2,222 Search Results

Using a sample of ( 2712.4 ± 14.3 ) × 10 6     ψ ( 3686 ) events collected with the BESIII detector, we perform a search for the isospin-violating decays χ c J → Λ Σ ¯   0 + c . c . ( J = 0 , 1 , 2 ) and η c → Λ Σ ¯   0 + c . c . No significant signal for χ c J or η c is observed in the Λ Σ ¯   0 invariant mass distribution. The upper limits on the branching fractions at the 90% confidencemore » level are set to be B ( χ c 0 → Λ Σ ¯   0 + c . c . ) < 1.5 × 10 − 6 , B ( χ c 1 → Λ Σ ¯   0 + c . c . ) < 1.6 × 10 − 6 , B ( χ c 2 → Λ Σ ¯   0 + c . c . ) < 1.7 × 10 − 6 , and B ( η c → Λ Σ ¯   0 + c . c . ) < 6.2 × 10 − 5 for the first time.« less

Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system’s radial expansion, has received less attention. Notably, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (𝑝_T) dependence of radial flow fluctuations (𝑣₀⁡(𝑝_T)) over 0.5 < 𝑝_T < 10 GeV and demonstrates its collective nature using a two-particle correlation method in Pb+Pb collisions at $$\sqrt{𝑠_{NN}}$$ = 5.02 TeV. The data revealmore » three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in 𝑝_T, and centrality-independent shape in 𝑝_T. The comparison with a hydrodynamic model demonstrates the sensitivity of 𝑣₀⁡(𝑝_T) to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.« less

The LEGEND Collaboration is searching for neutrinoless double-beta (0⁢𝜈⁢𝛽⁢𝛽) decay by operating high-purity germanium detectors enriched in ⁷⁶Ge in a low-background liquid argon environment. Building on key technological innovations from the GERmanium Detector Array (GERDA) experiment and the MAJORANA DEMONSTRATOR experiment, LEGEND-200 has performed a first 0⁢𝜈⁢𝛽⁢𝛽 decay search based on 61.0 kg yr of data. Over half of this exposure comes from our highest performing detectors, including newly developed inverted-coaxial detectors, and is characterized by an estimated background level of 0.5$$^{+0.3}_{−0.2}$$ ⁢cts/(keV ton yr) in the 0⁢𝜈⁢𝛽⁢𝛽 decay signal region. A combined analysis of data from GERDA, the MAJORANAmore » DEMONSTRATOR, and LEGEND-200, characterized by a 90% confidence level exclusion sensitivity of 2.8 ×1026 yr on the half-life of 0⁢𝜈⁢𝛽⁢𝛽 decay, reveals no evidence for a signal and sets a new observed lower limit at 𝑇$$^{⁢0𝜈}_{1/2}$$ > 1.9 × 10²⁶ yr (90% confidence level). Assuming the decay is mediated by Majorana neutrinos, this corresponds to an upper limit on the effective Majorana mass in the range 𝑚_𝛽⁢𝛽< 75–200 meV, depending on the adopted nuclear matrix element.« less

The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair ($$t\overline{t}$$) invariant mass in proton-proton collisions using 140 fb⁻¹ of data at $$\sqrt{s}=13$$ TeV collected in 2015–2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the $$t\overline{t}$$ invariant mass spectrum is sensitive to electroweak virtual corrections, including contributions from Higgs boson exchange, thereby providing sensitivity to the top-quark Yukawa coupling. This is the first measurement in ATLAS that aims to obtain this coupling exploiting this approach. The $$t\overline{t}$$ system is reconstructed in the single-lepton final state, requiring exactly onemore » isolated electron or muon and at least four jets with at least two identified as originating from b-quarks. The measured Yukawa coupling is found to be in good agreement with the Standard Model prediction. An upper limit on the top-quark Yukawa coupling strength of Y_t < 2.1 relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.« less

Jet flavour tagging enables the identification of jets originating from heavy-flavour quarks in proton–proton collisions at the Large Hadron Collider, playing a critical role in its physics programmes. This paper presents GN2, a transformer-based flavour tagging algorithm deployed by the ATLAS Collaboration that represents a different methodology compared to previous approaches. Designed to classify jets based on the flavour of their constituent particles, GN2 processes low-level tracking information in an end-to-end architecture and incorporates physics-informed auxiliary training objectives to enhance both interpretability and performance. Its performance is validated in both simulation and collision data. The measured c-jet (light-jet) rejection inmore » data is improved by a factor of 3.5 (1.8) for a 70% b-jet tagging efficiency, compared to the previous algorithm. GN2 provides substantial benefits for physics analyses involving heavy-flavour jets, such as measurements of Higgs boson pair production and the couplings of bottom and charm quarks to the Higgs boson, and demonstrates the impact of advanced machine learning methods in experimental particle physics.« less

We present the result of a search for the charged-lepton-flavor violating decays τ⁻ → e^∓ℓ^±ℓ⁻, where ℓ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb⁻¹ recorded by the Belle II experiment at the SuperKEKB e⁺e⁻ collider. The selection of e⁺e⁻ → τ⁺τ⁻ events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 × 10⁻⁸ are set at the 90% confidence level. These results are the most stringent bounds to date formore » four of the modes.« less

This paper presents a search for physics beyond the Standard Model targeting a heavy resonance visible in the invariant mass of the lepton-jet system. The analysis focuses on final states with a high-energy lepton and jet, and is optimised for the resonant production of leptoquarks — a novel production mode mediated by the lepton content of the proton originating from quantum fluctuations. Four distinct and orthogonal final states are considered: e+light jet, μ+light jet, e+b-jet, and μ+b-jet, constituting the first search at the Large Hadron Collider for resonantly produced leptoquarks with couplings to electrons and muons. Events with an additionalmore » same-flavour lepton, as expected from higher-order diagrams in the signal process, are also included in each channel. The search uses proton-proton collision data from the full Run 2, corresponding to an integrated luminosity of 140 fb⁻¹ at a centre-of-mass energy of $$\sqrt{s}=13$$ TeV, and from a part of Run 3 (2022–2023), corresponding to 55 fb⁻¹ at $$\sqrt{s}=13.6$$ TeV. No significant excess over Standard Model predictions is observed. The results are interpreted as exclusion limits on scalar leptoquark ($$\tilde{S}$$₁) production, substantially improving upon previous ATLAS constraints from leptoquark pair production for large coupling values. The excluded $$\tilde{S}$$₁ $$\tilde{S}$$₁ mass ranges depend on the coupling strength, reaching up to 3.4 TeV for quark-lepton couplings y_de = 1.0, and up to 4.3 TeV, 3.1 TeV, and 2.8 TeV for y_sμ, y_be, and y_bμ couplings set to 3.5, respectively.« less

We report a search for an axion-like particle a in B → K^(*)a decays using data collected with the Belle detector at the KEKB asymmetric-energy electron-positron collider. The search is based on a 711 fb⁻¹ data sample collected at the Υ(4S) resonance energy, corresponding to a sample of 772 × 10⁶ Υ(4S) events. In this study, we search for the decay of the axion-like particle into a pair of photons, a → γγ. We scan the two-photon invariant mass in the range 0.16 GeV–4.50 GeV for the K modes and 0.16 GeV–4.20 GeV for the K^* modes. No significant signalmore » is observed in any of the modes, and 90% confidence level upper limits are established on the coupling to the W boson, g_aW, as a function of a mass. The limits range from 3 × 10⁻⁶ GeV⁻¹ to 3 × 10⁻⁵ GeV⁻¹, improving the current constraints on g_aW by a factor of two over the most stringent previous experimental results.« less

A search for R-parity-conserving supersymmetry in events with large missing transverse momentum, jets and at least one hadronically decaying $$\tau$$-lepton is presented. Both gluino and squark pair production are considered, with the cascade decay of each gluino or squark producing either a $$\tau$$-slepton or a $$\tau$$-sneutrino. Three channels are examined, requiring either exactly one hadronically decaying $$\tau$$-lepton and no other leptons, exactly one hadronically decaying $$\tau$$-lepton and at least one other lepton, or two or more hadronically decaying $$\tau$$-leptons. Analyses in the three channels are optimised independently and combined statistically. Two separate analysis strategies, either a cut-and-count or machine-learning approach,more » are used. The search uses 140 and 51.8 of pp collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015–2018 at TeV and 2022–2023 at TeV, respectively. Gluino masses below 2.25 TeV and squark masses up to 1.7 TeV are excluded« less

Measurements of $$t\bar{t}\ell ^{+}\ell ^{-}$$ production in the region of high dilepton invariant mass with effective field theory (EFT) interpretations are presented. They are performed using final states with three isolated leptons (electrons or muons) and are based on $$\sqrt{s} = 13$$ TeV proton–proton collision data with an integrated luminosity of $$140\,\textrm{fb}^{-1}$$, recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. Measurements of the $$t\bar{t}\ell ^{+}\ell ^{-}$$ signal strength and cross-section upper-limits are performed inclusively in lepton flavour and separately for electrons and muons. The study also aims to probe anomalous four-fermion interactions including tomore » test for possible lepton flavor universality violation. No significant deviations from the Standard Model predictions are observed and the measurements are interpreted through the EFT formalism to provide new constraints on the relevant operators.« less