所有结果

Decipher the short-distance component of $X(3872)$ in $B_c$ decays

Wei Wang; Qiang Zhao分类： 物理学 >> 核物理学

A foremost task in understanding the nature of the X(3872) involves the discrimination of the two-quark and multiquark configurations. In this work, we propose a method to probe the short-distance component of the X(3872) by measuring the ratio between the Bc semileptonic and nonleptonic decays into the X(3872). We demonstrate that if the X(3872) production mechanism is through the c¯c component, the ratios would be universal and could be reliably predicted in theory. Measurements of these ratios at LHC and the next-generation electron-positron colliders are capable of validating/invalidating this production mechanism and providing deeper insights into the nature of the X(3872). |

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

Wen-Long Sang; Feng Feng; Yu Jia; Shuang-Ran Liang分类： 物理学 >> 核物理学

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 Xiong分类： 物理学 >> 核物理学

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. |

The nature of near-threshold XYZ states

Martin Cleven; Feng-Kun Guo; Christoph Hanhart; Qian Wang; Qiang Zhao分类： 物理学 >> 核物理学

We demonstrate that the recently observed X,Y,Z states cannot be purely from kinematic effect. Especially the narrow near-threshold structures in elastic channels call for nearby poles of the S-matrix which are qualified as states. We propose a way to distinguish cusp effects from genuine states and demonstrate that (not all of) the recently observed X,Y,Z states cannot be purely from kinematic effects. Especially, we show that the narrow near-threshold structures in elastic channels call for nearby poles of the S-matrix, since the normal kinematic cusp effect cannot produce that narrow structures in the elastic channels in contrast to genuine S-matrix poles. In addition, it is also discussed how spectra can be used to distinguish different scenarios proposed for the structure of those poles, such as hadro-quarkonia, tetraquarks and hadronic molecules. The basic tool employed is heavy quark spin symmetry. |

Photoproduction of hidden charm pentaquark states $P_c^+(4380)$ and $P_c^+(4450)$

Qian Wang; Xiao-Hai Liu; Qiang Zhao分类： 物理学 >> 核物理学

We propose to study the pentaquark candidates of P+c(4380) and P+c(4450) in J/ψ photoproduction and look for further experimental evidence for their nature. Since the photoproduction process does not satisfy the so-called "anomalous triangle singularity" condition their presence in J/ψ photoproduction would conclude that they should be genuine states and provide further evidence for their existence. |

Understanding the newly observed heavy pentaquark candidates

Xiao-Hai Liu; Qian Wang; Qiang Zhao分类： 物理学 >> 核物理学

We find that several thresholds can contribute to the enhancements of the newly observed heavy pentaquark candidates P+c(4380) and P+c(4450) via the anomalous triangle singularity (ATS) transitions in the specific kinematics of Λb→J/ψK−p. Apart from the observed two peaks we find that another peaks around 4.5 GeV can also be produced by the ATS. We also show that the Σ(∗)c can be produced at leading order in Λb decay. This process is different from the triangle diagram and its threshold enhancement only appears as CUSP effects if there is no pole structure or the ATS involved. The threshold interaction associated with the presence of the ATS turns out to be a general phenomenon and plays a crucial role in the understanding of candidates for exotic states. |

Searching for observable effects induced by anomalous triangle singularities

Xiao-Hai Liu; Makoto Oka; Qiang Zhao分类： 物理学 >> 核物理学

We investigate the anomalous triangle singularity (ATS) and its possible manifestations in various processes. We show that the ATS should have important impact on our understanding of the nature of some newly observed threshold states. Discussions on how to distinguish the ATS phenomena from genuine dynamic pole structures are presented. |

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

M. Jansen; H.-W. Hammer; Yu Jia分类： 物理学 >> 核物理学

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 Sang分类： 物理学 >> 核物理学

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. |

Employing spin symmetry to disentangle different models for the XYZ states

Martin Cleven; Feng-Kun Guo; Christoph Hanhart; Qian Wang; Qiang Zhao分类： 物理学 >> 核物理学

In order to test different models proposed for some states discovered recently in the charmonium mass range that do not fit into the pattern predicted by the conventional quark model, we derive predictions for the spectrum within the hadro-charmonium picture, the tetraquark picture as well as the hadronic molecular approach. We exploit heavy quark spin symmetry for the hadro-charmonium and hadronic molecule scenarios. The patterns that emerge from the different models turn out to be quite distinct. For example, only within the hadro-charmonium picture a pseudoscalar state emerges that is lighter than the Y(4260). Possible discovery channels of these additional states are discussed. |