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تسجيل مستخدم جديدObservation of an excited $B_c^+$ state
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Using $pp$ collision data corresponding to an integrated luminosity of $8.5,mathrm{fb}^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of $sqrt{s} = 7$, $8$ and $13mathrm{,Tekern -0.1em V}$, the observation of an excited $B_c^+$ state in the $B_c^+pi^+pi^-$ invariant-mass spectrum is reported. The observed peak has a mass of $6841.2 pm 0.6 {,rm (stat),} pm 0.1 {,rm (syst),} pm 0.8,(B_c^+) mathrm{,MeV}/c^2$, where the last uncertainty is due to the limited knowledge of the $B_c^+$ mass. It is consistent with expectations of the $B_c^{*}(2^{3}S_{1})^+$ state reconstructed without the low-energy photon from the $B_c^{*}(1^{3}S_{1})^+ to B_c^+ gamma$ decay following $B_c^{*}(2^{3}S_{1})^+ to B_c^{*}(1^{3}S_{1})^+ pi^+ pi^-$. A second state is seen with a global (local) statistical significance of $2.2,sigma$ ($3.2,sigma$) and a mass of $6872.1 pm 1.3 {,rm (stat),} pm 0.1 {,rm (syst),} pm 0.8,(B_c^+) mathrm{,MeV}/c^2$, and is consistent with the $B_c(2^{1}S_{0})^+$ state. These mass measurements are the most precise to date.
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A search is performed in the invariant mass spectrum of the $B_c^{+}pi^{+}pi^{-}$ system for the excited $B_c^{+}$ states $B_c(2^{1}S_{0})^+$ and $B_c(2^{3}S_{1})^+$ using a data sample of $pp$ collisions collected by the LHCb experiment at the centr
e-of-mass energy of $sqrt{s} = 8 ,{mathrm{TeV}}$, corresponding to an integrated luminosity of $2 ,{mathrm{fb^{-1}}}$. No evidence is seen for either state. Upper limits on the ratios of the production cross-sections of the $B_c(2^{1}S_{0})^+$ and $B_c(2^{3}S_{1})^+$ states times the branching fractions of ${B_c(2^{1}S_{0})^+} to {B_c^{+}pi^{+}pi^{-}}$ and ${B_c(2^{3}S_{1})^+} to {B_c^{*+}pi^{+}pi^{-}}$ over the production cross-section of the $B_c^{+}$ state are given as a function of their masses. They are found to be between 0.02 and 0.14 at $95%$ confidence level for $B_c(2^{1}S_{0})^+$ and $B_c(2^{3}S_{1})^+$ in the mass ranges $[6830, 6890] ,{mathrm{MeV}}/c^{2}$ and $[6795,6890] ,{mathrm{MeV}}/c^{2}$, respectively.
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