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Register a new userObservation of structure in the $J/psi$-pair mass spectrum
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Using proton-proton collision data at centre-of-mass energies of $sqrt{s} = 7$, $8$ and $13mathrm{,TeV}$ recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of $9mathrm{,fb}^{-1}$, the invariant mass spectrum of $J/psi$ pairs is studied. A narrow structure around $6.9mathrm{,GeV/}c^2$ matching the lineshape of a resonance and a broad structure just above twice the $J/psi$ mass are observed. The deviation of the data from nonresonant $J/psi$-pair production is above five standard deviations in the mass region between $6.2$ and $7.4mathrm{,GeV/}c^2$, covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow $X(6900)$ structure are measured assuming a Breit--Wigner lineshape.
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Very recently, the LHCb Collaboration reported the observation of several enhancements in the invariant mass spectrum of a $J/psi$ pair between 6.2 and 7.4 GeV. In this work, we propose the dynamical mechanism to mimic the experimental data of a di-$J/psi$ mass spectrum given by LHCb, which is based on the reactions, where all the possible combinations of a double charmonium directly produced by a proton-proton collision are transferred into a final state $J/psi J/psi$. We find that the LHCb experimental data can be well reproduced. We further extend our framework to study a di-$Upsilon(1S)$ system, and give the line shape of a differential cross section of a partner process in a $bbar{b}$ system on the invariant mass of $Upsilon(1S)Upsilon(1S)$, which shows that there should exist possible enhancements near $m_{Upsilon(1S)Upsilon(1S)}=$19.0, 19.3, 19.7 GeV in the $Upsilon(1S)$-pair invariant mass spectrum. These predictions can be tested in LHCb and CMS, which can be as a potential research issue in near future.
Structure in the di-$J/psi$ mass spectrum observed by the LHCb experiment around 6.9 and 7.2 GeV is interpreted in terms of $J^{PC}=0^{++}$ and $2^{++}$ resonances between a $cc$ diquark and a $bar c bar c$ antidiquark, using a recently confirmed string-junction picture to calculate tetraquark masses. The main peak around 6.9 GeV is likely dominated by the $0^{++}(2S)$ state, a radial excitation of the $cc$-$bar c bar c$ tetraquark, which we predict at $6.871pm 0.025$ GeV. The dip around 6.75 GeV is ascribed to the opening of the Swave di-$chi_{c0}$ channel, while the dip around 7.2 GeV could be correlated with the opening of the di-$eta_c(2S)$ or $Xi_{cc} bar Xi_{cc}$ channel. The low-mass part of the di-$J/psi$ structure appears to require a broad resonance consistent with a predicted $2^{++}(1S)$ state with invariant mass around $M_{rm inv} = 6400$ MeV. Implications for $bb bar b bar b$ tetraquarks are discussed.
The production of $J/psi$ pairs in proton-proton collisions at a centre-of-mass energy of 7 TeV has been observed using an integrated luminosity of $37.5 pb^{-1}$ collected with the LHCb detector. The production cross-section for pairs with both jpsi in the rapidity range $2<y^{J/psi}<4.5$ and transverse momentum $p_{T}^{J/psi}<10 GeV/c$ is $$ sigma^{J/psi J/psi} = 5.1pm1.0pm1.1 nb,$$ where the first uncertainty is statistical and the second systematic.
We report the first observation of the decay J/psi --> 3 gamma. The signal has a statistical significance of 6 sigma and corresponds to a branching fraction of B(J/psi --> 3 gamma) = (1.2 +- 0.3 +- 0.2) x 10^-5, in which the errors are statistical and systematic, respectively. The measurement uses psi(2S) --> pi+ pi- J/psi events acquired with the CLEO-c detector operating at the CESR e+e- collider.
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