Category: Theses Opportunities

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Ricardo Gonçalo

CO-SUPERVISOR: Yvonne Peters

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade de Coimbra

ABROAD-INSTITUTION: University of Manchester

ABSTRACT

After an intense search, the associated production of the Higgs boson with a top quark pair was finally observed in 2018. This production channel provides the best way to directly measure the coupling between Higgs and top, the heaviest fundamental particles in the Standard Model (SM). But it also provides a way to look beyond the SM, in particular to search for signs of a non-standard Higgs boson, leading to CP-violation in the Higgs sector. Such a component is well justified in scenarios like the two-Higgs doublet model, and finding it would constitute a major discovery. In particular, it could lead to understanding why there is a huge asymmetry between matter and antimatter in the Universe. In this leading edge research, the selected student will analyze data collected by the ATLAS experiment during the LHC Run 3, to start in 2021. He or she will have the opportunity to participate in the operation of the experiment at CERN. The student will integrate the Portuguese ATLAS team and will use recent techniques developed in our group to enhance the experimental sensitivity of the analysis. He or she will be co-supervised by a colleague from the University of Manchester, where the student will spend part of the time of the PhD, as foreseen in the grant. Besides the Manchester group, the student would work in a vibrant international team within the ATLAS collaboration.  

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Rute Pedro

CO-SUPERVISOR: Nuno Castro

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade do Minho

ABSTRACT

The Standard Model (SM) of Particle Physics is notably descriptive and predicted new particles well in advance, from which the Higgs boson discovered at CERN’s Large Hadron Collider (LHC) is a remarkable recent case. However, there is paramount evidence for the need of beyond-Standard Model (BSM) physics, namely to provide dark matter candidates, explain the matter/dark-matter asymmetry, address the hierarchy problem and others. The LHC has a rich program on searches for New Physics (NP) but clues from new particles or interactions have not yet been located. Typical searches are guided by specific BSM candidates and would benefit from a complementary model-independent strategy, augmenting the scope of searches to signs of NP not even framed by theory. This proposal is to perform a novel generic search for NP within the ATLAS/LHC experiment using anomaly detection (AD) techniques based on generative Deep Learning (DL). The DL model will learn SM physics from simulated data and then look for anomalous non-SM like events in the real collision data. Detector effect anomalies can mislead the NP detection and a detailed study of this background will be considered to construct a high fidelity AD. Moreover, the impact of sources of theoretical and experimental uncertainties on the AD performance will be assessed. Benchmark NP signals will be used as tests throughout the AD development. The project will be integrated into the ATLAS Portuguese group, and collaboration with several international groups is foreseen. Synergies with the LIP Competence Centre for Simulation and Big Data and with the LIP Phenomenology group will be explored, namely to investigate approaches for experimental result interpretability and recasting into theory exclusion limits.  

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Patricia Conde Muino

CO-SUPERVISOR: Nuno Castro

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade de Lisboa

ABSTRACT

The non-abelian structure of the gauge theory in the SM implies the existence of triple and quartic gauge boson couplings (TGC and QGC, respectively) fully constrained by the gauge symmetry. The measurement QGC provide a window into the Electroweak symmetry breaking mechanism, given the fact that the longitudinal modes of the W and Z bosons are Goldstone bosons. Deviations from the SM predictions can appear due to interchange of new particles, integrated out in the effective interaction, in new physics theories. Models with a new heavy scalar singlet interacting with the Higgs sector can modify the quartic gauge boson couplings but not the triple ones. It is, therefore, essential to probe this missing part of the SM, measuring both the TGC and QGC.

  The ATLAS sensitivity to anomalous couplings in the γγWW, γγγγ and γγZZ vertices can be improved two orders of magnitude by using the ATLAS Forward Proton tagging detectors (AFP) [1]. AFP effectively converts the LHC into a photon-photon collider: the two scattered protons emit two photons that annihilate to produce a pair of vector bosons (two W’s, for instance). The protons, that stay intact after the interaction, are scattered through very small angles and they are detected at the AFP stations. Since there is no underlying event, the two vector bosons are the only particles produced centrally. If they decay to leptons they can be easily triggered and identified. The invariant mass of the vector boson pair can be measured precisely by determining the proton energy loss with the AFP detectors, even in the case of neutrinos in the final state. The presence of anomalous quartic gauge boson couplings could be observed as an increase in the number of detected vector boson pairs with large invariant masses. This project proposes the search for anomalous couplings of the type γγWW using the AFP detectors. The same final state can be used to search for dark matter in photon-induced processes, using also the capability of the forward proton tagging detectors [2]. The search for dark matter is challenged due to the low transverse momentum of the leptons produced. An adequate strategy for triggering this kind of processes is therefore needed. It implies the combination of proton tagging information with muon/electron triggers reconstructed with the ATLAS central detectors, already at the first level trigger and probably making use of the topological trigger processors. The development and optimisation of such a trigger strategy is also an objective of this project. The student is will develop the work in the framework of the ATLAS international collaboration. S/he is expected to contribute to the ATLAS data taking activities and also the commissioning and performance studies of the AFP detector, fundamental for the success of this project. Frequent presentations of the results achieved are expected, either in collaboration meetings at CERN or by videoconference. 7.- References [1] E. Chapon, C. Royon, and O. Kepka, Anomalous quartic WWγγ, ZZγγ, and trilinear WWγ couplings in two-photon processes at high luminosity at the LHC, Phys.Rev. D81 (2010) 074003, arXiv:0912.5161 [hep-ph]. [2] L.A. Harland-Lang, V.A. Khoze, M.G. Ryskin and M. Tasevsky, LHC Searches for Dark Matter in Compressed Mass Scenarios: Challenges in the Forward Proton Mode, arXiv:1812.04886 [hep-ph]. 

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Patricia Conde Muino

CO-SUPERVISOR: Rui Santos

CO-SUPERVISOR: Valerio Dao

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade de Lisboa

ABROAD-INSTITUTION: CERN

ABSTRACT

Since the discovery of the Higgs boson, the precise measurement of its properties has become a fundamental part of the ATLAS Physics programme. The recent announcement of the observation of the Higgs decay to b-quarks and the associated production of the Higgs with top quarks, done by the ATLAS and CMS collaborations at CERN, probe directly the coupling of the Higgs to quarks and constituted an important step forward in the understanding of the Higgs mechanism. As the LHC continues to take data and more luminosity is accumulated, more precise measurements of the Higgs boson properties are possible, opening the door to search for new physics in the Higgs sector. Along this line of research, the study of the high transverse momentum (high-pT) Higgs production, in the associated production channel with a W boson, is sensitive to new physics in the hWW vertex and constitutes one of the measurements to be done in the near future by the ATLAS collaboration. The main objective of this research project is to search for anomalous Spin and CP components of the interaction vertex between the Higgs and the W bosons. The measurement will be done in the associated production channel of the Higgs with a W, when the W decays to leptons (𝓵𝜈) and the Higgs decays to b-quark pairs. Being affected by different systematics and kinematic constraints with respect to the h→WW decay, this channel allows probing the spin and CP properties of the hWW vertex in a complementary way to the studies done so far. The work plan foresees the identification of the most sensitive observables for anomalous couplings, followed by the analysis of the full Run-2 ATLAS dataset. The results will be interpreted in the framework of the Effective Field Theory (EFT) approach. At the same time the results will be interpreted in terms of extensions of the Standard Model with extra sources of CP-violation. The student will be part of the ATLAS team participating in this analysis. The work will be developed in an international collaboration and the results obtained will be presented at CERN. The student is expected to contribute in addition to the data taking and detector operation activities, both at CERN and at LIP. This PhD grant foresees a two-year stay at CERN to develop the proposed research project. 

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Ricardo Gonçalo

CO-SUPERVISOR: Konstantinos Nikolopoulos

CO-SUPERVISOR: Filipe Veloso

HOST INSTITUTION: LIP

DEGREE INSTITUTION: Universidade de Coimbra

ABROAD-INSTITUTION: University of Birmingham

ABSTRACT

Since its discovery, the Higgs boson became a prime tool to search for physics beyond the Standard Model (SM). At the current level of precision, the Higgs boson is compatible with SM expectations. A number of open questions suggest the existence of new physics, that could be unveiled as we explore the LHC data. A wealth of experimental results from the ATLAS and CMS experiments, probe the region around the minimum of the Higgs potential, or vacuum. But the shape of this potential is not constrained experimentally. This shape is intimately connected to the breaking of the electroweak gauge symmetry, which resulted in the fundamental forces we experience today. To experimentally constrain this shape we must measure the Higgs boson self-coupling, which is accessible at the LHC through the simultaneous production of two Higgs bosons. The selected student will join the Portuguese ATLAS team, working in close collaboration with theorists and integrated into our international collaboration. He or she will be able to contribute to enhancing our current knowledge in this important area, which will become one of the most important measurements of LHC experiments. The student will also employ the latest theory developments and the most recent advances in reconstruction techniques: from boosted object identification to machine learning. Part of this research will be done at the University of Birmingham, in the United Kingdom, in co-supervision with a colleague from the ATLAS collaboration. The successful student will be able to participate in the operation of the ATLAS experiment during the LHC Run 3 to start in 2021, and travel to CERN will be required. 

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Filipe Veloso

CO-SUPERVISOR: Ricardo Gonçalo

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade de Coimbra

ABSTRACT

The top quark is the heaviest elementary particle known, with a mass close to the electroweak symmetry breaking, and it can provide clues about the symmetry breaking and the Higgs mechanisms. It is thus an excellent object to test the Standard Model of Particle Physics (SM). There is an important effort to study the top quark properties, like its mass, production cross-sections, electric charge, spin, decay asymmetries, rare decays, etc… Deviations from SM predictions of the production and decay properties of the top quark provide model-independent tests for physics beyond the SM. According to the SM, the top quark decays nearly 100% of the time to a W boson and a b quark. The Cabbibo-Kobayashi-Maskawa (CKM) quark mixing matrix is related to the rates of the Flavour Changing Charged Current (FCCC) decay modes. Some of the elements, e.g. Vts, were not yet directly measured but are determined from the unitarity conditions of the matrix. A direct measurement of these elements put strict conditions on the assumptions behind the matrix properties, as the existence of only three families on the SM. This research program will be developed within the Portuguese ATLAS group. It aims to measure the decay rate of the top quark into a W boson plus a s-quark with LHC data collected by the ATLAS detector using computational tools such as machine learning techniques. This result will then be used to measure the amplitude of the CKM element Vts. In addition the student will participate in the maintenance and operation of the ATLAS detector, namely in the calibration of the TileCal hadronic calorimeter. Short periods at CERN may also be required in order to collaborate in working meetings and/or shifts.

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Helena Santos

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade de Lisboa

ABSTRACT

Context: Ultrarelativistic nucleus-nucleus collisions at the Large Hadron Collider (LHC) provide an unique opportunity to recreate the Quark Gluon Plasma (QGP) in the laboratory energy frontier. This plasma of quarks and gluons, which is known to behave as a nearly perfect liquid, was the prevailing state of the Universe shortly after the Big Bang. The capabilities of ATLAS, namely large acceptance and high granularity calorimeters, afford excellent handles for QGP studies. A major goal of the Heavy Ion Program of the LHC is the understanding of the effects of the QGP on jets, namely the study of the nature of the energy loss suffered by the quarks and gluons while crossing the QGP. The bottom quark, in particular, constitutes an excellent probe. Due to its large virtuality, Q, it has a formation time,∆t ≈1/Q ≈ 0.1 fm/c, much smaller than the formation time of the QGP at the LHC (≈10 fm/c). The understanding of the nature of the energy loss (either collisional or gluon radiative), by its turn, is crucial to infer the properties of the QGP. The HI ATLAS/LIP group is contributing with important developments preceding the b-jet physics analysis, namely on b-jet reconstruction, b-tagging and b-jet triggers in HI collisions. 

Objectives: The student will participate in Pb+Pb data acquisition at CERN already during Run 3 (expected for the Falls of 2021-22-23-24) and will analyse the data. The analysis will explore strategies, namely machine learning techniques, to separate the b-quarks jets from light jets (mostly u- and d-quarks) and further separation of b-quarks jets originating directly from the hard process from those arising from gluon splitting with the aim of devising novel experimental observables sensitive to the different energy loss of quarks and gluons. 

Requirements: This is an experimental PhD program. The work will be developed at LIP – Laboratorio de Instrumentacao e Fisica Experimental de Particulas. The student should have solid computing skills, namely in C++ programming. Furthermore, she/he will concurrently participate in the technical activities in which the ATLAS/LIP group is involved, namely in the Tile calorimeter and/or in Trigger systems.

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Helena Santos

CO-SUPERVISOR: Patricia Conde Muino

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade de Lisboa

ABSTRACT

Context: Ultrarelativistic nucleus-nucleus collisions at the Large Hadron Collider (LHC) provide an unique opportunity to recreate the Quark Gluon Plasma (QGP) in the laboratory energy frontier. This plasma of quarks and gluons, which is known to behave as a nearly perfect liquid, was the prevailing state of the Universe shortly after the Big Bang. The capabilities of ATLAS, namely large acceptance and high granularity calorimeters, afford excellent handles for QGP studies. The ATLAS experiment is strongly committed with the HI program of LHC and great expectations on the capabilities of the Upgrade to bring deeply understanding on the nature of the QGP are raised. A major goal of the Heavy Ion Program of the LHC is the understanding of the effects of the QGP on jets, namely the study of the nature of the energy loss suffered by the quarks and gluons while crossing the QGP. The bottom quark, in particular, constitutes an excellent probe. Due to its large virtuality, Q, it has a formation time,∆t ≈1/Q ≈ 0.1 fm/c, much smaller than the formation time of the QGP at the LHC (≈10 fm/c). The understanding of the nature of the energy loss (either collisional or gluon radiative), by its turn, is crucial to infer the properties of the QGP. The HI ATLAS/LIP group is contributing with important developments preceding the b-jet physics analysis, namely on b-jet reconstruction, b- tagging and b-jet triggers in HI collisions. 

Objectives: The goal of the proposed thesis is the prospective study of the Heavy Flavour jets production in the HL-LHC phase (expected to start in 2027) benefiting from the 1 order of magnitude increased luminosity foreseen for the Pb+Pb runs. The most important ATLAS upgraded components for the proposed project are the calorimeters front-end electronics and the new tracker detector. Currently jets are reconstructed using the transverse energy of calorimeter towers (piled cells) as input signals, after subtracting the QCD underlying event. The new readout electronics of the calorimeters will provide support for a more sophisticated detector signal processing. The remaining part of jet reconstruction regards the identification of the b-jets, i.e. b-tagging. This technique aims the highest possible efficiency for tagging b-jets, with the largest possible rejection of light jets. In ATLAS the most used techniques take advantage of the relatively long lifetime of hadrons containing bottom quarks (ctau 450 mm), as well as of the hard fragmentation and the high mass of these hadrons. These properties lead to tracks in the ITk with large impact parameters (i.e., transverse and longitudinal distances of closest approach of the track to the primary and secondary vertices), on contrary to the tracks from light jets. Such a feature allows to disentangle heavy flavour jets from light jets, but it requires excellent impact parameter resolution. This is ensured by the ITk. Machine learning techniques using the properties of both the impact parameters and the secondary vertices have proven to increase significantly the b-tagging performance in pp collisions and the development in Pb+Pb is ongoing. Analysis of data taken in Run 2 and Run 3 of LHC will provide not only results on Heavy Flavour jets in HI collisions per se, but will also contribute preciously to the validation of the prospective study in the HL-LHC. 

Requirements: This is an experimental PhD program. The work will be developed at LIP – Laboratorio de Instrumentacao e Fisica Experimental de Particulas. The student should have solid computing skills, namely in C++ programming. Furthermore, she/he will concurrently participate in the technical activities in which the ATLAS/LIP group is involved, namely in the Tile calorimeter and/or in Trigger systems.