1. charm baryon spectroscopy and decays at Belle
Recent studies of
charm baryon spectroscopy and decays at Belle
Y. Kato (KMI, Nagoya)
2018/3/6
Moriond QCD

2. Physics of charmed baryons (1)
・Charm quark is heavier than u,d,s quarks
・spin-spin interaction∝1/m1m2
・Di-quark structure in light quarks
- Di-quark as new degree of freedom.
- Easier to understand baryons.
Nucleon
Charmed baryon
Every pair can not be distinguished.
Light di-quark and charm quark.
2018/3/20
Moriond QCD

3. Physics of charmed baryons (2)q c s c s W- q q q W W c s q q q q q q q q q q q q q q
Spectator
Internal W
W-exchange
・ The weak decay of charmed baryon is not understood well.
- Three diagrams contribute in the tree level, strength of each is not known.
・ Ground state charm baryon is a good laboratory to study strange baryons
as decay proceed via c->s transition.
・ Belle has collected ~1 ab-1 e+e- data samples (mainly at Υ(4S)).
- 109 e+e-→ cc samples
- 7.7× 108 BB samples
・ Huge data sample enable to study various charmed baryons.
2018/3/20
Moriond QCD

4. Contents (4) Confirmation of excited Ωc (3) Observation of Ξc(2930)
(2) Ωc decay
(5) Production rates of various baryons
(1) Search for strange pentaquark via Λc+→π0pφ decay
2018/3/6
Moriond QCD

5. (1) Search for pentaquark via Λc+→π0φp
・ LHCb observed hidden charm pentaquark state in the
Λb0 → K- Pc+ → K- (J/ψ p).
・ Analogue search for hidden-strange pentaquark by
switching b->c (Λb0 →Λc+), c->s (J/ψ -> φ)
:Λc+→ π0 Ps+ → π0 (φ p)
・ Λc+→π0φp decay itself is not observed so far.
2018/3/6
Moriond QCD

6. Search for pentaquark via Λc+→π0φp (result)
M(K+K-pπ0)
Phys. Rev. D 96, (R)
M(K+K-)
M(φp)
・ Perform 2D fit on M(K+K-pπ0) and M(K+K-) plane.
No significant Λc+ signals is observed.
New upper limits:
- Br (Λc+→φ p π0) < 15.3x10-5
- Br (Λc+→K+K-pπ0 )NR < 6.3x10-5
・ Also perform 2D fit in each M(φp) bin.
No significant Ps+ signal observed.
- Br(Λc+→π0Ps+)xBr(Ps+→φp) < 8.3×10-5
2018/3/6
Moriond QCD

7. (2) Hadronic decays of Ωc
・ Among 4 ground state charmed baryons (Λc, Ξc0 Ξc+, Ωc),
information of Ωc (css) is not studied well as cross section is small.
・Only Ωc has the same flavor light quarks (ss).
Constructive interference is thought to be the origin
of its short life time.
・ Precise measurements will shed light on the
dynamics of baryon weak decays.
・ Belle performed measurements of 8 decay modes
relative to the bench mark mode: Ω-π+
Life time (ps)
Phys. Rev. D 97,
Λc+
Ξc0
Ξc+ Ωc
200±6
442±26
69±12
M(Ωπ+)
どの崩壊をstudyしたか良く分からない。
2018/3/20
Moriond QCD

8. Mass distributions for eight decay modes
Ω-π+π-π+
Ω-π+π0
Ξ-K-π+π+　
Ξ0K-π+
Ξ-KSπ+ (New!)　
Ξ0KS (New!)
ΛKSKS (new!)
Σ+K-K-π+
Phys. Rev. D 97,
2018/3/20
Moriond QCD

9. Intermediate states K*0 K*0 Ξ(1530) ρ-
M(Ξ-π+) in
Ξ-K-π+π+
M(π+π0) in
Ω-π+π0
Ξ(1530) ρ-
M(K-π+) in
Ξ-K-π+π+
M(K-π+) in
Ξ0K-π+
K*0
K*0
Phys. Rev. D 97,
・ Intermediate resonances are studied for the first time in 3 decays.
2018/3/20
Moriond QCD

10. Results of the branching fraction
Phys. Rev. D 97,
New!
・Precision improved by ~factor 2 for already measured modes.
・First measurements of intermediate resonances.
- Dominant contributions.
・Three first observations.
2018/3/20
Moriond QCD

11. (3) Observation of Ξc(2930)
・ A 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 study with ~3 times statistics.
Previous study by BaBar
New study by Belle
arXiv:
(Appear in EPJC)
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
2018/3/20
Moriond QCD

12. Some comments on Ξc(2930)
・ Ξc(2930)0 is the first charmed charmed baryon established in the B-decay.
・Candidates of corresponding Λc, Σc states?
・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.
No signal.
B→(Λc+π) p by BaBar
Mass = (2846±8±10) MeV/c2, Width = MeV
Possibly Σc(2800), may be a different state.
The Ξc’ -Σc mass difference is ~120 MeV.
Good candidate as partner of Ξc(2930).
Need confirmation by Belle.
/PhysRevD
2018/3/6
Moriond QCD

13. (4) Excited Ωc states
Phys. Rev. Lett. 118,
Ωc(3050)
Ωc(3066)
Ωc(3000)
Ωc(3090)
Ωc(3119)
Ωc(3188)?
・ LHCb reported 5 (+1) excited Ωc states in the Ξc+K- final states.
・ Belle studied the same final state to confirm existence.
・ Though statistical sensitivity of Belle is much lower than LHCb,
study with different production may shed light on the nature of these states.
2018/3/20
Moriond QCD

14. Confirmation by Belle ・ The Ξc+ is reconstructed in 7 decay modes:
(Ξ-π+π+, ΛK-π+π+, Ξ0π+, Ξ0π+π-π+,
Σ+K-π+, ΛKSπ+, ad Σ0Ksπ+)
・ Widths are fixed to the value
obtained by LHCb in the fit.
・ 4 of 5 states are confirmed (>3σ)
Only Ξc(3119) was not confirmed.
・ The mass is consistent with LHCb.
Ξc+K+ (wrong sign)
Ξc+ sideband
arXiv:
(appear in PRD)
LHCb
Belle
Ωc(3000)
3000.4±0.2±
3000.7±1.0±0.2 (3.9σ)
Ωc(3050)
3050.2±0.1±0.1＋
3050.2±0.4±0.2 (4.6σ)
Ωc(3066)
3065.6±0.1±
3064.9±0.6±0.2 (7.2σ)
Ωc(3090)
3090.2±0.3±
3089.3±1.2±0.2 (5.7σ)
Ωc(3119)
3119.1±0.3±
3119.0±0.3±0.9 (0.4σ)
2018/3/20
Moriond QCD

15. (5) Production rate of various baryons
arXiv: (Appear in PRD)
Charmed baryons
Hyperons
・ Production ratio = σ/(2J+1) for various baryons in e+e- collision.
- Lie on exp(-αm). Explained by string fragment model.
- First measurement for charmed baryons.
- Feed down from higher resonance subtracted.
・ A factor three suppression of Σc is observed (not for hyperons).
- If charmed baryon production proceeds via diquark-anti-diquark pair production,
the mass difference of spin-0 di-quark (Λc) and spin-1 di-quark (Σc) can explain this.
Suggesting a di-quark structure in the charmed baryon.
2018/3/20
Moriond QCD

16. Conclusion ・ Belle is still actively working on the charmed baryons.
- Hidden-strange pentaquark in Λc+ -> pφπ0
- Branching fractions of hadronic decay of Ωc
- Observation of Ξc(2930)
- Confirmation of excited Ωc
- Production rate of various baryons
・ Belle II will enable us to discover more states,
determination of quantum numbers!
2018/3/6
Moriond QCD