The Fast Track Trigger Upgrade for the ATLAS Experiment High Rate Trigger Algorithms

At LHC, in the context of the ATLAS experiment, for the so-called Phase-II, the instantaneous luminosity is expected to reach $5\cdot 10^{34} cm^{-2} s^{-1}$. In such conditions the overlapping collisions and the QCD multi-jet production makes triggering on heavy fermions (such as $b$ quarks and $\tau$ leptons) an extremely difficult challenge. The standard methods used at offline levels to select these objects such as $b$-tagging and primary vertex reconstruction require full granularity information from the tracker of the ATLAS detector and are therefore too time consuming to be implemented at trigger level. Within the ATLAS experiment, the FTK project has developed a custom processor that is capable of reconstructing all the tracks within an event after the Level-1 trigger. In fact FTK can provide real time tracking at 100 kHz using a highly parallelized and high performance system. In my thesis I will describe the FTK system in detail, in the context of the the ATLAS experiment itself. I will also summarize the FTK performances showing that the early availability of reconstructed tracks immediately after the Level-1 increases the signal efficiency while maintaining the same background rate. I will also describe my work on the development of modules that allow to monitor and test the digital logic of the Associative Memory (AM) Board, the core of the FTK processing unit. The AM board in fact, is equipped with custom memory chips (AM chips) capable of ultra-parallized pattern matching. The memories receive the hit information from the tracker they give in output coarse reconstructed tracks that make the fast high quality track fitting, done by the FTK processing unit, possible. Once the FTK system will be fully functional the High Level Trigger performance will be highly improved and it becomes important to optimize the Level-1 trigger to be able to profit at best of the improved trigger performance. For this reason a new approach on Level-1 selection of purely hadronic decays of the Higgs boson will be described in the second part of the thesis. The new strategies rely on global features of the event and not only in single thresholds. The Randall-Sundrum Graviton with a mass of 500 GeV and decaying to a couple of Standard Model Higgs each one decaying to $b\bar{b}$ has been used as benchmark signal. The trigger algorithms developed follow two major strategies: the tagging of calorimetric clusters at Level-1 with muons from the semileptonic decay of the $b$ quark, done trough the exploitation of the newly installed topological trigger, and the selection based on clean multi-jet topological and kinematic configuration. Efficiencies on signal selection and background selection are calculated for both selection algorithms

PolarLight: a CubeSat X-ray Polarimeter based on the Gas Pixel Detector

The gas pixel detector (GPD) is designed and developed for high-sensitivity astronomical X-ray polarimetry, which is a new window about to open in a few years. Due to the small mass, low power, and compact geometry of the GPD, we propose a CubeSat mission Polarimeter Light (PolarLight) to demonstrate and test the technology directly in space. There is no optics but a collimator to constrain the field of view to 2.3 degrees. Filled with pure dimethyl ether (DME) at 0.8 atm and sealed by a beryllium window of 100 micron thick, with a sensitive area of about 1.4 mm by 1.4 mm, PolarLight allows us to observe the brightest X-ray sources on the sky, with a count rate of, e.g., ~0.2 counts/s from the Crab nebula. The PolarLight is 1U in size and mounted in a 6U CubeSat, which was launched into a low Earth Sun-synchronous orbit on October 29, 2018, and is currently under test. More launches with improved designs are planned in 2019. These tests will help increase the technology readiness for future missions such as the enhanced X-ray Timing and Polarimetry (eXTP), better understand the orbital background, and may help constrain the physics with observations of the brightest objects.Comment: Accepted for publication in Experimental Astronom

Author Correction: Re-detection and a possible time variation of soft X-ray polarization from the Crab

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X-ray Polarimetry of the accreting pulsar 1A~0535+262 in the supercritical state with PolarLight

The X-ray pulsar 1A 0535+262 exhibited a giant outburst in 2020, offering us a unique opportunity for X-ray polarimetry of an accreting pulsar in the supercritical state. Measurement with PolarLight yielded a non-detection in 3-8 keV; the 99% upper limit of the polarization fraction (PF) is found to be 0.34 averaged over spin phases, or 0.51 based on the rotating vector model. No useful constraint can be placed with phase resolved polarimetry. These upper limits are lower than a previous theoretical prediction of 0.6-0.8, but consistent with those found in other accreting pulsars, like Her X-1, Cen X-3, 4U 1626-67, and GRO J1008-57, which were in the subcritical state, or at least not confidently in the supercritical state, during the polarization measurements. Our results suggest that the relatively low PF seen in accreting pulsars cannot be attributed to the source not being in the supercritical state, but could be a general feature.Comment: accepted for publication in Ap

The Imaging X-ray Polarimetry Explorer (IXPE): Technical Overview

The Imaging X-ray Polarimetry Explorer (IXPE) will expand the information space for study of cosmic sources, by adding linear polarization to the properties (time, energy, and position) observed in x-ray astronomy. Selected in 2017 January as a NASA Astrophysics Small Explorer (SMEX) mission, IXPE will be launched into an equatorial orbit in 2021. The IXPE mission will provide scientifically meaningful measurements of the x-ray polarization of a few dozen sources in the 2-8 keV band, including polarization maps of several x-ray-bright extended sources and phase-resolved polarimetry of many bright pulsating x-ray sources

DESIGN AND DEVELOPMENT OF A NOVEL ELECTRONIC SYSTEM FOR THE IMAGING X-RAY POLARIMETRY EXPLORER SPACE TELESCOPE

La Tesi di dottorato descrive le attività svolte dal candidato nell’ambito della missione spaziale NASA Imaging X-ray Polarimetry Explorer (IXPE) che si propone di lanciare un telescopio spaziale per raggi X in orbita nel dicembre del 2021. Le attività descritte consistono nello sviluppo di un sistema di acquisizione e processamento dei dati provenienti da un rivelatore di fotoni X, il Gas Pixel Detector (GPD) sviluppato dall’INFN (Istituto Nazionale di Fisica Nucleare) di Pisa. Questo rivelatore è in grado di misurare la polarizzazione dei raggi X incidenti, ed il sistema sviluppato, premetterà di digitalizzare e comprimere i dati provenienti dal GPD, tramite l’impiego di una FPGA resistente alle radiazioni, per il successivo invio a terra. Oltre alle attività di progettazione sono anche descritte le attività di sviluppo del sistema di test e la successiva campagna di caratterizzazione del sistema. The Doctoral Thesis describes the activity done by the candidate in the context of the Imaging X-ray Polarimetry Explorer (IXPE) NASA space mission. The mission objective is to put in orbit an X-ray telescope by December 2021. The thesis describes the development of the data acquisition and processing system for the Gas Pixel Detector (GPD), i.e. a novel detector capable of measuring the polarization of X-rays which has been developed by the (National Institute of Nuclear Physics) INFN of Pisa. The developed system uses a radiation-hardened FPGA to perform data processing, before the downlink to earth. Besides the design activities, the thesis also describes the activity done for the development of the ground test equipment and the measurement campaign which aimed to the characterization of the data acquisition system