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Toponium: The smallest bound state and simplest hadron in quantum mechanics

Jing-Hang Fu1, Yu-Ji Li2, Hui-Min Yang3, Yu-Bo Li4, Yu-Jie Zhang1,5,*, and Cheng-Ping Shen6,7,†

  • *Contact author: zyj@buaa.edu.cn
  • †Contact author: shencp@fudan.edu.cn
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Phys. Rev. D 111, 114020 – Published 20 June, 2025

DOI: https://doi.org/10.1103/fqc9-k315

Abstract

We explore toponium, the smallest known quantum bound state of a top quark and its antiparticle, bound by the strong force. With a Bohr radius of 8×10−18  m and a lifetime of 2.5×10−25  s, toponium uniquely probes microphysics. Unlike all other hadrons, it is governed by ultraviolet freedom. This distinction offers novel insights into quantum chromodynamics. Our analysis reveals a toponium signal exceeding 5σ in the distribution of the cross section ratio between e+e−→bb¯ and e+e−→qq¯ (q=b, c, s, d, u), based on 400  fb−1 of data collected at s≈341  GeV. This discovery enables a top quark mass measurement with an uncertainty reduced by a factor of ten compared to current precision levels. Moreover, this method improves the systematic uncertainty by at least a factor of 2.7 compared to any other possible methods.

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Phys. Rev. D 111, 114020– Published 20 June, 2025
  • Received 25 February 2025
  • Accepted 27 May 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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