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: This paper presents two searches for the electroweak production of higgsinos with compressed mass spectra using 140 fb(-1) of root s = 13TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider. Events are required to feature an energetic jet, large missing transverse momentum, and at least one low-momentum charged particle that serves as a candidate higgsino decay product. In the first search, targeting higgsino mass splittings in the range of 0.3-1 GeV, the higgsinos are expected to predominantly decay into pions that are identified as low-momentum charged particles with large transverse impact parameters due to the long higgsino lifetime (c tau approximate to O(0.1-10mm)), and neural networks are used to discriminate between signal and background processes. The second search targets larger mass splittings in the range of 1-3 GeV, where the higgsinos are expected to decay promptly into low-momentum leptons, one of which is identified by dedicated low-momentum electron or muon taggers based on neural networks utilising tracking and calorimeter information. No significant excess above the Standard Model prediction is observed in either search and the results are interpreted within simplified models, to set lower limits on the masses of the higgsino-like charginos and neutralinos. Together, these searches exclude chargino masses below 126 GeV at 95% confidence level for mass splittings between the chargino and lightest neutralino in the range of 0.3-2 GeV. This represents the first ATLAS constraints in a portion of this parameter space and surpasses the limits previously set by other experiments.

Abstract: Low-lying states of Zr-94 were investigated via low-energy multi-step Coulomb excitation. From the measured gamma-ray yields, 16 reduced E2 transition probabilities between low-spin states were determined, together with the spectroscopic quadrupole moments of the 2(1,2)+ states. Based on this information, for the first time in the Zr isotopic chain, the shapes of the 0(1,2)(+) states including their deformation softness were inferred in a model-independent way using the quadrupole sum rules approach. The ground state of Zr-94 possesses a rather diffuse shape associated with a spherical configuration, while the 0(2)(+) state is triaxial tending towards oblate and more strongly deformed. The observed features of shape coexistence in Zr-94 are consistent with both Monte-Carlo shell-model predictions and IBM-CM calculations, and provide model-independent constraints on the shape character assigned in the IBM-CM to the intruder configuration in Zr92-96.