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First observation of the direct production of a non-vector state in e+e- annihilation

2022-09-13 Author:
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   The BESIII Collaboration has observed the direct production of an axial vector particle,  χc1(1P), in electron-positron (e+e-) annihilation. The paper has been published in Physical Review Letters [Phy. Rev. Lett. 129 (2022) 122001]. This is the first experimental observation of the direct production of a non-vector state in e+e- annihilation process.

Particle production at e+e- colliders has been playing an outstanding role in the development of the Standard Model of particle physics and for our understanding of hadrons as bound objects of quarks and anti-quarks. At e+e- colliders, particles are typically created in the annihilation of a positron, e+, and an electron, e-, via the exchange of a virtual photon, thus have the same quantum numbers (JPC=1--) as the virtual photon, such as  ρ  and J/ψ. The  χc1(1P) is however one of the P-wave charmonium triplets with the quantum numbers JPC=1++. It can only be produced via the exchange of two or more virtual photons, which makes reactions of this kind extremely rare and difficult to observe. Hadron production via two-photon annihilation was discussed theoretically already 40 years ago, but so far all experimental attempts to find them were unsuccessful.

Figure 1: The energy-dependent cross sections of e+e-   γJ/ψ γμ+μ- including (blue and green curves) and not including (red curve) the signal process e+e-  χc1(1P). The gray curve denotes the signal strength in the hypothetical case of no interference. The black dots with error bars are measured results from data.

The BESIII experiment carried out an energy scan of four data samples around the  χc1(1P) mass, and searched for the reaction e+e-  χc1(1P). By exploring the quantum mechanical interference of the signal process e+e-  χc1(1P) γJ/ψ  γμ+μ- and the physical background process with Initial State Radiation (ISR) photon e+e-γISRμ+μ-  (see Figure 1), the signal process is observed with a significance of 5.1σ (the probability that the signal is caused by the fluctuation of the background contribution is 3*10-7). For the first time, the electronic width of  χc1(1P) is determined to be Γee=(0.12+0.13-0.08) eV, in contrast of a few keV for vector states, which is 4 orders of magnitude smaller.

The new BESIII result confirms the theoretical framework and, furthermore, demonstrates a new production mechanism for hadronic particles at e+e- colliders and hence opens a new avenue for detailed studies of hadrons at presently running and future e+e- colliders, such as super tau-charm factories. The method can for instance be applied to study the properties, such as electronic width and line shape, of other non-vector particles, which is especially important in the case of so-called exotic particles. It is worth pointing out that the exploitation of the interference effect will not only be useful for future precision studies of hadrons at e+e- colliders, but has also been suggested for instance for Higgs studies at the Large Hadron Collider (LHC) at CERN.



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