Ortho-positronium observation in the Double Chooz experiment


The Double Chooz experiment measures the neutrino mixing angle θ 13 by detecting reactor \( {\overline{\nu}}_e \) via inverse beta decay. The positron-neutron space and time coincidence allows for a sizable background rejection, nonetheless liquid scintillator detectors would profit from a positron/electron discrimination, if feasible in large detector, to suppress the remaining background. Standard particle identification, based on particle dependent time profile of photon emission in liquid scintillator, can not be used given the identical mass of the two particles. However, the positron annihilation is sometimes delayed by the ortho-positronium (o-Ps) metastable state formation, which induces a pulse shape distortion that could be used for positron identification. In this paper we report on the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events. The o-Ps formation fraction and its lifetime were measured, finding the values of 44 % ±12 % (sys.) ±5 % (stat.) and 3.68 ns ±0.17 ns (sys.) ±0.15 ns (stat.) respectively, in agreement with the results obtained with a dedicated positron annihilation lifetime spectroscopy setup.

A preprint version of the article is available at ArXiv.


  1. [1]

    Double CHOOZ collaboration, Y. Abe et al., Improved measurements of the neutrino mixing angle θ 13 with the Double CHOOZ detector, arXiv:1406.7763 [INSPIRE].

  2. [2]

    Double CHOOZ collaboration, Y. Abe et al., First measurement of θ 13 from delayed neutron capture on hydrogen in the Double CHOOZ experiment, Phys. Lett. B 723 (2013) 66 [arXiv:1301.2948] [INSPIRE].

    Google Scholar 

  3. [3]

    Double CHOOZ collaboration, Y. Abe et al., Background-independent measurement of θ 13 in Double CHOOZ, Phys. Lett. B 735 (2014) 51 [arXiv:1401.5981] [INSPIRE].

    Google Scholar 

  4. [4]

    Daya Bay collaboration, F.P. An et al., Spectral measurement of electron antineutrino oscillation amplitude and frequency at Daya Bay, Phys. Rev. Lett. 112 (2014) 061801 [arXiv:1310.6732] [INSPIRE].

    ADS  Article  Google Scholar 

  5. [5]

    RENO collaboration, J.K. Ahn et al., Observation of reactor electron antineutrino disappearance in the RENO experiment, Phys. Rev. Lett. 108 (2012) 191802 [arXiv:1204.0626] [INSPIRE].

    Article  Google Scholar 

  6. [6]

    G. Ranucci, A. Goretti and P. Lombardi, Pulse-shape discrimination of liquid scintillators, Nucl. Instrum. Meth. A 412 (1998) 374 [INSPIRE].

    ADS  Article  Google Scholar 

  7. [7]

    D. Franco, G. Consolati and D. Trezzi, Positronium signature in organic liquid scintillators for neutrino experiments, Phys. Rev. C 83 (2011) 015504 [arXiv:1011.5736] [INSPIRE].

    ADS  Google Scholar 

  8. [8]

    H.J. Ache, Positronium and muonium chemistry, Adv. Chem. 175 (1979) 1, American Chemical Society, U.S.A. (1979).

  9. [9]

    Y. Kino et al., Positron annihilation in liquid scintillator for electron antineutrino detection, J. Nucl. Radiochem. Sci. 1 (2000) 63.

    Article  Google Scholar 

  10. [10]

    G. Consolati et al., Characterization of positronium properties in doped liquid scintillators, Phys. Rev. C 88 (2013) 065502 [arXiv:1308.0493] [INSPIRE].

    ADS  Google Scholar 

  11. [11]

    Borexino collaboration, G. Bellini et al., First evidence of pep solar neutrinos by direct detection in Borexino, Phys. Rev. Lett. 108 (2012) 051302 [arXiv:1110.3230] [INSPIRE].

    ADS  Article  Google Scholar 

  12. [12]

    Double CHOOZ collaboration, Y. Abe et al., Reactor electron antineutrino disappearance in the Double CHOOZ experiment, Phys. Rev. D 86 (2012) 052008 [arXiv:1207.6632] [INSPIRE].

    Google Scholar 

  13. [13]

    C. Aberle et al., Large scale Gd-beta-diketonate based organic liquid scintillator production for antineutrino detection, 2012 JINST 7 P06008 [arXiv:1112.5941] [INSPIRE].

  14. [14]

    C. Aberle, C. Buck, F.X. Hartmann and S. Schonert, Light yield and energy transfer in a new Gd-loaded liquid scintillator, Chem. Phys. Lett. 516 (2011) 257 [INSPIRE].

    ADS  Article  Google Scholar 

  15. [15]

    Y. Abe et al., The waveform digitiser of the Double CHOOZ experiment: performance and quantisation effects on photomultiplier tube signals, 2013 JINST 8 P08015 [arXiv:1307.4917] [INSPIRE].

  16. [16]

    Particle Data Group collaboration, J. Beringer et al., Review of particle physics (RPP), Phys. Rev. D 86 (2012) 010001 [INSPIRE].

    Google Scholar 

  17. [17]

    R. Brun and F. Rademakers, ROOT: an object oriented data analysis framework, Nucl. Instrum. Meth. A 389 (1997) 81 [INSPIRE].

    ADS  Article  Google Scholar 

Download references

Open Access

This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.

Author information




Corresponding author

Correspondence to C. Jollet.

Additional information

ArXiv ePrint: 1407.6913

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0), which permits use, duplication, adaptation, distribution, and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

The Double Chooz collaboration., Abe, Y., dos Anjos, J.C. et al. Ortho-positronium observation in the Double Chooz experiment. J. High Energ. Phys. 2014, 32 (2014). https://doi.org/10.1007/JHEP10(2014)032

Download citation


  • Neutrino Detectors and Telescopes