Dr Eleonora Rossi

Abstract:

Measurements of longitudinal flow decorrelations in 13-TeV $pp$ and 5.44-TeV $\mathrm{Xe}+\mathrm{Xe}$ collisions with the ATLAS detector are presented. The measurements are performed using the two-particle correlation method, combining charged-particle tracks within $\left|\eta \right|<2.5$ with either calorimeter energy clusters or towers within $4.0<\left|\eta \right|<4.9$ . A template-based subtraction procedure is used to remove nonflow effects in both the $pp$ and the $\mathrm{Xe}+\mathrm{Xe}$ analyses. The dependence of the longitudinal flow decorrelations on the pseudorapidity separation between the particles is characterized via the slope parameter $F_n$ for the elliptic ( $n=2$ ) and triangular ( $n=3$ ) harmonic moments. The results are reported as a function of charged-particle multiplicity for the $pp$ and $\mathrm{Xe}+\mathrm{Xe}$ collision systems. Comparing the data to a color string-based model of the initial geometry indicates that in $pp$ and peripheral $\mathrm{Xe}+\mathrm{Xe}$ collisions, subnucleonic structure, and fluctuations in longitudinal energy deposition are needed to describe the data.

Abstract:

This paper presents a search for semi-visible Higgs boson decays in final states with a photon (γ) and missing transverse momentum pTmiss$$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$. 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 (pTγ$$ {p}_{\mathrm{T}}^{\gamma } $$) and 70 GeV on the pTmiss$$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$, in combination with a selection on the transverse mass of the pTmiss$$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$-γ system (mT) above 80 GeV. The results are interpreted in terms of Higgs boson decays into a γ and a dark photon γd (H → γγd). 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 pTmiss$$ {p}_{\mathrm{T}}^{\mathrm{miss}} $$ 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 mT 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 → γγd 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%).

Abstract:

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 s=13$$ \sqrt{s}=13 $$ 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.