Strange Quarkonium States at BES *

Chinese Physics C Vol. 39, No. 8 (2015) 082001

Strange quarkonium states at BES0 *

LIU Pei-Lian(4ê)1;1) FANG Shuang-Shi(V)1;2) LOU Xin-Chou(£"Î)1,2 1 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China 2 University of Texas at Dallas, Richardson, Texas 75080-3021, USA

Abstract: We present physics opportunities and topics with the s¯s states (strangeonia) that can be studied with the BES0 detector operating at the BEPC/ collider. Though the φ and η/η0 states have long been established experimentally, only a handful of strangeonia are well known, in contrast to the rich c¯c charmoium system. An overview of the s¯s states and their experimental status is presented in this paper. The BES0 experiment has collected the world’s largest samples of J/ψ, ψ(2S), ψ(3770), and direct e+e− annihilations at energies below the J/ψ and above 3.8 GeV, and will continue to accumulate high quality, large integrated luminosity in the τ-charm energy region. These data, combined with the excellent performance of the BES0 detector, will oﬀer unprecedented opportunities to explore the s¯s system. In this paper we describe the experimental techniques to explore strangeonia with the BES0 detector.

Key words: strangeonium state, BES0 detector, charmonium decay, e+e− annihilation PACS: 13.25.Gv, 13.66.Bc DOI: 10.1088/1674-1137/39/8/082001

1 Introduction heavy quark effective theory is not applicable. The study of strangeonium mesons is of particular Half a century ago the quark model was introduced to interest since they are a bridge between the light u, d describe the large array of hadrons, within which mesons quarks and the heavy c, b quarks. In addition, the are composed of qq¯ bound together by the strong inter- strangeonium spectrum also helps identify the exotics action, and baryons consist of three quarks. Since then (e.g., glueball, hybrid, multi-quark states) which can the quark model has offered a useful tool in understand- decay into the same final states. Within the frame- ing the substructures of hadrons, and led to the advent work of the relativistic quark model with QCD [1], a of Quantum Chromodynamics (QCD) for describing the spectroscopy similar to heavy quarkonia is expected for strong interaction. With the development of accelerators strangeonium (s¯s) states. Their decays were studied in 3 and detectors, hundreds of hadrons have been discovered detail with the P0 model [2], which is a phenomelogical and the quark model provides a good description of the theory for describing light meson decays. The s¯s spec- 3 observed hadrons, in particular the ground states and troscopy with spin J < 4 predicted by the P0 model is the heavy quarkonium states. Taking charmonium spec- shown in Fig. 1. Only seven states (underlined with the troscopy, bound states of the charmed quark and anti- solid lines) have been established and many members of quark (c¯c), as an example, it reflects a beautiful regular- the spectrum are still missing. ity and numerous electromagnetic and strong transitions. The poor experimental situation is largely due to the However, there are still many unseen states predicted by small fractions of s¯s states produced among hadrons, the quark model, in particular the s¯s states, for which and their large width. At present the experimental the experimental situation is not well understood. information on the strangeonium states mainly came Being the bound state of a light s and anti-s quark, a from the diffractive photoproduction reactions γp Xp, → strangeonium state is related to long-range (i.e., confine- strangeness exchange reactions K−p XΛ, and e+e− col- → ment) interactions, which provide information on non- lisions. Given our unsatisfactory knowledge of strangeo- perturbative QCD in the low energy region, where the nium, the search for these missing states is critical for

Received 15 May 2015 ∗ Supported by CAS/SAFEA International Partnership Program for Creative Research Teams, CAS and IHEP grants for the Thou- sand/Hundred Talent programs and National Natural Science Foundation of China (11175189) 1) E-mail: [email protected] 2) E-mail: [email protected] Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd

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high statistics measurements, the study of η0 decays has already been listed in the physics programs of many experiments, including CLAS, Crystal Ball, WASA-at- COSY and KLOE-2. In addition to τ-charm physics, BES0 also has the capability to investigate η0 decays via J/ψ radiative or hadronic decays. Based on a sample of 1.3 109 J/ψ events, BES0 has made many important con×tributions to the study of η0 decays, including the measurements of η0 hadronic decays [5], and the search for rare or forbidden decays [6].

3 2.2 φ (1 S1) Another well established strangeonium is the 3 φ(1020), which is the 1 S1 state. The φ meson was first seen in a bubble chamber experiment at Brookhaven in 1962, was subsequently determined to be a vector meson with J P C =1−−, and could be accommodated as an SU(3) singlet, supplementing an octet of other vector mesons (e.g., ρ, K∗ and ω). To explain that φ prefers Fig. 1. The strangeonium family. to decay into two kaons but not ρπ, the Okubo-Zweig- Iizuka (OZI) rule [7] was introduced. The φ contains understanding the qq¯ interaction. a s¯s pair; when it decays, the strange quarks have to In this paper, we will review the experimental status go somewhere, and the kaon pair route is the only pos- of strangeonia, show the possible J/ψ and ψ0 decays best sibility. Since it has a quite narrow width and could suited for the search and study of new strangeonia, and be directly produced in e+e− collisions, an φ factory, propose approaches to search for new strangeonium-like named DAΦNE, was built to study its properties using states similar to the charmonium-like states (X, Y, Z), the abundant production of φ collected with the KLOE which were discovered at the Babar, Belle and BESIII detector, which had made a significant contribution to experiments. We identify the e+e− (s¯s)(s¯s) interaction kaon and hadronic physics in the low QCD energy re- → as an especially important data sample to reconstruct gion. An upgrade of DAΦNE and the KLOE detector new s¯s mesons with high efficiency in a mode indepen- were performed. The luminosity was designed to be im- dent way. proved by a factor of 3 and the detector was also up- graded accordingly. A review of the physics with the 2 Strangeonia status KLOE-2 detector at DAΦNE is given in Ref. [8].

1 0 1 1 P1 2.1 η (1 S0) 2.3 h (1380) ( ) 1 The strangeonium spectrum is shown in Fig. 1. Start- The P1 strangeonium state, h1(1380), is still a poorly 1 0 known meson. In 1988, the first evidence of the 1P ing with the 1 S0 state, the η was discovered a half cen- 1 tury ago in bubble chamber experiments [3]. Since then strangeonium state was seen by the LASS spectrometer − the η0, being a pure SU(3) singlet, has attracted both at SLAC, from a Partial Wave Analysis (PWA) of K p K0 Kπ∓Λ [9]. It was confirmed in the pp¯ K K π0π→0 theoretical and experimental attention due to its special S → L S role in understanding low energy QCD. Because of the by the crystal barrel detector at LEAR with a mass of 0 M =(1440 60) MeV/c2 and a width of Γ =(170 80) MeV chiral anomaly, the η is a non-Goldstone boson which distinguishes itself from the others in the ground pseu- [10]. Due to the severe suppression by the phase space, ∗ doscalar nonet. Therefore the η0 provides a unique field its dominant decay is K K. To date only these two exper- to test the fundamental symmetries and the predictions iments have observed this state, and further confirmation for chiral perturbative theory. In addition, the η η0 mix- from other experiments is strongly needed. There is some − ing issue remains. Of interest is the gluonic content in theoretical interest in h1(1380). In particular, Ref. [11] η0, which makes this case more complicated. Many in- discusses the origin of this state in chiral dynamics from teresting and important issues are extensively discussed the interaction of the vector-pseudoscalar. in Ref. [4]. 3 2.4 f1(1420) ( P1) In general our present knowledge of the η0 is based on limited statistics; only its most dominant decays have The first evidence for the f1(1420) was seen in the been observed, and are not well measured. To perform K∗K¯ mass spectrum in π−p reactions [12], but it is

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3 questionable because the structures around 1.4 1.5 GeV 2.6 φ(1680) (2 S1) (e.g., η(1405), η(1475)) are found to be complicated.− De- The φ(1680) is well established in e+e− production. tailed analyses [13] confirmed its existence and the spin- It was reported in e+e− K Kπ∓ [24] and subsequent parity is determined to be J P C =1++. The observations S analyses [25] found the →small rate of φ(1680) coupling of f (1420) in central production [14] provides unambigu- 1 to K+K−. The absence of φ(1680) in e+e− ωπ+π− ous evidence that f (1420) has spin 1 and suggests a non- 1 indicates that the φ(1680) is the promising 23→S s¯s can- strange quark component in f (1420) despite its decay 1 1 didate because it prefers to decay into strange mesons to K∗Kπ. In addition, the f (1420) is not seen in the 1 as expected from the OZI rule. The B factories [26] ob- strange exchange K−p interactions, in which another me- served the φ(1680) via the ISR process and found a new son, the f (1510), possibly having large s¯s components, 1 decay mode of φ(1680) K+K−π+π−. is observed. Therefore, it has been suggested that the The φ(1680) is also→expected to be observed in pho- f (1420) could be a hybrid [15], a four quark [16] or a 1 toproduction experiments. However, the latest results molecular-like state [17]. on γp K+K− show a clear structure in the K+K− mass This state has also been investigated in radiative and spectrum→ with a mass of M =1753 3 MeV/c2 and a width hadronic decays of the J/ψ. MARK 0 [18] reported a of Γ =122 63 MeV, which is in good agreement with re- structure around 1.42 GeV observed in the K∗K¯ mass sults fromprevious photoproduction experiments. The spectrum in J/ψ ωK∗K¯ π, but no similar peak was mass is much higher than the 1680 MeV/c2 reported from seen in J/ψ φK→K¯ π. These results were confirmed e+e− experiments. No structure was observed around by the BES/→experiment [19] using the 58 million J/ψ the 1.68 GeV/c2 or 1.75 GeV/c2 mass regions in the events. The mass and width obtained from a fit to the γp K Kπ∓p interaction. The discrepancy on the K∗Kπ mass are consistent with those of the f (1420). S 1 measured→ mass between experiments could be explained The spin-parity is not determined here due to the large by the interference with other mesons. The probability background. The amplitude analyses of J/ψ γKK¯π that the structure around 1.75 GeV/c2 observed in the indicates that the structure observed around 1.42→ GeV photoproduction process is a new state cannot be ruled in the KK¯ π mass spectrum is a mixture of two or three out [27]. states [20]. Therefore, study of the φ(1680) is necessary to clar- The f (1510) could be the 3P s¯s state instead of the 1 1 ify its nature. Based on the prediction of the 3P model, f (1420) due to its production in hadronic K−p experi- 0 1 the branching fraction of φ(1680) decaying into φη is ments. However, the absence of the f (1510) in hadronic 1 smaller than those of KK¯ and K∗K. This has not been and radiative J/ψ decays makes this case complicated. studied yet, but will play an important role in addressing Given the complexity in the KK¯π0 system, it is impor- the discrepancies. The data accumulated at BES0 allow tant to extract these states in J/ψ decays using high us to investigate the φ(1680) produced in charmonium statistics data which will enable PWA. In addition to decays. K∗K, the observation of new decay modes, e.g., π+π−η 3 and 4π, could also provide valuable information on the 2.7 φ3(1850) (1 D3) f1(1420). Recently, BES0 reported evidence for f1(1510) ψ γπ+π−η0 The φ3(1850) was first reported with a mass of observed in J/ decays [21]. 2 +40 → 1850 10 MeV/c and a width of 80−30 MeV by a bubble chamber experiment [28] in the KK¯ mass spectra in 0 3P 2.5 f2(1525) ( 2) the K−p KK¯Λ interaction. A study of the K∗K¯ mass spectrum→in the same experiment also indicated a struc- 0 3 2 The f2(1525) is widely accepted as the P2 state, ture around 1.85 GeV/c and production rate compatible − 0 0 ¯ which was first seen in π p KSKSn collisions with lim- with the KK decay mode. The Omega experiment [29] ited statistics. Since then this→ state has been observed observed a similar structure in the hypercharge exchange in many different production processes, including K−p reaction K−p KK¯(Λ/Σ0). Its spin-parity was deter- →P C collisions, e+e− annihilations, pp¯ annihilations, and ep mined to be J = 3−−, which is consistent with the − 0 0 collisions. Given that it decays dominantly into KK¯ in- absence of φ3(1850) in the reaction K p K K Λ. The → S S stead of ππ and is only observed in J/ψ φKK¯ decays, high statistics data from the LASS experiment [30] con- → 0 it seems to be an s¯s meson. A new idea that the f2(1525) firmed the existence of φ3(1850) and the spin-parity of is dynamically generated from the vector-vector interac- J P C =3−−. The state is interpreted as the φ like of the P C − tion has been introduced in Ref. [22]. Support for this J =3−− nonet. idea can be found in Ref. [23], in which approaches to 3S 3D 0 2.8 φ(2170) (3 1 or 2 1) study the production of f2(1525) and other resonances ∗ 0 (f0(1370), f0(1710), f2(1270), K2(1430)) in the J/ψ, ψ Another possible strangeonium is the φ(2170), which and Υ(nS) decays are broadly discussed. was first discovered with a mass of 2175 10 15 MeV/c2

082001-3 Chinese Physics C Vol. 39, No. 8 (2015) 082001 and a width of Γ = 58 16 20 MeV in the initial state many interpretations. radiation (ISR) processe+e− γφf (980) [31]. The BES 0 2.9 Other strangeonium states experiment confirmed this resonance→ in the hadronic de- 3 cay of J/ψ φf0(980)η [32] and the resonance parame- Based on the expectations of the P model, orbital → 0 ters are measured to be M = 2186 10 6 MeV/c2 and and radial excited strangeonium states are summarized Γ =63 23 17 MeV. In 2007, the Babar experiment up- in Table 1, where the masses and widths are from the + − dated the results with both decays of f0(980) π π world average values in Ref. [27] or Ref. [2]. Most of the 0 0 → 2 and f0(980) π π [33]. In addition Babar also ob- unobserved strangeonium states are in the 1 2 GeV/c → served an evident structure in the process of e+e− mass range; their widths are expected to be ∼broad. Due → γφη [34]. Belle [35] also observed this structure. The to the overlap with the qq¯ (q=u, d) states, it is hard for +79 2 mass, 2079 13−28 MeV/c , is consistent with previous an experiment to observe them by bump hunting alone. +25 reports, while the width 192 23−61 MeV, is broader. The Thus, high statistics experiments with large acceptance discovery of φ(2170) triggered many theoretical specu- spectrometers and full PWA in several different channels lations on its nature. Ref [36] performs Faddeev cal- are required to sort out the multi-states and to deter- culations for the three mesons system, φKK¯, and ob- mine their quantum numbers. The application of the tains a neat resonance peak around a total mass of PWA technique necessarily includes information about 2150 MeV and an invariant mass for the KK¯ system normal qq¯ mesons, both established states and undiscov- around 970 MeV. This finding provides a natural expla- ered ones. This also applies to the identification of the nation for the φ(2170). At present, this state is listed hybrid mesons with non-exotic quantum numbers that in the PDG [27] as the excited φ meson, which is also can mix with normal qq¯ states. Thus, as part of the referred to as Y(2175) in the literature. program of identifying hybrid mesons, the high statistics The theoretical models [37] present many possible de- data sets will enable the study of ground state qq¯ mesons cay modes of the φ(2170). For most of the dominant de- as well. ∗ ∗ ∗ cays, e.g., K K¯ , K(1460)K¯, K (1410)K¯ and K1(1270)K¯, the mass threshold is quite close to the mass of the 2.10 Current experiments studying strangeonia φ(2170), and the broad strange meson also makes it dif- Different experiments, using e+e− collision, photo- ficult to see a clear structure with the complicated final production (γp), and hadron production (Kp), have con- states. Besides KK¯ ∗, the study of the decay modes φη tributed greatly to the understanding of the s¯s spectrum. and φη0 is very helpful to distinguish φ(2170) among However, as discussed above, our present information on

Table 1. Summary of strangeonium states and their dominant decays. 2S+1 2 N LJ state mass (MeV/c ) width (MeV) dominant decays 1 0 + − 1 S0 η (957.780.06)exp. (0.1980.009)exp. ππη, γπ π 3 + − 0 1 S1 φ (1019.4610.019)exp. (4.2660.031)exp. KK, π π π 3 ∗ 2 S1 φ(1680) (168020)exp. (15050)exp. KK,¯ K K,¯ φη 1 ∗ 2 S0 η(1440) ∼ 1440 11∼100 K K¯ 3 ∗ ∗ ∗ ∗ 3 S1 φ(2050) ∼2050 ∼380 K K,¯ K K¯ , K1(1270)K¯ , K1(1402)K¯ , K (1414)K¯ , φη 1 ∼ ∼ ∗ ¯ ∗ ¯ ∗ ∗ ¯ ∗ ¯ 3 S0 ηs(1950) 1950 175 K K, K K , K0(1430)K, K (1414)K 3 1 P0 f0(1500) ∼1500 279 KK¯ , ηη 3 ∗ 1 P1 f1(1420) (1426.40.9)exp. (54.92.6)exp. K K¯ 3 0 +6 ¯ ∗ ¯ 1 P2 f2(1525) (1525 5)exp. (73−5)exp. KK, K K, ηη 1 ∗ 1 P1 h1(1380) (138619)exp. (9130)exp. K K¯ 1 ∗ ∗ ∗ 2 P1 h1(1850) ∼1850 193 K K,¯ K K¯ , φηs 3 ∗ ∗ 2 P0 f0(2000) ∼2000 ∼800 KK¯ , K K¯ , K1(1270)K¯ , ηηs 3 ∗ ∗ ∗ ∗ 2 P1 f1(1950) ∼1950 ∼300 K K¯ K K¯ , K1(1270)K¯ , K (1410)K¯ 3 ∼ ∼ ∗ ¯ ∗ ¯ ∗ ¯ ∗ ¯ 0 2 P2 f2(2000) 2000 400 K K, K K , K1(1270)K, K2(1430)K, ηη, ηη 1 ∗ ∗ ∗ 1 D2 η2(1850) ∼1850 129 K K,¯ K K¯ 3 ∗ 1 D1 φ(1850) ∼1850 ∼650 KK,¯ K K,¯ K1(1270)K¯ , φη 3 ∗ 1 D2 φ2(1850) ∼1850 214 K K,¯ φη 3 +28 ¯ ∗ ¯ 1 D3 φ3(1850) (1854 7)exp. (87−23)exp. KK, K K 1 ∼ ∼ ∗ ¯ ∗ ¯ ∗ ∗ ∗ 1 F3 h3(2200) 2200 250 K K, K K , K2(1430)K, K K1(1270), φη 3 ∼ ¯ ∗ ¯ ∗ ¯ ∗ ∗ ∗ 1 F2 f2(2200) 2200 425 KK, K K, K K , K1(1270)K, K2(1430)K, K K1(1270) 3 ∼ ∼ ∗ ¯ ∗ ¯ ∗ ∗ ∗ 1 F3 f3(2200) 2200 300 K K, K K , K1(1270)K, K2(1430)K, K K1(1270) 3 ∗ ∗ ∗ 1 F4 f4(2200) ∼2200 ∼150 KK¯ K K¯ , K K¯

082001-4 Chinese Physics C Vol. 39, No. 8 (2015) 082001 the strangeonium spectrum is still far from complete. gion. Recently a preliminary result on the observation Even for established strangeonium states, there are still of φ(1680) was reported by CMD-3. The alternative many unresolved issues. Due to the low production rates, strategy is to use B-factory data, after radiation of a broad widths and complex final states, an experiment hard initial state photon, to investigate the light hadron must have both much higher statistical sensitivity and spectroscopy; many nice results from Belle and Babar good acceptance to make a significant contribution to have been summarized in Ref. [39]. In the near future, the strangeonium sector. Table 2 summarizes current the Belle-II detector will collect an enormous amount of and future experiments which have the capability of ex- data for the study of heavy flavor physics, as well as the ploring the strangeonium states. study of the strangeonium sector with high precision. On hadron production, the PANDA experiment be- Table 2. Summary of current or future experi- ing constructed at FAIR is a general purpose spectrome- ments studying strangeonium physics. ter that will map the hadron spectrum, using antiproton experiments timeline production beams colliding with an internal proton target. Based on BES0 2008 charmonia decays, e+e− annihilations simulations and the studies of specific physics channels, CMD-3 2010 e+e− annihilations PANDA will be excellent [40] at distinguishing states of GlueX 2015 photoproduction interest from the huge number of background events. CLAS12 2015 photoproduction BelleII 2016 ISR PANDA 2018? hadronic production 3 Study of s¯s states through charmonium decays The BES0 detector [21] fulfils these requirements 3 with its excellent performance. The superconduction According to the predictions from the P0 model, solenoid and the helium-based drift chamber, cover- most of these states are broad and the decay final states ing 93% of the 4π stereo angle, give high acceptance are complicated, so they are usually indistinguishable in and good momentum resolution; the combination of the the mass spectrum. We therefore need to have an exper- Time-Of-Flight (TOF) and dE/dx measurements pro- iment with excellent charged and neutral detection and vide good particle identification; the high-performance high statistics to hunt for those states from a large back- CsI calorimeter has an energy resolution of 2% for 1 GeV ground. The BES0 detector at the BEPC/, running photons. The available high statistics data offer a unique at the energy range of 2–4.6 GeV, may provide an im- opportunity to study strangeonium spectroscopy. portant contribution to this field. The BES0 detector In the case of photoproduction, the Jefferson Lab [38] is a large solid-angle magnetic spectrometer with high is completing its upgrade of the CEBAF accelerator to acceptance, full primary vertex reconstruction and sec- 12 GeV electron beam, along with new installations to ondary vertex reconstruction for long lived particles such 0 study meson spectroscopy. The GlueX detector in Hall- as KS and Λ, high particle identification efficiency, good D is designed to have a uniform acceptance overall and momentum resolution of charged particles and excellent cover all decay angles, which is essential for amplitude energy resolution of electrons and photons. In addition, analysis. Using the linearly polarized photon produced the designed peak luminosity of BEPC/, 1033 cm−2s−1 by the coherent Bremsstrahlung of the primary electron at 3.773 GeV, is about 100 times better than its prede- beam on a diamond radiator, GlueX is capable of search- cessor, which allows us to accumulate the very large data ing for exotics as well as performing conventional meson samples in a short period of time. About 1.3 billion J/ψ spectroscopy. events1) and 0.5 billion ψ(2S) events have been collected With the Forward Tagger Facility, CLAS12 in Hall- by the BES0 detector, which will provide a great oppor- B, which has been designed to determine the Generalized tunity to perform experimental studies of strangeonium Parton Distributions (GPDs), will also have the capabil- produced in J/ψ and ψ(2S) decays. ity to study the meson spectrum using virtual photons. The world’s largest direct data J/ψ, ψ(2S) samples, In the last decade, the KLOE experiment at DAΦNE collected by the BES0 experiment, offer excellent oppor- played an important role in the study of φ, η and η0 tunities to detect new s¯s mesons and new states through decays. The KLOE-2 experiment has a plan to collect the decays of the J/ψ and ψ(2S). Many J/ψ, ψ(2S) about 50 fb−1 in several years, which will investigate φ, decays to φ and η have been well measured. Examples η and η0 decays with unprecedented precision. The other of such decays are J/ψ ηK∗K¯ ∗, φK∗K¯ , φKK,¯ φf, φππ, two e+e− experiments, CMD-3 and SND at VEPP-2000, φη, ηππ. Possible new→s¯s states can be probed through are collecting data at the center-of-mass of 0.3 2.0 GeV, the recoil masses of the K∗K¯ ∗, K∗K¯ , KK¯, f, ππ, η/φ, re- which mainly focus on the physics in the low∼energy re- spectively, based on the expectation that the same pro-

1) With the same approach as for J/ψ events in 2009 [41], the preliminary number of J/ψ events is determined to be 1086.7×106. The total number of J/ψ events taken in 2009 and 2012 is determined to be (1310.610.5)×106 .

082001-5 Chinese Physics C Vol. 39, No. 8 (2015) 082001 cesses that produce the φ, η in the J/ψ, ψ(2S) decays At BES0, different processes, including ISR, J/ψ de- will couple to new s¯s mesons which are kinematically al- cays and data taken at the peak of the φ(2170) could be lowed. used to make an extensive study of the φ(2170). First, 0 + − 0 The J/ψ γη is a two-body decay and the photon the ISR process e e γISR(φ ) could also be used to energy is mono→chromatic, which makes it easy to distin- study φ(2170) φππ. →The most significant data samples guish the η0 from the background. Considering the large recorded by BES→ 0 are 2.9 fb−1 ψ(3770) and 3 fb−1 branching fraction of J/ψ γη0, the J/ψ data sample above 3.8 GeV. They are∼ not sufficient for an extensiv∼ e offers a clean environment→to investigate η0 anomalous study of the φ(2170). decays. For the φ(2170) in J/ψ ηφπ+π−, the background The φ production rate in J/ψ hadronic decays is at level is quite high under the→ φ(2170), which makes it a level of 3 10−4 2 10−3. The φ could be easily recon- hard to investigate the φπ for any states with a small structed with× its ∼dominan× t K+K− decay mode. Of inter- production rate. Compared to the other e+e− exper- est is to investigate the mass spectrum recoiling against iments, BEPC/ has an advantage in taking data di- φ to search for the rare decays of η/η0 via the two-body rectly at 2.2 GeV. With the assumption of 100 pb−1 0 decays of J/ψ φη/η [42]. In addition, the precision data at the peak of φ(2170) and (φ(2170) Zsπ) 10% measurement of→the full set of J/ψ decays into a vector (φ(2170) φf (980)), the observB ed n→umber ∼of Z ×B → 0 s and a pseudoscalar pair are also allowed to investigate events is estimated to be L 10% σ(φ(2170) φf0(980)) the pseudoscalar mixing and the gluonic content of the ε (φ K+K−) 300, where× the× efficiency→is estimated× η0. to×Bbe ab→out 50%.∼This may enable us to investigate the 3 According to the P0 model, the h1(1380) dominantly existence of Zs. decays into K∗K, whereas ωη, and ρπ are suppressed due to the OZI rule [43]. The φη mode could be its fa- 5 Search for new s¯s mesons in e+e− → vorable decay mode, but is strongly suppressed because (s¯s)(s¯s) interactions of the limited phase space. Therefore the J/ψ K∗Kη → and J/ψ K∗Kη0 modes would be the most preferable We propose to search for new (s¯s) mesons produced → 0 channels to study the h1(1380) at BES . In addition, in association with a well established (s¯s) state such as 0 0 the radiative decays of h1(1380) to η or η may be re- the φ or η/η . This is motivated by the large (c¯c)(c¯c) constructed in J/ψ data, though there is no theoretical rates reported by the Belle [46] and BaBar [47] exper- prediction for their branching ratios. iments which have helped establish the ηc(2S) state in the recoil mass spectra against an J/ψ or an ψ(2S). Though the mechanism through which large e+e− 4 Hunt for strangeonium-like particles → at BES0 J/ψ, ψ(2S)+c¯c interactions occur is not well understood, if the same process works for the e+e− (s¯s)(s¯s) process, we can probe an (s¯s) system with a fully→ reconstructed In 2013, BES0 and Belle observed a charged φ, η or η0 with the BES0 data by examining the recoil charmonium-like Z+(3900) [44] state, and subsequently c side of the event, for which the recoil mass can be calcu- several similar structures were reported by the BES0 ex- lated by M = (√s E∗ )2 p∗2. Here the center of periment. These observations inspired an extensive dis- recoil q − φ − φ cussion on their internal substructures. Most recently, a mass energy √s is well measured, and the φ, η and η0 neutral partner of the Zc (3900) has been observed, which mesons are all narrow, resulting in a very good resolution + indicates that Zc (3900) is part of an isotriplet and sug- on the Mrecoil. This approach can probe the (s¯s) system gests a new hadron spectroscopy. and detect (s¯s) states on the recoil side without the need We propose a search for the charged strangeonium- to reconstruct specific final states of the (s¯s) meson in like structure in the decay φ(2170) π+π−φ, as the recoil. in Ref. [45]. Similar to Y(4260) →π+π−J/ψ and → Υ(10860) π+π−Υ(1S,2S), two charged strangeonium- 6 Summary like strucutures→ are expected to be observed in φ(2170) π+π−φ. Therefore, φπ is an ideal channel to detect→ Though a substantial number of hadrons have been strangeonium-like states. Since the isospin of the φπ established experimentally, there are predicted light system is 1, the isosinglet s¯s state decaying into φπ is hadrons in the mass region of 1 to 2 GeV/c2, which have suppressed by isospin violation. For the conventional not been observed yet. Many of the missing hadrons are mesons composed of u,d quarks, the φπ decay mode is strangeonium states. Techniques for searching for new strongly suppressed by the OZI rule. Therefore the ob- s¯s mesons with the BES0 detector, and topics related servation of a φπ decay mode may imply an exotic na- to the s¯s states, have been outlined in this paper. ture. High statistics data accumulated with the BES0 de-

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tector are important for the investigation of the strangeo- from conventional mesons. BES0 data can enable prob- nium spectroscopy, and will help distinguish exotic states ing of the meson spectrum with unprecedented detail.

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