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  • Open Access

Constraining axionlike particles from rare pion decays

Wolfgang Altmannshofer, Stefania Gori, and Dean J. Robinson
Phys. Rev. D 101, 075002 – Published 2 April 2020
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  • INTRODUCTION
  • ALP-PION MIXING
  • ALP LIFETIME
  • PION SEMILEPTONIC DECAYS TO ALPS
  • PIENU RESIDUALS BOUND
  • PIBETA DIPHOTON BOUND
  • CONCLUSIONS AND OUTLOOK
  • ACKNOWLEDGMENTS
    • References

    Abstract

    Ultraviolet completions for axionlike particles (ALPs) lighter than the neutral pion generically induce ALP-neutral pion mixing and are therefore sensitive to direct constraints on the mixing angle. For ALPs below the pion mass, we demonstrate that strong and novel bounds on the ALP-pion mixing angle can be extracted from existing rare pion decay data, measured by the PIENU and PIBETA experiments.

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    • Received 2 October 2019
    • Accepted 17 March 2020

    DOI:https://doi.org/10.1103/PhysRevD.101.075002

    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.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Particle phenomena
    Particles & Fields

    Authors & Affiliations

    Wolfgang Altmannshofer1,*, Stefania Gori1,†, and Dean J. Robinson2,1,‡

    • 1Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, California 95064, USA
    • 2Ernest Orlando Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA

    • *waltmann@ucsc.edu
    • †sgori@ucsc.edu
    • ‡drobinson@lbl.gov

    Article Text

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    Issue

    Vol. 101, Iss. 7 — 1 April 2020

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    Images

    • Figure 1
      Figure 1

      Left: schematic PIENU detector configuration of the target, tracking, and calorimeter elements (gray). Overlaid are typical event topologies for the prompt (blue) and invisible (green) ALP scenarios. Right: schematic cross section of the PIBETA detector configuration including the target, tracking, and calorimeter elements (gray). Overlaid are typical event topologies at the minimum truth-level opening angle configuration of a π0 (red) and lighter prompt ALP (orange) diphoton decay.

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    • Figure 2
      Figure 2

      PIENU fit bin residuals (gray) for the Ecal distribution in the low-energy regime, normalized against the π+→eν (“πe2”) rate. Overlaid are π+→aeν (“πa3”) binned spectra for the prompt (thin solid) and invisible (thick solid) regimes, with ma=40  MeV (orange) and 80 MeV (red). The spectra include acceptance corrections, with total acceptance εa, but are normalized such that Γ[πa3]/Γ[πe2]=5×10−4.

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    • Figure 3
      Figure 3

      Left: 95% CL exclusion regions from the PIENU fit residuals for the prompt (blue) and invisible (green) ALP regimes. Also shown is the PIBETA exclusion from the θγγ spectrum for prompt ALPs (purple). Right: combined PIENU and PIBETA exclusion regions for the models with gaγeff≃sinϑgπγ (“pure mixing scenario”). In this case, the mean characteristic decay length regions ⟨βγ⟩cτ<1  cm (dot-dashed red line and above) and ⟨βγ⟩cτ>1  m (dot-dashed red line and below) approximately delineate where the prompt and invisible regime exclusions apply, respectively. Also shown are exclusions from CHARM/Nu-Cal, E137, E141, and LEP (gray), and projected reaches for SeaQuest (yellow line and below) [25], Belle II (green line and below) [26], PrimEx (red line and above), and GlueX (orange line and above) [27].

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    • Figure 4
      Figure 4

      PIBETA reconstructed diphoton opening angle distribution (black) for π+→(π0→γγ)eν, normalized to unity. Also shown are π+→aeν binned spectra for the prompt regime, with ma=110, 120, 130 MeV and mπ. The spectra are normalized such that Γ[π+→aeν]/Γ[π+→π0eν]=1.

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