Charmed Baryon Spectroscopy and Decays using the Belle Detector John Yelton University of Florida yelton@ufl.edu I review recent results on charmed baryon decay, mass and width measurements using the world’s largest data set of e+e- annihilation events that have been taken in the Upsilon energy range.

Most of the data was taken at the Upsilon(4S) energy, but most charmed baryon studies are made using particles produced in the charm continuum. These charmed baryons are generally easier to detect, and have better signal:noise than those in B decays.

H L L H L Why Study Charmed Baryon Spectroscopy? In the Heavy Quark Effective Theory charmed baryons are considered to be a combination of a heavy c quark, rather loosely bound to a light di-quark. All the couplings and masses we measure can be extrapolated using HQET, up to the B-baryons system (substitute c->b), and also (though not so precisely), the strange system (c->s) . H L L H is c quark for us, but can be b or s L is u, d, or s quark The charmed baryons offer us a fascinating quantum mechanical system, with more experimentally accessible states than the b-system, narrower and better defined states than the hyperon system, and more variety of possible states than for mesons. We would like to identify the states, and measure their widths and masses as accurately possible in order to understand the underlying structure of baryons.

Why Study Charmed Baryons Decays? Still much work to be done to understand the interplay between the various types of decay diagrams for weakly decaying charmed baryons. For instance, here are three completely different decay diagrams with the same final state. e.g. c* u d S Λ* π Λ* π π Λ* d s u d s u d c u u c d C u d Simple Spectator Internal W W-exchange Large samples of charmed baryons, with well-defined JP in their initial state, can be used to look for, and study, hyperons in the resonant substructure of their decays.

- - N=145744 fully reconstructed Number of c produced 36,447432 ABSOLUTE branching fraction measurement of c pK-+ The PDG lists around 40 branching ratios for c decays, but to get the branching fractions you need to know the absolute branching fraction for one of them – this has been a problem for 30 years! Nothing special about pK-+ Easy! Number of c pK-+ detected Total number of c produced BF= How can we know this? Belle noted that there is a large signal in: e+e- cc D(*)- p + c+ - - - Detect the D(*)-p + with no c+ and look for peak in the missing mass plot. Number of c produced 36,447432 N=145744 fully reconstructed

B(Λc+ →pK-π+) = 6.84±0.24 % BESIII have measured 5.84±0.27±0.23% PRL 113, 042002 (2014) +0.27 -0.21 B(Λc+ →pK-π+) = 6.84±0.24 % BESIII have measured 5.84±0.27±0.23% modest disagreement. Note that BESIII also measure other modes directly, and these will have to be carefully averaged by the PDG

Instead of pK-π+ look for pK+π- Doubly Cabibbo-Suppressed Decays of the Λc+ Instead of pK-π+ look for pK+π- M(pK-π+) M(pK+π-) 1,450,000 events 3587±380 events

Double mis-ID is not a big problem (it would produce a very wide peak), but there is unavoidable background from singly-Cabibbo-suppressed Λc→ΛK+ which needs to be subtracted. Final yield is 3379±380±78 ( > 9 σ) Finding the efficiencies of the final states is complicated by the fact that the detector efficiency depends on the location in the Dalitz plot. For the Cabibbo-allowed decay, we define the efficiency in small 2D “pixels”, and correct the yield in each pixel. For the DCS, this is not useful because of the lack of statistics and poor signal:noise

Can accurately find the efficiency for CF, but for DCS, the possible substructure introduces a systematic uncertainty.

B(Λc+ →pK+π-)/ B(Λc+ →pK-π+) = (2.35±0.27±0.21)x10-3 = (0.82±0.12)tan4Ɵc (note that the “W-exchange” diagram cannot give DCS decays, so this is surprisingly close to tan4Ɵc )

Spectroscopy of Excited Ξc (csu and csd) States Lowest mass excited states seen in Ξc n(π) (threshold = 2.61 GeV) Higher mass resonances seen in Λc+K- π+ (threshold = 2.92 GeV) why not also ΛD? (threshold = 2.97 GeV) We can reconstruct Λ decays using their long lifetime, reconstruct D mesons (1 mode for D+, 3 for D0), cut at high momentum, and look for resonant peaks

Large c(3055)0 Small c(3080)0 Large c(3055)+ ΛD+ (D+→Kππ)

Units are MeV Mass (ΛD) Width(ΛD) Significance c(3055)0 FIRST OBSERVATION 3059.00.50.6 6.42.11.1 8.6 c(3055)+ 3055.80.40.2 7.01.21.5 11.7 c(3080)+ 3079.60.40.1 <6.3 4.8 We can remind ourselves how this compares with the Λc+K- π+ mode, and then combine the results to get more precise numbers for the masses and widths

M[c(2455)++K-] M[c(2520)++K-] (well-known in ) c(3055)+ c(3080)+ NOTHING at 3123 Limit much tighter than observation reported by BaBar (Phys. Rev. D 77 012002) Phys. Rev. D 89 052003

Numbers are still preliminary! Combining the results of the different decay modes: Units are MeV Mass (BELLE) Mass(BaBar) Width(BELLE) Width(BaBar) c(3055)0 3059.00.50.6 6.42.11.1 c(3055)+ 3055.90.4 3054.21.20.5 7.81.21.5 17611 c(3080)0 3081.61.10.2 4.41.81.9 5.92.31.5 c(3080)+ 3077.90.9 3077.00.40.2 3.00.70.4 5.51.30.6 B(c(3055)+ →ΛD+)/B(c(3055)+ →Σc++K-) = 5.09±1.01±0.76 B(c(3080)+ →ΛD+)/B(c(3080)+ →Σc++K-) = 1.29±0.30±0.15 B(c(3080)+ →ΛΣc(2520)/B(c(3080)+ →Σc++K-) = 1.07±1.01±0.76 Clearly the c(3055) likes to decay to D The c(3080) likes to decay cK This is a challenge to theory! Numbers are still preliminary!

What are these states? In d-wave c baryons (i.e. 2 units of orbital angular momentum), the two units can be in two very different places. Between the heavy quark and light di-quark (-modes, low mass excitation) Between the two lighter quarks (-mode, higher mass excitation). In principle, 2 units can be 2 of either or 1 of each. Each can then combine with the light quark spin (0, or 1) and the charm quark spin to make many (34) different states!

Use many c decay modes to investigate the excited c baryons c/(Jp=1/2+) ; c(2645)(JP=3/2+) ; c(2790)(JP=1/2-) ; c(2815)(JP=3/2-) ; c(2980)(JP=?) c(2815)0 c(2645)+- c(2815)+ c(2645)0+ PRELIMINARY Cut on c(2815)0 Plot c0+ to see c(2645)+ Cut on c(2815)+ Plot c+- to see c(2645)0

c(2980)0 c(2645)+- c(2980)+ c(2645)0+ PRELIMINARY c/0 c/+

PRELIMINARY c(2790) c/+ c(2815) c/+ c’+- c’0+ c’0+

Particle Yield Mass Width c(2645)+ PDG c(2645)0 1260 975 2645.58 ± 0.06 ± 0.07+0.28 -0.40 2645.9 ± 0.5 2646.43 ± 0.07 ± 0.07+0.28 -0.40 2.06 ± 0.13 ± 0.13 2.6 ± 0.2 ± 0.4 2.35 ± 0.18 ± 0.13 < 5.5 c(2815)+ c(2815)0 941 1258 2816.73 ± 0.08 ± 0.06+0.28 -0.40 2816.6 ± 0.9 2820.20 ± 0.08 ± 0.07+0.28 -0.40 2819.6 ± 1.2 2.43 ± 0.20 ± 0.17 < 3.5 2.54 ± 0.18 ± 0.17 < 6.5 c(2980)+ c(2980)0 916 1443 2966.0 ± 0.8 ± 0.2+0.28 -0.40 2970.7 ± 2.2 2970.8 ± 0.7 ± 0.2+0.28 -0.40 2968.0 ± 2.6 28.1 ± 2.4+1.0 -2.0 17.9 ± 3.5 30.3 ± 2.3+1.0-5.0 20.7 ± 0.7 c/+ c/0 7055 11560 2578.4 ± 0.1 ± 0.4+0.28 -0.40 2575.6 ± 3.0 2579.2 ± 0.1 ± 0.4+0.28 -0.40 2577.9 ± 2.9 c(2790)+ c(2790)0 2231 1241 2791.6 ± 0.2 ± 0.1 ± 0.4+0.28-0.40 2789.8 ± 3.2 2794.9 ± 0.3 ± 0.1 ± 0.4+0.28 -0.40 2791.9 ± 3.3 8.9 ± 0.6 ± 0.8 < 15 10.0 ± 0.7 ± 0.8 < 12 ALL NUMBERS ARE PRELIMINARY! Values are in MeV

ISOSPIN SPLITTING? Particle M(c+)-M(c0) MeV/c2 c(2645) -0.85 ± 0.09 ± 0.08 ± 0.48 c(2815) -3.47 ± 0.12 ± 0.05 ± 0.48 c(2980) -4.8 ± 0.1 ± 0.2 ± 0.5 c/ -0.8 ± 0.1 ± 0.1 ± 0.5 c(2790) -3.3 ± 0.4 ± 0.1 ± 0.5 ALL numbers are PRELIMINARY!

PRELIMINARY c/c(2765) Ongoing Analysis c/c(2765)c+- (Off scale!) c(2880) c/c(2765) PRELIMINARY c(2625) c(2593) M(c+- ) The (2765) is a big, wide, enhancement found by CLEO in 2000. Until now, there have not been any further studies of its properties, and neither its mass or width is well measured. We don’t even know if it is a c or a c!

We DO know that the (2765) state resonates through an intermediate c(2455) M(c++-) +M(c0+) PDG M =2766.6  2.4 (stat)  ~50 MeV BELLE Preliminary Data Belle is working on an exhaustive analysis of this state, with a view to measuring its mass and width and knowing its quantum numbers. What is this particle that is so copiously produced? Could it be a radial excitation?

Summary and Conclusions The charmed baryon system remains a fertile ground of research Belle has long history of discoveries and measurements on the charmed baryon system I have shown results on: First measurement of doubly-Cabibbo-suppressed decays of the c Investigation of excited c states, observation of D states including the first observation of the c (3055)0 Systematic survey of the masses and widths of 5 iso-doublets, with much greater precision than before. First transitions of the c(2815) and c (3055)0 to c/ Plenty of new results in the pipeline, including analysis of the c (2765) Many results still to come using the 1ab-1 of Belle data. Other results will emerge once Belle II starts to take data. There will be many talks with the same title in the next decade.