Observation of _ _ ηc decay into Σ+Σ- and Ξ-Ξ+ final states Phy. Rev. D87, 012003 (2013) 耿聪(Cong Geng) 中国科学技术大学 近代物理系 2014/4/20 武汉 晨光杯答辩 1 Outline Motivation • Intermediate charm meson loop mechanism • Prediction for ηc→baryon pairs _ _ Search for ηc→Σ+Σ-, Ξ-Ξ+ at BESIII Results and Comparison Discussion 2014/4/20 武汉 晨光杯答辩 2 Motivation 2014/4/20 武汉 晨光杯答辩 3 QCD - A big picture Confinement Asymptotic freedom Nobel Prize 2004 2014/4/20 武汉 晨光杯答辩 4 Charmonium • Annihilation decays of charmonia below open charm. _ • c and c annihilate at the origin _ of the (cc) wave function Relatively large energy scale ~ 2mc Short-distance, Perturbative QCD ？ 2014/4/20 武汉 晨光杯答辩 5 Helicity Selection Rule Relatively large energy scale ~ 2mc pQCD asymptotic behaviors are expected Helicity Selection Rule (HSR) !!! • An alternative description of this selection rule with the quantum number named “naturalness” • The selection rule requires that The masses of light quarks from the gluons could be ignored. Helicity conservation is not a consequence of a particular symmetry but is a dynamical consequence of leading twist pQCD Chernyark and Zitnitsky, Phys. Rept. 112, 173 (1984); Brodsky and Lepage, PRD24, 2848 (1981). 2014/4/20 武汉 晨光杯答辩 6 1. “ρπ puzzle” pQCD expects : However, the measured value is orders of magnitude smaller than the approximate “12%”(even consider the HSR) _ 2. ψ(3770) non-DD decay Unclarified!!!! 2014/4/20 武汉 晨光杯答辩 7 Intermediate charm meson loop mechanism (IML) _ cc pair first couples to an intermediate charm meson pair, and then these two mesons re-scatter into two light mesons. IML to account for “ρπ puzzle” IML for ψ(3770) non-DD decay Y. J. Zhang et al, PRL102, 172001 (2009) Q. Wang et al, PRD85, 074015 (2012) 2014/4/20 武汉 _ 晨光杯答辩 8 c1 VV Long-distance contribution via intermediate meson loops X. H. Liu and Q. Zhao, PRD81, 014017 (2010) Quite well consistent ! 2014/4/20 武汉 晨光杯答辩 9 Prediction for ηc→baryon pairs X. H. Liu and Q. Zhao J. Phys. G38, 035007,2011 2014/4/20 武汉 IML for baryon pair production in ηc 晨光杯答辩 10 _ _ Search for ηc→Σ+Σ-, Ξ-Ξ+ at BESIII 2014/4/20 武汉 晨光杯答辩 11 BESIII and Data Sets 𝟐. 𝟐𝟓 × 𝟏𝟎𝟖 J/ψ events collected with BESIII detector at BEPCII from June 12nd to July 28th in the year of 2009. The analysis depends on the decay mode J/ψ→γηc. 2014/4/20 武汉 晨光杯答辩 12 Event Selections (I) 𝑱/𝝍 → 𝜸𝜼𝒄 , 𝜼𝒄 → 𝜮+ 𝜮− , 𝜮 → 𝒑𝝅𝟎 𝑱/𝝍 → 𝜸𝜼𝒄 , 𝜼𝒄 → 𝚵 − 𝚵+ , 𝚵 → 𝚲𝝅 Common criteria for 𝜮 pair and 𝜩 pair: • Vertex constraints for charged tracks and the requirement on their solid angles • dE/dx and TOF are combined to do particle identification • Photon pre-selections : requirements on minimum energy and time information in EMC 2014/4/20 武汉 晨光杯答辩 13 Event Selections (II) For 𝜮 pair : For 𝜩 pair : 𝝅𝟎 Kinematic fit on energy-momentum conservation is applied on J/𝜓 →∗→ 𝛾𝑝𝑝𝜋 0 𝜋 0 𝜮 2014/4/20 武汉 Vertex fit and Second vertex fit tool is used to reconstruct the long-life Λ and Ξ to distinguish the two pions Kinematic fit on energy-momentum conservation is applied on J/𝜓 → 𝛾𝛯 − 𝛯+ → 𝛾𝑝𝑝2(𝜋 + 𝜋 − ) 晨光杯答辩 14 Invariant mass 𝚵 𝚺 ηc 2014/4/20 武汉 ηc 晨光杯答辩 15 Background Studies Kinematic Fit to suppress the background with one more or one mis-detected photon. The background can be well understood ! 2014/4/20 武汉 晨光杯答辩 16 Number of Signals Fit Function 2014/4/20 武汉 晨光杯答辩 17 Systematic Uncertainties Dominant PDG 2014/4/20 武汉 晨光杯答辩 18 Results and Comparison Theoretical prediction Consistent ! 2014/4/20 武汉 晨光杯答辩 19 Discussion Theoretical predictions are on the basis of intermediate charm meson loop mechanism. First Experimental observation of 𝜼𝒄 → 𝜮+ 𝜮− and 𝜼𝒄 → 𝚵− 𝚵+ . The BRs are order of magnitude 10-3 and agree with with the predictions. Future, to improve the precisions of both experiment and theory. Theory also predicted the similar decays in χc0 and hc. This is the opportunity for the BESIII after getting high statistics. 2014/4/20 武汉 晨光杯答辩 20 My Major Contribution at BESIII Publications: International Conference: 2014/4/20 武汉 晨光杯答辩 21 Thanks for your attention ! 2014/4/20 武汉 晨光杯答辩 22
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