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| Format: | Preprint |
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2026
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| Accès en ligne: | https://arxiv.org/abs/2605.09140 |
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| _version_ | 1866917477516050432 |
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| author | Roig, Pablo |
| author_facet | Roig, Pablo |
| contents | We ask whether a measured Michel distribution, apparently in excellent agreement with the Standard Model interpretation of the $\ell_i \to \ell_j ν\barν$ decay, could instead arise from a different invisible sector. Within a general low-energy effective field theory, we analyze lepton decays $\ell_i \to \ell_j + X\bar X$ for electrically neutral, color-singlet, mutually conjugate invisible pairs $X\bar X$ of spin up to $2$, allowing (pseudo)scalar, (axial)vector, and antisymmetric tensor interactions in the lepton current, focusing on the massless limit relevant for exact degeneracies. We formulate a criterion for indistinguishability based on the full set of measurable differential distributions. Under these assumptions, besides the obvious spin $1/2$ case, there is a unique nontrivial solution: a massless complex scalar pair coupled through a purely left-handed vector current exactly reproduces the standard Michel pattern, including its extensions to daughter-lepton polarization and radiative channels. All other cases studied here are distinguishable, in particular because higher-spin invisible pairs produce additional kinematic prefactors. These results isolate the only nonstandard and nontrivial invisible sector that can remain hidden in Michel-type lepton decay measurements. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2605_09140 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Can a Nonstandard Invisible Pair Mimic the Michel Distribution? Roig, Pablo High Energy Physics - Phenomenology High Energy Physics - Experiment We ask whether a measured Michel distribution, apparently in excellent agreement with the Standard Model interpretation of the $\ell_i \to \ell_j ν\barν$ decay, could instead arise from a different invisible sector. Within a general low-energy effective field theory, we analyze lepton decays $\ell_i \to \ell_j + X\bar X$ for electrically neutral, color-singlet, mutually conjugate invisible pairs $X\bar X$ of spin up to $2$, allowing (pseudo)scalar, (axial)vector, and antisymmetric tensor interactions in the lepton current, focusing on the massless limit relevant for exact degeneracies. We formulate a criterion for indistinguishability based on the full set of measurable differential distributions. Under these assumptions, besides the obvious spin $1/2$ case, there is a unique nontrivial solution: a massless complex scalar pair coupled through a purely left-handed vector current exactly reproduces the standard Michel pattern, including its extensions to daughter-lepton polarization and radiative channels. All other cases studied here are distinguishable, in particular because higher-spin invisible pairs produce additional kinematic prefactors. These results isolate the only nonstandard and nontrivial invisible sector that can remain hidden in Michel-type lepton decay measurements. |
| title | Can a Nonstandard Invisible Pair Mimic the Michel Distribution? |
| topic | High Energy Physics - Phenomenology High Energy Physics - Experiment |
| url | https://arxiv.org/abs/2605.09140 |