arXiv:1609.08928v1 [hep-ph] 28 Sep 2016

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A review of the open charm and open bottom mesons Hua-Xing Chen1 a , Wei Chen2 b , Xiang Liu3,4 c , Yan-Rui Liu5 d , and Shi-Lin Zhu6,7,8 1

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arXiv:1609.08928v1 [hep-ph] 28 Sep 2016

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School of Physics and Beijing Key Laboratory of Advanced Nuclear Materials and Physics, Beihang University, Beijing 100191, China Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E2, Canada School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China Research Center for Hadron and CSR Physics, Lanzhou University and Institute of Modern Physics of CAS, Lanzhou 730000, China School of Physics and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan 250100, China School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China Collaborative Innovation Center of Quantum Matter, Beijing 100871, China Center of High Energy Physics, Peking University, Beijing 100871, China Received: date / Revised version: date Abstract. Since the discovery of the first charmed meson in 1976, many open-charm and open-bottom ∗ mesons were observed. In 2003 two narrow charm-strange states Ds0 (2317) and Ds1 (2460) were discovered by the BaBar and CLEO Collaborations, respectively. After that, more excited heavy mesons were reported. In this work, we review the experimental and theoretical progress in this field. PACS. 14.40.Lb Charmed mesons – 14.40.Nd Bottom mesons – 14.40.Rt Exotic mesons

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Introduction . . . . . . . . . . . . . . . . . . . . . . . 1.1 Quark level models . . . . . . . . . . . . . . . . 1.2 Heavy quark symmetry and effective Lagrangians 1.3 Unsettled issues . . . . . . . . . . . . . . . . . . Experimental progress on the heavy meson/baryons states . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 The charmed mesons . . . . . . . . . . . . . . . 2.2 The charmed-strange mesons . . . . . . . . . . 2.3 The bottom mesons . . . . . . . . . . . . . . . 2.4 The bottom-strange mesons . . . . . . . . . . . 2.5 The charmed baryons . . . . . . . . . . . . . . 2.6 The bottom baryons . . . . . . . . . . . . . . . 2.7 The doubly-charmed baryons . . . . . . . . . . 2.8 The X(5568) . . . . . . . . . . . . . . . . . . . Candidates of the conventional excited mesons . . . 3.1 The charmed mesons . . . . . . . . . . . . . . . 3.2 The charmed-strange mesons . . . . . . . . . . 3.3 The bottom mesons . . . . . . . . . . . . . . . 3.4 The bottom-strange mesons . . . . . . . . . . . Candidates of the exotic states . . . . . . . . . . . . ∗ 4.1 The Ds0 (2317) and Ds1 (2460). . . . . . . . . . 4.2 The X(5568) . . . . . . . . . . . . . . . . . . .

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Outlook and summary . . . . . . . . . . . . . . . . .

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1 Introduction According to the conventional quark model (QM), the mesons are composed of the quark-antiquark pair and baryons composed of three-quarks. Such a simple model has been very successful in explaining hadron properties. However, recent progress on hadron spectra is challenging the naive quark model [1, 2]. The challenges mainly come from the hadrons containing heavy quarks, i.e. charmoniumlike XYZ mesons, Q¯ q -type mesons, and Qqq-type baryons (Q denotes the heavy charm/bottom quark, and q denotes the light up/down/strange quark). The presence of the heavy quark degrees of freedom provides a useful handle to explore the candidates of the exotic hadrons. For the excited states, more decay channels are allowed and the coupled channel effects due to the nearby hadron-hadron thresholds affect significantly the hadron properties. For ∗ example, the low mass of the Ds0 (2317) is difficult to understand if one does not consider the contributions from the DK channel [3]. Up to now, all types of the QM mesons including q q¯, ¯ have been found. But for the baryons, even the Q¯ q and QQ lowest QQq baryon (Ξcc ) has not been confirmed, and no QQQ baryon is observed at all. Although there are good candidates of exotic hadrons beyond the QM assignment,

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Hua-Xing Chen et al.: A review of the open charm and open bottom mesons

e.g. the glueballs and hybrid mesons, their confirmation screening effect are sometimes important. In this section is still on the way. The study of hadron spectra helps us we first give an outline of the widely used quark level understand how the strong interaction binds the quarks methods and hadron level methods and present the deand gluons into matter fields and find out the relation tailed results in Sec. 3 together with reviews on the conventional excited charmed and bottom mesons. In Sec. 4 between QM and quantum chromodynamics (QCD). There have accumulated huge experimental data in we will discuss the candidates of the exotic states, i.e., the ∗ hadron spectroscopy in the past decade. The theoretical Ds0 (2317), Ds1 (2460) and the recently observed X(5568). progress is also significant. New phenomena on the higher For the recent theoretical progress about the heavy baryons, hadrons provide us a good opportunity to understand the we refer to Ref. [5]. Before reviewing the widely used quark level methstrong interaction deeper. We have noticed that there exods and hadron level methods, we would like to note that ist recent nice reviews for different types of hadrons in the literature: for example, reviews on baryons [4, 5], hybrid the basic scales in QCD are the ΛQCD , the quark masses 0 mesons [6], heavy quark pentaquarks and tetraquarks [2, mq s and the scale of chiral symmetry breaking Λχ . Several symmetries of QCD are hidden behind these scales. 7], exotic hadrons [8, 9], and heavy hadrons in nuclear matFor example, in the limit mu,d,s → 0, QCD has the chiral ter [10]. See also reviews in Refs. [11, 12,13,14, 15, 16, 17]. symmetry which is spontaneously broken below the scale In this review, we mainly focus on open-flavored heavy hadrons including the charmed and bottom mesons. The Λχ ∼ 1 GeV. The c/b quark is much heavier than the u/d/s quark. Contrary to the chiral symmetry, there is antop quark decays weakly before it transforms into a hadron [18, 19,20]. For the Bc system, the readers may consult the ref- other symmetry in the heavy quark sector. In the infinitely heavy limit of the heavy quark mass, the QCD Lagrangian erences [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33]. has a heavy quark symmetry which has two meanings: (1) In recent years, the development on experimental meaheavy quark flavor symmetry (HQFS) which is a symmesurements makes it possible to investigate excited hadrons. try for the exchange of heavy quark flavors b ↔ c; and (2) New open-flavored hadrons (HQ ), especially the mesons, heavy quark spin symmetry (HQSS) which is a symmehave been observed at ee, pp, and ep colliders. There, try for the exchange of heavy quark spins ↑Q ↔↓Q . This the produced hard heavy quark becomes a softer heavy spin-flavor symmetry plays a crucial role in understanding quark by emitting gluons or massive gauge bosons and the properties of heavy quark hadrons. Both the quark then fragments into HQ nonperturbatively. The hadrons level and hadron level investigations involve this imporare usually detected in B (Bs ) decays or inclusive protant symmetry. + − ¯ ductions, i.e. e e → QQ → HQ + X, pp → HQ + X, ep → HQ +X. Many interesting states were observed such ∗ as the charmed-strange mesons Ds0 (2317) and Ds1 (2460), 1.1 Quark level models which are lower than the QM prediction and were discussed widely in terms of the various configurations like The basic approach to study hadron spectra is the quark the molecule, tetraquark, and coupled channel effect. potential model. Generally speaking, the potential includes Since the mass splittings between the higher states are the contributions from the color Coulomb interaction, spinsmaller than those of the lower states, different assign- orbit interaction, spin-spin interaction, and quark confinements (orbital or radial excitation states) are possible and ment. The first three parts result from the one-gluontheir nature needs detailed investigations. Very recently, exchange force [36] between free quarks while the last the D0 Collaboration reported the observation of a four- part is added phenomenologically to meet the fact that quark candidate X(5568) but the LHCb and CMS anal- the quark interaction becomes stronger and stronger with yses do not support its existence. Some theoretical inves- the increasing distance and thus no colored hadron exists. tigations also find it difficult to understand its low mass Since the potential is not an experimental observable, any and production mechanism. On the other hand, there are versions of the potential model that can reproduce the also developments on the charmed and bottom baryons in hadron masses are acceptable. The confinement potential recent years. All these experimental information will be cannot be obtained analytically from QCD now. There reviewed in Sec. 2. exist various types, e.g. the linear potential [37, 38], logaBecause of the difficulty in understanding the nonper- rithmic [39], power-law [40], or error-function [41]. When turbative nature of QCD at low energy, one has to rely on considering the electromagnetic properties, one needs the the effective theoretical approaches to study hadron prop- additional one-photon-exchange interaction terms. Before ∗ (2317), Ds1 (2460), and erties. Various methods reflecting several aspects of QCD the observation of the exotic Ds0 have been proposed, such as the relativistic quark model, X(3872), the quark model gives satisfactory descriptions the constitute quark model, the chiral quark model, the for the hadron spectra except a few exceptions, e.g. the quark pair creation (QPC) model, the Regge trajectory Roper resonance and Λ(1405). The interpretation of these phenomenology, the chiral unitary model, the QCD sum candidates of the exotic mesons requires the important rule, and some effective Lagrangian theories/approaches, coupled channel effects. The quark potential model has etc. Among these models, the most famous one is the to be improved to account for the properties of the new Godfrey-Isgur (GI) relativized quark model [34, 35], which hadrons. The most famous potential is given in the Godfreywe shall pay particular attention to in the present review. Besides these models, the coupled-channel effect and the Isgur (GI) relativized quark model [34, 35]. Its Hamilto-

Hua-Xing Chen et al.: A review of the open charm and open bottom mesons

nian includes a relativistic kinetic term and a momentumdependent potential Vef f (p, r). The potential in the nonrelativistic limit becomes  X Veff (r) → H conf + H hyp + H SO(cm) + H SO(tp) , i