BLED WORKSHOPS
IN PHYSICS
VOL. 13, NO. 1
p. 62
Proceedings of the Mini-Workshop
Hadronic Resonances
Bled, Slovenia, July 1 - 8, 2012
News from Belle: Recent Spectroscopy Results
M. Bračko⋆
University of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia and
Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia
Abstract. This paper reports on some of the latest spectroscopicmeasurements performed
with the experimental data collected by the Belle spectrometer, which has been operating
at the KEKB asymmetric-energy e+e− collider in the KEK laboratory in Tsukuba, Japan.
1 Introduction
The Belle detector [1] at the asymmetric-energy e+e− collider KEKB [2] has accu-
mulated about 1 ab−1 of data by the end of its operation in June 2010. The KEKB
collider, called a B-factory, most of the time operated near the Υ(4S) resonance,
but it has accumulated substantial data samples also at other Υ resonances, like
Υ(1S), Υ(2S) and Υ(5S), as well as in the nearby continuum. In particular, the
data samples at the Υ(4S) and Υ(5S) resonances are by far the largest available
in the world, corresponding to integrated luminosities of 798 fb−1 and 123 fb−1,
respectively. Large amount of collected experimental data and excellent detector
performance enabled many interesting spectroscopic results, including discov-
eries of new hadronic states and studies of their properties. This report covers
most recent and interesting spectroscopic measurements—performedwith either
charmonium(-like) and bottomonium(-like) states.
2 Bottomonium and Bottomonium-like States
The Belle collaboration used a data sample at the CM energy around the Υ(5S)
mass 10.89 GeV, and found large signals for decays into π+π−Υ(1S), π+π−Υ(2S)
and π+π−Υ(3S) final states [3]. If these transitions are only from the Υ(5S) reso-
nance, then the corresponding partial widths are more than two orders of mag-
nitude larger than the corresponding partial widths for Υ(4S), Υ(3S) and Υ(2S)
decays to π+π−Υ(1S). These results motivate a search for the hb(mP) resonances
in theΥ(5S) data.hb(1P) andhb(2P) states are observed in themissingmass spec-
trum of π+π− pairs for the Υ(5S) decays, with significances of 5.5σ and 11.2σ, re-
spectively [4]. This is the first observation of the hb(1P) and hb(2P) spin-singlet
bottomonium states in the reaction e+e− → hb(mP)π+π− at the Υ(5S) energy.
Later hb(1P) and hb(2P) were studied in the Υ(5S) → hbπ+π− → γηb(1S)π+π−
⋆ Representing the Belle Collaboration.
News from Belle: Recent Spectroscopy Results 63
Decay mode Branching fraction in %
hb(1P) → γηb(1S) 49.2±5.7+5.6−3.3
hb(2P) → γηb(1S) 22.3±3.8+3.1−3.3
hb(2P) → γηb(2S) 47.5±10.5+6.8−7.7
Table 1. The branching fractions for hb → γηb decays, as measured by Belle.
decay [5]. In the same final state, Belle observes [5] also the first evidence for a
ηb(2S) in Υ(5S) → hb(2P)π+π− → γηb(2S)π+π− decay. The width of ηb(2S) is
small, with Γ = (4±8)MeV. Branching fractions for observed radiative hb decays
are summarized in Table 1.
Comparable rates of hb(1P) and hb(2P) production indicate a possible exotic
process that violates heavy quark spin-flip and this motivates a further study of
the resonant structure in Υ(5S) → hb(mP)π+π− and Υ(5S) → Υ(nS)π+π− de-
cays [6]. Due to the limited statistics, only the study ofM(hb(mP)π) distribution
is possible for hb(mP)π+π−, while in the case of Υ(nS)π+π− decay modes the
Dalitz plot analysis can be performed. As a result, two charged bottomonium-
like resonances, Zb(10610) and Zb(10650), are observed with signals in five dif-
ferent decay channels, Υ(nS)π± (n = 1, 2, 3) and hb(mP)π± (m = 1, 2). The av-
eraged values for the mass and widths of the two states are calculated to be:
M(Zb(10610)) = (10607.2 ± 2.0) MeV, Γ(Zb(10610)) = (18.4 ± 2.4) MeV and
M(Zb(10650)) = (10652.2 ± 1.5) MeV, Γ(Zb(10650)) = (11.5 ± 2.2) MeV. The
measured masses are only a few MeV above the thresholds for the open beauty
channels B∗B (10604.6 MeV) and B∗B
∗
(10650.2 MeV) [9], which could indicate
a molecular nature of the two observed states. Angular analysis of charged pion
distributions favours the JP = 1+ spin-parity assignment for both Zb(10610) and
Zb(10650).
3 Charmonium and Charmonium-like States
There has been a renewed interest in charmonium spectroscopy since 2002. The
attention to this field was drawn by the discovery of the two missing cc states
below the open-charm threshold, ηc(2S) and hc(1P) [7,8] with JPC=0−+ and 1+−,
respectively, but even with the discoveries of new new charmonium-like states
(so called “XYZ” states).
3.1 The X(3872) news
The storyabout the so called “XYZ” states began in 2003, when Belle reported
on B+ → K+J/ψπ+π− analysis, where a new state decaying to J/ψπ+π− was
discovered [10]. The new state, called X(3872), was soon confirmed and also in-
tensively studied by the CDF, DØ and BABAR collaborations [11–19]. So far it has
been established that this narrow state (Γ = (3.0+1.9−1.4 ± 0.9) MeV) has a mass of
64 M. Bračko
(3872.2± 0.8) MeV, which is very close to theD0D∗0 threshold [9]. The intensive
studies of several X(3872) production and decay modes suggest two possible JPC
assignments, 1++ and 2−+, and establish the X(3872) as a candidate for a loosely
bound D0D∗0 molecular state. However, results provided substantial evidence
that the X(3872) state must contain a significant cc component as well.
Recently, Belle performed a study of B→ (ccγ)K using the final data sample
with 772 million of BB pairs collected at the Υ(4S) resonance [20]. Pure D0D∗0
molecular model [21] predicts B(X(3872) → ψ′γ) to be less than B(X(3872) →
J/ψγ). Results by the BABAR collaboration [19] show that B(X(3872) → ψ′γ) is
almost three times that of B(X(3872) → J/ψγ), which is inconsistent with the
pure molecular model, and can be interpreted as a large cc −D0D∗0 admixture.
We observe X(3872) → J/ψγ together with an evidence for χc2 → J/ψγ in B± →
J/ψγK± decays, while in our search for X(3872) → ψ′γ no significant signal is
found. We also observe B → χc1K decays in both, charged as well as neutral
B decays. The obtained results suggest that the cc-D0D∗0 admixture in X(3872)
may not be as large as discussed above.
New results for the X(3872) →J/ψπ+π− decay modes in B+→K+X(3872)
and B0→K0 (→π+π−)X(3872) decays are obtained with the complete Belle data
set of 772 million BB pairs collected at the Υ(4S) resonance [22]. The results for
the X(3872) mass and width are obtained by a 3-dimensional fit to distributions
of the three variables: beam-constrained-mass Mbc=
√
(Ecmsbeam)
2 − (pcmsB )
2 (with
the beam energy Ecmsbeam and the B-meson momentum p
cms
B both measured in the
centre-of-mass system), the invariant mass Minv(J/ψπ+π−) and the energy dif-
ference ∆E=EcmsB −E
cms
beam (where E
cms
B is the B-meson energy in the centre-of-mass
system). As a first step, the fit is performed for the reference channel
ψ ′→J/ψπ+π−, and the resolution parameters are then fixed for the fit of the
X(3872). The mass, determined by the fit, is (3871.84±0.27±0.19) MeV. Including
the new Belle result, the updated world-average mass of the X(3872) is
mX=(3871.67±0.17) MeV. If the X(3872) is an S-wave D∗0D
0
molecular state, the
binding energy Eb would be given by the mass differencem(X)−m(D∗0)−m(D0).
With the current value ofm(D0)+m(D∗0)=(3871.79± 0.30) MeV [9], a binding en-
ergy of Eb=(−0.12±0.35)MeV can be calculated, which is surprisingly small and
would indicate a very large radius of the molecular state.
The best upper limit for the X(3872) width was 2.3 MeV (with 90% C.L.), ob-
tained by previous Belle measurement [10]. The 3-dimensional fits aremore sensi-
tive to the naturalwidth, which is smaller than the detector resolution (σ ∼4MeV).
Due to the fit sensitivity and the calibration performed on the reference channel
ψ ′→J/ψπ+π−, the updated upper limit for the X(3872) width is about 1/2 of the
previous value: Γ(X(3872)) < 1.2MeV at 90% C.L.
Previous studies performed by several experiments suggested two possible
JPC assignments for the X(3872), 1++ and 2−+. In the recent Belle analysis [20],
the X(3872) quantum numbers were also studied with the full available data sam-
ple collected at the Υ(4S) resonance. At the current level of statistical sensitivity
it is not possible to distinguish completely between the two possible quantum
number assignments, so both hypotheses are still allowed. Possible C-odd neu-
News from Belle: Recent Spectroscopy Results 65
tral partners of X(3872) are also searched, but no signal is found for this type of
states.
4 Summary and Conclusions
Many new particles have already been discovered during the operation of the
Belle experiment at the KEKB collider, and some of them are mentioned in this
report. Some recent Belle results also indicate that analogs to exotic charmonium-
like states can be found in bb systems. Although the operation of the experiment
has finished, data analyses are still ongoing and thereforemore interesting results
on charmonium(-like) and bottomonium(-like) spectroscopy can still be expected
from Belle in the near future.
References
1. Belle Collaboration, Nucl. Instrum. Methods A 479, 117 (2002).
2. S. Kurokawa and E. Kikutani, Nucl. Instrum. Methods A 499, 1 (2003), and other papers
included in this Volume.
3. Belle Collaboration, Phys. Rev. Lett. 100, 112001 (2008); Phys. Rev. D 82, 091106 (2010).
4. Belle Collab., Phys. Rev. Lett. 108, 032001 (2012).
5. Belle Collab., arXiv:1205.6351 [hep-ex], to appear in Phys. Rev. Lett. .
6. Belle Collaboration, Phys. Rev. Lett. 108, 122001 (2012).
7. Belle Collaboration, Phys. Rev. Lett. 89, 102001 (2002).
8. Cleo Collaboration, Phys. Rev. Lett. 95, 102003 (2005).
9. K. Nakamura et al. (Particle Data Group), J. Phys. G 37, 075021 (2010).
10. Belle Collaboration, Phys. Rev. Lett. 91, 262001 (2003).
11. CDF Collaboration, Phys. Rev. Lett. 93, 072001 (2004); DØCollaboration, Phys. Rev. Lett.
93, 162002 (2004); BABAR Collaboration, Phys. Rev. D 71, 071103 (2005).
12. Belle Collaboration, arXiv:hep-ex/0505037, arXiv:hep-ex/0505038; submitted to the
Lepton-Photon 2005 Conference.
13. Belle Collaboration, Phys. Rev. Lett. 97, 162002 (2006).
14. BABAR Collaboration, Phys. Rev. D 74, 071101 (2006).
15. Belle Collaboration, arXiv:0809.1224v1 [hep-ex]; contributed to the ICHEP 2008 Con-
ference.
16. Belle Collaboration, arXiv:0810.0358v2 [hep-ex]; contributed to the ICHEP 2008 Con-
ference.
17. CDF Collaboration, Phys. Rev. Lett. 98, 132002 (2007).
18. BABAR Collaboration, Phys. Rev. D 77, 011102 (2008).
19. BABAR Collaboration, Phys. Rev. Lett. 102, 132001 (2009).
20. Belle Collaboration, Phys. Rev. Lett. 107, 091803 (2011).
21. E. S. Swanson, Phys. Rep. 429, 243 (2006).
22. Belle Collaboration, Phys. Rev. D 84, 052004(R) (2011).
23. CDF Collaboration, Phys. Rev. Lett. 103, 152001 (2009).
24. BABAR Collaboration, Phys. Rev. D 77, 111101(R) (2008).
25. LHCb Collab., Proc. XIX International Workshop on Deep-Inelastic Scattering and Re-
lated Subjects (DIS2011), LHCb-CONF-2011-021.