Zhi-Guo He

A bstract: In this work, we comprehensively study the total and differential decay widths of the radiative Dalitz decays H → l ⁺ l ⁻ γ up to QCD next-to-leading order (NLO) accuracy within the framework of NRQCD factorization. Our calculation includes the decays of S -wave states ( η _c , η _b ) and the P -wave triplets ( χ _cJ , χ _bJ for J = 0 , 1 , 2) to both electron ( l = e ) and muon ( l = μ ) final states. To match realistic experimental detection thresholds, systematic kinematic cuts are implemented on the final-state photon energy. Our analysis of the lepton-pair invariant mass distribution shows distinct singular behaviors across the multiplets originating from the lepton-pair threshold region, which is regularized by the lepton mass, and the soft-photon region, respectively. For the S -wave states, there is only one peak near the lepton-pair threshold region, while for the J = 0 and J = 2 P -wave states, the peaks show up in both regions. However, for the J = 1 P -wave states, the peak only appears in the soft-photon region. Such features provide a rich venue to probe the γ ^* → l ⁺ l ⁻ form factor in heavy quarkonium decay. Integrating over the bounded phase spaces reveals a distinct hierarchy among the χ _QJ states in the sensitivity of our theoretical predictions to the soft-photon energy cuts, ordered as χ _{Q 1} > χ _{Q 2} > χ _{Q 0} . In the χ _cJ → l ⁺ l ⁻ γ cases, as the energy cut increases from 100 MeV to 500 MeV the theoretical predictions at QCD NLO are reduced by 6%( χ _{c 0} ), 66%( χ _{c 1} ), and 21%( χ _{c 2} ) for l = e , and by 17%( χ _{c 0} ), 66%( χ _{c 1} ), and 39%( χ _{c 2} ) for l = μ . Comparing our predictions with the upcoming high-precision experimental tests at BESIII will definitely deepen our understanding of the predictive power of perturbative calculations.

The color-octet (CO) mechanism is a cornerstone of nonrelativistic QCD, yet knowledge of its long-distance matrix elements remains limited, preventing stringent tests of the theory. We demonstrate that the Artru-Collins asymmetry in hadronic decays of the P -wave bottomonium state χ b 2 provides a direct probe of CO dynamics. At leading order, the asymmetry arises exclusively from the CO decay channel, whereas the color-singlet (CS) contribution affects only the unpolarized rate, so that a nonzero signal constitutes unambiguous evidence of the CO mechanism. This observable provides a novel way to extract the ratio ρ 8 between CO and CS matrix elements. Focusing on e + e − → ϒ ( 2 S ) → γ χ b 2 at Belle, we show that the asymmetric beam configuration preserves the asymmetry in the laboratory frame and avoids the strong suppression present in the center-of-mass frame. With the Belle II dataset, ρ 8 could be determined with sufficient precision to address the long-standing discrepancy between the lattice calculations and phenomenological determinations.

Within the framework of nonrelativistic-QCD factorization, we perform a comprehensive investigation of the color singlet (CS) contribution to prompt J / ψ pair production at the CERN Large Hadron Collider at next-to-leading order (NLO) in α s . Specifically, we compare our NLO predictions with measurements from the LHCb, CMS, and ATLAS collaborations. We find that the CS contribution itself can well describe the LHCb data in most of the experimental bins, except in the regions where the fixed-order calculation is spoiled by the emission of soft or hard-collinear gluons. In the CMS and ATLAS cases, however, the CS predictions greatly undershoot both the measured total and differential cross sections, despite sizable K factors, of 2–3, for the total cross sections. Published by the American Physical Society 2025