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Register a new userNew Spectroscopy with Charm and Beauty Multiquark States
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Authors
Luciano Maiani
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Exotic charmonium and bottonomium resonances recently discovered are discussed and interpreted as diquark-antidiquark states containing a pair of charm quarks and a pair of light, up and down, quarks. Successes, shortcomings and predictions of the model are illustrated.
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Why do we see certain types of strongly interacting elementary particles and not others? This question was posed over 50 years ago in the context of the quark model. M. Gell-Mann and G. Zweig proposed that the known mesons were $q bar q$ and baryons $qqq$, with quarks known at the time $u$ (up), $d$ (down), and $s$ (strange) having charges (2/3,-1/3,-1/3). Mesons and baryons would then have integral charges. Mesons such as $qq bar q bar q$ and baryons such as $qqqq bar q$ would also have integral charges. Why werent they seen? They have now been seen, but only with additional heavy quarks and under conditions which tell us a lot about the strong interactions and how they manifest themselves. The present article describes recent progress in our understanding of such exotic mesons and baryons.
We study the dependence on the quark mass of the compositeness of the lowest-lying odd parity hyperon states. Thus, we pay attention to $Lambda-$like states in the strange, charm and beauty, sectors which are dynamically generated using a unitarized meson-baryon model. In the strange sector we use an SU(6) extension of the Weinberg-Tomozawa meson-baryon interaction, and we further implement the heavy-quark spin symmetry to construct the meson-baryon interaction when charmed or beauty hadrons are involved. In the three examined flavor sectors, we obtain two $J^P=1/2^-$ and one $J^P=3/2^-$ $Lambda$ states. We find that the $Lambda$ states which are bound states (the three $Lambda_b$) or narrow resonances (one $Lambda(1405)$ and one $Lambda_c(2595)$) are well described as molecular states composed of $s$-wave meson-baryon pairs. The $frac{1}{2}^-$ wide $Lambda(1405)$ and $Lambda_c(2595)$ as well as the $frac{3}{2}^-$ $Lambda(1520)$ and $Lambda_c(2625)$ states display smaller compositeness and so they would require new mechanisms, such as $d$-wave interactions.
The mass spectra of all-charm tetraquark states with the [cc][$bar{c}bar{c}$] quark configuration are investigated. The coulomb plus linear potential is used in conjunction with the relativistic mass correction term $mathcal{O}(frac{1}{m})$. To determine the fitting parameters for all-charm tetraquarks states [cc][$bar{c}bar{c}$], we first calculate the mass spectra of charmonia [c$bar{c}$] and its decay constants ($f^{2}_{P/V}$). We estimated the masses of the tetraquark states in their ground and radially excited states. For mass spectra of tetraquark states, we also included spin-spin, spin-orbital, and tensor interactions. The mass spectra of charmonia produced in this study are reasonably consistent with experimental and theoretical predictions made by others, whilst the mass spectra of the tetraquark states are consistent with previous theoretical predictions. We propose that the X(6900) state, which has a mass range of 6.2 - 6.9 GeV and was recently detected by LHCb, has the quantum numbers $0^{-+}$, $1^{-+}$, $2^{-+}$ and belongs to the P-wave of the all-cham tetraquark state.
Results on open charm and beauty production and on the search for top production in high-energy electron-proton collisions at HERA are reviewed. This includes a discussion of relevant theoretical aspects, a summary of the available measurements and measurement techniques, and their impact on improved understanding of QCD and its parameters, such as parton density functions and charm- and beauty-quark masses. The impact of these results on measurements at the LHC and elsewhere is also addressed.
Selected new results from the H1 and ZEUS collaborations on $ep$ interactions at 300 - 318 GeV centre-of-mass energy are presented. The full pre-upgrade integrated luminosity of HERA of 110 pb$^{-1}$ is used. Charm cross sections are measured up to high values of $x_B$ and $Q^2$ and are found to be well described by NLO QCD in the 3 flavour scheme. Orbitally excited $D$ mesons are observed; radial excitations are searched for, but are not seen. The first $b$ cross section measurement is confirmed with a lifetime based method, establishing the excess over NLO QCD.
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