OPMD

Abstract:

MALTA2 is a depleted monolithic active pixel sensor (DMAPS) designed for tracking at high rates and is produced in the modified Tower 180 nm CMOS imaging technology. The pixel pitch is 36.4 µm and the sensing layer consists of either high resistivity epitaxial or Czochralski silicon. The implementation of a small collection electrode features a small pixel capacitance and offers low noise. Typically a detection threshold of around 200 e⁻ is used. It is calibrated making use of a dedicated charge injection circuit on chip and an Fe-55 source. In this contribution, MALTA2 sensors are characterized in terms of hit detection efficiency inside the pixel at fine threshold steps. A comparison is made for samples before and after irradiation at different substrate voltages and with different doping concentration of the internal n-layer. Data was taken at CERN SPS test beam campaigns in 2023 and 2024, using the MALTA beam telescope consisting of six MALTA tracking planes with 4 µm spatial and 2 ns timing resolution. A reconstruction of the most probable signal amplitude from binary hit data is performed. A two-dimensional map of the collected charge is obtained with sub-pixel resolution. The presented method provides an in-beam alternative to grazing angle studies or Edge-TCT for determining a charge collection profile.

Abstract:

This article reports on a search for dijet resonances using $132{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded at $\sqrt{s}=13\mathrm{TeV}$ by the ATLAS detector at the Large Hadron Collider. The search is performed solely on jets reconstructed within the ATLAS trigger to overcome bandwidth limitations imposed on conventional single-jet triggers, which would otherwise reject data from decays of sub-TeV dijet resonances. Collision events with two jets satisfying transverse momentum thresholds of $p_{\mathrm{T}}\ge 85\mathrm{GeV}$ and jet rapidity separation of $|y^{*}|<0.6$ are analysed for dijet resonances with invariant masses from 375 to $1800\mathrm{GeV}$ . A data-driven background estimate is used to model the dijet mass distribution from multijet processes. No significant excess above the expected background is observed. Upper limits are set at 95% confidence level on coupling values for a benchmark leptophobic axial-vector $Z^{\prime }$ model and on the production cross section for a new resonance contributing a Gaussian-distributed line-shape to the dijet mass distribution.

Abstract:

A search for events with one displaced vertex from long-lived particles using data collected by the ATLAS detector at the Large Hadron Collider is presented, using $140{\mathrm{fb}}^{-1}$ of proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ recorded in 2015–2018. The search employs techniques for reconstructing vertices of long-lived particles decaying into hadronic jets in the muon spectrometer displaced between 3 m and 14 m from the primary interaction vertex. The observed number of events is consistent with the expected background and limits for several benchmark signals are determined. A scalar-portal model and a Higgs-boson-portal baryogenesis model are considered. A dedicated analysis channel is employed to target Z-boson associated long-lived particle production, including an axionlike particle and a dark photon model. For the Higgs boson model, branching fractions above 1% are excluded at 95% confidence level for long-lived particle proper decay lengths ranging from 5 cm to 40 m. For the photophobic axionlike particle model considered, this search produces the strongest limits to date for proper decay lengths greater than $\mathcal{O}(10)\mathrm{cm}$ .

Abstract:

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 p p data, both at s 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 AA ). The jet R AA is measured as a function of jet p T , the charged k t splitting scale ( d 12 ), and the angular separation ( Δ R 12 ) of two leading sub-jets. The jet R AA gradually decreases with increasing d 12 , implying significantly stronger suppression of large-radius jets with larger k t splitting scale. The jet R AA gradually decreases for Δ R 12 in the range 0.01 − 0.2 and then remains consistent with a constant for Δ R 12 ≳ 0.2 . The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.

Abstract:

The CP properties of the Higgs boson are studied in the vector-boson fusion production mode. The analysis exploits the decay mode of the Higgs boson into two τ-leptons using 140 fb−1 of proton-proton collision data at s=13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Results are obtained using the Optimal Observable method. CP-violating interactions between the Higgs boson and electroweak gauge bosons are considered in the effective field theory framework, with the interaction strength described in the HISZ basis by d~, and in the Warsaw basis by cHW~, cHB~, and cHW~B. No deviations relative to the Standard Model are observed, and limits are obtained on the strength parameters. The d~ parameter is constrained to the interval [−0.012, 0.044] at the 95% confidence level while cHW~ is constrained to [−0.24, 0.83], when considering both linear and quadratic effects of physics beyond the Standard Model.