EPJ Web of Conferences 72, 00020 ( 2014) DOI: 10.1051/epjconf/20147200020 C Owned by the authors, published by EDP Sciences, 2014 Exotic Hadron Spectroscopy Alessandro Pilloni1,a 1Dipartimento di Fisica and INFN, “Sapienza” Università di Roma P.le Aldo Moro 2 - I-00185 Roma (Italy) Abstract. Since ten years ago experiments have been observing a host of exotic states decaying into heavy quarkonia. The interpretation of most of them still remains uncertain and, in some cases, controversial. Notwith- standing, a considerable progress has been made on the quality of the experimental information available and a number of ideas and models have been put forward to explain the observations. We will review the most promis- ing theoretical interpretations of exotic states, in particular we will discuss the nature of the charmonium-like resonances in the region 3850−4050 MeV, and propose a new mechanism to explain prompt X(3872) production cross section at hadron colliders. Finally, the phenomenology of doubly charmed states is commented. 1 Introduction • Tetraquarks: a quark pair (diquark) in the 3¯c (attrac- tive) configuration, neutralizing its color with an anti- Heavy quarkonium sector has proved to be extremely use- quark pair [6, 7]. A full nonet of states is predicted for ful for the understanding of QCD. In the limit mQ →∞, each spin-parity, i.e. a large number of states are ex- the gauge field dynamics can be simplified in terms of ef- pected. There is no need for these states to be close to fective potential models [1], or in terms of effective the- any threshold. Isospin-violating decay are predicted in a ories as NRQCD [2], which allow us to make accurate natural way. The lack of observations of predicted states predictions about quarkonia spectra, production cross sec- constitutes the main drawback to this model. tions and decay rates. However, in the last ten years a lot of unexpected quarkonium-like resonances have been dis- • Hybrids: bound states of a quark-antiquark pair in the covered. There are several hints that these states cannot fit 8c configuration and of a number of constituent glu- standard heavy quarkonium interpretations, whence they ons. The lowest-lying state is expected to have quantum +− were called X, Y, Z. A graphic summary of the situation in numbers JPC = 0 . Since a quarkonium state cannot charm sector is reported in Fig. 1. Despite a lot of efforts, have these quantum numbers, this is a unique signature a comprehensive framework for a unified description of for hybrids. An additional signature is the preference for all these states is still missing. The actual identification of these states would represent a major progress in our under- standing of strong interaction dynamics, and might imply Y (4660) the prediction of a large number of additional states that 4500 ψ(4415) Y (4360) have not yet been observed. Y (4260) ∗ ∗ ψ(4160) X(4160)Ds Ds The most likely possible states beyond ordinary 1 2 D2 D D∗ ± s s 4000 Z (3900) χc1(2P ) X(3940)D∗D∗ mesons and baryons are: ηc(3S) ψ(4040) c χc0(2P ) DsDs ∗ ψ(3770) h (2P ) χc2(2P ) DD • c X(3872) 1 Molecules: bound states of two mesons, just like deu- 1 D2 DD terium is the bound state of a proton and a neutron. The η (2S) ψ(2S) c χ Mass (MeV) 3500 χ c2 binding mechanism involves the exchange of a light me- hc c1 χc0 son (π, σ) [3], or even a 0-order contact interaction [4]. The mass of these states is expected to be near the sum 3000 J/ψ of the two meson masses. In particular, the system ηc PC would be stable if the binding energy is negative, so J 0−+ 1−− 1+− 0++ 1++ 2++ 2−+ ?? that the mass of the states would be set below meson threshold. The smaller is this binding energy, the larger Figure 1. Charmonium sector. Black lines represent observed is the spatial extension of the molecule, up to 10 fm for charmonium levels, blue lines represent predicted levels accord- X(3872). The bound state should decay mainly into its ing to [1], red line are exotic states. The open charm threshold constituents [5]. are reported on the right. ae-mail: [email protected] . This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20147200020 EPJ Web of Conferences State Mass (MeV) Width (MeV) Decay channels IG JPC X(3872) 3871.68 ± 0.17 < 1.2 see Ref. [14] (∗∗) 1++ Y(4260) 4250 ± 9 108 ± 12 see Ref. [14] (0−) 1−− + +− Zc(3900) 3899 ± 6 46 ± 22 J/ψ π likely (1 ) 1 ∗ + +− Zc(3885) 3883.9 ± 4.5 24.8 ± 11.5 DD (1 ) 1 ± ± + ?− Zc(4020) 4022.9 2.8 7.9 3.7 hc π (1 ) 1 ± ± ∗ ∗ + +− Zc(4025) 4026.3 4.5 24.8 9.5 D D (1 ) 1 ∗ + +− Zb(10610) 10607.2 ± 2.0 18.4 ± 2.4 Υ(nS ) π, hb(nP) π, BB (1 ) 1 ± ± Υ ∗ ∗ + +− Zb(10650) 10652.2 1.5 11.5 2.2 (nS ) π, hb(nP) π, B B (1 ) 1 Table 1. Mass, width and quantum numbers of exotic states analysed. IG is undefined for X(3872) because of isospin-violating decay. DD* D*D* DD* D*D* 90 90 80 80 2 70 2 70 60 60 50 50 40 40 30 30 Events/0.01 GeV/c Events/0.01 GeV/c 20 20 10 10 0 0 3.65 3.70 3.75 3.80 3.85 3.90 3.95 4.00 4.05 3.65 3.70 3.75 3.80 3.85 3.90 3.95 4.00 4.05 π± ψ 2 π± ψ 2 Mmax( J/ ) (GeV/c ) Mmax( J/ ) (GeV/c ) Figure 2. Result of the fit to the BES and Belle [10] data reported in Ref. [17], in the presence of a lighter resonance (left panel) or heavier resonance (right panel). The former fit has χ2/DOF = 41/65, whereas the latter fit has χ2/DOF = 47/65. Red curves: fit to data. Green curves: Zc(3900). Purple curves: additional resonances. Blue dashed curves: background. a hybrid to decay into quarkonium and a state that can Z (10650) [13], which look very much like the b-partners + − b be produced by the excited gluons (e.g. π π pairs) [8]. of Zc and Zc. Mass, width and quantum numbers are re- ported in Tab. 1. • Hadrocharmonium (hadrobottomomium): a color- singlet proto-quarkonium QQ¯ core is supposed to inter- act with a light quark cloud via a Van-der-Waals resid- 2 Exotic resonances near X(3872) and ual force [9]. Quantum numbers are the same of a pair tetraquark model quarkonium-light meson. The Zc(3900) is a charged state seen by BES and Belle [10] In this talk we would like to summarize the recent re- → + − → + − 1 as a resonance in Y(4260) Zc (3900) π J/ψ π π , search on exotic hadrons by our group. In Sec. 2 we will +− likely carrying quantum number JPC = 1 . Some hints talk about Z states, and about its interpretation in terms of c for the existence of an almost degenerate Z0 neutral part- tetraquark model. In Sec. 3 we will show a possible com- c ner seem encouraging [15]. The Zc is described in sev- prehensive framework for most of known states in terms ∗ eral papers as a DD molecule, the I = 1 partner of the of Feshbach resonances. In Sec. 4 we will propose a new X(3872) [16]. However, a real bound state must have neg- mechanism to explain huge X(3872) prompt production ative binding energy, whereas Zc is about 25 MeV above cross section at hadron colliders. Finally, in Sec. 5 we pro- ∗ DD threshold. Such above-threshold states should be pose a new tetraquark particle with peculiar quantum num- considered a sort of virtual unstable states. However, the bers and experimental signatures, whose existence can be mechanism for which such resonances are pushed above checked in lattice simulations and at LHCb. threshold is still unclear. In this review we will rely on the set of best es- In Ref. [7], assuming X(3872) to be a 1++ tetraquark, tablished states: in the charm sector we consider the two 1+− states were predicted at 3882 MeV and 3755 MeV. X(3872), Y(4260) (both seen by many experiments), and +− In Ref. [17] the Zc(3900) was identified as the 1 heav- the charged states Zc(3900) [10]. Furthermore, we ex- ier tetraquark state. The presence of a lighter state would tend our analysis to some states recently announced by support the tetraquark hypothesis, whereas molecular pic- BES, i.e. Zc(3885) [11], Zc(4020) and Zc(4025) [12]. Fi- 1 nally, in the bottom sector we will consider Zb(10610) and Hereafter the charge-conjugated modes are understood. 00020-p.2 Lepton and Hadron Physics at Meson-Factories State Hadro-cc¯,-bb¯ Open c/b Γ (MeV) ν (MeV) A (MeV1/2) X(3872) J/ψ ρ0 D0D¯ ∗0 0 0 − + ∗0 Zc(3900) ψ(3770) π D D¯ 46 ± 22 23 ± 6 10 ± 5 ∗+ ¯ ∗0 ± ± ± Zc(4020) hc(2P) πP-wave D D 24.8 9.5 9 4.5 8 4 ∗+ ¯ ∗0 ± ± ± Zc(4025) hc(2P) πP-wave D D 7.9 3.7 5.6 2.8 3 2 + ∗0 Zb(10610) χb0(1P) ρP-wave B B¯ 18.4 ± 2.4 3 ± 2 11 ± 4 ∗+ ¯ ∗0 ± ± ± Zb(10650) χb0(1P) ρP-wave B B 11.5 2.2 1.8 1.5 8 4 √ Table 2.