ATLAS

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:

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$$ \sqrt{s}=13 $$ 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~$$ \overset{\sim }{d} $$, and in the Warsaw basis by cHW~$$ {c}_{H\overset{\sim }{W}} $$, cHB~$$ {c}_{H\overset{\sim }{B}} $$, and cHW~B$$ {c}_{H\overset{\sim }{W}B} $$. No deviations relative to the Standard Model are observed, and limits are obtained on the strength parameters. The d~$$ \overset{\sim }{d} $$ parameter is constrained to the interval [−0.012, 0.044] at the 95% confidence level while cHW~$$ {c}_{H\overset{\sim }{W}} $$ is constrained to [−0.24, 0.83], when considering both linear and quadratic effects of physics beyond the Standard Model.

Abstract:

A search for the production of a Higgs boson in association with a single top quark, tH, is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb−1 at a centre-of-mass energy of 13 TeV, collected by the ATLAS detector at the LHC. The search targets Higgs-boson decays into bb¯$$ b\overline{b} $$, WW*, ZZ*, and ττ, accompanied by an isolated lepton (electron or muon) from the top-quark decay. Multivariate techniques are employed to enhance the separation between signal and background processes. The observed signal strength, μtH, defined as the ratio between the measured cross-section and the predicted Standard Model value, is μtH = 8.1 ± 2.6 (stat.) ± 2.0 (syst.). The significance of the observed (expected) signal above the background-only expectation is 2.8 (0.4) standard deviations. The corresponding observed (expected) upper limit at the 95% confidence level on the tH cross-section is found to be 13.9 (6.1) times the value predicted by the Standard Model. An interpretation with an inverted sign of the top-quark Yukawa coupling is performed, and the signal strength and corresponding limit are reported.

Abstract:

The production cross-section of high-mass τ-lepton pairs is measured as a function of the dilepton visible invariant mass, using 140 fb−1 of s=13$$ \sqrt{s}=13 $$ TeV proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider. The measurement agrees with the predictions of the Standard Model. A fit to the invariant mass distribution is performed as a function of b-jet multiplicity, to constrain the non-resonant production of new particles described by an effective field theory or in models containing leptoquarks or Z′ bosons that couple preferentially to third-generation fermions. The constraints on new particles improve on previous results, and the constraints on effective operators include those affecting the anomalous magnetic moment of the τ-lepton.

Abstract:

A search for exotic decays of the Higgs boson $H$ into new scalar or pseudoscalar particles that subsequently decay into $b$ -quarks is presented. The search considers $ZH$ production with several decay scenarios for the Higgs boson; first to a pair of identical scalars, $H\to 2a\to 4b$ , second to a pair of scalars with different masses ( $m_{a1}<m_{a2}$ ), either directly, $H\to a_1{a}_2\to 4b$ , or via a longer decay chain, $H\to a_1{a}_2\to 3a_1\to 6b$ . The analysis uses proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ . No significant excess above the Standard Model prediction is observed. The search sets upper limits at 95% confidence level on the ratio of the Higgs boson production cross section to the SM prediction times the branching ratio of Higgs bosons decaying into $4b$ or $6b$ , between 4% and 25% for $\sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to 2a\to 4b)$ , between 24% and 38% for $\sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to a_1{a}_2\to 4b)$ , and between 10% and 20% for $\sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to a_1{a}_2\to 3a_1\to 6b)$ , depending on the masses of the scalar particles.