Charmed Baryons. Flavor Physics and CP Violation John Yelton University of Florida

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1 May 22 1 FPCP U. Penn Charmed Baryons Flavor Physics and CP Violation 22 University of Florida A review of the experimental results on charmed baryons with an accent on recent results.

2 May 22 2 FPCP U. Penn Why Study Charmed Baryons? The charmed baryon sector offers the richest spectroscopy of quark combinations. Compared with mesons there are more states as there are more possibilities for orbital excitations. Compared with mesons, the extra mass associated with orbital excitations is less, leading to less phase space for decays and narrower states. Because the charm quark has a large mass, the states can be described as a combination of a heavy quark and a light di-quark - this picture does not work well for strange baryons. Compared with the B sector, charmed baryons are much easier to study experimentally. Once charmed baryons are understood, extrapolation to B baryons is easy. There are four weakly decaying charmed baryons - good laboratory for studying weak decays.

3 May 22 3 FPCP U. Penn How To Study Charmed Baryons Two main techniques: Fixed Target Experiments, e.g. FOCUS, SELEX, E-791. Advantage: Long pathlength can be used measure the lifetimes, and as a tag of charm. Disadvantage: Charm particles are a small minority of events. Experiments at e + e Machines, e.g. CLEO, BELLE. Advantage: 4% of continuum events are charm. Disadvantage: Short pathlength, difficult to separate vertices. Running at Λ + c Λ + c threshold would be exciting. Maybe CLEO c in 26?

4 May 22 4 FPCP U. Penn Naming Charmed Baryons We always consider a charmed baryon as being the combination of a charmed quark and a light diquark with its own J P LIGHT to give a state of a particular J P. If the two light quarks are u and/or d, then the particle is Λ c or a Σ c. If the wave-function is antisymmetric under interchange of the two light quarks, then the particle is a Λ c and is I=. If it is symmetric, then it is aσ c and it is I=1. The lowest lying state is therefore the Λ + c with J P = 1 +,J P cud 2 LIGHT = + Next lowest is the isotriplet of Σ c s with J P = 1 +,J P cqq 2 LIGHT =1+. Then is the isotriplet of Σ c s with J P = 3 +,J P cqq 2 LIGHT =1 +.

5 Λ + c (Ground State) More than thirty decay modes measured. Recent contribution from BELLE on Cabibbo-suppressed and W-exchange modes. Phys. Lett. B526, 258 (22). Mode Yield Mode 2 B 1 /B 2 Previous ΛK ± 35 Λπ +.74 ±.1 ±.12 ΣK + 75 ± 18 Σ π +.56 ±.14 ±.8 Σ + K + π 15 ± 24 Σ + π + π.47 ±.11 ± Σ + K + K 246 ± 2 Σ + π + π.76 ±.7 ±.9.94 ±.17 ±.19 Σ + φ 129 ± 17 Σ + π + π.85 ±.12 ± ±.33 ±.25 Ξ(Σ + K )K + 75 ± 16 Σ + π + π.23 ±.5 ±.5 Ξ(ΛK )K + 75 ± 16 ΛK K +.26 ±.8 ±.3 pk + K 676 ± 89 pk π +.14 ±.2 ±.2.39 ±.9 ±.7 pφ 345 ± 43 pk π +.15 ±.2 ±.2.24 ±.6 ±.3 Some of these decays can proceed only by W-exchange diagrams: c d u s s s u u

6 Λ + c Lifetime New measurements from: SELEX τ(λ + c ) = ± 7. ± 5.6 fs. PRL (21). FOCUS τ (Λ + c ) = 24.6 ± 3.4 ± 2.4 fs. PRL (22) Events/2 fs (b) s Reduced Proper Time (ps) (c.f. PDG 21 value of 188 ± 7 fs from 6 experiments.) Short lifetime of the Λ + c presumably due to W-exchange decays.

7 May 22 7 FPCP U. Penn Recent Σ ++ c and Σ c Results All 3 masses well measured. Measurement of widths from CLEO and FOCUS Events/.5 MeV/c Events/.5 MeV/c Γ = MeV/c Γ = MeV/c M(Λ + GeV/c c π ) - M(Λ + c ) M(Λ + GeV/c c π + ) - M(Λ + c ) 2 CLEO FOCUS Γ(Σ ++ c )=2.3±.2±.3MeV, Γ(Σ ++ c )= ±.38 MeV, Γ(Σ c)=2.5±.2±.3mev. Γ(Σ c)= ±.38 MeV.

8 May 22 8 FPCP U. Penn Σ + c and Σ + c Production Events / 2 MeV M (MeV) 3 35 M(Λ + c π ) M(Λ + c ) CLEO finds a signal of Events. M(Σ + c ) = ± 1.1 ± 2. MeV.

9 May 22 9 FPCP U. Penn Σ ( ) c Summary In MeV + ++ M(Σ c ) CLEO ±.1 ± ±.2 ± ±.1 ±.2 FOCUS ±.19 ± ±.19 ±.12 PDG ± ± ±.19 Γ(Σ c ) CLEO 2.5 ±.2 ±.3 < ±.2 ±.3 FOCUS ± ±.38 M(Σ c ) CLEO ± 1. ± ± 1.1 ± ± 1.1 ±.8 FOCUS (232.7 ± 1.2) (234.2 ± 1.5) Γ(Σ c) CLEO ± 1. < ± 4. FOCUS (9.4± 3.7) (23.6 ± 4.5) Note small isosping splitting (but singly charged state lowest mass?) Note Γ(Σ c) 7 Γ(Σ c ).

10 May 22 1 FPCP U. Penn

11 May FPCP U. Penn Search for Higher States

12 May FPCP U. Penn Lower resonance: Yield= , M ππ = 48.1 ± 2.4 MeV,σ=2.9±2.6MeV. Upper resonance: Yield= , M ππ = ±.8 MeV,σ=4.2±.7MeV Events / 5 MeV M (MeV) 67 77

13 May FPCP U. Penn Higher Level States Conclusion CLEO explanation: Lower resonance is two of the first orbital excitations of the Σ c, These are L=1( ), where the J P c (ud) LIGHT =1 diquark combines with the charm quark to give a J P = 1, 3 pair. 2 2 Upper resonance could be the first orbital excitation where the excitation is between the two light quarks. ( L=1 ), where the J P c(u d) LIGHT = diquark combines with the charm quark to give a J P = 1 Λ 2 c particle.

14 May FPCP U. Penn Ξ c States - Predicted

15 May FPCP U. Penn Ξ c States - Discovered

16 I+ I+ May FPCP U. Penn Ξ + c Lifetime Events / (MeV/c 2 ) ( c ) m( ) (MeV/c 2 ) CLEO II.V 9. fb 1 N c c I I + = FWHM = 1 MeV/c 2 Events / (1 fs) CLEO II.V 9. fb 1 N I c + = c I m( ) (MeV/c 2 ) Proper Time (fs) 6 Events/(5 MeV/c 2 ) a) GeV/c 2 M(Ξ c + ) All Corrected Events/(4 fs) b) τ =.439±.22±.9 ps ps Reduced Proper Time PDG has fs CLEO 53 ± 47 ± 18 fs FOCUS 439 ± 22 ± 9fs

17 May FPCP U. Penn The Ω c Ω c is css combination. Many sightings over the years, in particular E-687 in Σ + K K π +. CLEO (21) finds a signal in the sum of 5 modes: (Ω π +,Ω π + π π +,Ω π + π,ξ K π +,Ξ K π + π +.) BELLE has the best signal yet, using Ω π CLEO measures: M= ± 2.6 ± 1.9MeV/c 2 BELLE measures: M= ± MeV/c2

18 May FPCP U. Penn Ω c Ω e + ν Result To look for semi-leptonic decays of the Ω c, CLEO first looks for inclusive Ω production and finds a yield of 76 ± 32 events K Combinations / (1MeV / c 2 ) M K (GeV / c 2 ) Ke Combinations / (1MeV / c 2 ) ( a ) ( b ) Ke Combinations / (1MeV / c 2 ) I M K (GeV / c 2 ) Look for Ω s with a) right sign lepton, and b) wrong sign lepton. After subtracting measured backgrounds, excess of: 11.4 ± 3.8 events due to Ω c Ω e + ν. Find Ω c Ω π + Ω c =.41 ±.19 ±.4 Ω e + ν Ξ Note: c Ξ π + =.44 ±.9 and Λ c Λπ + =.3±.1 Ξ c Ξ e + ν Λ + c Λe + ν

19 May FPCP U. Penn Looking for the Ξ ++ cc SELEX VERY preliminary...looking in Λ + c K π + π +

20 May 22 2 FPCP U. Penn CONCLUSIONS and hopes for the future There are 22 charmed baryons found. Many of them need confirmation, but they give a spectroscopic picture that is complex, but orderly and understandable. Still work to be done on spectroscopy. I anticipate more discoveries such as Ω c Ωγ, Ω c1 Ξ c K,Ξ c1 Ξ c ππ, moreσ c1 states, Λ c1 pd. Much work being done details - Ω c lifetimes, production mechanisms, precise masses, widths, absolute branching fractions, more semi-leptonic decays etc. etc. BELLE and BaBar are in a good position to contribute to this research. If CESR c runs at Λ + c threshold, so can CLEO c.