Pentaquarks and doubly heavy exotic mesons

Abstract

Abstract. I discuss the experimental evidence for and theoretical interpretation of the new mesons and baryons with two heavy quarks. These include doubly-heavy baryons, exotic hadronic quarkonia and most recently a manifestly exotic pentaquark-like doubly heavy baryon with a minimal quark content uudc discovered by LHCb, whose mass, decay mode and width are in agreement with a prediction based on a physical picture of a deuteron-like Σ cD * "hadronic molecule". This strongly suggests a parallel with X(3872), whose mass is almost exactly at the D * D threshold. It also raises the possibility that such states might have a complementary description as deuteron-like "molecule" of two heavy mesons quasi-bound by pion exchange [5][6][7]. First observation of manifestly exotic hadronsThe attraction due to π exchange is 3 times weaker in the I=1 channel than in the I= 0 channel. Consequently, in the charm system the I=1 state is expected to be well above the D * D threshold and the I=0 X(3872) is at the threshold. 1 In the bottom system the attraction due to π exchange is essentially the same, but the kinetic energy is much smaller by a factor of ∼m(B)/m(D)≈2.8 . Therefore the net binding in the bottom system is stronger than in the charm system. This expectation has been confirmed by the discovery by BESIII [8] of charged exotic charmonium Z c (3900) at 3899.0 ± 3.6 ± 4.9 MeV with Γ = 46 ± 10 ± 20 MeV, 27 MeV aboveDD * threshold. e-mail: marek@proton.tau.ac.il 1 For simplicity we treat X(3872) as an isoscalar, since it has no charged partners, and we ignore here the issue of isospin breaking in its decays. A more refined treatment results in the same conclusions.

Tetraquark and two-meson states at large $N_{\mathrm{c}}$

“…For recent reviews on the experimental properties of tetraquark candidates and their theoretical interpretations, we refer the reader to Refs. [25][26][27][28][29][30][31][32][33][34][35].…”

Holographic tetraquarks and the newly observed Tcc+ at LHCb

“…This heavy diquark-anti-quark supersymmetry allows for mass relations not only between heavy baryons and mesons such as Qq and QQq, but also between heavy baryons and tetraquarks with hidden heavy flavor such as Qqq and Q Q qq. The recent quark model estimates for this last state are remarkable [27,28]. It is suggested that for a compact b b qq tetraquark, the binding energy is significant and about 200 MeV.…”