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Towards a new spectroscopy
Nuclear Physics B (Proc. Suppl.) 185 (2008) 99–106 www.elsevierphysics.com
Towards a new spectroscopy R. Faccini1 1
University ”La Sapienza” and INFN Rome, Dipartimento di Fisica, 2 Ple Aldo Moro, I-00185, Rome, Italy I.
INTRODUCTION
Although the Standard Model of elementary particles is well established, strong interactions are not yet fully under control. We believe QCD is the field theory capable of describing them, but we are not yet capable, in most of the cases, to make exact predictions. Among the critical issues that are yet unknown, the possible existence of bound states of more than three quarks, and therefore the existence of a new spectroscopy, is an extremely interesting one. It has been shown recently how the diquarkantidiquark interpretation of the sub-GeV scalar meson nonet made of f0 (980), a0 (980), κ(800), σ(500) can lead to a remarkable description of the decay properties of these particles [1], adding a rather strong confidence that they are indeed tetraquark objects. On one side therefore the search has extended to further light mesons to search for higher mass tetraquark states. On the other side systems that include heavy quark-antiquark pairs (quarkonia) are ideal and unique laboratories to proble both the high energy regimes of QCD, where an expansion in terms of the coupling constant is possible, and the low energy regimes, where nonperturbative effects dominate. In the last years this field is experiencing a rapid expansion with a wealth of new data coming in from diverse sources: data on quarkonium formation from dedicated experiments (BES at BEPC, KEDR at VEPP-4M CLEO-c at CESR), clear samples produced by high luminosity B-factories (PEP and KEKB), and very large samples produced from gluon-gluon fusion in p¯ p annihilations at Tevatron (CDF and D0 experiments). In this review I will first summarize recent developments in the search for excited states of the scalar nonet among the light mesons. Next, the core of the paper will be spent to review the experimental evidences of new states that might be aggregations of more than just a heavy quark-antiquark pair. The currently most credited possible states beyond the mesons and the baryons are (you can find a review in [2]):
FIG. 1: Charmonium states with L <= 2. The theory predictions are according to the potential models described in Ref. [2].
FIG. 2: Bottomonium with L <= 2. The theory predictions are according to the potential models described in Ref. [2].
• hybrids: bound states of a quark-antiquark pair and a number of gluons. The lowest lying state is expected to have quantum numbers J P C = 0+− . The impossibility of a quarkonium state to assume these quantum numbers (see below) makes this a unique signature for hybrids. Alternatively a good signature would be the preference to decay into a 0920-5632/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2008.10.021
quarkonium and a state that can be produced by the excited gluons (e.g. π + π − pairs). • molecules: bound states of two mesons, usually rep¯ where Q is the heavy quark. resented as [Q¯ q ][q Q], The system would be stable if the binding energy would set the mass of the states below the sum of the two meson masses. While this could be the case for when Q = b, this does not apply for Q = c, where most of the current experimental data are.
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TABLE I: Most recent determination of the J P C = 1−− charmonium states from BES [3], compared to the 2006 edition of the PDG [4] M (MeV/c2 ) Γtot (MeV)
ψ(3770) ψ(4040) ψ(4160) ψ(4415) PDG2006 3771.1±2.4 4039±1.0 4153±3 4421±4 BES ’07 3771.4±1.8 4038.5±4.6 4191.6±6.0 4415.2±7.5 PDG2006 23.0±2.7 80±10 103±8 62±20 BES ’07 25.4±6.5 81.2±14.4 72.7±15.1 73.3±21.2
In this case the two mesons can be bound by pion exchange. This means that only states decaying strongly into pions can bind with other mesons (e.g. there could be D ∗ D states), and that the bound state could decay into it’s constituents. • tetraquarks: a quark pair bound with an antiquark ¯ A full nonet pair, usually represented as [Qq][q¯ Q]. of states is predicted for each spin-parity, i.e. a large amount of states is expected. There is no need for these states to be close to any threshold.
meson sector. The ¯ model of light-scalars is very effective at explaining the most striking feature of these particles, namely their inverted pattern, with respect to that of ordinary q q¯ mesons, in the mass-versus-I3 diagram [9]. Such pattern is not explicable using a q q¯ model. For
II.
LIGHT MESON SPECTROSCOPY
The problem of the interpretation of the light scalar mesons, namely f0 , a0 , κ, σ, is one of the oldest problems in hadronic physics, since the times of the paper by Gell-Mann and L´evy on the formulation of the Linearσ-Model [5]. For many years the question about the existence of the σ meson as a resonance in ππ scattering has been debated [6] and only recently a thorough analysis of ππ scattering amplitudes shows that σ(500) and κ(800) are poles on the second Riemann sheet [7], in other words, proper resonances. On the experimental side the reconsideration of σ was triggered by the E791 analysis of D → 3π data [8]. It has been recently stressed that light scalar mesons are most likely the lightest particles with an exotic structure, i.e., they cannot be classified as q q¯ mesons. Furthermore, their dynamics is tightly connected with instanton physics. It is indeed recently discussed how the consideration of instantonic effects allows for the building of a rather consistent model for the description of light scalar meson dynamics, provided that the hypothesis is made that these particles are diquark-antidiquark mesons [1]. Therefore, new ways of aggregation of quark matter could be established by the experimental/theoretical investigation of these particles. The idea of four-quark mesons dates back to the pioneering papers of Jaffe [9]. The suggestion of discussing exotic mesons and hadrons in terms of diquarks was instead introduced in [9] and extended in [10] to the scalar
MMeV
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FIG. 3: Vector mesons (q q¯ states) and the sub-GeV scalar mesons in the I3 − m plane.
example, in the q q¯ model, the f0 (980) should be an s¯ s state [11] while the I = 1, a0 (980), should be a u¯ u + dd¯ state. If this were the case, the degeneracy of the two particles would need some convincing explanation. Given the success of this identification, the quest for more such exotic states has opened. A relative of the lowest lying scalar mesons could have been found very recently by BaBar: the Y (2175), a particle first observed in the decay Y → φf0 (980) [12]. A recent study [13], investigating the mass spectrum of tetraquarks and reanalizing BaBar data, has shown that the Y (2175) has a mass compatible with diquarks bound states with two to four strange quarks and that it might have a large decay ¯ pair. The latter observation is an indication rate in a ΛΛ that the Y (2175) be a bound state of two strange and two light quarks.
III.
HEAVY QUARKONIUM SPECTROSCOPY
The heavy quark inside these bound states has low enough energy that the corresponding spectroscopy is close to the non-relativistic interpretations of the atoms. The quantum numbers that are more appropriate to characterize a state are therefore, in decreasing order of energy splitting among different eigenstates, the radial excitation (n), the spatial angular momentum L, the spin S and the total angular momentum J. Given this set of quantum numbers, the parity and charge conjugation of the states are derived by P = (−1)(L+1) and
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C = (−1)(L+S) . Figures 1 and 2 show the mass and quantum number assignments of the well established charmonium and bottomonium states.
A.
Charmonium spectroscopy
Figure 1 shows that all the predicted states below open charm threshold have been observed, leaving the search open only to states above the threshold. In this field the latest developments concern the measurement of the paramaters and the quantum number assignment for the J P C = 1−− states. The BES collaboration has recently performed a fit to the R scan results which takes into account interference between resonances more accurately [3]. The updated parameters are reported in Tab. I, compared with the most recent determinations. The J P C = 1−− assignment does not univocously identify the state, since both 2S+1 LJ =3 D1 and 3 D1 states would match it. The recent observation from Belle of the first exclusive decay of the ψ(4415) → DD2∗ (2460) [14], shows that this meson is predominantly D wave. At the same time the study from CLEO-c of the ψ(3770) → χcJ γ [15] confirms the dominance of the D wave also in this meson. Both these assignments confirm the theoretical predictions as shown in Fig. 1.
B.
Bottomonium spectroscopy
Figure 2 shows that the panorama in the bottomonium sector is much less complete, since there is a large number of states below the open bottom threshold which have not yet been observed. A significant step forward has been achieved recently BaBar which has observed the ground state, ηb in the radiative decays of the Υ(3S) [16]. The measured mass m(ηb ) = 9388.9+3.1 −2.3 ± 2.7 is in agreement with the expectations from the models, thus strengthening our understanding of the meson spectroscopy.
IV.
NON-STANDARD CHARMONIUM STATES A.
The X(3872)
The X(3872) was the first state that was found not to easily fit charmonium spectroscopy. It was initially observed decaying into J/ψπ + π − with a mass just beyond the open charm threshold [17]. The π + π − invariant mass distribution preferred the hypothesis of a X(3872) → J/ψρ decay, which would have indicated that if this were a charmonium state, the decay would have violated the isospin. Since it would be quite unusual to have the dominant decay to be isospin violating, a search of the isospin partner X + → J/ψρ was conducted invain by BaBar [18]. In the meanwhile the decay X → J/ψγ
Belle J/ψππKS
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FIG. 4: Measured mass of the X(3872) particle. The different production modes (B 0 → XKS and B − → XK − ) and the different decay modes (X → J/ψππ and X → D ∗0 D0 ) are separated.
was observed [19], implying positive intrinsic charge conjugation. The most recent developments concern the final assessment of the J P C of this particle and the indication that the X(3872) is a actually a doublet. The CDF collaboration has infact performed the full angular analysis of the X → J/ψππ decay [20] concluding that J P C = 1++ and 2−+ are the only assignments consistent with data. It also confirmed that the decays has a ρ as intermediate state. Combining this information with the preliminary result from Belle [21] which rules out the 2−+ hypothesis, the only possible assignment is J P C = 1++ . As far as the mass and width of the X(3872) are concerned, BaBar has published an analysis of the B → XK decays with X → D ∗0 D0 [23] while Belle has updated the mass measurements in X → J/ψππ decays [25]. The summary of all available mass measurements is shown in Fig. 4 where the measurements are separated by production and decay channel. There is an indication that the particle decaying into J/ψππ is different from the one decaying into D ∗0 D0 , their masses differing by about 4.5 standard deviations. In addition, the BaBar paper contains also a first measurement of the X(3872) width, Γ =(3.0+1.9 −1.4 ± 0.9)MeV. Finally the measurements of X the branching fractions in J/ψππ and D ∗0 D0 are summarized in Tab. II. B.
The 1−− family
The easiest way to assign a value for J P C to a particle is to observe its production via e+ e− annihilation, where the quantum numbers must be the same as the the photon: J P C = 1−− . B factories can investigate a large range of masses for such particles by looking for events where the initial state radiation brings the e+ e− centerof-mass energy down to the particle’s mass (the so-called
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TABLE II: Measured X(3872) branching fractions, separated by production and decay mechanism. The ratio of the measurements in the two production mechanisms is also reported as R0/+ = BF (B → XK − )/BF (B → XK 0 ). A ’∗ ’ indicates numbers which are derived from the published values by assuming gaussian uncorrelated errors. BaBar Belle combined BF(B → XK − )BF(X → J/ψππ)×105 1.01±0.25 ± 0.10 [22] 1.05±0.18 [17] 1.04±0.15∗ BF(B → XK 0 )BF(X → J/ψππ)×105 0.51±0.28 ± 0.07 [22] 0.99 ± 0.33∗ 0.72 ± 0.22∗ +1.9 − ∗0 0 5 BF(B → XK )BF(X → D D )×10 17 ± 4 ± 5 [23] 10.7±3.1−3.3 [24] 13 ± 3∗ BF(B → XK 0 )BF(X → D∗0 D0 )×105 22 ± 10 ± 4 [23] 17±7+3 19 ± 6∗ −5 [24] R0/+ with X → J/ψππ 0.50 ± 0.30 [22] 0.94 ± 0.26 0.75 ± 0.20∗ R0/+ with X → D∗0 D0 1.4 ± 0.6 [23] 1.6 ± 0.6∗ 1.5 ± 0.4∗
’ISR’ events). Alternatively, dedicated e+ e− machines, like CESR and BEP scan directly the certer-of-mass energies of interest.
FIG. 5: J/ψπ + π − invariant mass in ISR production.
Y (4260) → J/ψπ 0 π 0 and some events of Y (4260) → J/ψK + K − . While investigating whether the Y (4260) decayed to ψ(2S)π + π − BaBar found that such decay did not exist but discovered a new 1−− state, the Y (4350) [29]. While the absence of Y (4260) → ψ(2S)π + π − decays could be explained if the pion pair in the J/ψπ + π − decay were produced with an intermediate state that is to amssive to be produced with a ψ(2S) (e.g. an f 0 ), the absence of Y (4350) → J/ψπ + π − is still to be understood, more statistics might be needed in case the Y (4260) decay hides the Y (4350). Recently Belle has published the confirmation of all these 1−− states [30, 31] and at the same time has unveiled a new states that was not visible in BaBar data due to the limited statistics: the Y (4660). Figures 5 and 6 show the published invarianet mass spectra for both the J/ψπ + π − and the ψ(2S)π + π − decays. A critical information for the unravelling of the puzzle is whether the pion pair comes from a resonant state. Figure 7 shows the di-pion invariant mass spectra published by Belle for all the regions where new resonances have been observed. Although the subtraction of the continuum is missing, there is some indication that only the Y (4660) has a well defined intermediate state (most likely an f0 ), while others have a more complex structure. A discriminant measurement between Charmonium states and new aggregation forms is the relative decay rate between these decays into Charmonium and the decays into two charm mesons. Searches have therefore been carried out for Y → D (∗) D(∗) decays [32– 34] without any evidence for a signal. The most strin¯ gent limit is [34] BF (Y (4260) → D D)/BF (Y (4260) → J/ψπ + π − ) < 1.0@ 90% confidence level.
C. FIG. 6: ψ(2S)π + π − invariant mass in ISR production.
The observation of new states in these processes started with the discovery of the Y (4260) → J/ψπ + π − by BaBar [26], promptly confirmed both in the same production process [27] and in direct production by CLEOc [28]. The latter paper also reported evidence for
The 3940 family
Three different states have been observed in the past years by the Belle collaboration with masses close to 2 3940Mev/c : one, named X, observed in continuum events (i.e. not in Y (4S) decays) produced in pair with a J/ψ meson and decaying into DD ∗ [35]; a second one, named Y , observed in B decays and decaying into J/ψω [36]; a third one, named Z produced in two-photon
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FIG. 7: Di-pion invariant mass distribution in Y (4260) → J/ψπ + π − (left), Y (4350) → ψ(2S)π + π − (center), and Y (4660) → ψ(2S)π + π − (right) decays.
sured in this paper are lower than when observed, albeit 2 consitent (mY = 3914.6+3.8 −3.4 (stat.) ± 1.9(sys.)Mev/c , ΓY = 33+12 (stat.) ± 5(sys.)MeV), opens the interesting −8 possibility that the X and the Y particles be the same, thus solving the two abovementioned open issues.
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The X(4160)
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FIG. 8: The J/ψω distribution in a) B → J/ψωK + and b) B → J/ψωKS decays. The superimposed line is the result of the fit to the data.
reactions and decaying into D-pairs [37]. While the X is consistent with both J P C = 0+− and 1++ , the quantum number assignment of the Y and the Z states is clear: J P C = 1++ and 2++ respectively. Finally the Y is the only apparently broad state (Γ = 87 ± 34MeV). Because of these quantum number assignments and their masses these states are good candidates for the radial eccitation of the χ mesons, in particular the Z(3940) meson could be identified with the χc0 (2P ) and the Y (3940) with the χc1 (2P ). The unclear points are the identification of the X(3940) state and the explanation of why the Y (3940) state does not decay preferentially in D mesons. The most recent develpment on this topic is the confirmation from the BaBar collaboration of the Y (3940) → J/ψω decays [38]. The analysis utilizes the decay properties of the ω meson to extract a clean signal (see Fig. 8). The interesting part is that he mass and the width mea-
As we have already discussed, it is critical to investigate decay channels of the new states into D meson pairs. Unfortunately the detection efficiency for D mesons in low, due to the large number of possible decays. The Belle collaboration has developed a partial reconstruction technique that allows to overcome this limitation in the case of new states produced in continuum paired with known Charmonium states [39]. The Charmonium is fully reconstructed, while only one of the two D mesons is reconstructed. The kinematics of the other is inferred from the known center-of-mass energy and the different possibile D mesons are discriminated on the basis of the missing mass. This technique has allowed the confirmation of the X(3940) production and decay, and, most interestingly, the observation of the X(4160) state, decaying into DD ∗ . Given the fact that, for reasons yet to be understood, continuum events seem to produce J P C = 0−+ or 1++ states in pair with the J/ψ and since the measured mass is consistent with the expectations of a radial excitation of the ηc , this new state is likely to be an ηc (3S).
E.
The first charged state: Z(4430)
The real turning point in the query for states beyond the Charmonium was the observation by the Belle Collaboration of a charged state decaying into ψ(2S)π ± [40]. Figure 9 shows the fit to the ψ(2S)π invariant mass dis-
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picture can be drawn.
V.
FIG. 9: The ψ(2S)π invariant mass distribution in B → ψ(2S)πK decays.
tribution in B → ψ(2S)πK decays, returning a mass M = 4433 ± 4MeV/c2 and a width Γ = 44+17 −13 MeV. Due to the relevance of such an observation a large number of tests has been performed, breaking the sample in several subsamples and finding consistent results in all cases. Also, the possibility of a reflection of a B → ψ(2S)K ∗ ∗ decay has been falsified by explicitely vetoing windows in the Kπ invariant mass. In terms of quarks, such a state must contain a c and a c¯, but given its charge it must also contain at least a u ¯ The only open options are the tetraquark, the and a d. molecule or the threshold effects. The latter two options are possible due to the closeness of the D1 D∗ threshold.
FIG. 10: Measured masses of the newly observed states, positioned in the spectroscopy according to their most likely quantum numbers. The charged state (Z(4430)) has clearly no C quantum number.
Finding the corresponding neutral state, observing a decay mode of the same state or at least having a confirmation of its existence, are critical before a complete
NON-STANDARD BOTTOMONIUM STATES
Possible new forms of aggregations found in states with two charm quarks would apply also in states with two bottom quarks. In this respect Bottomonium spectroscopy is a very good test stand for the speculations made on the Charmonium one. On the other side, searching Bottomonium states is more challenging since they tend to be broader and there are more possible decay channels. Among the known states there is already one with unusual behaviour: there has been a recent observation [41] of an anomalous enhancement, by two orders of magnitude, of the rate of Y (5S) decays to a Y (1S) or a Y (2S) and two pions. This indicates that either the Y (5S) itself or a state very closeby has a decay mechanism which enhances the amplitudes for these proceeses. The presence of two decay channels to other bottomonium states excludes the possibility of this state being a molecular aggregrate, but all other models are possible and would predict a large variety of not yet observed states. In order to discriminate whether the exotic state coincides with the Y (5S) both Belle and BaBar have performed high luminosity scans of the center of mass energy of the collisions above the Υ(4S) mass. In December 2007 Belle performed a scan to further investigate their observation on the exclusive decays of the Υ(5S). They integrated a very high luminosity at six energy points in the Υ(5S) and Υ(6S) region, between √ S = 10.86 and 11.02 GeV [42]. The measured rate of observed decays into Υ(nS)π + π − presents a resonant structure which is higher and narrower than the previous measurements of the Υ(5S) properties (see Fig. 11): they measure as Breit-Wigner parameters m = 10889.6± 1.8(stat.) ± 1.5(sys.) MeV and Γ = 54.7+8.5 −7.2 (stat) ± 2.5(sys.), to be compared to m = 10865 ± 8 MeV and Γ = 110 ± 13 in PDG [4]. Few months later, in MarchApril 2008 BaBar has performed a high statistics scan of center-of-mass energy from the Υ(4S) mass to 11.2 GeV, the maximum energy achieavable by PEP-II. The capability to integrate significant luminosities (∼ 25pb−1 ) at energy points in only 5 MeV steps allowed a very accurate R = σhad /σμ0 scan which highlights a very rich structure of threshold openings. On top of such a structure it is extremely difficult to define resonance parameters. An attempt, published in [43] to fit the Υ(5S) parameters taking into account interference effects returns BreitWigner parameters which are closer to the ones observed by Belle:m = 10876 ± 2 MeV and Γ = 43 ± 4. Unfortunately for the quest for new states, these studies seem rather to indicate that the resonance parameters as fitted in inclusive R scans in presence of threshold openings present a bias.
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(a) Fit with common μ and Γ, χ2/n.d.f. = 39.4/16
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FIG. 11: Left: e+ e− → Υ(nS)π + π − cross-sections measured by Belle with the Breit-Wigner fit superimposed. The adshed line corresponds to the maximum of the inclusive cross-section. Right: R scan as measured by BaBar with a fit to two Breit-Wigners and two flat components, one interfering with both resonant structures and one not.
VI.
CONCLUSIONS
long established mesons.
More than 40 years after its first formulation, the quark model is still open to new observations. In the field of light mesons the identification of the scalar nonet with tetraquark states is becoming stronger and there are candidates for the corresponding excitations. Moreover the heavy-quarkonium is still a valid test ground for understanding QCD. The study of well established quarkonium states yields information on low energy QCD while the undestanding of the quarkonium spectroscopy, predictable in potential models, allows searches for different aggregation states than the
The high statistics and quality data from B-Factories have produced a very large number of new states whose interpretation is still a matter of debate. This paper attempted a categorized review of all these states. A full summary, including the most likely quantum number assignment is shown in Fig. 10. Lots of theoretical models have been developed to interpret the situation but the picture is far from complete: more precise predictions are needed from theory and a systematic experimental exploration of all possibile production and decay mechanisms of these new states is still in the works.
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[12] BABAR, B. Aubert et al., “A structure at 2175-MeV in e+ e- –¿ Phi f0(980) observed via initial-state radiation”, Phys. Rev. D74, 091103 (2006), [hep-ex/0610018]. [13] N. V. Drenska, R. Faccini and A. D. Polosa, “Higher Tetraquark Particles”, 0807.0593. [14] Belle, G. Pakhlova, “Observation of ψ(4415) → ¯ ∗2 (2460) decay using initial-state radiation”, DD arXiv:0708.3313 [hep-ex]. [15] CLEO, R. A. Briere et al., “Observation of ψ(3770) → γchic0 ”, Phys. Rev. D74, 031106 (2006), [hepex/0605070]. [16] BABAR, B. Aubert et al., “Observation of the bottomonium ground state in the decay υ(3S) → γηb ”, Phys. Rev. Lett. 101, 071801 (2008), [0807.1086]. [17] Belle, S. K. Choi et al., “Observation of a new narrow charmonium state in exclusive B − → K − π + π − J/ψ decays”, Phys. Rev. Lett. 91, 262001 (2003), [hepex/0309032]. [18] BaBar, B. Aubert et al., “Search for a charged partner of the X(3872) in the B meson decay B → X − K, X − → J/ψπ − π 0 ”, Phys. Rev. D71, 031501 (2005), [hepex/0412051]. [19] BABAR, B. Aubert et al., “Search for B + → X(3872)K + , X(3872) → J/ψγ”, Phys. Rev. D74, 071101 (2006), [hep-ex/0607050]. [20] CDF, A. Abulencia et al., “Analysis of the quantum numbers J(PC) of the X(3872)”, Phys. Rev. Lett. 98, 132002 (2007), [hep-ex/0612053].
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[21] Belle, K. Abe et al., “Experimental constraints on the possible J(PC) quantum numbers of the X(3872)”, hepex/0505038. [22] BABAR, B. Aubert et al., “Study of J/ψπ + π − states produced in B 0 → J/ψπ + π − K 0 and B − → J/ψπ + π − K − ”, Phys. Rev. D73, 011101 (2006), [hepex/0507090]. [23] BABAR, B. Aubert et al., “Study of Resonances in Exclusive B Decays to Dbar(*)D(*)K”, arXiv:0708.1565 [hep-ex]. [24] Belle, G. Gokhroo et al., “Observation of a nearthreshold D0 anti-D0 pi0 enhancement in B → D0 antiD0 pi0 K decay”, Phys. Rev. Lett. 97, 162002 (2006), [hep-ex/0606055]. [25] Belle, “Study of X(3872) in B meson decays”, BelleCONF-0711 (2007). [26] BABAR, B. Aubert et al., “Observation of a broad structure in the π + π − J/ψ mass spectrum around 4.26GeV/c2 ”, Phys. Rev. Lett. 95, 142001 (2005), [hepex/0506081]. [27] CLEO, Q. He et al., “Confirmation of the Y(4260) resonance production in ISR”, Phys. Rev. D74, 091104 (2006), [hep-ex/0611021]. [28] CLEO, T. E. Coan et al., “Charmonium decays of Y(4260), psi(4160), and psi(4040)”, Phys. Rev. Lett. 96, 162003 (2006), [hep-ex/0602034]. [29] BABAR, B. Aubert et al., “Evidence of a broad structure at an invariant mass of 4.32 GeV/c**2 in the reaction e+ e− → π + π − ψ(2S) measured at BaBar”, Phys. Rev. Lett. 98, 212001 (2007), [hep-ex/0610057]. [30] Belle, C. Z. Yuan et al., “Measurement of e+e- to pi+piJ/psi Cross Section via Initial State Radiation at Belle”, arXiv:0707.2541 [hep-ex]. [31] Belle, X. L. Wang et al., “Observation of Two Resonant Structures in e+e- to pi+ pi- psi(2S) via Initial State Radiation at Belle”, arXiv:0707.3699 [hep-ex]. [32] Belle, K. Abe et al., “Measurement of the near-threshold e+ e− → D(∗)± D∗∓ cross section using initial-state radiation”, Phys. Rev. Lett. 98, 092001 (2007), [hep-
ex/0608018]. [33] Belle, G. e. a. Pakhlova, “Measurement of the near¯ cross section using initial-state threshold e+ e− → DD radiation”, arXiv:0708.0082 [hep-ex]. [34] BABAR, B. Aubert, “Study of the Exclusive InitialState-Radiation Production of the DDbar System”, arXiv:0710.1371 [hep-ex]. [35] Belle, K. Abe et al., “Observation of a charmoniumlike state produced in association with a J/psi in e+ e- annihilation at s**1/2 approx. 10.6-GeV”, Phys. Rev. Lett. 98, 082001 (2007). [36] Belle, K. Abe et al., “Observation of a near-threshold omega J/psi mass enhancement in exclusive B –¿ K omega J/psi decays”, Phys. Rev. Lett. 94, 182002 (2005), [hep-ex/0408126]. [37] Belle, S. Uehara et al., “Observation of a chi’/c2 candidate in gamma gamma –¿ D anti-D production at Belle”, Phys. Rev. Lett. 96, 082003 (2006), [hep-ex/0512035]. [38] Babar, T. B. collaboration and B. Aubert, “Observation of Y (3940) → J/ψω in B → J/ψωK at BABAR”, arXiv:0711.2047 [hep-ex]. [39] Belle, K. A. e. al., “Search for new charmonium states in the processes e+ e- –¿ J/psi D(*) D(*) at sqrts 10.6 GeV”, arXiv:0708.3812 [hep-ex]. [40] Belle, K. Abe et al., “Observation of a resonance-like structure in the π ± ψ mass distribution in exclusive B → Kπ ± psi decays”, arXiv:0708.1790. [41] Belle, K. F. Chen et al., “Observation of anomalous Υ(1S)π + π − and Υ(2S)π + π − production near the Υ(5S) resonance”, Phys. Rev. Lett. 100, 112001 (2008), [0710.2577]. [42] The Belle, . I. Adachi, “Observation of an enhancement in e+e- to Upsilon(1S)pi+pi- , Upsilon(2S)pi+pi-, and Upsilon(3S)pi+pi- production at Belle”, 0808.2445. [43] BABAR, B. Aubert et al., “Measurement of the e+e- –¿ b anti-b cross section between sqrt(S)=10.54 and 11.20 GeV”, 0809.4120.
Towards a new spectroscopy R. Faccini1 1
University ”La Sapienza” and INFN Rome, Dipartimento di Fisica, 2 Ple Aldo Moro, I-00185, Rome, Italy I.
INTRODUCTION
Although the Standard Model of elementary particles is well established, strong interactions are not yet fully under control. We believe QCD is the field theory capable of describing them, but we are not yet capable, in most of the cases, to make exact predictions. Among the critical issues that are yet unknown, the possible existence of bound states of more than three quarks, and therefore the existence of a new spectroscopy, is an extremely interesting one. It has been shown recently how the diquarkantidiquark interpretation of the sub-GeV scalar meson nonet made of f0 (980), a0 (980), κ(800), σ(500) can lead to a remarkable description of the decay properties of these particles [1], adding a rather strong confidence that they are indeed tetraquark objects. On one side therefore the search has extended to further light mesons to search for higher mass tetraquark states. On the other side systems that include heavy quark-antiquark pairs (quarkonia) are ideal and unique laboratories to proble both the high energy regimes of QCD, where an expansion in terms of the coupling constant is possible, and the low energy regimes, where nonperturbative effects dominate. In the last years this field is experiencing a rapid expansion with a wealth of new data coming in from diverse sources: data on quarkonium formation from dedicated experiments (BES at BEPC, KEDR at VEPP-4M CLEO-c at CESR), clear samples produced by high luminosity B-factories (PEP and KEKB), and very large samples produced from gluon-gluon fusion in p¯ p annihilations at Tevatron (CDF and D0 experiments). In this review I will first summarize recent developments in the search for excited states of the scalar nonet among the light mesons. Next, the core of the paper will be spent to review the experimental evidences of new states that might be aggregations of more than just a heavy quark-antiquark pair. The currently most credited possible states beyond the mesons and the baryons are (you can find a review in [2]):
FIG. 1: Charmonium states with L <= 2. The theory predictions are according to the potential models described in Ref. [2].
FIG. 2: Bottomonium with L <= 2. The theory predictions are according to the potential models described in Ref. [2].
• hybrids: bound states of a quark-antiquark pair and a number of gluons. The lowest lying state is expected to have quantum numbers J P C = 0+− . The impossibility of a quarkonium state to assume these quantum numbers (see below) makes this a unique signature for hybrids. Alternatively a good signature would be the preference to decay into a 0920-5632/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2008.10.021
quarkonium and a state that can be produced by the excited gluons (e.g. π + π − pairs). • molecules: bound states of two mesons, usually rep¯ where Q is the heavy quark. resented as [Q¯ q ][q Q], The system would be stable if the binding energy would set the mass of the states below the sum of the two meson masses. While this could be the case for when Q = b, this does not apply for Q = c, where most of the current experimental data are.
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TABLE I: Most recent determination of the J P C = 1−− charmonium states from BES [3], compared to the 2006 edition of the PDG [4] M (MeV/c2 ) Γtot (MeV)
ψ(3770) ψ(4040) ψ(4160) ψ(4415) PDG2006 3771.1±2.4 4039±1.0 4153±3 4421±4 BES ’07 3771.4±1.8 4038.5±4.6 4191.6±6.0 4415.2±7.5 PDG2006 23.0±2.7 80±10 103±8 62±20 BES ’07 25.4±6.5 81.2±14.4 72.7±15.1 73.3±21.2
In this case the two mesons can be bound by pion exchange. This means that only states decaying strongly into pions can bind with other mesons (e.g. there could be D ∗ D states), and that the bound state could decay into it’s constituents. • tetraquarks: a quark pair bound with an antiquark ¯ A full nonet pair, usually represented as [Qq][q¯ Q]. of states is predicted for each spin-parity, i.e. a large amount of states is expected. There is no need for these states to be close to any threshold.
meson sector. The ¯ model of light-scalars is very effective at explaining the most striking feature of these particles, namely their inverted pattern, with respect to that of ordinary q q¯ mesons, in the mass-versus-I3 diagram [9]. Such pattern is not explicable using a q q¯ model. For
II.
LIGHT MESON SPECTROSCOPY
The problem of the interpretation of the light scalar mesons, namely f0 , a0 , κ, σ, is one of the oldest problems in hadronic physics, since the times of the paper by Gell-Mann and L´evy on the formulation of the Linearσ-Model [5]. For many years the question about the existence of the σ meson as a resonance in ππ scattering has been debated [6] and only recently a thorough analysis of ππ scattering amplitudes shows that σ(500) and κ(800) are poles on the second Riemann sheet [7], in other words, proper resonances. On the experimental side the reconsideration of σ was triggered by the E791 analysis of D → 3π data [8]. It has been recently stressed that light scalar mesons are most likely the lightest particles with an exotic structure, i.e., they cannot be classified as q q¯ mesons. Furthermore, their dynamics is tightly connected with instanton physics. It is indeed recently discussed how the consideration of instantonic effects allows for the building of a rather consistent model for the description of light scalar meson dynamics, provided that the hypothesis is made that these particles are diquark-antidiquark mesons [1]. Therefore, new ways of aggregation of quark matter could be established by the experimental/theoretical investigation of these particles. The idea of four-quark mesons dates back to the pioneering papers of Jaffe [9]. The suggestion of discussing exotic mesons and hadrons in terms of diquarks was instead introduced in [9] and extended in [10] to the scalar
MMeV
1
1000
1000
0
a0980 f0980
1020 800
900
Κ900
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Ω782
800
700
In setting after these states one must also beware of threshold effects, where amplitudes might be enhanced when new hadronic final states become possible.
MMeV
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FIG. 3: Vector mesons (q q¯ states) and the sub-GeV scalar mesons in the I3 − m plane.
example, in the q q¯ model, the f0 (980) should be an s¯ s state [11] while the I = 1, a0 (980), should be a u¯ u + dd¯ state. If this were the case, the degeneracy of the two particles would need some convincing explanation. Given the success of this identification, the quest for more such exotic states has opened. A relative of the lowest lying scalar mesons could have been found very recently by BaBar: the Y (2175), a particle first observed in the decay Y → φf0 (980) [12]. A recent study [13], investigating the mass spectrum of tetraquarks and reanalizing BaBar data, has shown that the Y (2175) has a mass compatible with diquarks bound states with two to four strange quarks and that it might have a large decay ¯ pair. The latter observation is an indication rate in a ΛΛ that the Y (2175) be a bound state of two strange and two light quarks.
III.
HEAVY QUARKONIUM SPECTROSCOPY
The heavy quark inside these bound states has low enough energy that the corresponding spectroscopy is close to the non-relativistic interpretations of the atoms. The quantum numbers that are more appropriate to characterize a state are therefore, in decreasing order of energy splitting among different eigenstates, the radial excitation (n), the spatial angular momentum L, the spin S and the total angular momentum J. Given this set of quantum numbers, the parity and charge conjugation of the states are derived by P = (−1)(L+1) and
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C = (−1)(L+S) . Figures 1 and 2 show the mass and quantum number assignments of the well established charmonium and bottomonium states.
A.
Charmonium spectroscopy
Figure 1 shows that all the predicted states below open charm threshold have been observed, leaving the search open only to states above the threshold. In this field the latest developments concern the measurement of the paramaters and the quantum number assignment for the J P C = 1−− states. The BES collaboration has recently performed a fit to the R scan results which takes into account interference between resonances more accurately [3]. The updated parameters are reported in Tab. I, compared with the most recent determinations. The J P C = 1−− assignment does not univocously identify the state, since both 2S+1 LJ =3 D1 and 3 D1 states would match it. The recent observation from Belle of the first exclusive decay of the ψ(4415) → DD2∗ (2460) [14], shows that this meson is predominantly D wave. At the same time the study from CLEO-c of the ψ(3770) → χcJ γ [15] confirms the dominance of the D wave also in this meson. Both these assignments confirm the theoretical predictions as shown in Fig. 1.
B.
Bottomonium spectroscopy
Figure 2 shows that the panorama in the bottomonium sector is much less complete, since there is a large number of states below the open bottom threshold which have not yet been observed. A significant step forward has been achieved recently BaBar which has observed the ground state, ηb in the radiative decays of the Υ(3S) [16]. The measured mass m(ηb ) = 9388.9+3.1 −2.3 ± 2.7 is in agreement with the expectations from the models, thus strengthening our understanding of the meson spectroscopy.
IV.
NON-STANDARD CHARMONIUM STATES A.
The X(3872)
The X(3872) was the first state that was found not to easily fit charmonium spectroscopy. It was initially observed decaying into J/ψπ + π − with a mass just beyond the open charm threshold [17]. The π + π − invariant mass distribution preferred the hypothesis of a X(3872) → J/ψρ decay, which would have indicated that if this were a charmonium state, the decay would have violated the isospin. Since it would be quite unusual to have the dominant decay to be isospin violating, a search of the isospin partner X + → J/ψρ was conducted invain by BaBar [18]. In the meanwhile the decay X → J/ψγ
Belle J/ψππKS
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3875.1± 0.7±0.5
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3872.0± 0.6±0.5
+
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BaBar J/ψππK
3872.2± 0.4
Average 2
χ =27/5 DOF 3860
3865
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MX(MeV)
FIG. 4: Measured mass of the X(3872) particle. The different production modes (B 0 → XKS and B − → XK − ) and the different decay modes (X → J/ψππ and X → D ∗0 D0 ) are separated.
was observed [19], implying positive intrinsic charge conjugation. The most recent developments concern the final assessment of the J P C of this particle and the indication that the X(3872) is a actually a doublet. The CDF collaboration has infact performed the full angular analysis of the X → J/ψππ decay [20] concluding that J P C = 1++ and 2−+ are the only assignments consistent with data. It also confirmed that the decays has a ρ as intermediate state. Combining this information with the preliminary result from Belle [21] which rules out the 2−+ hypothesis, the only possible assignment is J P C = 1++ . As far as the mass and width of the X(3872) are concerned, BaBar has published an analysis of the B → XK decays with X → D ∗0 D0 [23] while Belle has updated the mass measurements in X → J/ψππ decays [25]. The summary of all available mass measurements is shown in Fig. 4 where the measurements are separated by production and decay channel. There is an indication that the particle decaying into J/ψππ is different from the one decaying into D ∗0 D0 , their masses differing by about 4.5 standard deviations. In addition, the BaBar paper contains also a first measurement of the X(3872) width, Γ =(3.0+1.9 −1.4 ± 0.9)MeV. Finally the measurements of X the branching fractions in J/ψππ and D ∗0 D0 are summarized in Tab. II. B.
The 1−− family
The easiest way to assign a value for J P C to a particle is to observe its production via e+ e− annihilation, where the quantum numbers must be the same as the the photon: J P C = 1−− . B factories can investigate a large range of masses for such particles by looking for events where the initial state radiation brings the e+ e− centerof-mass energy down to the particle’s mass (the so-called
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TABLE II: Measured X(3872) branching fractions, separated by production and decay mechanism. The ratio of the measurements in the two production mechanisms is also reported as R0/+ = BF (B → XK − )/BF (B → XK 0 ). A ’∗ ’ indicates numbers which are derived from the published values by assuming gaussian uncorrelated errors. BaBar Belle combined BF(B → XK − )BF(X → J/ψππ)×105 1.01±0.25 ± 0.10 [22] 1.05±0.18 [17] 1.04±0.15∗ BF(B → XK 0 )BF(X → J/ψππ)×105 0.51±0.28 ± 0.07 [22] 0.99 ± 0.33∗ 0.72 ± 0.22∗ +1.9 − ∗0 0 5 BF(B → XK )BF(X → D D )×10 17 ± 4 ± 5 [23] 10.7±3.1−3.3 [24] 13 ± 3∗ BF(B → XK 0 )BF(X → D∗0 D0 )×105 22 ± 10 ± 4 [23] 17±7+3 19 ± 6∗ −5 [24] R0/+ with X → J/ψππ 0.50 ± 0.30 [22] 0.94 ± 0.26 0.75 ± 0.20∗ R0/+ with X → D∗0 D0 1.4 ± 0.6 [23] 1.6 ± 0.6∗ 1.5 ± 0.4∗
’ISR’ events). Alternatively, dedicated e+ e− machines, like CESR and BEP scan directly the certer-of-mass energies of interest.
FIG. 5: J/ψπ + π − invariant mass in ISR production.
Y (4260) → J/ψπ 0 π 0 and some events of Y (4260) → J/ψK + K − . While investigating whether the Y (4260) decayed to ψ(2S)π + π − BaBar found that such decay did not exist but discovered a new 1−− state, the Y (4350) [29]. While the absence of Y (4260) → ψ(2S)π + π − decays could be explained if the pion pair in the J/ψπ + π − decay were produced with an intermediate state that is to amssive to be produced with a ψ(2S) (e.g. an f 0 ), the absence of Y (4350) → J/ψπ + π − is still to be understood, more statistics might be needed in case the Y (4260) decay hides the Y (4350). Recently Belle has published the confirmation of all these 1−− states [30, 31] and at the same time has unveiled a new states that was not visible in BaBar data due to the limited statistics: the Y (4660). Figures 5 and 6 show the published invarianet mass spectra for both the J/ψπ + π − and the ψ(2S)π + π − decays. A critical information for the unravelling of the puzzle is whether the pion pair comes from a resonant state. Figure 7 shows the di-pion invariant mass spectra published by Belle for all the regions where new resonances have been observed. Although the subtraction of the continuum is missing, there is some indication that only the Y (4660) has a well defined intermediate state (most likely an f0 ), while others have a more complex structure. A discriminant measurement between Charmonium states and new aggregation forms is the relative decay rate between these decays into Charmonium and the decays into two charm mesons. Searches have therefore been carried out for Y → D (∗) D(∗) decays [32– 34] without any evidence for a signal. The most strin¯ gent limit is [34] BF (Y (4260) → D D)/BF (Y (4260) → J/ψπ + π − ) < 1.0@ 90% confidence level.
C. FIG. 6: ψ(2S)π + π − invariant mass in ISR production.
The observation of new states in these processes started with the discovery of the Y (4260) → J/ψπ + π − by BaBar [26], promptly confirmed both in the same production process [27] and in direct production by CLEOc [28]. The latter paper also reported evidence for
The 3940 family
Three different states have been observed in the past years by the Belle collaboration with masses close to 2 3940Mev/c : one, named X, observed in continuum events (i.e. not in Y (4S) decays) produced in pair with a J/ψ meson and decaying into DD ∗ [35]; a second one, named Y , observed in B decays and decaying into J/ψω [36]; a third one, named Z produced in two-photon
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FIG. 7: Di-pion invariant mass distribution in Y (4260) → J/ψπ + π − (left), Y (4350) → ψ(2S)π + π − (center), and Y (4660) → ψ(2S)π + π − (right) decays.
sured in this paper are lower than when observed, albeit 2 consitent (mY = 3914.6+3.8 −3.4 (stat.) ± 1.9(sys.)Mev/c , ΓY = 33+12 (stat.) ± 5(sys.)MeV), opens the interesting −8 possibility that the X and the Y particles be the same, thus solving the two abovementioned open issues.
a) 400 200 0
D.
b)
The X(4160)
400 200 0 3.8
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mJ/ψω (GeV/c2)
FIG. 8: The J/ψω distribution in a) B → J/ψωK + and b) B → J/ψωKS decays. The superimposed line is the result of the fit to the data.
reactions and decaying into D-pairs [37]. While the X is consistent with both J P C = 0+− and 1++ , the quantum number assignment of the Y and the Z states is clear: J P C = 1++ and 2++ respectively. Finally the Y is the only apparently broad state (Γ = 87 ± 34MeV). Because of these quantum number assignments and their masses these states are good candidates for the radial eccitation of the χ mesons, in particular the Z(3940) meson could be identified with the χc0 (2P ) and the Y (3940) with the χc1 (2P ). The unclear points are the identification of the X(3940) state and the explanation of why the Y (3940) state does not decay preferentially in D mesons. The most recent develpment on this topic is the confirmation from the BaBar collaboration of the Y (3940) → J/ψω decays [38]. The analysis utilizes the decay properties of the ω meson to extract a clean signal (see Fig. 8). The interesting part is that he mass and the width mea-
As we have already discussed, it is critical to investigate decay channels of the new states into D meson pairs. Unfortunately the detection efficiency for D mesons in low, due to the large number of possible decays. The Belle collaboration has developed a partial reconstruction technique that allows to overcome this limitation in the case of new states produced in continuum paired with known Charmonium states [39]. The Charmonium is fully reconstructed, while only one of the two D mesons is reconstructed. The kinematics of the other is inferred from the known center-of-mass energy and the different possibile D mesons are discriminated on the basis of the missing mass. This technique has allowed the confirmation of the X(3940) production and decay, and, most interestingly, the observation of the X(4160) state, decaying into DD ∗ . Given the fact that, for reasons yet to be understood, continuum events seem to produce J P C = 0−+ or 1++ states in pair with the J/ψ and since the measured mass is consistent with the expectations of a radial excitation of the ηc , this new state is likely to be an ηc (3S).
E.
The first charged state: Z(4430)
The real turning point in the query for states beyond the Charmonium was the observation by the Belle Collaboration of a charged state decaying into ψ(2S)π ± [40]. Figure 9 shows the fit to the ψ(2S)π invariant mass dis-
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picture can be drawn.
V.
FIG. 9: The ψ(2S)π invariant mass distribution in B → ψ(2S)πK decays.
tribution in B → ψ(2S)πK decays, returning a mass M = 4433 ± 4MeV/c2 and a width Γ = 44+17 −13 MeV. Due to the relevance of such an observation a large number of tests has been performed, breaking the sample in several subsamples and finding consistent results in all cases. Also, the possibility of a reflection of a B → ψ(2S)K ∗ ∗ decay has been falsified by explicitely vetoing windows in the Kπ invariant mass. In terms of quarks, such a state must contain a c and a c¯, but given its charge it must also contain at least a u ¯ The only open options are the tetraquark, the and a d. molecule or the threshold effects. The latter two options are possible due to the closeness of the D1 D∗ threshold.
FIG. 10: Measured masses of the newly observed states, positioned in the spectroscopy according to their most likely quantum numbers. The charged state (Z(4430)) has clearly no C quantum number.
Finding the corresponding neutral state, observing a decay mode of the same state or at least having a confirmation of its existence, are critical before a complete
NON-STANDARD BOTTOMONIUM STATES
Possible new forms of aggregations found in states with two charm quarks would apply also in states with two bottom quarks. In this respect Bottomonium spectroscopy is a very good test stand for the speculations made on the Charmonium one. On the other side, searching Bottomonium states is more challenging since they tend to be broader and there are more possible decay channels. Among the known states there is already one with unusual behaviour: there has been a recent observation [41] of an anomalous enhancement, by two orders of magnitude, of the rate of Y (5S) decays to a Y (1S) or a Y (2S) and two pions. This indicates that either the Y (5S) itself or a state very closeby has a decay mechanism which enhances the amplitudes for these proceeses. The presence of two decay channels to other bottomonium states excludes the possibility of this state being a molecular aggregrate, but all other models are possible and would predict a large variety of not yet observed states. In order to discriminate whether the exotic state coincides with the Y (5S) both Belle and BaBar have performed high luminosity scans of the center of mass energy of the collisions above the Υ(4S) mass. In December 2007 Belle performed a scan to further investigate their observation on the exclusive decays of the Υ(5S). They integrated a very high luminosity at six energy points in the Υ(5S) and Υ(6S) region, between √ S = 10.86 and 11.02 GeV [42]. The measured rate of observed decays into Υ(nS)π + π − presents a resonant structure which is higher and narrower than the previous measurements of the Υ(5S) properties (see Fig. 11): they measure as Breit-Wigner parameters m = 10889.6± 1.8(stat.) ± 1.5(sys.) MeV and Γ = 54.7+8.5 −7.2 (stat) ± 2.5(sys.), to be compared to m = 10865 ± 8 MeV and Γ = 110 ± 13 in PDG [4]. Few months later, in MarchApril 2008 BaBar has performed a high statistics scan of center-of-mass energy from the Υ(4S) mass to 11.2 GeV, the maximum energy achieavable by PEP-II. The capability to integrate significant luminosities (∼ 25pb−1 ) at energy points in only 5 MeV steps allowed a very accurate R = σhad /σμ0 scan which highlights a very rich structure of threshold openings. On top of such a structure it is extremely difficult to define resonance parameters. An attempt, published in [43] to fit the Υ(5S) parameters taking into account interference effects returns BreitWigner parameters which are closer to the ones observed by Belle:m = 10876 ± 2 MeV and Γ = 43 ± 4. Unfortunately for the quest for new states, these studies seem rather to indicate that the resonance parameters as fitted in inclusive R scans in presence of threshold openings present a bias.
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(a) Fit with common μ and Γ, χ2/n.d.f. = 39.4/16
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FIG. 11: Left: e+ e− → Υ(nS)π + π − cross-sections measured by Belle with the Breit-Wigner fit superimposed. The adshed line corresponds to the maximum of the inclusive cross-section. Right: R scan as measured by BaBar with a fit to two Breit-Wigners and two flat components, one interfering with both resonant structures and one not.
VI.
CONCLUSIONS
long established mesons.
More than 40 years after its first formulation, the quark model is still open to new observations. In the field of light mesons the identification of the scalar nonet with tetraquark states is becoming stronger and there are candidates for the corresponding excitations. Moreover the heavy-quarkonium is still a valid test ground for understanding QCD. The study of well established quarkonium states yields information on low energy QCD while the undestanding of the quarkonium spectroscopy, predictable in potential models, allows searches for different aggregation states than the
The high statistics and quality data from B-Factories have produced a very large number of new states whose interpretation is still a matter of debate. This paper attempted a categorized review of all these states. A full summary, including the most likely quantum number assignment is shown in Fig. 10. Lots of theoretical models have been developed to interpret the situation but the picture is far from complete: more precise predictions are needed from theory and a systematic experimental exploration of all possibile production and decay mechanisms of these new states is still in the works.
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