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تسجيل مستخدم جديدObservation of an exotic narrow doubly charmed tetraquark
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Conventional hadronic matter consists of baryons and mesons made of three quarks and quark-antiquark pairs, respectively. The observation of a new type of hadronic state, a doubly charmed tetraquark containing two charm quarks, an anti-$u$ and an anti-$d$ quark, is reported using data collected by the LHCb experiment at the Large Hadron Collider. This exotic state with a mass of about 3875 MeV$/c^2$ manifests itself as a narrow peak in the mass spectrum of $D^0D^0pi^+$ mesons just below the $D^{*+}D^0$ mass threshold. The near threshold mass together with a strikingly narrow width reveals the resonance nature of the state.
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An exotic narrow state in the $D^0D^0pi^+$ mass spectrum just below the $D^{*+}D^0$ mass threshold is studied using a data set corresponding to an integrated luminosity of 9 fb$^{-1}$ acquired with the LHCb detector in proton-proton collisions at cen
tre-of-mass energies of 7, 8 and 13 TeV. The state is consistent with the ground isoscalar $T^+_{cc}$ tetraquark with a quark content of $ccbar{u}bar{d}$ and spin-parity quantum numbers $mathrm{J}^{mathrm{P}}=1^+$. Study of the $DD$ mass spectra disfavours interpretation of the resonance as the isovector state. The decay structure via intermediate off-shell $D^{*+}$ mesons is confirmed by the $D^0pi^+$ mass distribution. The mass of the resonance and its coupling to the $D^{*}D$ system are analysed. Resonance parameters including the pole position, scattering length, effective range and compositeness are measured to reveal important information about the nature of the $T^+_{cc}$ state. In addition, an unexpected dependence of the production rate on track multiplicity is observed.
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.
Hadron spectroscopy provides direct physical measurements that shed light on the non-perturbative behavior of quantum chromodynamics (QCD). In particular, various exotic hadrons such as the newly observed $T_{cc}^+$ by the LHCb collaboration, offer u
nique insights on the QCD dynamics in hadron structures. In this letter, we demonstrate how heavy ion collisions can serve as a powerful venue for hadron spectroscopy study of doubly charmed exotic hadrons by virtue of the extremely charm-rich environment created in such collisions. The yields of $T_{cc}^+$ as well as its potential isospin partners are computed within the molecular picture for Pb-Pb collisions at center-of-mass energy $2.76~mathrm{TeV}$. We find about three-order-of-magnitude enhancement in the production of $T_{cc}^+$ in Pb-Pb collisions as compared with the yield in proton-proton collisions, with a moderately smaller enhancement in the yields of the isospin partners $T_{cc}^0$ and $T_{cc}^{++}$. The $T_{cc}^+$ yield is comparable to that of the $X(3872)$ in the most central collisions while shows a considerably stronger decrease toward peripheral collisions, due to a threshold effect of the required double charm quarks for $T_{cc}^+$. Final results for their rapidity and transverse momentum $p_T$ dependence as well as the elliptic flow coefficient are reported and can be tested by future experimental measurements.
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}
In the present work, we investigate the axialvector doubly-charmed tetraquark molecular states without strange, with strange and with doubly-strange via the QCD sum rules, and try to make assignment of the $T^+_{cc}$ from the LHCb collaboration in th
e scenario of molecular states. The predictions favor assigning the $T^+_{cc}$ to be the heavier $DD^{*}$ molecular state with the spin-parity $J^P=1^+$, while the lighter $DD^{*}$ molecular state with the spin-parity $J^P=1^+$ still escapes experimental detections. All the predicted doubly-charmed tetraquark molecular states can be confronted to the experimental data in the future.
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