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تسجيل مستخدم جديدNews & Views on a new search for the doubly charmed baryon $Xi_{cc}^+$ at the LHC
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Xing-Gang Wu
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Invited News & Views for the journal, SCIENCE CHINA: Physics, Mechanics & Astronomy, on a recently new search for the doubly charmed baryon $Xi_{cc}^+$ at the LHC, which is done by the LHCb Collaboration, arXiv:1909.12273.
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A search for the doubly charmed baryon $Xi_{cc}^{+}$ is performed through its decay to the $Lambda_c^+ K^- pi^+$ final state, using proton-proton collision data collected with the LHCb detector at centre-of-mass energies of 7, 8 and 13$mathrm{,Tekern
-0.1em V}$. The data correspond to a total integrated luminosity of $9,mathrm{fb}^{-1}$. No significant signal is observed in the mass range from 3.4 to 3.8$mathrm{,Gekern -0.1em V}/c^2$. Upper limits are set at $95%$ credibility level on the ratio of the $Xi_{cc}^{+}$ production cross-section times the branching fraction to that of $Lambda_c^+$ and $Xi_{cc}^{++}$ baryons. The limits are determined as functions of the $Xi_{cc}^{+}$ mass for different lifetime hypotheses, in the rapidity range from 2.0 to 4.5 and the transverse momentum range from 4 to 15$mathrm{,Gekern -0.1em V}/c$.
A highly significant structure is observed in the $Lambda_c^+K^-pi^+pi^+$ mass spectrum, where the $Lambda_c^+$ baryon is reconstructed in the decay mode $pK^-pi^+$. The structure is consistent with originating from a weakly decaying particle, identi
fied as the doubly charmed baryon $Xi_{cc}^{++}$. The difference between the masses of the $Xi_{cc}^{++}$ and $Lambda_c^+$ states is measured to be $1334.94 pm 0.72 (mathrm{stat}) pm 0.27 (mathrm{syst}~mathrm{MeV}/c^2$, and the $Xi_{cc}^{++}$ mass is then determined to be $3621.40 pm 0.72 (mathrm{stat}) pm 0.27 (mathrm{syst} pm 0.14 , (Lambda_c^+)~mathrm{MeV}/c^2$, where the last uncertainty is due to the limited knowledge of the $Lambda_c^+$ mass. The state is observed in a sample of proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.7 $mathrm{fb}^{-1}$, and confirmed in an additional sample of data collected at 8 TeV.
The first measurement of the lifetime of the doubly charmed baryon $Xi_{cc}^{++}$ is presented, with the signal reconstructed in the final state $Lambda_c^+ K^- pi^+ pi^+$. The data sample used corresponds to an integrated luminosity of $1.7,mathrm{f
b}^{-1}$, collected by the LHCb experiment in proton-proton collisions at a centre-of-mass energy of $13mathrm{,Tekern -0.1em V}$. The $Xi_{cc}^{++}$ lifetime is measured to be $0.256,^{+0.024}_{-0.022}{,rm (stat),} pm 0.014 {,rm(syst)}mathrm{,ps}$.
A search for the doubly charmed baryon $it{Omega}_{cc}^{+}$ with the decay mode $it{Omega}_{cc}^{+}toit{Xi}_c^{+}K^{-}pi^{+}$ is performed using proton-proton collision data at a centre-of-mass energy of 13$mathrm{,Tekern -0.1em V}$ collected by the
LHCb experiment from 2016 to 2018, corresponding to an integrated luminosity of $5.4,mathrm{fb}^{-1}$. No significant signal is observed within the invariant mass range of 3.6 to 4.0$mathrm{,Gekern -0.1em V}/c^2$. Upper limits are set on the ratio $R$ of the production cross-section times the total branching fraction of the $it{Omega}_{cc}^{+}toit{Xi}_c^{+}K^{-}pi^{+}$ decay with respect to the $it{Xi}_{cc}^{++}toit{Lambda}_c^{+}K^{-}pi^{+}pi^{+}$ decay. Upper limits at 95% credibility level for $R$ in the range $0.005$ to $0.11$ are obtained for different hypotheses on the $it{Omega}_{cc}^{+}$ mass and lifetime in the rapidity range from 2.0 to 4.5 and transverse momentum range from 4 to 15$mathrm{,Gekern -0.1em V}/c$.
The doubly charmed baryon decay $Xi_{cc}^{++} rightarrow Xi_{c}^{+} pi^{+}$ is observed for the first time, with a statistical significance of $5.9sigma$, confirming a recent observation of the baryon in the $Lambda_c^{+} K^{-} pi^{+} pi^{+}$ final s
tate. The data sample used corresponds to an integrated luminosity of $1.7,mathrm{fb}^{-1}$, collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of $13mathrm{,Tekern -0.1em V}$. The $Xi_{cc}^{++}$ mass is measured to be begin{equation} onumber 3620.6pm 1.5~(text{stat})pm 0.4~(text{syst}) pm 0.3~(Xi_{c}^{+})~text{MeV}/it{c}^{2}, end{equation} and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be begin{equation} onumber frac{mathcal{B} (Xi_{cc}^{++} rightarrow Xi_{c}^{+} pi^{+}) times mathcal{B}(Xi_{c}^{+} rightarrow pK^{-}pi^{+})} {mathcal{B} (Xi_{cc}^{++} rightarrow Lambda_c^{+} K^{-} pi^{+} pi^{+}) times mathcal{B}(Lambda_c^{+} rightarrow pK^{-}pi^{+})} = 0.035pm 0.009~(text{stat}) pm 0.003~(text{syst}). end{equation}
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