Physicists Measure Rare Lambda Particle Decay With New Precision
Researchers at the LHCb experiment have pinned down how often lambda particles decay into protons and muons—a measurement that tests fundamental predictions about weak nuclear forces. The result, the most precise to date, helps validate the standard model of particle physics and could reveal cracks where new physics might hide.
Originaltitel: Branching fraction measurement of the $$ \Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu } $$ decay
A bstract A measurement of the branching fraction for the decay $$ \Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu } $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> <mml:mo>→</mml:mo> <mml:mi>p</mml:mi> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>−</mml:mo> </mml:msup> <mml:msub> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>μ</mml:mi> </mml:msub> </mml:math> is presented using pp collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV. The analysis is based on data recorded between 2016 and 2018, corresponding to an integrated luminosity of 5.4 fb − 1 . The result is obtained using Λ → pμ − decays as a normalisation channel. The measured branching fraction is $$ \mathcal{B}\left(\Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu}\right)=\left(1.462\pm 0.016\pm 0.100\pm 0.011\right)\times {10}^{-4} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>B</mml:mi> <mml:mfenced> <mml:mrow> <mml:mi>Λ</mml:mi> <mml:mo>→</mml:mo> <mml:mi>p</mml:mi> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>−</mml:mo> </mml:msup> <mml:msub> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>μ</mml:mi> </mml:msub> </mml:mrow> </mml:mfenced> <mml:mo>=</mml:mo> <mml:mfenced> <mml:mrow> <mml:mn>1.462</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.016</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.100</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.011</mml:mn> </mml:mrow> </mml:mfenced> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> , where the uncertainties are statistical, systematic, and due to the limited knowledge of the normalisation mode branching fraction, respectively. This result improves the precision of the branching fraction measurement by a factor of two compared to the previous best measurement and sets a more stringent bound on lepton flavour universality in s → u quark transitions. It is consistent with previous measurements, and the extracted lepton flavour universality test observable, $$ {R}^{\mu e}=\frac{\Gamma \left(\Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu}\right)}{\Gamma \left(\Lambda \to p{e}^{-}{\overline{\nu}}_e\right)}=0.175\pm 0.012 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>R</mml:mi> <mml:mi>μe</mml:mi> </mml:msup> <mml:mo>=</mml:mo> <mml:mfrac> <mml:mrow> <mml:mi>Γ</mml:mi> <mml:mfenced> <mml:mrow> <mml:mi>Λ</mml:mi> <mml:mo>→</mml:mo> <mml:mi>p</mml:mi> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>−</mml:mo> </mml:msup> <mml:msub> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>μ</mml:mi> </mml:msub> </mml:mrow> </mml:mfenced> </mml:mrow> <mml:mrow> <mml:mi>Γ</mml:mi> <mml:mfenced> <mml:mrow> <mml:mi>Λ</mml:mi> <mml:mo>→</mml:mo> <mml:mi>p</mml:mi> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>−</mml:mo> </mml:msup> <mml:msub> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>e</mml:mi> </mml:msub> </mml:mrow> </mml:mfenced> </mml:mrow> </mml:mfrac> <mml:mo>=</mml:mo> <mml:mn>0.175</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.012</mml:mn> </mml:math> , agrees with the Standard Model prediction.