Search for doubly charmed baryons at Belle

1. Search for doubly charmed baryons at Belle N-lab Yuji Kato

2. Hadron physics Higgs particle gives mass of elementary particles. However, it explains only several % of the mass of the matter... Almost of the nucleon mass comes from dynamics of quark and gluon Bound state but mass increased...? Understand these two features simultaneously Confinement of the quark Sub-questions to be answered ・Why “very simple” constituent quark model works so well? ・What is the real degree of freedom to describe a hadron? Only constituent quark is enough?

3. Heavy baryon spectroscopy in B-factory Heavy quark brings simplicity! C e- c ・A heavy quark behaves like static potential source. →Light “di-quark” with color 3bar and a heavy quark bounded by a string. ・Confirmed to works very well. →Suggesting “di-quark” structure in the baryon. Λc+ u d e+ c Heavy baryon spectroscopy driven by B-factories 16/21 single charmed baryon has was discovered by B-factories ・All the ground states are discovered. ・Many excited states are also discovered.

4. c c c c Doubly charmed baryon u d 1.Non-relativistic potential works well for heavy quarks Charmonium spectroscopy→Understanding of QQbar potential to be “Linear+Coulomb type” c c Double charmed baryon(Ξcc) spectroscopy→QQ potential Colored object → more deep insight of non-perturbative QCD 2.Comparison with single charmed baryon Λc+ Ξcc+ C C C Good comparison between light quark and heavy quark →Understanding of “constituent mass” d u d

5. Past experimental search Evidence from SELEX in 2003 Mass:3519±1.0MeV/c2 B-factories Babar@232fb-1 Belle@461.5fb-1 M(Λc+K-π+) hep-ex/0606051 M(Λc+K-π+) hep-ex/0605075 hep-ex/0208014 ・Evidence by SELEX was not supported by B-factories. ・No established state even for grand state.

6. Ξcc decay modes Grand state baryon decays via weak interaction. A c-quark transfers to s-quark with W+ emission. Ξcc+(+) is expected to decay into ΛcK-π+(π+) and Ξc0π+(π+) π+ π+ u u d W+ d W+ Ξcc+ Ξcc+ c s K- c s c u c c d u Ξc0 c d d Λc+ d ・Background: inclusive Λc+ or Ξc0 production. ・σ(e+e-→Λc+X)/σ(e+e-→Ξc0X) >~10 ・Belle did not search for decay modes with Ξc0

7. Belle: The best place for Ξcc search Belle detector cavity e+ source The world highest luminosity! In this study.. ・Use full statistics of Belle data. (2×previous Belle, 4×previous BaBar) ・Decay modes including Ξc0 is also studied. ・Use several sub decays of Λc+ and Ξc0. → About 8 times statistics compared with previous BaBar’s for decay modes with Ξc0.

8. Belle Detector SC solenoid 1.5T Aerogel Cherenkov cnt. n=1.015~1.030 3.5 GeVe+ CsI(Tl) 16X0 TOF conter m / KL detection 14/15 lyr. RPC+Fe 8 GeVe- ・General purpose detector ・Momentum measurement by SVD, CDC ・p/K/π ID by CDC (dE/dx), TOF, ACC → cover p<~4 GeV/c Central DriftChamber Si vtx. det. 3/4 lyr. DSSD

9. Reconstruction of Λc+ M(pK-π+) M(pKs) ・Peaks corresponds to Λc+are clearly seen. ・Apply mass constraint on Λc+→Resolution for Ξcc is 2.5-3.5 MeV/c2 ・S/N ratio is comparable.→Combine two decay modes

10. Result for decay mode with Λc+ M(ΛcK-π+) M(ΛcK-π+π+) Data MC expectation for signal (height is arbitrary) ・The significance from -2sqrt(ln(Lmax/L0)) for whole mass region. →No point exceeds 3σ. ・95 % C.L upper limit of σ(e+e-→ΞccX)×Br(Ξcc→ΛcK-π+(π+)) 4.6-26.0 fb⇔88 fb by Babar ・Theoretical prediction ~1.0-200 fb⇔ 90-520 fb assuming Br=5%

11. Reconstruction of Ξc0 M(Ξ-π+) M(ΛK-π+) M(pK-K-π+) ・Yield is comparable for 3 modes ・S/N largely depends on the decay mode. →Simultaneous fitting with fixing signal yield gives better sensitivity. However, no measurement on branching ratio

12. Study of the Ξc+(2645) Ξc0π+ is a strong decay mode of Ξc+(2645) as well as weak decay of Ξcc. →Use yield of Ξc+(2645) to determine signal ratio. Ξ-π+ ΛK-π+ Ξc+(2645) feed down of Ξc(2790)+ ・Ξ-π+ :pK-K-π+:ΛK-π+:=1298±51:974±47:1444±58 ・First measurement of the width of the Ξc+(2645) is also performed: 2.6±0.2(stat)±0.4 MeV/c2⇔ <3.1 MeV/c2 pK-K-π+

13. Result:Ξc0π+ Ξ-π+ ΛK-π+ Data MC expectation for signal (height is arbitrary but relative scale is correct) ・No significant signal. ・95% C.L upper limit of σ(e+e-→Ξcc+X)×Br(Ξcc+→Ξc0π+)×Br(Ξc0→Ξ-π+) =0.076-0.26 fb⇔ 1.3 fb by Babar. ・Theoretical prediction ~1.0-200 fb ⇔ 30-104 fb assuming both of Br=5% pK-K-π+

14. Summary ・Heavy baryon is a good environment for hadron physics. ・Belle is the world best experiment for Ξcc search. ・The Ξcc was not discovered. UL of the cross section becoming closer to the theoretical prediction. ・First precise measurement of the width of the Ξc+(2645). To find Ξcc .......

15. Prospect: BelleⅡexperiment ・Physics beyond the SM ・~50 times statistics ・Belle→ Belle2 detector 8m Development of novel PID counter: TOP →Talk by Y. Aritain this session and T. Hayakawa, K. Matsuoka in the parallel session tomorrow.

16. Backup

17. Reconstruction of Ξ- (real data) p π- M(pπ-) Decay topologyof Ξ-→Λπ-→pπ-π- Λ π- Ξ- IP M(Λπ-) Before cut After cut ・Λ is selected by goodLambda()>0. ・Closest distance of Λ and π- in z-direction<3mm. ・cos(Θ) of momentum vector of Ξ- andvertex vector of Λ-π- >0.95. ・χ2 of vertex fitting of Λ π- <50. Efficiency of these cut is ~93%. 17