Future Colliders

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:

A search for a pseudoscalar a produced in association with a top-quark pair, or in association with a single top quark plus a W boson, with the pseudoscalar decaying into b-quarks (a→bb¯$$a\rightarrow b\bar{b}$$), is performed using the full Run 2 data sample using a dileptonic decay mode signature. The search covers pseudoscalar boson masses between 12 and 100 GeV and involves both the kinematic regime where the decay products of the pseudoscalar are reconstructed as two standard b-tagged small-radius jets, or merged into a large-radius jet due to its Lorentz boost. No significant excess relative to expectations is observed. Assuming a branching ratio BR(a→bb¯)=100%$$\text {BR}(a\rightarrow b\bar{b})=100\% $$, the range of pseudoscalar masses between 50 and 80 GeV is excluded at 95% confidence level for a coupling of the pseudoscalar to the top quark of 0.5, while a coupling of 1.0 is excluded at 95% confidence level for the masses considered, with the coupling defined as the strength modifier of the Standard Model Yukawa coupling.

Abstract:

Measurements of the total and differential Higgs boson production cross-sections, via WH and ZH associated production using H → WW* → ℓνℓν and H → WW* → ℓνjj decays, are presented. The analysis uses proton-proton events delivered by the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector between 2015 and 2018. The data correspond to an integrated luminosity of 140 fb−1. The sum of the WH and ZH cross-sections times the H → WW* branching fraction is measured to be 0.44−0.09+0.10stat.−0.05+0.06syst. pb, in agreement with the Standard Model prediction. Higgs boson production is further characterised through measurements of the differential cross-section as a function of the transverse momentum of the vector boson and in the framework of Simplified Template Cross-Sections.

Abstract:

Jets with different radius parameters R are an important tool for probing quantum chromodynamics processes at different angular scales. Jets with small R=0.2$$R=0.2$$ are instrumental in measurements of the substructure of large-R jets resulting from collimated hadronic decays of energetic W,  Z,  and Higgs bosons, top quarks, and of potential new resonances. This paper presents measurements of the energy scale, resolution, and associated uncertainties of jets with radius parameters R=0.2$$R=0.2$$ and 0.6,  obtained using the ATLAS detector. The results are based on 37fb-1$$37~\text {fb}^{-1}$$ of proton–proton collision data from the Large Hadron Collider at a centre-of-mass energy of s=13$$\sqrt{s}=13$$ TeV. A new in situ method for measuring jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets (|η|<1.2)$$(|\eta | < 1.2)$$ typically vary from 1% to about 5% as a function of |η|$$|\eta |$$ at very low transverse momentum, pT,$$p_{\text {T}},$$ of around 20 GeV for both R=0.2$$R=0.2$$ and 0.6 jets. The relative energy resolution ranges from (35±6)%$$(35 \pm 6)\%$$ at pT=20$$p_{\text {T}} = 20$$ GeV to (6±0.5)%$$(6 \pm 0.5)\%$$ at pT=300$$p_{\text {T}} = 300$$ GeV for central R=0.2$$R=0.2$$ jets, and is found to be slightly worse for R=0.6$$R=0.6$$ jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.

Abstract:

A search is performed for long-lived heavy neutral leptons (HNLs), produced through the decay of a W boson along with a muon or electron. Two channels are explored: a leptonic channel, in which the HNL decays into two leptons and a neutrino, and a semi-leptonic channel, in which the HNL decays into a lepton and a charged pion. The search is performed with 140 fb−1 of s = 13 TeV proton-proton collision data collected by ATLAS during Run 2 of the Large Hadron Collider. No excess of events is observed; Dirac-like and Majorana-like HNLs with masses below 14.5 GeV and mixing coefficients as small as 10−7 are excluded at the 95% confidence level. The results are interpreted under different assumptions on the flavour of the leptons from the HNL decays.