Journée des thèses : ATLAS 1ère partie
par , , , ,
Amphithéâtre
CPPM
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10:00 - 10:15 : Overview of the ATLAS activities (Yann Coadou)
10:15 - 11:15 : PhD student presentations
Reconstruction of the ttH(bb) single lepton channel using multivariate technique
Royer Edson TICSE TORRES
The observation of the associated production of a Higgs boson with a top quark pair would allow for a direct measurement of the top Yukawa coupling at the LHC. A method to reconstruct the single lepton ttH(bb) system finding good variables to discriminate the main background tt+bb will be presented.
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Study of the Higgs boson coupling to the top quark and of the b-jet identification with the ATLAS experiment at the Large Hadron Collider.
Thomas CALVET
In July 2012, the ATLAS and CMS collaborations announced the discovery of a Higgs boson using data collected at the Large Hadron Collider (LHC) at CERN, Geneva. The associated production of the Higgs with a top-quark pair, pp -> ttH, will be accessible for the first time with the ongoing Run 2 of the LHC. This is the only channel that allows a direct measurements of the top-Higgs coupling at the LHC. The subject of my thesis is to study the ttH(H->bb) channel, when the Higgs decay into two b-quarks. One of the main background of the ttH(H->bb) analysis is the tt+bb production. It is thus important to precisely understand this background. With that aim I participated at the comparison of different tt generator to better understand the tt+bb uncertainties.
For both these analysis, the identification of jets originating from b-quarks is crucial. I was involved in the b-tagging studies for the ATLAS collaboration. I participated to the study of the b-jet definition on which the b-tagging rely to estimate its performances. More specifically, the ttbb background can be produced with a b-quark pair merge in a single jet. This background is not well modeled by the currently available generators and should be constrained in data. I then worked on the optimization of an algorithm allowing to discriminate jets contain a single b-hadrons from jets containing two b-hadrons.
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Electron reconstruction efficiency and top Yukawa coupling measurement using ttH events with two same sign leptons in ATLAS
Asma HADEF
Precise measurements of the electron reconstruction efficiency in data are crucial for all physics analysis involving electrons in ATLAS.
The data-driven results are obtained from the Zee Tag and Probe (T&P) method and based on full statistics of 2015 data. Overall, the reconstruction efficiencies vary from 97% to ~100% and scale factors are consistent with 1 in all pT and eta bins. A layout of work foreseen on the tth analysis is also included.
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Search for the Higgs boson decaying to a pair of bottom quarks and produced in association with a pair of top quarks in the LHC Run 2 data with the ATLAS detector
Yulia RODINA
In 2012 the ATLAS and CMS collaborations announced the discovery of a Higgs boson by studying proton-proton collisions at center-of-mass energies of 7 and 8 TeV at the Large Hadron Collider (LHC). This particle was theoretically predicted by R. Brout, F. Englert and P.W. Higgs in 1964. Its discovery was the key achievement of the first LHC run (Run 1, 2010-2012) and represents a milestone in particle physics, as the Higgs boson was the last missing part of the Standard Model (SM).
One of the main goals of the LHC Run 2, which started in 2015, is to test the nature of this recently discovered particle. Further data will allow in-depth investigation of the boson's properties – in particular its couplings to SM particles. Of particular relevance is the coupling of the Higgs boson with the top quark. The top quark is the heaviest SM fermion and therefore presents the largest coupling to the Higgs boson. A direct measurement of the Higgs-top coupling can be achieved by measuring the rate of associated production of a Higgs boson with top quarks (ttH). This production mode will be accessible for the first time with the large datasets anticipated during the LHC Run 2 (2015-2018), amounting to about 100/fb of proton-proton collisions at a center-of-mass energy of 13TeV.
The proposed research project aims at establishing the observation of the ttH process with the Higgs boson decaying into a pair of bottom quarks, ttH(H→bb), using Run 2 data recorded with the ATLAS experiment. This search channel is very important since the H→bb branching ratio is the largest for Higgs decay in the SM. From such measurement, the related Higgs boson couplings can be extracted to test the nature of the discovered boson. Two key aspects of this measurement will be the identification of b-quark initiated jets and the ability to separate the signal of interest from the overwhelming background from tt+jets production.
During the first year of my PhD (Autumn 2014-Autumn 2015), I was involved in work on the optimization of b-tagging algorithms. Their goal is to identify jets coming from the fragmentation of b-quarks. B-tagging is used in a wide range of physics analyses involving b‐quarks in the final state: top physics, Higgs physics, and beyond Standard Model searches. It is a very important tool for the ttH(bb) analysis, as there are four b-jets in the final state. For this analysis, high b-jet efficiency with the highest-possible light-jet rejection is required. B-tagging algorithms rely on special properties of B-hadrons, especially their long lifetime (~1.5 ps), that results in charged particles from their decay appearing displaced from the collision point. There are different types of b-tagging algorithms, exploiting either the impact parameter of tracks, the reconstructed secondary vertices, the presence of soft leptons, and even combining all this information using multivariate techniques. I focussed on the optimization of the so-called IP2D/IP3D algorithms, which are based on the impact parameter – distance of closest approach between the primary vertex and the track. A key aspect of their optimization for Run 2 is to take advantage of the Insertable B-Layer (IBL) - the new layer of ATLAS pixel detector, which is located close to beam pipe (3.3 cm). Thanks to the IBL, the performance of IP2D/IP3D can be improved, but in order to do so, new track variables
should be taken into consideration and the algorithms should be optimized with respect to these variables. I worked on developing a new categorization of tracks for the IP3D algorithm, using no less than 14 categories, depending on the track's quality. This categorization is currently used by IP2D/IP3D for the first data taken at Run 2 instead of the 5 categories that were used in Run 1, and is expected to result in a significantly improved performance.
During the 2nd year, starting in Autumn 2015, I am starting work on the analysis of the first data taken in Run 2 towards the observation of the ttH(bb) process. This work will be conducted within a team of scientists from IFAE and CPPM. The increased center-of-mass energy in Run 2 results in a production cross-section for ttH about 4.5 times higher than in Run 1, yet it is not large. In addition, the presence of many jets in the final state results in a large combinatorial background, and the channel suffers from a very large tt+jets background, particularly from events where the top-quark pair is produced in association with a b-quark pair. Those challenges will have to be addressed, and I will be in charge of exploring several avenues to this end. In particular, studies will be performed to exploit the combination of b-tagging information and kinematic reconstruction in order to construct a high-performance discriminating variable between signal and background.
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Commissioning of the Pixel Detector after the IBL Insertion, and SUSY Search Using the Two Same Sign Leptons Channel
Mahmoud ALSTATI
I will talk about the commissioning studies that used cosmic data collected during before Run 2 started. The studies included different pixel cluster properties such as: cluster size, charge, ToT and Lorentz Angle. It also includes some studies that focus on temperature effects on Lorentz Angle. A study of the Pixel B-Layer response during Run 1 is also included. Finally, a brief word is said about SUSY with 2 same sign leptons search, an activity in which I was recently involved in.