We use angle-resolved photoemission to unravel the quasiparticle decoherence process in the high-$T_c$ cuprates. The coherent band is highly renormalized, and the incoherent part manifests itself as a nearly vertical ``dive'' in the $E$-$k$ intensity plot that approaches the bare band bottom. We find that the coherence-incoherence crossover energies in the hole- and electron-doped cuprates are quite different, but scale to their corresponding bare bandwidth. This rules out antiferromagnetic fluctuations as the main source for decoherence. We also observe the coherent band bottom at the zone center, whose intensity is strongly suppressed by the decoherence process. Consequently, the coherent band dispersion for both hole- and electron-doped cuprates is obtained, and is qualitatively consistent with the framework of Gutzwiller projection.

Using a sample of 1.06×108 ψ(2S) events collected with the BESIII detector at BEPCII, the decay ψ(2S)→pp̄η is studied. A partial wave analysis determines that the intermediate state N(1535) with a mass of 1524±5-4+10 MeV/c2 and a width of 130-24-10+27+57 MeV/c2 is dominant in the decay; the product branching fraction is determined to be B(ψ(2S)→N(1535)p̄)×B(N(1535)→pη)+c.c.=(5. 2±0.3-1.2+3.2)×10-5. Furthermore, the branching fraction of ψ(2S)→pp̄η is measured to be (6.4±0.2±0.6) ×10-5. Here, the first error is statistical and the second is systematic. © 2013 American Physical Society.

With a sample of 225.3 × 106 J/φ events taken with the BESIII detector, the decay J/φ → γ3 (π+ π-) is analyzed. A structure at 1.84 GeV/c2 is observed in the 3(π+ π-) invariant mass spectrum with a statistical significance of 7.6σ. The mass and width are measured to be M = 1842.2 ± 4.2-2.6+7.1 MeV/c2 and Γ = 83 ± 14 ± 11 MeV. The product branching fraction is determined to be B (J/φ → γX(1840)) × B(X(1840) → 3(π+ π-)) = (2.44 ± 0.36-0.74+0.60) × 10-5. No η′ signals are observed in the 3(π+ π-) invariant mass spectrum, and the upper limit of the branching fraction for the decay η′ → 3(π+ π-) is set to be 3.1 × 10-5 at a 90% confidence level.