Bled Workshops in Physics Vol. 7, No. 1
A
Proceedings of the Mini-Workshop Progress in Quark Models (p. 74)
Bled, Slovenia, July 10-17, 2006

New resonances and spectroscopy at Belle
M. Brackoa,b, representing the Belle Collaboration
QUniversity of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia b JoZef Stefan Institute, P.O.B. 3000, SI-1001 Ljubljana, Slovenia
Abstract. The Belle experiment at the KEKB asymmetric-energy electron-positron collider has proven to be an excellent environment for a wide variety of measurements. Besides its main goal - measurements of CP violation in the system of B mesons - other most important achievements are observations of several yet undiscovered particles and measurements of their properties. The discoveries were often surprising, since only some of the observed states were predicted in various models, while others were not. The existence of these resonances therefore still imposes theoretical questions regarding their nature and also represents a challenge for a proper description in terms of QCD. Selected experimental results together with their possible interpretations are reviewed in this paper.
1 Introduction
The Belle detector [1] at the asymmetric-energy e+ e- collider KEKB [2] has accumulated around 630 fb-1 by July 2006. The KEKB collider is often called the B-factory, since it operates at the energy of the T(4S) resonance, slightly above the BB-production threshold, and thus the accumulated data set contains a large number of BB pairs. While the main goal of both B-factories* are measurements of CP violation in the B-meson system, the excellent detector performances also enable searches for new hadronic (bound) states as well as studies of their properties. There are several possible mechanisms of the particle production at B-factories: production in the B-meson decays, fragmentation of quarks in e+e-annihilation or creation of C-even states in two photon processes. In this paper, we address some interesting discoveries of new hadronic states, produced by different mechanisms and observed by the Belle collaboration.
Several charmonium-like new states have been recently observed by Belle, namely: X(3872), Z(3930), Y(3940) and X(3940). The naming convention indicates the lack of knowledge about the structure and properties of these particles at the time of their discovery.
2 The X, Y, Z story
* Besides KEKB in Japan, there is a similar collider called PEP-II in the USA, delivering data to the BABAR [3] detector.
2.1 Observation and properties of X(3872)
In 2003 Belle reported on the analysis of B± —> K±n+n-J/^ decays, where a narrow charmonium-like state (X(3872)) decaying to n+n-J/^ was discovered [4]. For the most recent update from Belle [5], the fitted yield of both, charged and neutral B mesons reconstructed in B —> KX(3872)(—> n+n- J/^) decay mode is shown in Fig. 1(a) as a function of the n+n-J/^ invariant mass. The observation of X(3872) resonance was later confirmed by the CDF [6], D0 [7] and BABAR [8] experiments. Currently, the world average of the mass is M(X(3872)) = (3871.2 ± 0.5) MeV/c2 [9] and the upper limit on its width, as measured by Belle, is r(X(3872)) <2.3 MeV [4].
Several interpretations of X(3872) resonance have been suggested, including charmonium hypothesis [10,?], D°D*° molecule [12] and tetraquarks [13]. Various dedicated studies were performed at Belle in order to determine possible quantum numbers of X(3872) and its nature. In 2005 Belle reported a strong evidence for the radiative decay of X(3872) —> yJ/^ [14]. The fitted yield of reconstructed B mesons, as obtained from the simultaneous fit to the AE and Mbc distributions** for B —> KyJ/^ decay candidates is shown in Fig. 1(b) as a function of the M(yJ/^). The observed signal with a significance above 4ct can be converted to B(X(3872) yJ/^)/B(X(3872) —> n+n- J/^) = 0.14 ± 0.05, which is not in agreement with the expectations for charmonium interpretation of X (3872). However, the observation of this radiative decay establishes even charge-conjugation parity (C) of X(3872).
Furthermore, Belle examined possible JPC quantum number assignments of X(3872) by studying angular correlations between the final-state particles in X(3872) —> n+n- J/^ decays [5]. An example is presented in Fig. 1(c): the measured distribution of the angle between the negative of the B meson flight direction and n+ momentum from X(3872) in the X(3872) frame, is in agreement with the expectation for the 1 ++ state. Additionally, the n+n- invariant mass distribution for the events in the X(3872) —> n+n-J/^ signal region, shown in Fig 1(d), peaks at the upper kinematic limit indicating the p0J/^ intermediate state and favours S-wave over P-wave as the relative orbital angular momentum between the final-state dipion and J/^. As a consequence of these studies, JPC = 1++ is strongly favoured for the X(3872), but the 2++ can not be completely ruled out.
The latter possibility could be ruled out by the recent study [15] of B —> KD°D°7t° decays, where a near-threshold enhancement at the (3875.4±0.7(stat. )± 1.1 (syst.)) MeV/c2 for the invariant mass of the D°D°7t° system was observed (see Fig. 1(e, f)). If the observed enhancement - whose invariant mass is however about 2ct higher than the world average value for X(3872) - is indeed due to the X(3872), the JPC = 1 ++ quantum number assignment for the X(3872) would again be favoured, since near-threshold decays X(3872) —> D°D*0/D°D07t0 are expected to be strongly suppressed for J = 2.
Two kinematic variables are used to identify B-meson candidates: AE = EB — Ebeam and Mj,c = 1/c2	— (pec)2, where Eb and pe are the reconstructed energy and
momentum of the B candidate, and Ebeam is the beam energy, all expressed in the centre-of-mass (CM) frame.
Fig.1. (a) n+n-J/^ invariant mass for B —> Krt+rt-J/^ decays [5]; (b) Yield of B mesons for B —> KyJ/^ decay candidates as a function of M(yJ/^) [14]; (c) Distribution of angle in X(3872) decays described in the text. The full histogram represents the expectation for JPC = 1 + + assignment and the hatched histogram is the contribution of background as obtained from the scaled sidebands of M(n+n-J/^); (d) M(n+n-) distribution for events in X(3872) signal region. The histogram again indicates the sideband-determined background, while the solid (dashed) curve shows the result of the fit using Breit-Wigner function for p° —> n+n-, and assuming J/^ and p0 to be in a relative S-wave (P-wave); (e) and (f) AM = M(D°D07t°) - 2M(D°) - M(tt°) and At distributions for near-threshold D°D°7T0 enhancement in B -> KD°D07t° decay [15],
While currently available X(3872) data - the mass, possible 1++ quantum numbers and observed decay modes - are in agreement with the hypothesis that X(3872) is a D°D*° molecule [12], some spin assignments corresponding to more conventional interpretations can still not be ruled out (see for example Ref. [16]). Further experimental results and theoretical calculations are thus needed to resolve the puzzle about the nature of the X(3872) resonance.
2.2 Z(3930) resonance
A search for the xCj (I = 0 or J = 2) states and other C-even charmonium states in the mass range of 3.73 GeV/c2 - 4.3 GeV/c2 was performed for the two-photon production of DD pairs, yy —» DD [17]. The two-photon process was studied in the non-tagged mode, where final-state electron and positron produced in the reaction e+e~ —> e+e~DD are not detected, and the DD system has a very small transverse momentum w.r.t. the e+e~ axis. These requirements help selecting DD pairs produced exclusively in collisions of two quasi-real photons. The D mesons were reconstructed in decays of D0 —> K-K-n+n0, K-n+n-and D + —> K~7t+7t+ (and their charge conjugated modes). The obtained DD invariant-mass distribution is shown in Fig. 2(a). A clear peak with 5.3ct significance denoted as Z(3930) was observed with mass (3929 ± 5(stat.) ± 2(syst.)) MeV/c2 and width (29 ± 10(stat.) ± 2(syst.)) MeV. A product of the two-photon decay width and branching fraction of the Z(3930) is found to be P(Z(3930))5(Z(3930) DD) = 0.18 ±0.05 ±0.03 keV.
Fig. 2. (a) Invariant mass of DD pairs produced in non-tagged two-photon reactions. The curves indicate the result of the fit with a resonant component (solid) and without it (dashed), (b) The |cos0*| distribution for Z(3930) —» DD decays. Expected predictions for J = 2 and J = 0 are shown as a solid and a dash-dotted line, respectively, and contain the non-peaking background shown separately by the dotted curve.
An angular analysis was also performed by Belle collaboration [17]. Efficiency corrected cos 0* distribution, where 0* is the angle between D meson and the beam axis in the yy rest frame, shows that the spin-2 assignment for the observed resonance is strongly favoured over spin-0 assignment. All performed Z(3930) measurements are thus consistent with the expectations for the xC2, a radial excitation of 3 P2 charmonium.
2.3 Two new states at M « 3940 MeV/c2
After the observation of a sub-threshold decay of X(3872) —> wJ/^ [14], using B —> KJ/^n+n-n0 decays in a similar way as described for B —> KJ/^n+n- decays in Sec. 2.1, Belle performed an analysis of the wJ/^ system produced in exclusive B —> KwJ/^ decays [18], selecting events with M(n+n-n0) « m^ .Events with M(Kw) < 1.6 GeV/c2 are rejected in order to suppress Kx —> Kw contribution, where Kx denotes resonances such as Ki (1270), Ki (1400), and K2(1400) that are known to decay to Kw. The events clustering near the bottom of the Dalitz plot shown in Fig. 3(a) are responsible for a strong enhancement above the phase space expectation, which can be observed in the plot of signal yield of B decays, as obtained from the fit to the Mbc distribution, in bins of M(wJ/^) (see Fig. 3(b)). The fit with an S-wave Breit-Wigner function yields (58 ± 11) events with a statistical significance above 8ct, corresponding to a new resonance named Y(3940) with a mass of (3943 ± 11 (stat.) ± 13(syst.)) MeV/c2 and a total width r = (87 ± 22(stat.) ± 26(syst.)) MeV. The measured fraction for this state is B(B KY(3940))B(Y(3940) wJ/^) = (7.1 ± 1.3(stat.) ± 3.1(syst.)) ■ 10-5.
Due to rather intriguing properties, the nature of Y(3940) is still mysterious. Namely, any charmonium state with a mass around 3940 MeV/c2 is expected to
(a)	
	
%	
n bins of M(w l/y)
o150 >
£!100 z
Mrecoi Jy)
GeV/c
.»(J'y)
Fig.3. (a)	vs. M2(^K) Dalitz plot for B —>	decays. Vertical line indi-
cates the region selected by the requirement M(K^) > 1.6 GeV/c2. (b) Yield of B mesons in B —>	decays as a function of	(c) Spectrum of mass recoiling against
the J/i|). Same recoil mass for events tagged as (e) J/i]>DD and (e) J/i]> D D *.

(b)
30
50
20
M
GeV/c
3880	4080	4280
M(wJ/y) (MeV)
dominantly decay to DD and/or DD*, which for Y(3940) have not been observed yet. Adding that for a cc charmonium the hadronic transition to cu J/i|) should be heavily suppressed, one can conclude that the Y(3940) resonance is probably not a conventional radially excited P-wave charmonium state. As an alternative interpretation, it has been suggested that Y(3940) is one of cc-gluon hybrid charmonium states that were first predicted in 1978 [19] and are expected to be produced in B meson decays [20]. It has been shown that D^D^ decays for these exotic states are forbidden or heavily suppressed [21], so that such a hybrid state with a mass equal to that of the Y(3940) would have a large branching fraction for decays to J/^ or plus light hadrons [22]. However, while this interpretation is able to explain Y(3940) decay modes, predicted masses for cc-gluon hybrid states are between 4300 and 4500 GeV/c2 [23], substantially higher than the measured Y(3940) mass.
Another resonance with a similar mass above DD threshold - denoted as X(3940) - was also discovered by the Belle collaboration. This state was observed in the J/^ recoil mass spectrum for inclusive e+e- —> J/^X processes [24]. The mass recoiling against the J/^ —> l+l- is determined as Mrecoii(J/^) = ^fEcM - Hj/^, )2 - (cPjai, )2/c2, where E* is the J/i|) CM energy and ECm is the CM energy of the event. The new peak can be seen in a recoil mass spectrum at about 3940 MeV/c2, together with three known peaks corresponding to nc, Xco and nc (2S) (see Fig. 3(c)).
Searches for two exclusive decay modes of this newly observed state were performed: X(3940) DD(*] and X(3940) cuJA|>. For the former search, only a single D meson besides the J/^ was reconstructed to increase the efficiency. Only events with the recoil mass Mrecoii(DJ/^) close to D(*^ mass were retained for the analysis. The resulting mass recoiling against the J/^ - corresponding to the invariant mass of the DD and DD* system - is shown in Fig. 3(d, e)). While no significant signal was observed at the mass of about 3940 MeV/c2 for the e+ e- —>
Fig.4. Distributions of the invariant masses (a) M(A+K-n+) and (b) M(A+K§rt-) (both shown as points with error bars) together with the fitting function (the solid line). The dashed lines indicate the background contributions in both cases. (c) The distribution of A+ K§rt- invariant mass (points with error bars) in the region around the value of 3.52 GeV (marked with an arrow), shown together with the fit result (depicted as a solid line).
J/i|)DD events, there is a clear peak for events tagged as e+e~ —> J/i|)DD*. The mass of the X(3940) resulting from the fit shown in Fig. 3(e) is (3943 ± 6(stat.) ± 6(syst.)) MeV/c2 and the upper limit on the X(3940) total width is 52 MeV at the 90% confidence level.
No significant signal was found for X(3940) —> ^J/^ decays. Since X(3940) state does not share decay modes with the Y(3940), these two states appear not to be the same. A possible interpretation is that X(3940) state is a radially excited charmonium state nc(3S).
3 Observation of Scx(2980) and 3cx(3077)
Early this year, using the data sample of 461.5 fb-1 the Belle collaboration reported the first observation of two charmed baryons [25]. These two baryons, denoted as Ecx(2980)+ and Ecx(3077) +, are found to be decaying into a A+ K-n+ final state (see Fig. 4 (a)). Assuming that these states carry charm and strangeness, the above observation would represent the first example of a baryonic decay in which the initial c and s quarks are carried away by two different final state particles. Most naturally, these two states could be interpreted as excited charm-strange baryons Ec. This interpretation could be further justified by positive results of the search for neutral isospin related partners of the above states, performed in events with the A+ K^n- final state. The latter search results in the observation of the Ecx(3077)° together with a broad enhancement near the threshold, i.e. in the mass region corresponding to the Ecx(2980)° (see Fig. 4 (b)). Preliminary values of properties for the four observed states are collected in Table 1.
In the A+K-n+ final state, the SELEX collaboration reported the observation of a double charmed baryon E+c at the mass of about 3520 MeV/c2 [26], which has not been confirmed by other experiments. This result - together with the observation of new states mentioned in the previous section of this paper, and the surprisingly large cross section for double charmonium production at B-factories [27,?] - have generated renewed theoretical interest in the spectroscopy, decays and production of charmonium. It has been suggested that the the comparison with the production of double charm tetraquark Tcc = ccud [29] could shed some light on these experimental results. To search for the E+c state, Belle extended
New State Mass (MeV/c2) Width (MeV/c2) Yield (events) Signif. (ff)
Scx (2980) +	2978.5 ± 2.1 ± 2.0	43.5 ± 7.5 ± 7.0	405.3 ± 50.7	6.3
Sex (3077)+	3076.7 ± 0.9 ± 0.5	6.2 ± 1.2 ± 0.8	326.0 ± 39.6	9.7
Scx(2980)° 2977.1 ± 8.8 ± 3.5 43.5 (fixed) 42.3 ±23.8 2.0
Scx(3077)° 3082.8 ± 1.8 ± 1.5 5.2 ± 3.1 ± 1.8 67.1 ± 19.9 5.1
Table 1. Summary of the parameters of the new states in the A+ K-n+ and A+ K§rt+ final states: masses, widths, yields and statistical significance.
the range of M(A+K-n+) search to include the region around 3520 MeV/c2 (see Fig. 4 (c)). However, the study shows no evidence for this state, and as a result only an upper limit on the ratio of cross-sections for exclusive Ecc(3520) + production and inclusive A+ production is given at the 90% confidence level: ct(ecc(3520)+) x B(Scc(3520)+ A+ K"n+)/o-(A+) <1.5 ■ 10-4.
4 Conclusion
The large data sample collected by the Belle experiment at KEKB provides an excellent opportunity for the search of new particles. During the Belle operation more than ten new states have been discovered. In this paper we report on some of the most exciting, like X(3872), Y(3940), X(3940) and Z(3930). The latter two resonances can be interpreted as charmonium states, nc (3S) and xc2, respectively. None of the existing measurements contradicts the X(3872) interpretation as a D0D*0 molecule. The nature of Y(3940) remains to be addressed.
Recently, new charmed baryons, Ecx(2980) and Ecx(3077), have been observed by the Belle collaboration. These states are most naturally interpreted as the excited charmed strange baryons, Ec. However, in contrast to known excited Ec baryons, the observed new states decay into separate charmed (A±) and strange (K) hadrons. As further studies of all new states are ongoing, more interesting results on charm spectroscopy are expected soon from the Belle experiment.
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