1. Hadron spectroscopy results from Belle
Y. Kato (KMI, Nagoya University)
2019/2/14
KEK-FF 2019

2. Introduction Exotic hadrons Heavy hadrons
Success of constituent quark model
Mesons
Baryons
Int. J. Mod. Phys. A, 21, 5583 (2006)
Phys.Rev. D18 (1978) 4187 
”Constituent quark” must be a good
approximation… but not the end.
・Why does it work so well?
・Where is the adaptive limit?
・Any other degrees of freedom?
Exotic hadrons
Heavy hadrons
e+e- collider (especially B-factory) is a powerful probe!
2019/2/14
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3. “New hadrons” from B-factories
Hadron Type
Charmonium
Bottomonium
D, D(s)
Charmed baryon
Hyperon
B-decay
ηc(2S) ψ2(3823) X(3872) X(3915)
Zc(4050) Zc(4250)
Zc(4430) Zc(4200)
D*0(2400) D1(2430)
Ξc(2930)
Initial
State
Radiation
Y(4260) Z(3900)
Y(4008) Y(4360)
Y(4660)
Double charmonium
X(3860) ≒ χc0(2P) X(3940) X(4160)
Two-photon
χc2(2P)
e+e-→ccbar
D*s0 (2317) D0(2550) DJ*(2600) DJ(2740) D3*(2750) D*s1(2700)
D*s1(2860) DsJ(3040)
Σc(2800) Λc(2940) Ξc(2980) Ξc(3080)
Ωc(2770) Ξc(3055)
Y(nS) decay
Zb(10610) Zb(10650)
ηb(1S) ηb(2S)
hb(1P) hb(2P)
Ω(2012)
Charm baryon decay
Ξ(1620)
Belle
BaBar
3:Observation of Ξc(2930)
1:Br(B+→K+Xcc)
2:X(3872/3915)→π0χc1
4:Absolute branching fraction of Ξc
Reaction
~ 40 new hadrons!
(Some states may be missed)
5:Observation of Ξc→Ξ(1620) π
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4. X(3872)
・ The poster boy of XYZ exotics, firstly observed by Belle.
- JPC = 1++
- Decay into both of J/ψρ (I=1) and J/ψω (I=0): isospin breaking
- Mass consistent with DD* with O(0.1) MeV presicion.
- Differential cross section at LHC consistent with χc1(2P)
DD* molecule?
JHEP01(2017)117
cc?
χc1(2P)
Decays and production are essential to identify the nature
・Currently, only the product of two branching fractions are measured.
- (B → K+ X) and (X(3872) → f)
・Former is production, and latter is the decay.
・Belle performed measurement of Br(B+→K+X(3872))
- This also leads to the Br(X(3872)→f). K+ B+ X
Br(B+→K+X(3872)) × Br(X(3872)→f)
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5. Principle of the measurement
・Extract Br (B+→Xcc K+) → Do not look for Xcc decays
- Fully reconstruct B meson decaying hadronically.
- Reconstruct K+ mesons from the signal B meson.
- Reconstruct Xcc from Missing mass:Mx2 = (Pbeam-PBTag-PK+)2
・ Belle performed the analysis with full data sets and measured 9 Xcc states:
ηc, J/ψ, χc0, χc1, ηc(2S), ψ(2S), ψ(3770), X(3872), and X(3915)
Reconstruct
Detect K+
Hadronic decay B- B+ X
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6. K+ Missing Mass distributions
Phys. Rev. D 97,
Low mass
Signal PDFについて言及した方が良いのでは?
High mass
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7. Results Xcc Br (B+→XccK+) (10-4) BaBar PDG (10-4) ηc 14.2 12.0±0.8±0.7σ
Br (B+→XccK+) (10-4)
BaBar
PDG (10-4) ηc
14.2
12.0±0.8±0.7
8.4±1.3±0.8
9.6±1.1
ηc(2S)
4.1
4.8±1.1±0.3
3.4±1.8
X(3872)
1.1
1.2±1.1±1.1 (<2.6)
<3.2
(<3.2)
X(3915)
0.3
0.4±1.6±0.0 (<2.8) -
・The best precision for ηc.
・First significant measurement of ηc(2S).
: Br(ηc(2S)→KKπ) = (1.3±0.4±0.4)×10-2 (my personal calculation)
・The most stringent upper limit for X(3872).
: (Br(X3872)→J/ψπ+π-) >
・The first limit for X(3915)
: Br(X(3915)→J/ψω) > ~10%. Γ(J/ψω) > O( 1 MeV). Very large as OZI suppressed mode
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8. Search for X(3872/3915)→χc1π0 in B decays
・ BES III observed X(3872) → χc1π0 in e+e- → γX(3872) (arXiv: )
Br(π0χc1)/Br(J/ψπ+π-) = ±0.10
・ π0χc1 is isospin 1, which is suppressed in pure cc state.
- Another evidence for non-cc.
・ Belle previously measured B+→χc1π0K+ using full statistics and found no structure in M(χc1π0 )
(Phys. Rev. D 93, ). However, no upper limit is put.
・ Belle re-analyzed the same data and set upper limit for Br(X(3872/3915)→π0χc1).
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9. Upper limit on X(3872/3915)→χc1π0 in B decays
Zoomed M(χc1π0)
・ No X(3872/3915) signals.
・ Br(π0χc1)/Br(J/ψπ+π-) < 0.97 at 90% C.L.
BES III)
・ Belle neither confirms nor rejects BES III result.
・ First limit on the X(3915)→π0χc1
Preliminary
Yields
Significance
Br(B+X)×Br(X→π0χc1)
X(3872)
2.7±5.5
0.3
<8.1×10-6
X(3915)
42±14
2.3
<3.8×10-5
Preliminary
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10. Physics of charmed baryons
(1) Di-quark structure in the baryon
・Spin-spin interaction∝1/m1m2
・Di-quark + charm quark
- Di-quark as new degree of freedom.
- Easier to understand baryons.
Nucleon
Charmed baryon
Pairs can not be distinguished.
Light di-quark and charm quark.
(2) Charm baryon as a hyperon laboratory
・ Ground-state dominantly decay via c→s.
・ Variety of hyperons are produced.
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11. Currently observed charmed baryons
LHCb
Belle
BaBar
CLEO
- State with [] observed in single exp.
- JP with () is QM prediction
2019/2/14
2019/2/19
(udc)
KEK-FF 2019
KMI 2019
(udc,uuc,ddc)
(usc,dsc)
(ssc)
arXiv:

12. Observation of Ξc(2930)0
・ Weak evidence of Ξc(2930) was reported by BaBar in B-→Ξc(2930)Λc ,Ξc(2930)→Λc+K-
・ Assigned as one star in PDG (need confirmation) .
・ Belle performed the same study with ~3 times statistics.
Previous study by BaBar (2008)
New study by Belle (2018)
Eur. Phys. J. C (2018) 78: 252.
Phys. Rev. D 77, (R)
Mass (MeV/c2)
Width (MeV)
BaBar
2931±3±5
36±7±11
Belle
2928.9±
19.5±
・ The statistical significance of the peak at Belle is 5.1σ.
・ Mass and width are consistent each other
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13. Evidence for charged partner Ξc(2930)+
Belle also found 4.1σ evidence for charged partner Ξc(2930)+ in
B0→ Ξc(2930)+ Λc-, Ξc(2930)+→ Λc+KS0
Mass (MeV/c2)
Width (MeV)
Ξc(2930)0
2928.9±
19.5±
Ξc(2930)+
2942.3±4.4±1.5
14.8±8.8±2.5
Eur. Phys. J. C (2018) 78: 928
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14. B Λc Ξc* Some comments on Ξc(2930) J=0 J=1/2 J=? Jz = 1/2
・ Ξc(2930)0 is the first charmed baryon established in the B-decay.
・As it is two-body B-decay, spin can be determined un-ambiguously
from helicity constraint if we have enough statistics.
Phys. Rev. D 77,
M(Λc+K-) in e+e-→cc by BaBar. Λc B
Ξc*
J=1/2
J=0
J=?
Jz = 1/2
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15. Absolute branching fractions of Ξc0
・Absolute branching fractions for ground state charmed baryons are not determined except for Λc+
- For Ξc0, all the branching fractions are measured relative to Ξc0→Ξ-π+
・Belle determined Ξc0→ Ξ-π+, ΛK-π+, pK-K-π+ in following steps:
1): Measure Br(B-→Λc-Ξc0) using missing mass technique.
2): Measure Br(B-→Λc-Ξc0) × Br(Ξc0→f) by exclusively reconstructing Ξc0 (f = Ξ-π+, ΛK-π+, pK-K-π+).
3): Divide 2) by 1). 2) 1)
Reconstruct
Reconstruct
Λc-
Hadronic decay
Λc- B+ B- B-
Reconstruct
Ξ-π+
ΛK-π+
pK-K-π+
Ξc0
Ξc0
Missing mass
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16. Results on branching ratios1)
Missing mass against Btag Λc-
arXiv:
・Ξc0 signal is observed by missing mass for the first time
with 5.5σ (40.9±9.0 counts)
・Br(B-→Λc-Ξc0) = (9.51±2.10±0.88)×10-4
・ Signal yields are obtained by 2D fit on Mbc-ΔE after
selecting signal region in M(Ξc)-M(Λc) plane. 2)
M(Ξc)-M(Λc)
Mbc ΔE
Ξc0→Ξ-π+ 3)
Decay
Br(B→ΞcΛc)xBr(Ξc→f)
(10-5)
Br(Ξc0→f) (%)
Significance
Ξ-π+
1.71±0.28±0.15
1.8±0.5±0.14
9.5σ
ΛK-π+
1.11±0.26±0.10
1.17±0.37±0.09
6.8σ
pK-K-π+
0.547±0.178±0.057
0.58±0.23±0.05
4.6σ
Ξc0→ΛK-π+
Ξc0→pK-K-π+
2019/2/14
KEK-FF 2019

17. Ξc+→Ξ(1620/1690)π+, Ξ(1620/1690)→Ξ-π+
・Not much is known about the S=-2 baryon family(Ξ).
- Even first orbital excitation with JP=1/2- is not identified.
- In the S=-1, JP=1/2- = Λ(1405) is a candidate of meson-baryon molecule state.
- Ξ(1620/1690) are candidates of Ξ analogue of Λ(1405).
Evidence for Ξ(1620) is poor (1 star state) in PDG.
・ Belle studied Ξ* in Ξc+→(Ξ-π+)π+ decay chain.
Ξc+ from Ξ-π+π+ decay
Belle has ~50000 Ξc+ on ~11500 BG.
Ξ(1620)
Phys.Lett. 38B (1972)
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18. M(Ξ-π+) distribution
arXiv:
(Accepted by PRL)
・Peaks correspond to Ξ(1530), Ξ(1620), and Ξ(1690) can be seen.
- Ξ(1620) and Ξ(1690) are not visible in the Ξc sideband.
・Fit are performed with floating Ξ(1530) and Ξ(1620) masses/widths.
Ξ(1690) mass and width are fixed with PDG values.
・Interference with non-resonant background is included.
・Ξ(1620) is the first hyperon established by charm baryon decay!
Ξ(1530)
Ξ(1620)
Ξ(1690)
Mass (MeV/c2)
Width (MeV)
Significance
Ξ(1620)
1610.4±
59.9±
25σ
Ξ(1690)
fixed 4σ
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19. Summary ・After ~10 years from the end of data taking,
Belle is actively working on the hadron spectroscopy.
・Decay properties of X(3872), X(3915) are investigated.
・A Charmed baryon Ξc(2930) is observed,
and branching fractions of Ξc0 are determined for the first time.
・Ξ(1620)0 observed from Ξc+ decay.
・Still many analyses ongoing, and some of them will be taken over at Belle II.
2019/2/14
KEK-FF 2019