Probing Neutrinophilic Axion-Like Particles in Tritium Beta Decay

Author(s)

Chung, Yi, Goertz, Florian, Hager, Maya, Lauer, Joscha

Abstract

We investigate the prospects for constraining neutrinophilic axion-like particles via measurements of the tritium beta decay spectrum, as performed by the KATRIN experiment and its planned TRISTAN detector upgrade. We study in detail the resulting spectral modifications and the corresponding experimental sensitivity. The relevant complementary searches are also discussed for comparison and we derive the most up-to-date and robust constraints on keV-scale neutrinophilic axion-like particles, covering both lepton-number-conserving and lepton-number-violating interactions. Cosmological constraints are generally more stringent; however, this conclusion relies on the assumption that the particles remain unchanged from the early universe to the present day. We therefore construct a model in which the neutrinophilic axion-like particle, being a pseudo-Nambu-Goldstone boson of an extended scalar sector, emerges from a spontaneous symmetry breaking featuring a non-trivial thermal history. We show that the model naturally evades the conventional cosmological bounds, allowing tritium beta decay measurements to provide the leading constraints.

Figures

Tritium beta spectrum with emission of a neutrinophilic ALP $a$. The normalized beta spectrum $\Gamma^{-1}\,\mathrm{d} \Gamma/\mathrm{d} E$ (Eq.~\eqref{e:total_spectrum}) as a function of the electron kinetic energy $E$ is shown for different keV ALP masses $m_a$, indicated by the color (upper panel). A large coupling $g_\nu = 2 \pi$ is assumed to improve visibility. The dashed red line corresponds to the SM beta decay (Eq.~\eqref{e:beta_spectrum}). In the lower panel, the relative difference between the ALP spectrum and the SM prediction is plotted.
Caption Tritium beta spectrum with emission of a neutrinophilic ALP $a$. The normalized beta spectrum $\Gamma^{-1}\,\mathrm{d} \Gamma/\mathrm{d} E$ (Eq.~\eqref{e:total_spectrum}) as a function of the electron kinetic energy $E$ is shown for different keV ALP masses $m_a$, indicated by the color (upper panel). A large coupling $g_\nu = 2 \pi$ is assumed to improve visibility. The dashed red line corresponds to the SM beta decay (Eq.~\eqref{e:beta_spectrum}). In the lower panel, the relative difference between the ALP spectrum and the SM prediction is plotted.
Sensitivity of tritium beta decay and other probes to neutrinophilic ALPs. The boundary of the accessible coupling $g_\nu$ is shown as a function of the ALP mass $m_a$. For tritium beta decay, the resulting purely statistical sensitivity (90\%\,C.L.) of the TRISTAN benchmark setup (this work) is displayed in black. Other bounds from complementary searches (Sec.~\ref{sec:Other}) are shown in blue, green, and red for cosmological, astrophysical, and laboratory bounds, respectively. Dashed lines indicate the constraints from processes with lepton number violation (LNV).
Caption Sensitivity of tritium beta decay and other probes to neutrinophilic ALPs. The boundary of the accessible coupling $g_\nu$ is shown as a function of the ALP mass $m_a$. For tritium beta decay, the resulting purely statistical sensitivity (90\%\,C.L.) of the TRISTAN benchmark setup (this work) is displayed in black. Other bounds from complementary searches (Sec.~\ref{sec:Other}) are shown in blue, green, and red for cosmological, astrophysical, and laboratory bounds, respectively. Dashed lines indicate the constraints from processes with lepton number violation (LNV).
Relevant constraints and benchmark in the UV model. The blue shaded regions are excluded by neutrino self-interaction constraints, while the region in between represents the allowed parameter space considering the $\mathrm{SI}\nu$ mode (per Eq.\,\eqref{eq:SInu}). The red line shows the meson decay bound (Eq.\,\eqref{eq:mesonBound}) and the black line is the projected TRISTAN sensitivity benchmark described in Sec.\,\ref{sec:KATRIN}. As an outlook, we show $3\times$ the sensitivity as the dotted-dashed line, the minimal requirement to be sensitive to the model, as well as the dotted line which depicts the originally predicted sensitivity in \cite{Arcadi:2018xdd} ($\approx 7 \times$ updated sensitivity). The green cross is our benchmark point with a mass of $m_a = 3.5$ keV and a coupling of $g_\nu = 4 \times 10^{-3}$.
Caption Relevant constraints and benchmark in the UV model. The blue shaded regions are excluded by neutrino self-interaction constraints, while the region in between represents the allowed parameter space considering the $\mathrm{SI}\nu$ mode (per Eq.\,\eqref{eq:SInu}). The red line shows the meson decay bound (Eq.\,\eqref{eq:mesonBound}) and the black line is the projected TRISTAN sensitivity benchmark described in Sec.\,\ref{sec:KATRIN}. As an outlook, we show $3\times$ the sensitivity as the dotted-dashed line, the minimal requirement to be sensitive to the model, as well as the dotted line which depicts the originally predicted sensitivity in \cite{Arcadi:2018xdd} ($\approx 7 \times$ updated sensitivity). The green cross is our benchmark point with a mass of $m_a = 3.5$ keV and a coupling of $g_\nu = 4 \times 10^{-3}$.
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