LHCb Research Articles

CP asymmetry from the effect of the isospin symmetry breaking during B-meson decay

Abstract The direct CP asymmetry in quasi-two-body decays of $B \rightarrow (V\rightarrow \pi^{+}\pi^{-})P $ is investigated in the perturbative QCD method, where P represents a pseudoscalar meson and V refers to $\rho$, $\omega$ and $\phi$ mesons, respectively. We present the amplitude of the quasi-two-body decay process and investigate the effects of mixed resonances involving $\rho^{0}-\omega$, $\rho^{0}-\phi$ and $\omega-\phi$, while considering the impact of isospin symmetry breaking. We observe a significant CP asymmetry when the invariant mass of the $\pi^{+}\pi^{-}$ pair within the resonance ranges of $\rho$, $\omega$ and $\phi$ mesons. Consequently, we proceed to quantify the regional CP asymmetry in these resonance regions. A significant difference is observed when comparing results obtained with and without the interference of the three vector mesons and isospin conservation. The CP asymmetry results obtained from the three-body decay process, without interference due to isospin conservation by the perturbative QCD method, are in agreement with the newly updated data acquired by the LHCb experiment. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

The B±‐Meson Production in High Energy pp Collisions in the Color Dipole Transverse Momentum Representation

ABSTRACTThe color dipole formalism in the transverse momentum framework together with the unintegrated gluon distributions is considered to obtain the differential cross sections in collisions. The theoretical predictions for the B±‐meson production assume the center‐of‐mass energy of the Large Hadron Collider and the rapidities available at the LHCb experiment. The results are compared with the corresponding measurements in terms of the meson transverse momentum.

Charmonium pair production at the LHCb in the ICEM using the kT–factorization: DPS versus SPS

In this paper, we study pair production of [Formula: see text] and [Formula: see text] charmonium states in the forward rapidity region of the LHCb experiment at the [Formula: see text][Formula: see text]TeV using the improved color evaporation model and the parton Reggeization approach, which is a gauge invariant version of the [Formula: see text]–factorization approach. Taking into account single and double parton scattering mechanisms and using the set of model parameters which have been fixed early, we compare our theoretical predictions with the recent LHCb experimental data as well as with another theoretical predictions. The numerical calculations are performed using the parton–level Monte–Carlo event generator KaTie and modified KMRW model for unintegrated parton distribution functions. We have found absolutely dominant role of the double parton scattering mechanism in the charmonium pair production at the LHCb.

Production and decay of anticharmed pentaquark state with quark content c¯sudd

We use an effective Lagrangian approach to investigate the production of the anticharmed pentaquark state Pc¯s(∗)- with the minimal quark content c¯sudd in the Λb0 decay. In our calculation, the Pc¯s(∗)- is considered as the molecule state of the nDs0(s1)- in an S wave, and its production and decay occur via triangle loops at the hadron level. With the moderate model parameters, the computed results suggest that the branching fractions of Λb0→Pc¯s(∗)-D+ can reach approximately 10-4. The possible decay modes Pc¯s(∗)-→ΛD(∗)- and Pc¯s(∗)-→ΣD(∗)- are also investigated. It is hoped that these predictions could be helpful in searching for the anticharmed-strange pentaquark candidates in future LHCb experiments.

Baryon number transportation over full rapidity space in pp collisions at energies available at the CERN Large Hadron Collider

The estimation of the antibaryon to baryon ratio is considered to be a useful tool for studying baryon number transport in pp, pA, and AA collisions. For this paper, the event generator with various tunes is used to measure the Λ¯/Λ ratio as a function of rapidity (y), transverse momentum (pT ), and multiplicity within both ALICE and LHCb acceptances. The results obtained using various Monte Carlo data for pp collisions at s=0.9 and 7 TeV are compared with the experimentally measured values of Λ¯/Λ ratio of ALICE and LHCb experiments. Out of the various studied tunes of 8.3, the string junction model is found to be the most successful to describe the experimentally observed results. Evidence of a considerable amount of baryon number transportation from the beam fragmentation to the ALICE and LHCb acceptances could be recognized. Published by the American Physical Society 2025

Recent results from LHCb

The LHCb experiment is a fully instrumented detector in the forward region at the Large Hadron Collider. LHCb’s forward acceptance and unique detector capabilities enable a broad heavy-ion physics program. LHCb has studied colliding systems from pp to PbPb, as well as fixed-target collisions with a wide variety of target species. This contribution presents recent results from the LHCb experiment.

Open heavy flavor production at LHCb

The LHCb experiment is a dedicated heavy-flavor experiment at the LHC and is uniquely well-suited to studying heavy-flavor production in heavyion collisions. Open heavy-flavor production studies at LHCb provide strong constraints on nuclear parton densities and probe the hadronization process in the hot, dense, nuclear media produced in heavy-ion collisions. Recent measurements of heavy-flavor production in heavy-ion collisions with the LHCb detector are presented, including studies of charm and beauty baryon production.

Experimental overview on light (anti)(hyper)nuclei production at the LHC

For many years, significant efforts at the LHC have been put towards the measurements of the production of (anti)(hyper)nuclei in high-energy hadronic collisions. The ALICE experiment has contributed with measurements in pp, p–Pb, Xe–Xe, and Pb–Pb collisions, at various center-of-mass energies. Recently, also the LHCb experiment has measured the production of nuclei in both collider and fixed-target modes. The highlights of recent experimental measurements are discussed in the context of the production models used to describe the hadronization mechanism.

Measurement of Λ_{b}^{0}, Λ_{c}^{+}, and Λ Decay Parameters Using Λ_{b}^{0}→Λ_{c}^{+}h^{-} Decays.

A comprehensive study of the angular distributions in the bottom-baryon decays Λ_{b}^{0}→Λ_{c}^{+}h^{-}(h=π,K), followed by Λ_{c}^{+}→Λh^{+} with Λ→pπ^{-} or Λ_{c}^{+}→pK_{S}^{0} decays, is performed using a data sample of proton-proton collisions corresponding to an integrated luminosity of 9 fb^{-1} collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13TeV. The decay parameters and the associated charge-parity (CP) asymmetries are measured, with no significant CP violation observed. For the first time, the Λ_{b}^{0}→Λ_{c}^{+}h^{-} decay parameters are measured. The most precise measurements of the decay parameters α, β, and γ are obtained for Λ_{c}^{+} decays and an independent measurement of the decay parameters for the strange-baryon Λ decay is provided. The results deepen our understanding of weak decay dynamics in baryon decays.

Study of the rare decay J/ψ → μ+μ−μ+μ−

The rare electromagnetic J/ψ → μ+μ−μ+μ− decay is observed with a significance greatly exceeding the discovery threshold, using proton-proton collision data collected by the LHCb experiment during 2016–2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb−1. The rate of this decay is measured relative to that of the J/ψ → μ+μ− mode. Using the QED model for the four-muon decay in the efficiency estimation, its branching fraction is determined to beBJ/ψ→μ+μ−μ+μ−=1.13±0.10±0.05±0.01×10−6,where the uncertainties are statistical, systematic and due to the uncertainty on the branching fraction of the J/ψ → μ+μ− decay.

Measurement of the effective leptonic weak mixing angle

Using pp collision data at s = 13 TeV, recorded by the LHCb experiment between 2016 and 2018 and corresponding to an integrated luminosity of 5.4 fb−1, the forward-backward asymmetry in the pp → Z/γ* → μ+μ− process is measured. The measurement is carried out in ten intervals of the difference between the muon pseudorapidities, within a fiducial region covering dimuon masses between 66 and 116 GeV, muon pseudorapidities between 2.0 and 4.5 and muon transverse momenta above 20 GeV. These forward-backward asymmetries are compared with predictions, at next-to-leading order in the strong and electroweak couplings. The measured effective leptonic weak mixing angle issin2θeffℓ=0.23147±0.00044±0.00005±0.00023,where the first uncertainty is statistical, the second arises from systematic uncertainties associated with the asymmetry measurement, and the third arises from uncertainties in the fit model used to extract sin2θeffℓ from the asymmetry measurement. This result is based on an arithmetic average of results using the CT18, MSHT20, and NNPDF31 parameterisations of the proton internal structure, and is consistent with previous measurements and with predictions from the global electroweak fit.

Graph Neural Network-based track finding in the LHCb vertex detector

The next decade will see an order of magnitude increase in data collected by high-energy physics experiments, driven by the High-Luminosity LHC (HL-LHC). The reconstruction of charged particle trajectories (tracks) has always been a critical part of offline data processing pipelines. The complexity of HL-LHC data will however increasingly mandate track finding in all stages of an experiment's real-time processing. This paper presents a GNN-based track-finding pipeline tailored for the Run 3 LHCb experiment's vertex detector and benchmarks its physics performance and computational cost against existing classical algorithms on GPU architectures. A novelty of our work compared to existing GNN tracking pipelines is batched execution, in which the GPU evaluates the pipeline on hundreds of events in parallel. We evaluate the impact of neural-network quantisation on physics and computational performance, and comment on the outlook for GNN tracking algorithms for other parts of the LHCb track-finding pipeline.

Measurement of the D* longitudinal polarization in B0→D*−τ+ντ decays

The longitudinal polarization fraction of the D* meson is measured in B0→D*−τ+ντ decays, where the τ lepton decays to three charged pions and a neutrino, using proton-proton collision data collected by the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV and corresponding to an integrated luminosity of 5 fb−1. The D* polarization fraction FLD* is measured in two q2 regions, below and above 7 GeV2/c4, where q2 is defined as the squared invariant mass of the τντ system. The FLD* values are measured to be 0.52±0.07±0.04 and 0.34±0.08±0.02 for the lower and higher q2 regions, respectively. The first uncertainties are statistical and the second systematic. The average value over the whole q2 range is FLD*=0.41±0.06±0.03. These results are compatible with the Standard Model predictions. © 2024 CERN, for the LHCb Collaboration 2024 CERN

Search for the Σ(1380)1/2− state in Λc+→γπ+Λ decay by triangle singularity

In this work, we investigate the resonance production in Λc+→γπ+Λ decay through the triangle singularity (TS) mechanism, within an effective Lagrangian approach. We find that the appropriate loop decay process could develop a triangle singularity in the invariant mass MπΛ around 1.35 GeV, with the shape depending on the quantum numbers of Σ* states that couple to the final πΛ system. Especially, the Σ(1380)1/2− state newly predicted in the pentaquark model, if exists, significantly enhances the TS contribution and sharpens the TS peak due to the S-wave Σ*πΣ vertex in the loop. Therefore, the existence of the Σ(1380)1/2− state can be examined by measuring the TS signal in Λc+→γπ+Λ. Considering also possible tree-level diagrams, we additionally obtain the branching ratio Br(Λc+→γπ+Λ)≃1.5×10−5. We suggest the investigation of this reaction by future BESIII, LHCb, and Belle experiments. Published by the American Physical Society 2024

Precision measurement of the Ξb− baryon lifetime

A sample of pp collision data, corresponding to an integrated luminosity of 5.5 fb−1 and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the Ξb− baryon to that of the Λb0 baryon, rτ≡τΞb−/τΛb0. The value rτ=1.076±0.013±0.006 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 rτRun 1,2=1.078±0.012±0.007. Multiplying by the world-average value of the Λb0 lifetime yields τΞb−Run 1,2=1.578±0.018±0.010±0.011 ps, where the uncertainties are statistical, systematic, and due to the limited knowledge of the Λb0 lifetime. This measurement improves the precision of the current world average of the Ξb− 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

Characterization of neutron-irradiated SiPMs down to liquid nitrogen temperature

Photodetectors used in future high-energy physics experiments will need to keep sufficient performance during a few years of data-taking despite radiation load, which, for example, in the planned upgrade of the Ring Imaging Cherenkov detectors in Large Hadron Collider beauty (LHCb) experiment is estimated at about 1013\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{13}$$\\end{document} 1-MeV neutron equivalent per cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document} (neq/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document}). Silicon photomultipliers (SiPMs) are considered as candidates for photodetectors for this application. However, their sensitivity to neutron irradiation may seriously compromise their operation, with the increase in dark count rates being the primary limitation after the irradiation. In this work, 1 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 1 mm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document} 15 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu $$\\end{document}m pitch NUV-HD-RH silicon photomultipliers developed by the Fondazione Bruno Kessler were characterized before and after the irradiation. In total, 5 SiPMs were irradiated at the Jožef Stefan Institute TRIGA nuclear reactor with different fluences from 109\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{9}$$\\end{document} neq/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document} up to 1013\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{13}$$\\end{document} neq/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document}. The SiPMs were also annealed at high temperatures and re-characterized after the annealing. For the SiPM characterization in all the cases, current–voltage (I-V curve) measurements, dark count rate measurements, and waveform analysis, including single photon time resolution, were carried out at different controlled temperature steps from room temperature down to the liquid nitrogen temperature. While cooling to − 20∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}C was enough for the SiPM irradiated at 109\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{9}$$\\end{document} neq/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document} to recover the ability to resolve single photons, by 1013\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{13}$$\\end{document} neq/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document}, cooling to liquid nitrogen temperature was necessary. With sufficient cooling, the measured single photon time resolution was around 90 ps FWHM for all irradiation levels.

Observation of New Charmonium or Charmoniumlike States in B^{+}→D^{*±}D^{∓}K^{+} Decays.

A study of resonant structures in B^{+}→D^{*+}D^{-}K^{+} and B^{+}→D^{*-}D^{+}K^{+} decays is performed, using proton-proton collision data at center-of-mass energies of sqrt[s]=7, 8, and 13TeV recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 fb^{-1}. A simultaneous amplitude fit is performed to the two channels with contributions from resonances decaying to D^{*-}D^{+} and D^{*+}D^{-} states linked by C parity. This procedure allows the C parities of resonances in the D^{*±}D^{∓} mass spectra to be determined. Four charmonium or charmoniumlike states are observed decaying into D^{*±}D^{∓}: η_{c}(3945), h_{c}(4000), χ_{c1}(4010), and h_{c}(4300), with quantum numbers J^{PC} equal to 0^{-+}, 1^{+-}, 1^{++}, and 1^{+-}, respectively. At least three of these states have not been observed previously. In addition, the existence of the T_{c[over ¯]s[over ¯]0}^{*}(2870)^{0} and T_{c[over ¯]s[over ¯]1}^{*}(2900)^{0} resonances in the D^{-}K^{+} mass spectrum, already observed in the B^{+}→D^{+}D^{-}K^{+} decay, is confirmed in a different production channel.

Search for the rare decay of charmed baryon Λc+ into the pμ+μ− final state

A search for the nonresonant Λc+→pμ+μ− decay is performed using proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the LHCb experiment, corresponding to an integrated luminosity of 5.4 fb−1. No evidence for the decay is found in the dimuon invariant-mass regions where the expected contributions of resonances is subdominant. The upper limit on the branching fraction of the Λc+→pμ+μ− decay is determined to be 2.9(3.2)×10−8 at 90%(95%) confidence level. The branching fractions in the dimuon invariant-mass regions dominated by the η, ρ and ω resonances are also determined. © 2024 CERN, for the LHCb Collaboration 2024 CERN

Dark matter-electron scattering and freeze-in scenarios in the light of Z′ mediation

We investigate dark matter (DM-)electron scattering in a minimal U(1)X extension of the Standard Model (SM), where the DM can appear as a Majorana fermion, a complex singlet scalar, or a Dirac fermion. To study bounds on the U(1)X gauge coupling (gX) and new gauge boson mass (MZ′), from DM-electron scattering, we consider several direct search experiments like CDMS, DAMIC, SENSEI, PandaX-II, DarkSide-50, and XENON1T-S2 for different U(1)X charges. In this setup, we consider DM production via freeze-in in both radiation-dominated and modified cosmological background to project sensitivities onto gX−MZ′ plane satisfying observed relic abundance. DM-electron scattering could provide comparable, or even stronger, bounds compared to those obtained from the electron/muon (g−2), low-energy scattering, and intensity frontier experiments within 0.01 GeV≲MZ′≲0.1 GeV. Constrains from freeze-in could provide stronger sensitivities for MZ′≳O(1) GeV; however, these limits are comparable to those obtained from LHCb and LEP experiments for O(10) GeV≲MZ′≲150 GeV. In the future, electron-muon scattering (MUonE), proton (FASER and DUNE), and electron/positron (ILC) beam-dump experiments could probe these parameters. Published by the American Physical Society 2024

Purely leptonic decays of heavy-flavored charged mesons

We study the purely leptonic decays of heavy-flavored charged pseudoscalar (P) and vector (V) mesons (D(s)(*)+, B(c)(*)+) in the relativistic independent quark (RIQ) model based on an average flavor-independent confining potential in an equally mixed scalar-vector harmonic form. We first compute the mass spectra of the ground-state-mesons and fix the model parameters necessary for the present analysis. Using the meson wave functions derivable in the RIQ model, and model parameters so fixed from hadron spectroscopy, we predict the decay constants: fP(V), ratios of decay constants: fV/fP, fP1/fP2, fV1/fV2, and the branching fractions (BFs): B(P(V)→l+νl), l=e, μ, τ, which agree with the available experimental data and other Standard Model (SM) predictions. For the unmeasured decay constants, especially in the purely leptonic decays of the charged vector mesons, our predictions could be tested in the upcoming Belle-II, SCTF, CEPC, FCC-ee, and LHCb experiments in the near future. Published by the American Physical Society 2024