Shunan Zhang

Abstract:

It is shown that the timing capabilities of the LHCb detector operated during the LHC Run 2 can be used to identify light ion particles with momenta of a few GeV/c. This is achieved by estimating the particle time of flight through a newly developed technique. A dedicated reconstruction procedure and a neural-network-based estimator of the particle speed have been developed to enable deuteron identification by suppressing the abundant background from lighter particles. The performance of the identification procedure is demonstrated in a sample of proton-helium collisions at sNN$$\sqrt{s_{\scriptscriptstyle \text {NN}}} $$ = 110 GeV, where the production of deuteron and triton particles is observed. This novel approach opens the way to study deuteron and antideuteron production for different collision systems at different energy scales, exploiting the rich dataset collected by the LHCb experiment.

Abstract:

The LHCb detector underwent a comprehensive upgrade in preparation for the third data-taking run of the Large Hadron Collider (LHC), known as LHCb Upgrade I. With its increased data rate, Run 3 introduced considerable challenges in both data acquisition (online) and data processing and analysis (offline). The offline processing and analysis model was upgraded to handle the factor 30 increase in data volume and the associated demands of ever-growing datasets for analysis, led by the LHCb Data Processing and Analysis (DPA) project. This paper documents the LHCb “Sprucing" — the centralised offline data processing and selections — and “Analysis Productions" — the centralised and highly automated declarative nTuple production system. The DaVinci application used by analysis productions for tupling spruced data is described as well as the apd and lbconda tools for data retrieval and analysis environment configuration. These tools allow for greatly improved analyst workflows and analysis preservation. Finally, the approach to data processing and analysis in the High-Luminosity Large Hadron Collider (HL-LHC) era — LHCb Upgrade II — is discussed.

Abstract:

The first observation of the Ξb0 → J/ψΞ− π+ decay and the most precise measurement of the branching fraction of the Λb0 → J/ψΞ− 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, corresponding to an integrated luminosity of 5.4 fb−1. Using the Λb0 → J/ψΛ and Ξb0 → J/ψΞ− 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(Ξb0 → J/ψΞ−) = (11.9 ± 1.4 ± 0.6) × 10−2, 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.

Abstract:

This paper presents the first measurement of ψ(2S) and χc1(3872) meson production within fully reconstructed jets. Each quarkonium state (tag) is reconstructed via its decay to the J/ψ(→μ+μ−)π+π− final state in the forward region using proton-proton collision data collected by the LHCb experiment at the center-of-mass-energy of 13TeV in 2016, corresponding to an integrated luminosity of 1.64 fb−1. The fragmentation function, presented as the ratio of the quarkonium-tag transverse momentum to the full jet transverse momentum (pT(tag)/pT(jet)), is measured differentially in pT(jet) and pT(tag) bins. The distributions are separated into promptly produced quarkonia from protonproton collisions and quarkonia produced from displaced b-hadron decays. While the displaced quarkonia fragmentation functions are in general well described by parton-shower predictions, the prompt quarkonium distributions differ significantly from fixed-order non-relativistic QCD (NRQCD) predictions followed by a QCD parton showe