions where the proton stays intact (${m_Y{=}m_p}$) are statistically separated from those where the proton dissociates to a low-mass hadronic system ($m_p{<}m_Y{<}10$ GeV). The double-differential cross sections are measured as a function of the invariant mass $m_{pipi}$ of the decay pions and the squared $4$-momentum transfer $t$ at the proton vertex. The measurements are presented in various bins of the photon-proton collision energy $W_{gamma p}$. The phase space restrictions are $0.5 < m_{pipi} < 2.2$ GeV, ${vert tvert < 1.5}$ GeV${}^2$, and ${20 < W_{gamma p} < 80}$ GeV. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the $m_{pipi}$ dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the $rho^{0}(770)$ meson mass and width at $m_rho = 770.8 {}^{+2.6}_{-2.7}$ (tot) MeV and $Gamma_rho = 151.3 {}^{+2.7}_{-3.6}$ (tot) MeV, respectively. The model is used to extract the $rho^0(770)$ contribution to the $pi^{+}pi^{-}$ cross sections and measure it as a function of $t$ and $W_{gamma p}$. In a Regge asymptotic limit in which one Regge trajectory $alpha(t)$ dominates, the intercept $alpha(t{=}0) = 1.0654 {}^{+0.0098}_{-0.0067}$ (tot) and the slope $alpha^prime(t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }$ (tot) GeV${}^{-2}$ of the $t$ dependence are extracted for the case $m_Y{=}m_p$.
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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