Engineering topics

Aloisio, A. (ORCID:0000000238836693)

Publications and source records attributed to Aloisio, A. (ORCID:0000000238836693).

At least 19 records

DOE OSTI2026

Search for Higgs bosons produced in association with a high-energy photon via vector-boson fusion and decaying to a pair of b -quarks in the ATLAS detector

A search for Standard Model Higgs bosons produced in association with a high-energy photon and decaying to $b\bar{b}$ is performed using 133 fb −1 of $\sqrt{s}$ = 13 TeV pp collision data collected with the ATLAS detector at the Large Hadron Collider at CERN. The photon requirement reduces the multijet background, and the H → $b\bar{b}$ decay is the dominant decay mode. Event selection requirements target events produced by vector-boson fusion, the dominant production mode in this channel. Several improvements enhance the search sensitivity compared to previous measurements. These improvements include better background modelling and characterization, the use of a neural-network classifier, and an updated signal extraction strategy adopting a direct binned-likelihood fit to the classifier output. With these improvements, the Higgs boson signal strength is measured to be 0.2 ± 0.7 relative to the Standard Model prediction. This corresponds to an observed significance of 0.3 standard deviations, compared to an expectation of 1.5 standard deviations assuming the Standard Model.

ATLAS experiment↗

DOE OSTI2026

Search for a resonance decaying into a scalar particle and a Higgs boson in the final state with two bottom quarks and two photons with 199 fb −1 of data collected at $\sqrt{s}$ = 13 and 13.6 TeV with the ATLAS detector

A search for the resonant production of a heavy scalar X decaying into a lighter scalar S and a Higgs boson, through the process X → S (→ $b\bar{b}$) H (→ γγ) , where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using integrated luminosities of 140 and 59 fb −1 of proton–proton collision data at centre-of-mass energies of 13 and 13.6 TeV, respectively, recorded with the ATLAS detector at the LHC. The search is performed over the mass ranges of 170 ≤ m X ≤ 1000 GeV and 15 ≤ m S ≤ 500 GeV. No significant excess over the Standard Model background predictions is observed and limits at 95% confidence level are set on the product of cross section and branching fraction for the process X → S (→ $b\bar{b}$) H (→ γγ) at 13 TeV, ranging from 9 fb to 0.06 fb.

Aad, G. (ORCID:0000000266654934)↗

DOE OSTI2026

Study of Higgs boson pair production in the HH → b $\overline{b}$ γγ final state with 308 fb −1 of data collected at $\sqrt{s}$ = 13 TeV and 13.6 TeV by the ATLAS experiment

A search for Higgs boson pair production in the b $\overline{b}$ γγ −1, consisting of two samples, 140 fb −1 at a centre-of-mass energy of s = 13 TeV and 168 fb −1 at $\sqrt{s}$ = 13.6 TeV , recorded between 2015 and 2024 by the ATLAS detector at the CERN Large Hadron Collider. In addition to a larger dataset, this analysis improves upon the previous search in the same final state through several methodological and technical developments. The Higgs boson pair production cross section divided by the Standard Model prediction is found to be μ HH = 0.9$^{+1.4}_{−1.1}$ (μ HH = 1$^{+1.3}_{−1.0}$ expected), which translates into a 95% confidence-level upper limit of μ HH < 3.7. At the same confidence level the Higgs self-coupling modifier is constrained to be in the range −1.6 < κ λ < 6.6 (−1.8 < κ λ < 6.9 expected).

ATLAS↗

DOE OSTI2026

Search for the Higgs boson decay to a Z boson and a photon in pp collisions at $\sqrt{s}$ = 13 TeV and 13.6 TeV with the ATLAS detector

A search for the Higgs boson decay to a Z boson and a photon in the ℓℓγ (ℓ = 𝑒,𝜇) final state is performed using pp collisions at $\sqrt{s}$ = 13.6 TeV recorded with the ATLAS detector at the Large Hadron Collider during 2022–2024, corresponding to an integrated luminosity of 165 fb−1. The signal yield, normalised to the Standard Model prediction, is measured to be 𝜇 = 0.9$^{+0.7}_{−0.6}$, compatible with the expected value of 𝜇 = 1.0 ± 0.7. This corresponds to an observed (expected) signal significance of 1.4 (1.5) standard deviations under the background-only hypothesis. This result is combined with that of a similar search performed with 140 fb −1 of $\sqrt{s}$ = 13 TeV pp collisions to provide the best expected sensitivity to date to this rare decay, namely an observed (expected) signal strength of 𝜇 = 1.3$^{+0.6}_{−0.5}$ (𝜇 = 1.0$^{+0.6}_{−0.5}$), corresponding to an observed (expected) significance of 2.5 (1.9) standard deviations. The measurement is consistent with the Standard Model expectation.

Aad, G. (ORCID:0000000266654934)↗

DOE OSTI2026

Observation of $t\bar{t}γγ$ production at $\sqrt{s} = 13$ TeV with the ATLAS detector

This paper presents the first observation of top-quark pair production in association with two photons ($t\bar{t}γγ$). The measurement is performed in the single-lepton decay channel using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. The data correspond to an integrated luminosity of 140 fb −1 recorded during Run 2 at a centre-of-mass energy of 13 TeV. The $t\bar{t}γγ$ production cross section, measured in a fiducial phase space based on particle-level kinematic criteria for the lepton, photons, and jets, is found to be 2.42$_{− 0.53}^{+ 0.58}$ fb , corresponding to an observed significance of 5.2 standard deviations. Additionally, the ratio of the production cross section of $t\bar{t}γγ$ to top-quark pair production in association with one photon is determined, yielding (3.30$_{− 0.65}^{+ 0.70}$) × 10 −3 .

Aad, G. (ORCID:0000000266654934)↗

DOE OSTI2026

Transforming jet flavour tagging at ATLAS

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 in 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.

Characterization and analytical techniques↗

DOE OSTI2025

Search for the lepton-flavor-violating τ − → e ∓ ℓ ± ℓ − decays at Belle II

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 for four of the modes.

flavour physics↗

DOE OSTI2025

Search for an axion-like particle in B → K (*) a(→ γγ) decays at Belle

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 × 10 6 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 signal 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.

B physics↗

DOE OSTI2025

Observation of W + W – γ production in pp collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector and constraints on anomalous quartic gauge-boson couplings

This Letter reports the observation of W + W – γ triboson production in 140 fb –1 of data collected by the ATLAS detector from proton–proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV at the LHC. Events with an opposite-charge eμ pair, a high transverse-momentum photon, and significant missing transverse momentum are considered. The observed (expected) significance of the signal is 5.9 (6.0) standard deviations. The measured fiducial cross-section, defined for the W + W – γ → e ± μ ∓ $ν\bar{ν}$γ final state is 6.2 ± 0.8 (stat.) ± 0.6 (sys.) fb, in good agreement with the Standard Model prediction of 6.1$^{+1.0}_{–0.7}$ fb. Constraints on the Wilson coefficients of 13 dimension-8 operators describing physics beyond the Standard Model through anomalous quartic gauge-boson couplings are derived using the effective field theory framework.

Anomalous quartic gauge couplings↗

DOE OSTI2025

Model-agnostic likelihood for the reinterpretation of the 𝐵 + → 𝐾 + ⁢$𝑣\bar{𝑣}$ measurement at Belle II

We recently measured the branching fraction of the 𝐵 + → 𝐾 + ⁢$𝑣\bar{𝑣}$ decay using 362 fb −1 of on-resonance 𝑒 + ⁢𝑒 − collision data under the assumption of Standard Model kinematics, providing the first evidence for this decay. To facilitate future reinterpretations and maximize the scientific impact of this measurement, we publicly release the full analysis likelihood along with all necessary material required for reinterpretation under arbitrary theoretical models sensitive to this measurement. In this work, we demonstrate how the measurement can be reinterpreted within the framework of the weak effective theory. Using a kinematic reweighting technique in combination with the published likelihood, we derive marginal posterior distributions for the Wilson coefficients, construct credible intervals, and assess the goodness of fit to the Belle II data. For the weak effective theory Wilson coefficients, the posterior mode of the magnitudes |𝐶 VL +𝐶 VR |, |𝐶 SL +𝐶 SR |, and |𝐶 TL | corresponds to the point (11.3, 0.0, 8.2). The respective 95% credible intervals are [1.9, 16.2], [0.0, 15.4], and [0.0, 11.2].

bottom quark↗

DOE OSTI2025

Measurement of substructure-dependent suppression of large-radius jets with charged particles in Pb+Pb collisions with ATLAS

Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its dependence on the jet substructure. The measurement uses 1.72 nb$^{-1}$ of Pb+Pb data and 255 pb$^{-1}$ of $pp$ data, both at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering $R = 0.2$ calorimetric jets and are measured for transverse momentum above $200$ GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ($R_\mathrm{AA}$). The jet $R_\mathrm{AA}$ is measured as a function of jet $p_{\mathrm{T}}$, the charged $k_t$ splitting scale ($\sqrt{d_{12}}$), and the angular separation ($dR_{12}$) of two leading sub-jets. The jet $R_\mathrm{AA}$ gradually decreases with increasing $\sqrt{d_{12}}$, implying significantly stronger suppression of large-radius jets with larger $k_t$ splitting scale. The jet $R_\mathrm{AA}$ gradually decreases for $dR_{12}$ in the range $0.01{-}0.2$ and then remains consistent with a constant for $ΔR_{12} \gtrsim 0.2$. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2025

Charged-hadron identification at Belle II

The Belle II experiment’s ability to identify particles critically affects the sensitivity of its measurements. We describe Belle II’s algorithms for identifying charged particles and evaluate their performance in separating pions, kaons, and protons using $426\,\text {fb}^{-1}$ of data collected at the energy-asymmetric $e^{+}e^{-}$ collider SuperKEKB in 2019–2022 at center-of-mass energies at and near the mass of the $\Upsilon$ (4S).

Adachi, I. (ORCID:0000000322870173)↗

DOE OSTI2025

Measurement of the 𝐷 0 →𝐾 − ⁢𝜋 + ⁢𝑒 + ⁢𝑒 − branching fraction and search for 𝐷 0 → 𝜋 + ⁢𝜋 − ⁢𝑒 + ⁢𝑒 − and 𝐷 0 →𝐾 + ⁢𝐾 − ⁢𝑒 + ⁢𝑒 − decays at Belle

We present a study of the rare charm meson decays 𝐷 0 →𝐾 + ⁢𝐾 − ⁢𝑒 + ⁢𝑒 − , 𝜋 + ⁢𝜋 − ⁢𝑒 + ⁢𝑒 − , and 𝐾 − ⁢𝜋 + ⁢𝑒 + ⁢𝑒 − using a 942 fb −1 dataset collected by the Belle detector at the KEKB asymmetric-energy 𝑒 + ⁢𝑒 − collider. We identify 𝐷 0 candidates via the charge of the pion from 𝐷* + →𝐷 0 ⁢𝜋 + decays and normalize the branching fractions to 𝐷 0 → 𝐾 − ⁢𝜋 + ⁢𝜋 − ⁢𝜋 + decays. The branching fraction for decay 𝐷 0 → 𝐾 − ⁢𝜋 + ⁢𝑒 + ⁢𝑒 − is measured to be (39.6 ± 4.5 (stat) ± 2.9 (syst)) × 10 −7 , with the dielectron mass in the 𝜌/𝜔 mass region 675 < 𝑚 𝑒⁢𝑒 < 875 MeV/⁢𝑐 2 . We also search for 𝐷 0 → ℎ − ⁡ℎ (′)+ ⁡𝑒 + ⁢𝑒 − (ℎ (′) = 𝐾, 𝜋) decays with the dielectron mass near the 𝜂 and 𝜙 resonances, and away from these resonances for the 𝐾 + ⁢𝐾 − ⁢𝑒 + ⁢𝑒 − and 𝜋 + ⁢𝜋 − ⁢𝑒 + ⁢𝑒 − modes. For these modes, we find no significant signals and set 90% confidence level upper limits on their branching fractions at the 𝒪⁡(10 −7 ) level.

branching fraction↗

DOE OSTI2025

Search for B 0 → K *0 𝜏 + ⁢𝜏 − Decays at the Belle II Experiment

We present a search for the rare flavor-changing neutral-current decay 𝐵 0 →𝐾 *0 ⁢𝜏 + ⁢𝜏 − with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb −1 data sample recorded at the center-of-mass energy of the ϒ⁡(4⁢𝑆) resonance. One of the 𝐵 mesons produced in the ϒ⁡(4⁢𝑆)→𝐵 0 $⁢\overline{𝐵}$ 0 process is fully reconstructed in a hadronic decay mode, while its companion 𝐵 meson is required to decay into a 𝐾 *0 and two 𝜏 leptons of opposite charge. The 𝜏 leptons are reconstructed in final states with a single electron, muon, charged pion or charged 𝜌 meson, and additional neutrinos. We set an upper limit on the branching fraction of ℬ⁡(𝐵 0 →𝐾 *0 ⁢𝜏 + ⁢𝜏 − )<1.8×10 −3 at the 90% confidence level, which is the most stringent constraint reported to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Constraining off-shell Higgs boson production and the Higgs boson total width using WW → ℓνℓν final states with the ATLAS detector

A measurement of off-shell Higgs boson production is performed in the H* → WW channel. The measurement uses a proton–proton collision dataset with an integrated luminosity of 140 fb -1 collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Final states in which both W bosons decay leptonically are targeted, and events are categorised based on the flavour of the final-state leptons, the jet multiplicity, and the output of neural network-based classifiers. The data are found to be compatible with the Standard Model expectation. An observed (expected) upper bound on the 95 % symmetric confidence level interval is set on the rate of off-shell Higgs boson production at a value of 3.4 (4.4) times the Standard Model prediction. These results are combined with the results from the measurement of on-shell Higgs boson production in the same final states to obtain an observed (expected) upper bound at 95 % confidence level on the Higgs boson total width of 13.1 (17.3) MeV.

Aad, G. [Aix-Marseille Univ., Marseille (France)] ↗

DOE OSTI2025

Measurement of 𝐵 + →𝜏 + ⁢𝜈 𝜏 branching fraction with a hadronic tagging method at Belle II

We present a measurement of the branching fraction of 𝐵 + →𝜏 + ⁢𝜈 𝜏 decays using 387±6×10 6 ϒ⁡(4⁢𝑆) collected between 2019 and 2022 with the Belle II detector at the SuperKEKB 𝑒 + ⁢𝑒 − collider. We reconstruct the accompanying 𝐵 − meson using the hadronic tagging method, while 𝐵 + →𝜏 + ⁢𝜈 𝜏 candidates are identified in the recoil. We find evidence for 𝐵 + →𝜏 + ⁢𝜈 𝜏 decays at 3.0 standard deviations, including systematic uncertainties. The measured branching fraction is ℬ⁡(𝐵 + →𝜏 + ⁢𝜈 𝜏 )=[1.24±0.41⁢(stat)±0.19⁢(syst)]×10 −4 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Observation of double parton scattering in same-sign W boson pair production in pp collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

This letter reports the measurement of double parton scattering in same-sign W boson pair production with the ATLAS detector. The data set used corresponds to an integrated luminosity of 140 fb -1 of proton–proton collisions at a center-of-mass energy of 13 TeV, collected during Run 2 of the Large Hadron Collider. The study is performed in final states including two same-charge leptons, electron or muon, missing transverse momentum, and up to one jet. An excess of events is observed over the expected background contributions with a significance of 8.8 standard deviations. The measured fiducial cross section times leptonic branching fraction is 4.59 ± 0.64 fb. The measurement corresponds to a double parton scattering effective cross section of 10.6 ± 1.8 mb.

DPS↗

DOE OSTI2025

Search for a Dark Higgs Boson Produced in Association with Inelastic Dark Matter at the Belle II Experiment

Inelastic dark matter models that have two dark matter particles and a massive dark photon can reproduce the observed relic dark matter density without violating cosmological limits. The mass splitting between the two dark matter particles 𝜒 1 and 𝜒 2 , with 𝑚⁡(𝜒 2 ) >𝑚⁡(𝜒 1 ), is induced by a dark Higgs field and a corresponding dark Higgs boson ℎ′. We present a search for dark matter in events with two vertices, at least one of which must be displaced from the interaction region, and missing energy. Using a 365 fb −1 data sample collected at Belle II, which operates at the SuperKEKB 𝑒 + ⁢𝑒 − collider, we observe no evidence for a signal. We set upper limits on the product of the production cross section 𝜎⁡(𝑒 + ⁢𝑒 − → ℎ′⁡𝜒 1 ⁢𝜒 2 ), and the product of branching fractions ℬ⁡(𝜒 2 → 𝜒 1⁢ 𝑒 + ⁢𝑒 − ) ×ℬ⁡(ℎ′ → 𝑥 + ⁢𝑥 − ), where 𝑥 + ⁢𝑥 − indicates 𝜇 + ⁢𝜇 − , 𝜋 + ⁢𝜋 − , or 𝐾 + ⁢𝐾 − , as functions of ℎ′ mass and lifetime at the level of 10 −1 fb. We set model-dependent upper limits on the dark Higgs mixing angle at the level of 10 −5 and on the dark photon kinetic mixing parameter at the level of 10 −3 . This is the first search for dark Higgs bosons in association with inelastic dark matter.

dark matter↗