Roberto Cardarelli

A bstract: Yields of charmonia, bound states of $$ c\overline{c} $$ c c ¯ quarks, are observed to be strongly suppressed in heavy-ion collisions relative to proton-proton collisions as a result of their interaction with the quark-gluon plasma produced in such collisions. Understanding the mechanisms responsible for this suppression requires a detailed understanding of charmonium production. To constrain the production mechanisms, it is essential to quantify how charmonium production correlates with jet activity. Measurements of J/ψ production in the dimuon decay channel are presented for inclusive J/ψ mesons and for J/ψ mesons that are either isolated or not isolated from jets, separately for prompt and non-prompt production. Non-isolated J/ψ mesons are defined by matching J/ψ candidates to anti- k _t jets with R = 0 . 2 and p _T > 20 GeV, reconstructed without signal from the J/ψ meson, allowing the quantification of jet activity accompanying J/ψ production. The analysis uses Pb+Pb and pp collisions at $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ s NN = 5.02 TeV recorded by the ATLAS experiment at the LHC, corresponding to integrated luminosities of 1 . 82 nb ^{− 1} and 0 . 26 fb ^{− 1} , respectively. The J/ψ yields in Pb+Pb, cross sections in pp , and nuclear modification factors are presented for non-isolated, isolated, and inclusive J/ψ up to p _T = 60 GeV. Isolated fractions and non-prompt fractions are reported in both pp and Pb+Pb collisions. Strong J/ψ suppression is observed to persist up to the highest measured p _T values, with differences observed between non-isolated, isolated, and inclusive J/ψ . These results provide new constraints on charmonium production mechanisms and their in-medium suppression.

A bstract: This paper presents a search for semi-visible Higgs boson decays in final states with a photon ( γ ) and missing transverse momentum $$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$ p T miss . The analysis is optimised for the gluon-gluon fusion production channel and exploits a novel trigger deployed in 2023, with reduced thresholds down to 50 GeV on the photon transverse momentum ( $$ {p}_{\mathrm{T}}^{\gamma } $$ p T γ ) and 70 GeV on the $$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$ p T miss , in combination with a selection on the transverse mass of the $$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$ p T miss -γ system ( m _T ) above 80 GeV. The results are interpreted in terms of Higgs boson decays into a γ and a dark photon γ _d ( H → γγ _d ). The analysis is performed using 135 fb ^{− 1} of Run 3 data collected by ATLAS in 2023 and 2024. A boosted decision tree is used to identify events with misreconstructed primary vertices, which can affect the $$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$ p T miss calculation. Data-driven methods are employed to estimate the backgrounds from jets and electrons misidentified as photons, while the backgrounds with genuine photons are taken from Monte Carlo simulations, normalised to data in control regions with muons. A binned maximum-likelihood fit to the m _T distribution is performed to evaluate the compatibility of the observed data with the Standard Model expectation. No significant excess is observed over the Standard Model prediction, and an observed (expected) upper limit on the branching ratio of H → γγ _d of 1.4% (1.2%) is obtained at 95% confidence level. Combining this result with previous Run 2 searches improves the observed (expected) upper limit to 0.9% (0.9%).

A bstract: A search for heavy spin-1 and spin-2 resonances decaying into a top-antitop-quark pair has been performed with 140 fb ^{− 1} of proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of $$ \sqrt{s}=13 $$ s = 13 TeV. Final states with either exactly one electron or muon, or exactly two leptons ( ee , μμ or eμ ), large missing transverse momentum, and two jets, at least one of which must be identified as likely containing a b -hadron decay, are considered. The search targets resonances with both narrow and broad widths relative to the detector resolution, and with masses in the range of 0.4–5.0 TeV. No significant deviation from the Standard Model prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for hypothetical Z′ bosons, Kaluza-Klein gravitons, and Kaluza-Klein gluons that decay into top-quark pairs.

A bstract: Soft unclustered energy patterns (SUEPs) refer to high-multiplicity, isotropic distributions of low-momentum particles that could arise in strongly-coupled hidden sector theories. A search for SUEPs whose decay products contain muons in the final state is presented using 140 fb ^{− 1} of proton–proton collision data at $$ \sqrt{s}=13 $$ s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider during 2015–2018. The analysis targets SUEP signatures via gluon–gluon fusion production of a scalar mediator in Hidden Valley scenarios. No significant excess over the Standard Model expectation is observed. Exclusion limits are set on the product of the mediator production cross section and the branching fraction for its decay into a SUEP, down to 0.05 fb for a mediator mass of m _S = 750 GeV, 0.4 fb for m _S = 400 GeV, and 70 fb for m _S = 125 GeV; the last value, when identifying the mediator with the Standard Model Higgs boson, translates to an upper limit on the branching fraction of around 0.2%.

A bstract: This paper presents the first differential fiducial measurements of γγ → ττ using 1.93 nb ^{− 1} of Pb+Pb data at $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ s NN = 5.02 TeV recorded by the ATLAS detector. Events in which one of the τ -leptons decays into a muon and two neutrinos $$ \tau \to {\nu}_{\tau }{\overline{\nu}}_{\mu}\mu $$ τ → ν τ ν ¯ μ μ are selected and are categorized into three regions by the presence of an electron or either one or three charged-particle track(s) from the second τ -lepton decay. The measurement is performed in events where both Pb ions remain intact and no neutrons are emitted. Differential cross-sections are measured for seven variables in three fiducial regions at particle level. The measurements are compared to theory predictions with different photon flux models and spin correlation effects. For the fiducial region with one muon and one electron in the final state, comparisons to next-to-leading-order electroweak predictions are also made. The transverse momentum ( p _T ) of the decay muon, the p _T of the visible decay particles of the other τ -lepton, the total p _T , invariant mass, and pseudorapidity of the visible particles from the di- τ system, and the rapidity and acoplanarity of the visible decay particles from either τ -lepton are measured. A maximum-likelihood fit to the muon transverse-momentum distributions in the three regions before unfolding is performed to extract the τ -lepton anomalous magnetic moment a _τ and electric dipole moment d _τ , the latter for the first time in heavy ion collisions. The observed 95% confidence level intervals are − 0 . 057 < a _τ < 0 . 035 and |d _τ | < 2 . 7 × 10 ^{− 16} e cm.

A bstract: This paper presents a measurement of the production cross-section of a W boson associated with one or two jets, where at least one jet originates from a b -quark. The data are collected with the ATLAS detector at the LHC in proton-proton collisions at $$ \sqrt{s}=13 $$ s = 13 TeV, corresponding to an integrated luminosity of 140 fb ^-1 . Differential cross-sections are presented as a function of the transverse momentum of the b -jet for both the electron and muon decay modes of the W boson. The results corrected for all detector effects are presented in a fiducial region defined with basic lepton and jet kinematics, and compared with theoretical predictions. The combined fiducial cross-section of W + b -jets for the electron and muon channels is measured to be 16 . 6 ± 1 . 9 pb, which is consistent with the next-to-leading-order QCD prediction of 16 . 8 ± 2 . 3 pb. The relative precision of this measurement is improved by approximately a factor of two compared with the previous ATLAS measurement at $$ \sqrt{s}=7 $$ s = 7 TeV.