Punzi, G., Baldini, W., Bassi, G., Contu, A., Fantechi, R., He, J. B., et al. (2024). Detector-embedded reconstruction of complex primitives using FPGAs. Nucl. Instrum. Methods Phys. Res. A, 1069, 169782–4pp.
Abstract: The slowdown of Moore's law and the growing requirements of future HEP experiments with ever-increasing data rates pose important computational challenges for data reconstruction and trigger systems, encouraging the exploration of new computing methodologies. In this work we discuss a FPGA-based tracking system, relying on a massively parallel pattern recognition approach, inspired by the processing of visual images by the natural brain (“retina architecture”). This method allows a large efficiency of utilisation of the hardware, low power consumption and very low latencies. Based on this approach, a device has been designed within the LHCb Upgrade-II project, with the goal of performing track reconstruction in the forward acceptance region in real-time during the upcoming Run 4 of the LHC. This innovative device will perform track reconstruction before the event-building, in a short enough time to provide pre-reconstructed tracks (“primitives”) transparently to the processor farm, as if they had been generated directly by the detector. This allows significant savings in higher-level computing resources, enabling handling higher luminosities than otherwise possible. The feasibility of the project is backed up by the results of tests performed on a realistic hardware prototype, that has been opportunistically processing actual LHCb data in parallel with the regular DAQ in the LHC Run 3.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2025). First measurement of b-jet mass with and without grooming. Phys. Lett. B, 869, 139854–13pp.
Abstract: The LHCb collaboration presents a novel suite of heavy-flavour jet substructure measurements at forward rapidity in proton-proton collisions at a centre-of-mass energy of root s = 13 TeV. The jet mass is a perturbatively calculable probe of the virtuality of hard-scattered quarks and gluons, connecting small-distance quantum chromodynamics (QCD) with long-distance experimental measurement. The jet mass is dominated by nonperturbative corrections at small values, presenting an excellent test of QCD across a broad range of jet energies. Measuring heavy-flavour jet mass with a theoretically unambiguous flavour definition for the first time probes the gluon splitting mechanism for heavy-flavour production and offers tests of perturbative QCD at new levels of theoretical precision. Utilising the soft drop jet-grooming technique to access the perturbative jet core further enhances constraints on first-principles theory. Measurements of the jet mass for jets containing fully reconstructed B-+/- hadrons are reported with and without grooming. The unique phase space instrumented by LHCb offers a different quarkgluon fraction than at midrapidity. These results offer unparalleled tests of quark flavour and mass dependence in QCD and provide a baseline for future studies of heavy-flavour jet quenching in heavy-ion collisions.
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LHCb Collaboration(Aaij, R. et al), Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., et al. (2021). Observation of the Lambda(0)(b) -> Lambda+cK+K-pi(-) decay. Phys. Lett. B, 815, 136172–10pp.
Abstract: The Lambda(0)(b) -> Lambda+cK+K-pi(-) decay is observed for the first time using a data sample of proton-proton collisions at centre-of-mass energies of root s = 7 and 8 TeV collected by the LHCb detector, corresponding to an integrated luminosity of 3fb(-1). The ratio of branching fractions between the Lambda(0)(b) -> Lambda K-+(c)+ K-pi(-) and the Lambda(0)(b) -> Lambda D-+(c)s(-) decays is measured to be B(Lambda(0)(b) -> Lambda+cK+K-pi(-))/B(Lambda(0)(b) -> Lambda D-+(c)s(-)) = (9.26 +/- 0.29 +/- 0.46 +/- 0.26) x 10(-2), where the first uncertainty is statistical, the second systematic and the third is due to the knowledge of the D-s(-) -> K+K-pi(-) branching fraction. No structure on the invariant mass distribution of the Lambda K-+(c)+ system is found, consistent with no open-charm pentaquark signature.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Measurement of the CP-violating phase phi(s) from B-s(0) -> J/psi pi(+)pi(-) decays in 13 TeV pp collisions. Phys. Lett. B, 797, 134789–12pp.
Abstract: Decays of B-s(0) and (B) over bar (0)(s) mesons into J/psi pi(+)pi(-) final states are studied in a data sample corresponding to 1.9 fb(-1) of integrated luminosity collected with the LHCb detector in 13 TeV pp collisions. A time-dependent amplitude analysis is used to determine the final-state resonance contributions, the CP-violating phase phi(s) = -0.057 +/- 0.060 +/- 0.011 rad, the decay-width difference between the heavier mass B-s(0) eigenstate and the B-0 meson of -0.050 +/- 0.004 +/- 0.004 ps(-1), and the CP-violating parameter vertical bar lambda vertical bar = 1.01(-0.06)(+0.08) +/- 0.03, where the first uncertainty is statistical and the second systematic. These results are combined with previous LHCb measurements in the same decay channel using 7 TeV and 8 TeV pp collisions obtaining phi(s) = 0.002 +/- 0.044 +/- 0.012 rad, and vertical bar lambda vertical bar = 0.949 +/- 0.036 +/- 0.019.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2025). Observation of charge-parity symmetry breaking in baryon decays. Nature, 643(8074), 1223–1228.
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 Lambda(b)(0) to the pK(-)pi(+)pi(-) 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 experiment(1) at the European Organization for Nuclear Research (CERN). The results reveal significant asymmetries between the decay rates of the Lambda(b)(0) 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 mechanism(2), 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.
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Aiola, S., Amhis, Y., Billoir, P., Jashal, B. K., Henry, L., Oyanguren, A., et al. (2021). Hybrid seeding: A standalone track reconstruction algorithm for scintillating fibre tracker at LHCb. Comput. Phys. Commun., 260, 107713–5pp.
Abstract: We describe the Hybrid seeding, a stand-alone pattern recognition algorithm aiming at finding charged particle trajectories for the LHCb upgrade. A significant improvement to the charged particle reconstruction efficiency is accomplished by exploiting the knowledge of the LHCb magnetic field and the position of energy deposits in the scintillating fibre tracker detector. Moreover, we achieve a low fake rate and a small contribution to the overall timing budget of the LHCb real-time data processing.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). First Observation of the (B)over-bar0s → Λc+(Λ)over-barc- Decay and Evidence for the (B)over-bar0 → Λc+(Λ)over-barc- Decay. Phys. Rev. Lett., 136(6), 061802–14pp.
Abstract: A search is presented for the two-body charmed baryonic decays, (B) over bar (0)((s)) -> Lambda(+)(c)(Lambda) over bar (-)(c), using a data sample collected by the LHCb experiment during 2011-2012 and 2015-2018, corresponding to an integrated luminosity of 9 fb(-1). The first observation of the (B) over bar (0)(s) -> Lambda(+)(c)(Lambda) over bar (-)(c) decay is reported with 6.2 sigma significance along with 4.3 sigma evidence for the (B) over bar (0) -> Lambda(+)(c)(Lambda) over bar (-)(c) decay. The branching fractions are measured to be B((B) over bar (0) -> Lambda(+)(c)(Lambda) over bar (-)(c)) = (1.01(-0.28)(+0.27) +/- 0.08 +/- 0.15)x10(-5) and B((B) over bar (0)(s) -> Lambda(+)(c)(Lambda) over bar (-)(c)) = (5.0 +/- 1.3 +/- 0.5 +/- 0.8)x10(-5), where the first uncertainty is statistical, the second systematic, and the third due to external inputs. These results provide novel experimental inputs for the theoretical framework describing two-body baryonic decays of B mesons via W-emission and W-exchange mechanisms.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). Observation of Bc+ → Dh+ h- Decays. Phys. Rev. Lett., 136(2), 021804–12pp.
Abstract: Searches are presented for B-c(+) -> Dh(+)h(-) decays, where D is a charmed meson and h(+/-) is a charged pion or kaon, using pp collision data collected by the LHCb experiment corresponding to an integrated luminosity of 9 fb(-1). The decays B-c(+) -> D+K+pi(-), B-c(+) -> D*K-+(+)pi-, and B-c(+) -> D-s(+) K+K- are observed for the first time. Their branching fractions, expressed as ratios relative to that of the B-c(+) -> B-s(0)pi(+) decay, are determined to be R(B-c(+) -> D+K+pi(-)) = (1.96 +/- 0.23 +/- 0.08 +/- 0.10) x 10(-3), R(B-c(+) -> D*K-+(+)pi(-)) = (3.67 +/- 0.55 +/- 0.24 +/- 0.20) x 10(-3), B-c(+) -> D-s(+) K+K- = (1.61 +/- 0.35 +/- 0.13 +/- 0.07) x 10(-3), where the first uncertainty is statistical, the second is systematic, and the third is due to the limited precision on the D-meson branching fractions. The decay channels proceed primarily through excited K-0 or D-0 resonances or phi mesons, and open a new avenue for studies of charge-parity violation in beauty mesons.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). Observation of the Rare Baryonic Decay B+ → p(Λ)over-bar and Measurement of its Weak Decay Parameter. Phys. Rev. Lett., 136(5), 051802–14pp.
Abstract: The first observation of the decay B+ -> p (Lambda) over bar is presented using proton-proton collision data collected by the LHCb experiment between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb(-1). The signal significance exceeds seven standard deviations. Using the B+ -> K-S(0)pi(+) decay as a normalization channel, the branching fraction is measured and combined with previous LHCb results based on data collected at 7 and 8 TeV in 2011 and 2012, yielding B(B+ -> p (Lambda) over bar) = (1.24 +/- 0.17 +/- 0.05 +/- 0.03) x 10(-7); where the first uncertainty is statistical, the second is systematic, and the third comes from the uncertainty on the branching fraction of the normalization channel. The B+ -> p (Lambda) over bar weak decay parameter is measured to be alpha(B) = 0.87(-0.29)(+0.26) +/- 0.09, indicating the presence of comparable S-wave and P-wave decay amplitudes.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). First Observation of CP Violation and Measurement of Polarization in B+ → ρ(770)0K*(892)+ Decays. Phys. Rev. Lett., 136(2), 021803–16pp.
Abstract: An amplitude analysis of the B+ -> (pi(+)pi(-))(K-S(0)pi(+)) decay is performed in the mass regions 0.30 < m pi(+)pi(-) < 1.10 GeV/c(2) and 0.75 < m(KS)(pi+)(0) < 1.20 GeV/c(2), using pp collision data recorded with the LHCb detector corresponding to an integrated luminosity of 9 fb-1. The polarization fractions and CP asymmetries for B+ -> rho(770)K-0*(892)(+) decays are measured. Violation of the CP symmetry in the decay B+ -> rho(770)K-0*(892)(+) is observed for the first time, with a significance exceeding 9 standard deviations. The CP asymmetry is measured to be A(CP) = 0.507 +/- 0.062(stat) +/- 0.024(syst) and the CP-averaged longitudinal polarization fraction of f(L) = 0.720 +/- 0.028(stat) +/- 0.009(syst). The measurements help to shed light on the polarization puzzle of B mesons decaying to two vector mesons.
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