Pionium

Pionium is a composite particle consisting of one
π⁺
and one
π⁻
meson. It can be created, for instance, by interaction of a proton beam accelerated by a particle accelerator and a target nucleus. Pionium has a short lifetime, predicted by chiral perturbation theory to be 2.89×10⁻¹⁵ s (i.e. 2.89 femtoseconds). It decays mainly into two
π⁰
mesons, and to a smaller extent into two photons.

It has been investigated at CERN to measure its lifetime. The Dimeson Relativistic Atomic Complex (DIRAC) experiment at the Proton Synchrotron was able to detect 21,227 atomic pairs from a total of 1.5×10⁹ events, which allows the pionium lifetime to be determined to within statistical errors of 9%.^[1]

In 2006, the NA48/2 collaboration at CERN published an evidence for pionium production and decay in decays of charged kaons, studying mass spectra of daughter pion pairs in the events with three pions in the final state K^± → π^±(ππ)_atom → π^±π⁰π⁰.^[2] This was followed by a precision measurement of the S-wave pion scattering length, published by the collaboration in 2009.^[3]

The results of the above experiments will provide crucial tests of low-energy QCD predictions.^[4]

^ Adeva, B.; et al. (2011). "Determination of ππ scattering lengths from measurement of π⁺π⁻π⁺π⁻ atom lifetime". Physics Letters B. 704 (1–2): 24–29. arXiv:1109.0569. Bibcode:2011PhLB..704...24A. doi:10.1016/j.physletb.2011.08.074. S2CID 18091499.
^ Batley, J. R.; et al. (2006). "Observation of a cusp-like structure in the π⁰π⁰ invariant mass distribution from K^± → π^±π⁰π⁰ decay and determination of the ππ scattering lengths". Physics Letters B. 633 (2–3): 173–182. arXiv:hep-ex/0511056. Bibcode:2006PhLB..633..173N. doi:10.1016/j.physletb.2005.11.087. S2CID 14747304.
^ Batley, J. R.; et al. (2009). "Determination of the S-wave ππ scattering lengths from a study of K^±→π^±π⁰π⁰ decays". European Physical Journal C. 64 (4): 589–608. arXiv:0912.2165. Bibcode:2009EPJC...64..589B. doi:10.1140/epjc/s10052-009-1171-3. S2CID 14006827.
^ Santamarina, C.; Schumann, M.; Afanasyev, L. G.; Heim, T. (2003). "A Monte Carlo calculation of the pionium break-up probability with different sets of pionium target cross sections". Journal of Physics B. 36 (21): 4273–4287. arXiv:physics/0306161. Bibcode:2003JPhB...36.4273S. CiteSeerX 10.1.1.286.4685. doi:10.1088/0953-4075/36/21/007. S2CID 250810773.

[hep-ph/9808407-1] Adeva, B.; et al. (2011). "Determination of ππ scattering lengths from measurement of π⁺π⁻π⁺π⁻ atom lifetime". Physics Letters B. 704 (1–2): 24–29. arXiv:1109.0569. Bibcode:2011PhLB..704...24A. doi:10.1016/j.physletb.2011.08.074. S2CID 18091499.

[hep-ex/0511056-2] Batley, J. R.; et al. (2006). "Observation of a cusp-like structure in the π⁰π⁰ invariant mass distribution from K^± → π^±π⁰π⁰ decay and determination of the ππ scattering lengths". Physics Letters B. 633 (2–3): 173–182. arXiv:hep-ex/0511056. Bibcode:2006PhLB..633..173N. doi:10.1016/j.physletb.2005.11.087. S2CID 14747304.

[na48-3] Batley, J. R.; et al. (2009). "Determination of the S-wave ππ scattering lengths from a study of K^±→π^±π⁰π⁰ decays". European Physical Journal C. 64 (4): 589–608. arXiv:0912.2165. Bibcode:2009EPJC...64..589B. doi:10.1140/epjc/s10052-009-1171-3. S2CID 14006827.

[urlA_Monte_Carlo_calculation_of_the_pionium_break-up_probability-4] Santamarina, C.; Schumann, M.; Afanasyev, L. G.; Heim, T. (2003). "A Monte Carlo calculation of the pionium break-up probability with different sets of pionium target cross sections". Journal of Physics B. 36 (21): 4273–4287. arXiv:physics/0306161. Bibcode:2003JPhB...36.4273S. CiteSeerX 10.1.1.286.4685. doi:10.1088/0953-4075/36/21/007. S2CID 250810773.

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