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Next-to-next-to-leading-order QCD corrections to $\chi_c0,2\rightarrow \gamma\gamma$

Wen-Long Sang; Feng Feng; Yu Jia; Shuang-Ran LiangSubjects: Physics >> Nuclear Physics

We calculate the next-to-next-to-leading-order (NNLO) perturbative corrections to?P-wave quarkonia annihilation decay to two photons, in the framework of nonrelativistic QCD (NRQCD) factorization. The order-α2s?short-distance coefficients associated with each helicity amplitude are presented in a semi-analytic form, including the "light-by-light" contributions. With substantial NNLO corrections, we find disquieting discrepancy when confronting our state-of-the-art predictions with the latest \textsf{BESIII} measurements, especially fail to account for the measured?χc2→γγwidth. Incorporating the effects of spin-dependent forces would even exacerbate the situation, since it lifts the degeneracy between the nonperturbative NRQCD matrix elements of?χc0?and?χc2?toward the wrong direction. We also present the order-α2s?predictions to?χb0,2→γγ, which await the future experimental test. |

Quasi Distribution Amplitude of Heavy Quarkonia

Yu Jia; Xiaonu XiongSubjects: Physics >> Nuclear Physics

The recently-proposed quasi distributions point out a promising direction for lattice QCD to investigate the light-cone correlators, such as parton distribution functions (PDF) and distribution amplitudes (DA), directly in the?x-space. Owing to its excessive simplicity, the heavy quarkonium can serve as an ideal theoretical laboratory to ascertain certain features of quasi-DA. In the framework of non-relativistic QCD (NRQCD) factorization, we compute the order-αs?correction to both light-cone distribution amplitudes (LCDA) and quasi-DA associated with the lowest-lying quarkonia, with the transverse momentum UV cutoff interpreted as the renormalization scale. We confirm analytically that the quasi-DA of a quarkonium does reduce to the respective LCDA in the infinite-momentum limit. We also observe that, provided that the momentum of a charmonium reaches about 2-3 times its mass, the quasi-DAs already converge to the LCDAs to a decent level. These results might provide some useful guidance for the future lattice study of the quasi distributions. |

Finite volume corrections to the binding energy of the X(3872)

M. Jansen; H.-W. Hammer; Yu JiaSubjects: Physics >> Nuclear Physics

The quark mass dependence of hadrons is an important input for lattice calculations. We investigate the light quark mass dependence of the binding energy of the X(3872) in a finite box to next-to-leading order in an effective field theory for the X(3872) with perturbative pions (XEFT). At this order, the quark mass dependence is determined by a quark mass-dependent contact interaction in addition to the one-pion exchange. While there is only a moderate sensitivity to the light quark masses in the region up to twice their physical value, the finite volume effects are significant already at box length as large as 20 fm. |

Can NRQCD Explain the $\gamma\gamma^* \to \eta_c$ Transition Form Factor Data?

Feng Feng; Yu Jia; Wen-Long SangSubjects: Physics >> Nuclear Physics

Unlike the bewildering situation in the?γγ?→π?form factor, a widespread view is that perturbative QCD can decently account for the recent \textsc{BaBar} measurement of?γγ?→ηc?transition form factor. The next-to-next-to-leading order (NNLO) perturbative correction to the?γγ?→ηc,b?form factor, is investigated in the NRQCD factorization framework for the first time. As a byproduct, we obtain by far the most precise order-α2s?NRQCD matching coefficient for the?ηc,b→γγ?process. After including the substantial negative order-α2s?correction, the good agreement between NRQCD prediction and the measured?γγ?→ηc?form factor is completely ruined over a wide range of momentum transfer squared. This eminent discrepancy casts some doubts on the applicability of NRQCD approach to hard exclusive reactions involving charmonium. |

Relativistic correction to gluon fragmentation function into pseudoscalar quarkonium

Xiangrui Gao; Yu Jia; LiuJi Li; Xiaonu XiongSubjects: Physics >> Nuclear Physics

Inspired by the recent measurements of the?ηc?meson production at LHC, we investigate the relativistic correction effect for the fragmentation function of the gluon into?ηc, which constitutes the crucial nonperturbative elements to understand?ηc?production at high?pT. Employing three distinct methods, we calculate the leading relativistic correction to the?g→ηc?fragmentation function in the NRQCD factorization framework, as well as verify the existing NLO result for the?c→ηc?fragmentation function. We also study the evolution behavior of these fragmentation functions with the aid of DGLAP equation. |

Performance of new 8-inch photomultiplier tube used for the Tibet muon-detector array

Ying Zhang; Jing Huang; Ding Chen; Liu-Ming Zhai; Xu Chen; Xiao-Bin Hu; Yu-Hui Lin; Hong-Bo Jin; Xue-Yao Zhang; Cun-Feng Feng: Huan-Yu Jia; Xun-Xiu Zhou; DANZENGLUOBU; Tian-Lu Chen; LABACIREN; Mao-Yuan Liu; Qi Gao; ZHAXICIRENSubjects: Physics >> Nuclear Physics

A new hybrid experiment has been constructed to measure the chemical composition of cosmic rays around the "knee" in the wide energy range by the Tibet ASγ?collaboration at Tibet, China, since 2014. They consist of a high-energy air-shower-core array (YAC-II), a high-density air-shower array (Tibet-III) and a large underground water-Cherenkov muon-detector array (MD). In order to obtain the primary proton, helium and iron spectra and their "knee" positions in the energy range lower than?1016?eV, each of PMTs equipped to the MD cell is required to measure the number of photons capable of covering a wide dynamic range of 100 -?106?photoelectrons (PEs) according to Monte Carlo simulations. In this paper, we firstly compare the characteristic features between R5912-PMT made by Japan Hamamatsu and CR365-PMT made by Beijing Hamamatsu. This is the first comparison between R5912-PMT and CR365-PMT. If there exists no serious difference, we will then add two 8-inch-in-diameter PMTs to meet our requirements in each MD cell, which are responsible for the range of 100 - 10000 PEs and 2000 - 1000000 PEs, respectively. That is, MD cell is expected to be able to measure the number of muons over 6 orders of magnitude. |

Subjects: Physics >> Nuclear Physics

The gamma-ray observation of dwarf spheroidal satellites (dSphs) is an ideal approach for probing the dark matter (DM) annihilation signature. The latest Fermi-LAT dSph searches have set stringent constraints on the velocity independent annihilation cross section in the small DM mass range, which gives very strong constraints on the scenario to explain the the AMS-02 positron excess by DM annihilation. However, the dSph constraints would change in the velocity dependent annihilation scenarios, because the velocity dispersion in the dSphs varies from that in the Milky Way. In this work, we use a likelihood map method to set constraints on the velocity dependent annihilation cross section from the Fermi-LAT observation of six dSphs. We consider three typical forms of the annihilation cross section, i.e. p-wave annihilation, Sommerfeld enhancement, and Breit-Wigner resonance. For the p-wave annihilation and Sommerfeld-enhancement, the dSph limits would become much weaker and stronger compared with those for the velocity independent annihilation, respectively. For the Breit-Wigner annihilation, the dSph limits would vary depending on the model parameters. We show that the scenario to explain the AMS-02 positron excess by DM annihilation is still viable in the velocity dependent cases |

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