publications

2025

1. A measurement of the high-mass \(\tau\bar{\tau}\) production cross-section at \(\sqrt{s}=13\) TeV with the ATLAS detector and constraints on new particles and couplings

   The ATLAS Collaboration

   Mar 2025

   Abs DOI

   The production cross-section of high-mass \(\tau\)-lepton pairs is measured as a function of the dilepton visible invariant mass, using 140 fb\(^{-1}\) of \(\sqrt{s}=13\) 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 \(\tau\)-lepton.

2023

1. Detector Development and Analysis Techniques for Finding Leptoquarks with the ATLAS Detector at the LHC

   Roman Müller

   University of Bern, Mar 2023

   PhD thesis, presented 20th October 2023

   Abs DOI

   The LHC and its experiments at CERN constitute the largest particle physics research programme to date, allowing for extensive studies of the existing Standard Model (SM) and for potential evidence of physics beyond that of our current comprehension. This thesis presents an analysis technique to find third generation Leptoquarks (LQ) in high mass \(\tau^+\tau^-\) final states. The sensitivity on the coupling strength of a leptoquark with a chosen mass of 1.5 TeV (2 TeV) is \(g^2 = 6.61^{+1.13}_{-0.95}\) (\(g^2 = 10.98^{+1.92}_{-1.65}\)). This is calculated using the total transverse mass of an event, \(m_{T}^{\text{total}}\). In order to enhance the accuracy of tests on the Standard Model and to broaden the potential for uncovering new realms of physics such as leptoquarks, the LHC will be upgraded to the High-Luminosity LHC (HL-LHC). This thesis presents the development of the Optoboard System - the part of the ATLAS new Inner Tracker (ITk) Pixel Detector readout system that handles the transfer of data, command and trigger between the modules and the backend. All ITk Pixel Detector modules are assigned and mapped to the Optoboard System with Twinax cables length between 3016 mm and 5776 mm. Compatible with requirements in data transmission reliability, the Optoboard System is validated first with jitter measurements and bit error rate tests, reaching the desired \(BER_\text{95%} < 2.7 \times 10^{-12}\) and second, with Sr-90 radiation at the ITk Pixel system test site with the Outer Barrel (OB) demonstrator.

2. Test of the Optosystem for the ATLAS ITk data transmission chain

   D.D. Santo, A. O'Neill, L. Franconi, I. Mateu, R. Müller, L. Halser, S. Juillerat, M. Weber, F. Munoz Sanchez, and F. Rizatdinova

   Journal of Instrumentation, Mar 2023

   Abs DOI

   After Run III the ATLAS detector will undergo a series of upgrades to cope with the harsher radiation environment and increased number of proton interactions in the High Luminosity LHC. One of the key projects in this suite of upgrades is the ATLAS Inner Tracker (ITk). The pixel detector of the ITk must be read out accurately and with extremely high rate. The Optosystem performs electrical-to-optical conversion of signals from the pixel modules. We present a general overview on the design of the Optosystem and recent results related to the performance of the data transmission chain, pivoted on the Optoboards, and to the radiation hardness of the ASICs housed on it.

3. System tests of the ATLAS ITk planar and 3D pixel modules

   Roman Müller, and ATLAS ITk collaboration

   Journal of Instrumentation, Mar 2023

   Abs DOI

   In order to validate the design of the new all-silicon Inner Tracker (ITk) for ATLAS for the HL-LHC, a series of system tests has been performed, to assess the performance of prototype planar and 3D pixel modules arranged into serial power chains mounted onto realistic mechanical structures. In this paper, the prototype loaded local supports and test infrastructure is described and the key results presented.

2022

1. A facility for radiation hardness studies based on a medical cyclotron

   John Anders, Saverio Braccini, T.S. Carzaniga, Pierluigi Casolaro, Meghranjana Chatterjee, Gaia Dellepiane, Laura Franconi, Lea Halser, Armin Ilg, Isidro Mateu, Federico Meloni, Claudia Merlassino, Antonio Miucci, Roman Müller, Marco Rimoldi, and Michele Weber

   Journal of Instrumentation, Apr 2022

   Abs DOI

   The development of instrumentation for operation in high-radiation environments represents a challenge in various research fields, particularly in particle physics experiments and space missions, and drives an ever-increasing demand for irradiation facilities dedicated to radiation hardness studies. Depending on the application, different needs arise in terms of particle type, energy and dose rate. In this article, we present a versatile installation based on a medical cyclotron located at the Bern University Hospital (Inselspital), which is used as a controlled 18-MeV proton source. This accelerator is used for daily production of medical radioisotopes, as well as for multidisciplinary research, thanks to a 6.5-meter long beam transfer line that terminates in an independent bunker, dedicated only to scientific activities. The facility offers a wide range of proton fluxes, due to an adjustable beam current from approximately 10 pA to the micro-ampere range, together with a series of steering and focusing magnets along the beamline that allow for the beam spot to be focused down to a few mm\(^2\). The beamline can be instrumented with a variety of beam monitoring detectors, collimators, and beam current measurement devices to precisely control the irradiation conditions. The facility also hosts a well equipped laboratory dedicated to the characterisation of samples after irradiation. An experimental validation of the irradiation setup, with proton fluxes ranging from \(5 \times 10^9 cm^{-2}s^{-1}\) to \(4 \times 10^{11} cm^{-2}s^{-1}\), is reported.