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

A bstract 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 $$ {\overline{\lambda}}_d{v}_H/{m}_b $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mover> <mml:mi>λ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>d</mml:mi> </mml:msub> <mml:msub> <mml:mi>v</mml:mi> <mml:mi>H</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>b</mml:mi> </mml:msub> </mml:math> , which can be as large as $$ \mathcal{O}(40) $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>O</mml:mi> <mml:mfenced> <mml:mn>40</mml:mn> </mml:mfenced> </mml:math> for moderate Yukawa couplings. This leads to sizable deviation from the SM prediction even for $$ \mathcal{O}\left(\mathrm{TeV}\right) $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>O</mml:mi> <mml:mfenced> <mml:mi>TeV</mml:mi> </mml:mfenced> </mml:math> vector-like quark masses and small mixing angles. We find that $$ \mathrm{Br}\left(\overline{B}\to {X}_s\gamma \right) $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Br</mml:mi> <mml:mfenced> <mml:mrow> <mml:mover> <mml:mi>B</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mo>→</mml:mo> <mml:msub> <mml:mi>X</mml:mi> <mml:mi>s</mml:mi> </mml:msub> <mml:mi>γ</mml:mi> </mml:mrow> </mml:mfenced> </mml:math> provides the most stringent constraint on this scenario among a wide range of precision observables.