We demonstrate that a vector-like fourth family of quarks induces a genuine chiral enhancement in b → sγ, which is absent in the Standard Model (SM). The coexistence of doublet and singlet states allows the chirality flip to occur inside the loop, leading to contributions proportional to the heavy vector-like mass. The resulting amplitude is enhanced by a factor <InlineEquation ID="IEq1"> <EquationSource Format="MATHML"><math display="inline"> <msub> <mover accent="true"> <mi>λ</mi> <mo stretchy="true">¯</mo> </mover> <mi>d</mi> </msub> <msub> <mi>v</mi> <mi>H</mi> </msub> <mo>/</mo> <msub> <mi>m</mi> <mi>b</mi> </msub> </math></EquationSource> <EquationSource Format="TEX">$ {\overline{\lambda}}_d{v}_H/{m}_b $</EquationSource> </InlineEquation>, which can be as large as <InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math display="inline"> <mi mathvariant="script">O</mi> <mfenced close=")" open="("> <mn>40</mn> </mfenced> </math></EquationSource> <EquationSource Format="TEX">$ \mathcal{O}(40) $</EquationSource> </InlineEquation> for moderate Yukawa couplings. This leads to sizable deviation from the SM prediction even for <InlineEquation ID="IEq3"> <EquationSource Format="MATHML"><math display="inline"> <mi mathvariant="script">O</mi> <mfenced close=")" open="("> <mi>TeV</mi> </mfenced> </math></EquationSource> <EquationSource Format="TEX">$ \mathcal{O}\left(\mathrm{TeV}\right) $</EquationSource> </InlineEquation> vector-like quark masses and small mixing angles. We find that <InlineEquation ID="IEq4"> <EquationSource Format="MATHML"><math display="inline"> <mi>Br</mi> <mfenced close=")" open="("> <mrow> <mover accent="true"> <mi>B</mi> <mo stretchy="true">¯</mo> </mover> <mo>→</mo> <msub> <mi>X</mi> <mi>s</mi> </msub> <mi>γ</mi> </mrow> </mfenced> </math></EquationSource> <EquationSource Format="TEX">$ \mathrm{Br}\left(\overline{B}\to {X}_s\gamma \right) $</EquationSource> </InlineEquation> provides the most stringent constraint on this scenario among a wide range of precision observables.

Chiral enhancement in the vector-like fourth family: case of b → sγ

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2026年7月1日

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Junichiro Kawamura,
Yuji Omura

摘要

We demonstrate that a vector-like fourth family of quarks induces a genuine chiral enhancement in b → sγ, which is absent in the Standard Model (SM). The coexistence of doublet and singlet states allows the chirality flip to occur inside the loop, leading to contributions proportional to the heavy vector-like mass. The resulting amplitude is enhanced by a factor ${\bar{λ}}_{d} v_{H} / m_{b}$ $ {\overline{\lambda}}_d{v}_H/{m}_b $ , which can be as large as $O (40)$ $ \mathcal{O}(40) $ for moderate Yukawa couplings. This leads to sizable deviation from the SM prediction even for $O (TeV)$ $ \mathcal{O}\left(\mathrm{TeV}\right) $ vector-like quark masses and small mixing angles. We find that $Br (\bar{B} \to X_{s} γ)$ $ \mathrm{Br}\left(\overline{B}\to {X}_s\gamma \right) $ provides the most stringent constraint on this scenario among a wide range of precision observables.