Abstract: The inner tracking system of the ATLAS experiment will be upgraded to a full silicon detector in 2030 for HL-LHC. The new tracking system is called ITk, the Inner Tracker. The ITk requirements include operational efficiency higher than 99% and noise hit occupancy smaller than 0.1%. During the pre-production phase of the ITk project, many short-strip modules were observed to exhibit so-called “Cold Noise (CN)”, wherein clusters of strips displayed very high noise when the modules were operated at temperatures below-35 degrees C. To investigate the CN impact and ensure the quality of module production, huge amount of effort have been put in by the collaboration. This paper focuses on the impact of CN on the tracking performance by examining two short strip modules that exhibit CN: one is non-irradiated, while the other one has been irradiated to the maximum expected end-of-lifetime fluence. For each module, the global and single strip tracking performance are evaluated. The global performance study shows that the non-irradiated module can be operated within specifications with a threshold around 1fC, but it is not possible to operate the irradiated module as required. In the single strip analysis, it was found that while CN does not affect the charge collection, it reduces the operating window and leaves less margin for detector operation. In the non-irradiated module, less than 3% of strips fail the detector requirements in the CN regions. For the irradiated module, about 20% of strips fail the requirements in the low CN region and around 52% fail in the high CN region. The fraction of strips that cannot operate due to CN throughout their lifetime can be predicted according to the measured noise at the required noise occupancy level and its expected median collected charge. In cases when the noise hit occupancy caused by CN is kept below 1% with a threshold smaller than 0.45 fC, at least 60% of strips could meet the operating requirements by the end of detector's lifetime. Thus, the module is likely to satisfy the operating requirements in terms of global efficiency and global noise occupancy.

Abstract: An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in pp collisions at root s = 13 TeV at the large hadron collider, have been considered. An integrated luminosity of 3.2 fb(-1), corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 and 1050 GeV (1160 and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.

Abstract: A search for a pair of low-mass pseudoscalars a that promptly decay into tau-leptons is presented using 140 fb(-1) of proton-proton collision data at 13 TeV centre-of-mass energy recorded with the ATLAS detector at the Large Hadron Collider. The result is used to place constraints on exotic decays of the Higgs boson into four tau-leptons, H -> aa -> tau(+)tau(-)tau(+)tau(-). This search focuses on events with either one or two tau-leptons decaying into hadrons and neutrinos, and the remaining three or two tau-leptons decaying into either electron or muon and neutrinos. No significant excess is observed above the expected Standard Model background and upper limits at the 95% confidence level on B(H -> aa -> tau(+)tau(-)tau(+)tau(-)) are set ranging from 0.06 to 0.23, depending on the mass m(a) ranging from 15 to 60 GeV.

Abstract: This report presents a comprehensive collection of searches for new physics performed by the ATLAS Collaboration during the Run 2 period of data taking at the Large Hadron Collider, from 2015 to 2018, corresponding to about 140 fb(-1) of root s = 13 TeV proton-proton collision data. These searches cover a variety of beyond-the-standard model topics such as dark matter candidates, new vector bosons, hidden-sector particles, leptoquarks, or vector-like quarks, among others. Searches for supersymmetric particles or extended Higgs sectors are explicitly excluded as these are the subject of separate reports by the Collaboration. For each topic, the most relevant searches are described, focusing on their importance and sensitivity and, when appropriate, highlighting the experimental techniques employed. In addition to the description of each analysis, complementary searches are compared, and the overall sensitivity of the ATLAS experiment to each type of new physics is discussed. Summary plots and statistical combinations of multiple searches are included whenever possible. (c) 2024 CERN for the benefit of the ATLAS Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Abstract: A search for Higgs boson pair production in the b (b) over bar gamma gamma final state is performed. The proton-proton collision dataset corresponds to an integrated luminosity of 308 fb-1, consisting of two samples, 140 fb(-1) at a centre-of-mass energy of root s = 13 TeV and 168 fb(-1) at root 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 mu(HH) = 0.9(-1.1)(+1.4) (mu(HH) = 1(-1.0)(+1.3) expected), which translates into a 95% confidence-level upper limit of mu(HH) < 3.7. At the same confidence level the Higgs self-coupling modifier is constrained to be in the range -1.6 < kappa(lambda) < 6.6 (-1.8 < kappa(lambda) < 6.9 expected).