LHCb

Abstract:

The LHCb Ring-Imaging Cherenkov detectors are built to provide charged hadron identification over a large range of momentum. The upgraded detectors are also capable of providing an independent measurement of the luminosity for the LHCb experiment during LHC Run 3. The modelling of the opto-electronics chain, the application of the powering strategy during operations, the calibration procedures and the proof of principle of a novel technique for luminosity determination are presented. In addition, the preliminary precision achieved during the 2023 data-taking year for real-time and offline luminosity measurements is reported.

Abstract:

The first observation of the Ξb0→J/ψΞ-π+$${{{{\varXi } ^0_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {{\varXi } ^-} {{\pi } ^+} }}$$ decay and the most precise measurement of the branching fraction of the Λb0→J/ψΞ-K+$${{{\varLambda } ^0_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {{\varXi } ^-} {{K} ^+} }$$ decay are reported, using proton-proton collision data from the LHCb experiment collected in 2016–2018 at a centre-of-mass energy of 13\,TeV$$\text {\,Te\hspace{-1.00006pt}V}$$, corresponding to an integrated luminosity of 5.4\,fb-1$$\text {\,fb} ^{-1}$$. Using the Λb0→J/ψΛ$${{{{\varLambda } ^0_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {\varLambda } }}$$ and Ξb-→J/ψΞ-$${{{{\varXi } ^-_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {{\varXi } ^-} }}$$ decays as normalisation channels, the ratios of branching fractions are measured to be B(Λb0→J/ψΞ-K+)B(Λb0→J/ψΛ)=(1.17±0.14±0.08)×10-2,B(Ξb0→J/ψΞ-π+)B(Ξb-→J/ψΞ-)=(11.9±1.4±0.6)×10-2,$$\begin{aligned} {\frac{{{\mathcal {B}}}({{{\varLambda } ^0_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {{\varXi } ^-} {{K} ^+} })}{{{\mathcal {B}}}({{{{\varLambda } ^0_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {\varLambda } }})}}&= (1.17 \pm 0.14 \pm 0.08)\times 10^{-2} \, ,\\ {\frac{{{\mathcal {B}}}({{{{\varXi } ^0_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {{\varXi } ^-} {{\pi } ^+} }})}{{{\mathcal {B}}}({{{{\varXi } ^-_{b}} \!\rightarrow {{J \hspace{-1.66656pt}/\hspace{-1.111pt}\psi }} {{\varXi } ^-} }})}}&= (11.9 \pm 1.4 \pm 0.6)\times 10^{-2}\, , \end{aligned}$$where the first uncertainty is statistical and the second systematic.

Abstract:

The Standard Model of particle physics—the theory of particles and interactions at the smallest scale—predicts that matter and antimatter interact differently due to violation of the combined symmetry of charge conjugation (C) and parity (P). Charge conjugation transforms particles into their antimatter particles, whereas the parity transformation inverts spatial coordinates. This prediction applies to both mesons, which consist of a quark and an antiquark, and baryons, which are composed of three quarks. However, despite having been discovered in various meson decays, CP violation has yet to be observed in baryons, the type of matter that makes up the observable Universe. Here we report a study of the decay of the beauty baryon Λ0b$${\varLambda }_{0}^{b}$$ to the pK−π+π− final state, which proceeds through b → u or b → s quark-level transitions, and its CP-conjugated process, using data collected by the Large Hadron Collider beauty experiment1 at the European Organization for Nuclear Research (CERN). The results reveal significant asymmetries between the decay rates of the Λ0b$${\varLambda }_{0}^{b}$$ baryon and its CP-conjugated antibaryon, providing, to our knowledge, the first observation of CP violation in baryon decays and demonstrating the different behaviours of baryons and antibaryons. In the Standard Model, CP violation arises from the Cabibbo–Kobayashi–Maskawa mechanism2, and new forces or particles beyond the Standard Model could provide further contributions. This discovery opens a new path in the search for physics beyond the Standard Model.