A sample of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>p</a:mi><a:mi>p</a:mi></a:math> collision data, corresponding to an integrated luminosity of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>5.5</c:mn><c:mtext> </c:mtext><c:mtext> </c:mtext><c:msup><c:mi>fb</c:mi><c:mrow><c:mo>−</c:mo><c:mn>1</c:mn></c:mrow></c:msup></c:math> and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msubsup><e:mi mathvariant="normal">Ξ</e:mi><e:mi>b</e:mi><e:mo>−</e:mo></e:msubsup></e:math> baryon to that of the <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:msubsup><h:mi mathvariant="normal">Λ</h:mi><h:mi>b</h:mi><h:mn>0</h:mn></h:msubsup></h:math> baryon, <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msub><k:mi>r</k:mi><k:mi>τ</k:mi></k:msub><k:mo>≡</k:mo><k:msub><k:mi>τ</k:mi><k:msubsup><k:mi mathvariant="normal">Ξ</k:mi><k:mi>b</k:mi><k:mo>−</k:mo></k:msubsup></k:msub><k:mo>/</k:mo><k:msub><k:mi>τ</k:mi><k:msubsup><k:mi mathvariant="normal">Λ</k:mi><k:mi>b</k:mi><k:mn>0</k:mn></k:msubsup></k:msub></k:math>. The value <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msub><o:mi>r</o:mi><o:mi>τ</o:mi></o:msub><o:mo>=</o:mo><o:mn>1.076</o:mn><o:mo>±</o:mo><o:mn>0.013</o:mn><o:mo>±</o:mo><o:mn>0.006</o:mn></o:math> is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msubsup><q:mi>r</q:mi><q:mi>τ</q:mi><q:mrow><q:mi>Run</q:mi><q:mtext> </q:mtext><q:mn>1</q:mn><q:mo>,</q:mo><q:mn>2</q:mn></q:mrow></q:msubsup><q:mo>=</q:mo><q:mn>1.078</q:mn><q:mo>±</q:mo><q:mn>0.012</q:mn><q:mo>±</q:mo><q:mn>0.007</q:mn></q:math>. Multiplying by the world-average value of the <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:msubsup><s:mi mathvariant="normal">Λ</s:mi><s:mi>b</s:mi><s:mn>0</s:mn></s:msubsup></s:math> lifetime yields <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mrow><v:msubsup><v:mrow><v:mi>τ</v:mi></v:mrow><v:mrow><v:msubsup><v:mrow><v:mi mathvariant="normal">Ξ</v:mi></v:mrow><v:mrow><v:mi>b</v:mi></v:mrow><v:mrow><v:mo>−</v:mo></v:mrow></v:msubsup></v:mrow><v:mrow><v:mi>Run</v:mi><v:mtext> </v:mtext><v:mn>1</v:mn><v:mo>,</v:mo><v:mn>2</v:mn></v:mrow></v:msubsup><v:mo>=</v:mo><v:mn>1.578</v:mn><v:mo>±</v:mo><v:mn>0.018</v:mn><v:mo>±</v:mo><v:mn>0.010</v:mn><v:mo>±</v:mo><v:mn>0.011</v:mn><v:mtext> </v:mtext><v:mtext> </v:mtext><v:mi>ps</v:mi></v:mrow></v:math>, where the uncertainties are statistical, systematic, and due to the limited knowledge of the <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:msubsup><y:mi mathvariant="normal">Λ</y:mi><y:mi>b</y:mi><y:mn>0</y:mn></y:msubsup></y:math> lifetime. This measurement improves the precision of the current world average of the <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:msubsup><bb:mi mathvariant="normal">Ξ</bb:mi><bb:mi>b</bb:mi><bb:mo>−</bb:mo></bb:msubsup></bb:math> lifetime by about a factor of 2, and is in good agreement with the most recent theoretical predictions. © 2024 CERN, for the LHCb Collaboration 2024 CERN