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Register a new userAn overview of $eta_c(1S)$, $eta_c(2S)$ and $h_c(1P)$ physics at BESIII
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Qingping Ji Institutionn One
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With the help of the largest data samples of $J/psi$ and $psi(2S)$ events ever produced in $e^+e^-$ annihilations, the three singlet charmonium states, $eta_c(1S)$, $eta_c(2S)$ and $h_c(1P)$, have been extensively studied at the BESIII experiment. In this review, a survey on the most recent results, including a series of precision measurements and observations of their new decay modes, is presented, which indicates the further investigations on their decays are needed to understand their decay mechanisms and have precision tests of the theoretical models. At present, about eight times larger data samples of 10 billion $J/psi$ events and 3 billion $psi(3686)$ events were collected with the BESIII detector, and thus the prospects for the study of these three charmonium states is discussed extensively.
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We report the measurement of gamma gamma to eta_c(1S), eta_c(2S) to eta pi+ pi- with eta decays to gamma rho and eta pi+ pi- using 941 fb^{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. The eta_c(1S) mass and width are measured to be M = [2984.6pm0.7 (stat.)pm2.2 (syst.)] MeV/c^{2} and Gamma = [30.8^{+2.3}_{-2.2}~(stat.) pm 2.5~(syst.)] MeV, respectively. First observation of eta_c(2S) to eta pi+ pi- with a significance of 5.5sigma including systematic error is obtained, and the eta_c(2S) mass is measured to be M = [3635.1pm3.7~(stat.)pm2.9~(syst.)] MeV/c^{2}. The products of the two-photon decay width and branching fraction (B) of decays to etapi+ pi- are determined to be Gamma_{gamma gamma}B = [65.4pm2.6~(stat.)pm6.9~(syst.)] eV for eta_c(1S) and [5.6^{+1.2}_{-1.1}~(stat.)pm1.1~(syst.)] eV for eta_c(2S). A new decay mode for the eta_c(1S) to etaf_0(2080) with f_0(2080) to pi+ pi- is observed with a statistical significance of 20sigma. The f_0(2080) mass and width are determined to be M = [2083^{+63}_{-66}~(stat.)pm 32~(syst.)] MeV/c^{2} and Gamma = [178^{+60}_{-178}~(stat.) pm 55~(syst.)] MeV. The cross sections for gamma gamma to eta pi+ pi- and etaf_{2}(1270) are measured for the first time.
A Dalitz plot analysis of $B^0 to eta_c(1S) K^+pi^-$ decays is performed using data samples of $pp$ collisions collected with the LHCb detector at centre-of-mass energies of $sqrt{s}=7,~8$ and $13$ TeV, corresponding to a total integrated luminosity of $4.7~text{fb}^{-1}$. A satisfactory description of the data is obtained when including a contribution representing an exotic $eta_c(1S) pi^-$ resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are $4096 pm 20~^{+18}_{-22}$ MeV and $152 pm 58~^{+60}_{-35}$ MeV, respectively. The spin-parity assignments $J^P=0^+$ and $J^{P}=1^-$ are both consistent with the data. In addition, the first measurement of the $B^0 to eta_c(1S) K^+pi^-$ branching fraction is performed and gives $displaystyle mathcal{B}(B^0 to eta_c(1S) K^+pi^-) = (5.73 pm 0.24 pm 0.13 pm 0.66) times 10^{-4}$, where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions.
A search for the decays $B^+ rightarrow h_c K^+$ and $B^0 rightarrow h_c K_S^0$ is performed. Evidence for the decay $B^+ rightarrow h_c K^+$ is found; its significance is $4.8sigma$. No evidence is found for $B^0 rightarrow h_c K_S^0$. The branching fraction for $B^+ rightarrow h_c K^+$ is measured to be $(3.7^{+1.0}_{-0.9}{}^{+0.8}_{-0.8}) times 10^{-5}$; the upper limit for the $B^0 rightarrow h_c K_S^0$ branching fraction is $1.4 times 10^{-5}$ at $90%$ C.L. In addition, a study of the $p bar{p} pi^+ pi^-$ invariant mass distribution in the channel $B^+ to (p bar{p} pi^+ pi^-) K^+$ results in the first observation of the decay $eta_c(2S) to p bar{p} pi^+ pi^-$ with $12.1sigma$ significance. The analysis is based on the 711 $mathrm{fb}^{-1}$ data sample collected by the Belle detector at the asymmetric-energy $e^+ e^-$ collider KEKB at the $Upsilon(4S)$ resonance.
We report the results of a study of $B^{pm}to K^{pm}eta_c$ and $B^{pm}to K^{pm}eta_c(2S)$ decays followed by $eta_c$ and $eta_c(2S)$ decays to $(K_SKpi)^0$. The results are obtained from a data sample containing 535 million $Bbar{B}$-meson pairs collected by the Belle experiment at the KEKB $e^+e^-$ collider. We measure the products of the branching fractions ${mathcal B}(B^{pm}to K^{pm}eta_c){mathcal B}(eta_cto K_S K^{pm}pi^{mp})=(26.7pm 1.4(stat)^{+2.9}_{-2.6}(syst)pm 4.9(model))times 10^{-6}$ and ${mathcal B}(B^{pm}to K^{pm}eta_c(2S)){mathcal B}(eta_c(2S)to K_S K^{pm}pi^{mp})=(3.4^{+2.2}_{-1.5}(stat+model)^{+0.5}_{-0.4} syst))times 10^{-6}$. Interference with the non-resonant component leads to significant model uncertainty in the measurement of these product branching fractions. Our analysis accounts for this interference and allows the model uncertainty to be reduced. We also obtain the following charmonia masses and widths: $M(eta_c)=(2985.4pm 1.5(stat)^{+0.5}_{-2.0}(syst))$ MeV/$c^2$, $Gamma(eta_c)=(35.1pm 3.1(stat)^{+1.0}_{-1.6}(syst))$ MeV/$c^2$, $M(eta_c(2S))=(3636.1^{+3.9}_{-4.2}(stat+model)^{+0.7}_{-2.0}(syst))$ MeV/$c^2$, $Gamma(eta_c(2S))=(6.6^{+8.4}_{-5.1}(stat+model)^{+2.6}_{-0.9}(syst))$ MeV/$c^2$.
The production of the $eta_c (1S)$ state in proton-proton collisions is probed via its decay to the $p bar{p}$ final state with the LHCb detector, in the rapidity range $2.0 < y < 4.5$ and in the meson transverse-momentum range $p_T > 6.5$ GeV/c. The cross-section for prompt production of $eta_c (1S)$ mesons relative to the prompt $J/psi$ cross-section is measured, for the first time, to be $sigma_{eta_c (1S)}/sigma_{J/psi} = 1.74 pm 0.29 pm 0.28 pm 0.18 _{B}$ at a centre-of-mass energy $sqrt{s} = 7$ TeV using data corresponding to an integrated luminosity of 0.7 fb$^{-1}$, and $sigma_{eta_c (1S)}/sigma_{J/psi} = 1.60 pm 0.29 pm 0.25 pm 0.17 _{B}$ at $sqrt{s} = 8$ TeV using 2.0 fb$^{-1}$. The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the $eta_c (1S)$ and $J/psi$ decays to the $p bar{p}$ final state. In addition, the inclusive branching fraction of $b$-hadron decays into $eta_c (1S)$ mesons is measured, for the first time, to be $B ( b rightarrow eta_c X ) = (4.88 pm 0.64 pm 0.29 pm 0.67 _{B}) times 10^{-3}$, where the third uncertainty includes also the uncertainty on the $J/psi$ inclusive branching fraction from $b$-hadron decays. The difference between the $J/psi$ and $eta_c (1S)$ meson masses is determined to be $114.7 pm 1.5 pm 0.1$ MeV/c$^2$.
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