New physics in $$b\rightarrow s\ell \ell $$ transitions at one loop
Rupert Coy, Michele Frigerio, Federico Mescia, Olcyr Sumensari
Abstract
Abstract We investigate new-physics contributions to $$b\rightarrow s \ell \ell $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>b</mml:mi><mml:mo>→</mml:mo><mml:mi>s</mml:mi><mml:mi>ℓ</mml:mi><mml:mi>ℓ</mml:mi></mml:mrow></mml:math> transitions in the context of an effective field theory extension of the Standard Model, including operator mixing at one loop. We identify the few scenarios where a single Wilson coefficient, $$C/\Lambda ^2 \sim 1/\mathrm{TeV}^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>C</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mi>Λ</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>∼</mml:mo><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:msup><mml:mrow><mml:mi>TeV</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msup></mml:mrow></mml:math> , induces a substantial shift in the lepton flavour universality ratios $$R_K$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>R</mml:mi><mml:mi>K</mml:mi></mml:msub></mml:math> and $$R_{K^*}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>R</mml:mi><mml:msup><mml:mi>K</mml:mi><mml:mo>∗</mml:mo></mml:msup></mml:msub></mml:math> at one loop, while evading Z -pole precision tests, collider bounds, and other flavour constraints. Novel fits to the present data are achieved by a left-handed current operator with quark-flavour indices (2, 2) or (3, 3). Interestingly, the running of the Standard Model Yukawa matrices gives the dominant effect for these scenarios. We match the favoured effective-theory scenarios to minimal, single-mediator models, which are subject to additional stringent constraints. Notably, we recognise three viable instances of a leptoquark with one coupling to fermions only. If the anomalies were confirmed, it appears that one-loop explanations have good prospects of being directly tested at the LHC.