Measurement of $Z$-boson production cross sections at $\sqrt{s}$ = 13 TeV and $t\bar{t}$ to $Z$-boson cross-section ratios with the ATLAS detector at the LHC

The measurement of $Z$-boson production cross sections in $pp$ collisions at centre-of-mass energy of $\sqrt{s}$ = 13 TeV is presented. Results are based on data corresponding to an integrated luminosity of 3.16 $\text{fb}^{−1}$ recorded with the ATLAS detector in 2015 at the LHC operating with the 25 ns bunch spacing configuration. The selection criteria of the measurement are optimized to be consistent with the new measurement of the $t\bar{t}$ production cross section to maximize elimination of the experimental and theoretical systematic uncertainties for the ratio of the top-quark pair to Z-boson cross sections. Single ratios at the centre-of-mass energy of 13 TeV and double ratios at different centre-of-mass energies, including published ATLAS results for $Z$-boson and $t\bar{t}$ production at $\sqrt{s}$ = 7 TeV and 8 TeV, are reported. The results are compared to the predictions of perturbative quantum chromodynamics calculations at next-to-next-to-leading order using various sets of parton distribution functions (PDFs) and including electroweak corrections. The comparison is performed using the open-source xFitter package. The measured cross sections are used to estimate the impact on the uncertainties of the current PDF set. Moreover, they are used to extract the top-quark mass using the PDF profiling method. The latest scheduled upgrade, High-Luminosity LHC, foresees an increase of an instantaneous lumin- osity at the LHC up to $5 \times 10^{34} \text{cm}^{−2}\text{s}^{−1}$. The ATLAS tracker will need to be completely replaced with a new all-silicon tracker due to the higher radiation dose and track density. Highly modular structures will be used for the strip system, so-called staves for the barrel region and petals for the end-caps region. A small-scaled strip prototype for the petal, called petalet, consists of silicon sensors, powering and readout components placed into a low mass carbon-based core. Double-sided petalets with different sensor designs and with two different readout and powering electronics layouts are evaluated. The electrical performance of the prototypes and single modules are reported as well as a description of the assembly procedure and tools. The outcomes of the production of the petalets are summarized and the best layout is chosen as a baseline for the future production of the petal.