Light axial vector bosons, nuclear transitions, and the $^{8}\mathrm{Be}$ anomaly

Light axial vector bosons, nuclear transitions, and the ${}^{8}{Be}$ anomaly

Jonathan Kozaczuk, David E. Morrissey, and S. R. Stroberg

Phys. Rev. D 95, 115024 – Published 22 June 2017

Abstract

New hidden particles could potentially be emitted and discovered in rare nuclear transitions. In this work, we investigate the production of hidden vector bosons with primarily axial couplings to light quarks in nuclear transitions, and we apply our results to the recent anomaly seen in ${}^{8}{Be}$ decays. The relevant matrix elements for ${}^{8}{Be}^{*} (1^{+}) \to {}^{8}{Be} (0^{+})$ transitions are calculated using ab initio methods with internucleon forces derived from chiral effective field theory and the in-medium similarity renormalization group. We find that the emission of a light axial vector with mass $m_{X} ≃ 17 MeV$ can account for the anomaly seen in the $1^{+} \to 0^{+}$ isoscalar transition together with the absence of a significant anomaly in the corresponding isovector transition. We also show that such an axial vector can be derived from an anomaly-free ultraviolet-complete theory that is consistent with current experimental data.

Received 18 January 2017

DOI:https://doi.org/10.1103/PhysRevD.95.115024

Physics Subject Headings (PhySH)

Extensions of gauge sector Nuclear tests of fundamental interactions Particle interactions

Particles & FieldsNuclear Physics

Authors & Affiliations

Jonathan Kozaczuk^1,2,*, David E. Morrissey^2,†, and S. R. Stroberg^2,3,‡

¹Amherst Center for Fundamental Interactions, Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, USA
²TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada
³Reed College, 3203 SE Woodstock Blvd, Portland, Oregon 97202, USA

^*kozaczuk@umass.edu
^†dmorri@triumf.ca
^‡sstroberg@triumf.ca

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Vol. 95, Iss. 11 — 1 June 2017

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Figure 1
Experimental spectrum of ${}^{8}{Be}$ labeled with total angular momentum and parity, compared with calculated spectra using the following interactions and model space parameters (see text for details). The shaded gray bands on the experimental spectrum indicate the width of the state. The $1^{+}$ states of interest are highlighted in red. Binding energies in MeV are also given beneath the ground states.
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Figure 2
Reduced transition matrix elements for the $M 1$ , $σ_{p}$ , and $σ_{n}$ operators between the $| S ⟩$ ( ${}^{8}{Be}^{*}$ , left column) and the $| V ⟩$ ( ${}^{8}{Be}^{*'}$ , right column) $1^{+}$ excited states and the ground state of ${}^{8}{Be}$ as a function of the isospin mixing fraction $| α |^{2}$ . Approximate corrections for meson exchange currents have been included. Circles indicate results using the SRG 1.88 interaction, triangles indicate the SRG 2.0 interaction, and squares indicate the EM $1.8 / 2.0$ interaction. The single-particle basis truncations are indicated by different colors: $e_{\max} = 4$ (cyan), 6 (green), 8 (blue), 10 (magenta), 12 (red). We include points for oscillator frequencies $ℏ ω = 12$ , 16, 20, 24, and 28 MeV. The $M 1$ matrix element for the $T ≃ 0$ , $J^{P} = 1^{+}$ state in the upper left is used to constrain the range of the isospin mixing fraction, which is then used to make predictions for the other matrix elements, indicated by the hashed boxes.
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Figure 3
Quark couplings required to explain the MTA-Atomki ${}^{8}{Be}$ anomaly via a light axial vector assuming $g_{u} < 0$ , $g_{d} > 0$ , and electron couplings such that $BR (X \to e^{+} e^{-}) = 1$ is prompt. The hatched band was obtained by considering $m_{X} = 16.7$ , 17.3, 17.6 MeV, imposing the corresponding requirements in Eqs. (38)–(39) to explain the MTA-Atomki result, then varying the relevant nuclear matrix elements and nucleon coefficients $Δ u^{(p), (n)}$ , $Δ d^{(p), (n)}$ , $Δ s^{(p), (n)}$ across their allowed ranges. Points below the hatched region feature couplings too small to explain the observed ${}^{8}{Be}^{*}$ transition rate for the three masses considered. The orange region to the upper right is excluded by the nonobservation of an excess in the isovector ${}^{8}{Be}^{*'} \to {}^{8}{Be} + e^{+} e^{-}$ channel for $m_{X} = 16.7 - 17.6 MeV$ .
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Figure 4
Quark-level couplings required to explain the Atomki ${}^{8}{Be}$ anomaly along with the most important constraints in the UV complete scenario described in Sec. 6. For this specific model, $g_{u}$ and $g_{d}$ lie along the dashed black line. The experimentally allowed region is indicated as such, and includes values of the couplings consistent with an axial vector interpretation of the ${}^{8}{Be}$ anomaly, depicted by the hatched region.
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Physical Review D

covering particles, fields, gravitation, and cosmology

Light axial vector bosons, nuclear transitions, and the ${}^{8}{Be}$ anomaly

Jonathan Kozaczuk, David E. Morrissey, and S. R. Stroberg

Phys. Rev. D 95, 115024 – Published 22 June 2017

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Physical Review D

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Light axial vector bosons, nuclear transitions, and the Be8 anomaly

Jonathan Kozaczuk, David E. Morrissey, and S. R. Stroberg

Phys. Rev. D 95, 115024 – Published 22 June 2017

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Light axial vector bosons, nuclear transitions, and the ${}^{8}{Be}$ anomaly