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## Measurement of the Higgs boson properties and of its coupling to the quark bottom and W/Z bosons in the lepton(s)+bb+X final state

by - 25 October 2018

Title : ATLAS/HIGGS: Measurement of the Higgs boson properties and of its coupling to the quark bottom and W/Z bosons in the lepton(s)+bb+X final state

Team : Masses et Interactions Fondamentales ; Groupe ATLAS-Tracking

Description :

The ATLAS experiment is installed at CERN’s Large Hadron Collider (LHC) in Geneva. Analysis of the data collected at 7 and 8 TeV (Run 1: 2011-2012) led to the discovery by ATLAS and CMS of a 125 GeV Higgs boson. After this first phase, the machine passed in 2015-2018 to a second phase, characterized by 13 TeV collisions and a much larger integrated luminosity (about 140 fb-1 expected). A third (Run 3) data-taking phase will start in 2021 to integrate >150 fb-1 of pp collisions at 14 TeV in 3 years.

For the observed value of its mass, decays in a pair of b quarks should dominate the width of the Higgs boson, with a branching ratio predicted by the theory of 58%. At the same time, the main production modes of the Higgs boson (fusion of 2 gluons or 2 W / Z bosons), which correspond to about 94% of the total cross section, are not exploitable to isolate H->bb decays, because of the high background due to the production of quarks and gluons in QCD processes. The production mode that offers the best sensitivity is therefore the associated production VH of a Higgs boson with a vector boson V = W, Z decaying to leptons.

A first evidence of VH, V->leptons, H->bb events at more than 3 standard deviations was obtained by the ATLAS experiment in the summer of 2017 with the data collected in 2015 and 2016. The H->bb decay and the VH production were observed for the first time with statistical significance greater than 5 standard deviations by the ATLAS experiment in the summer of 2018, with data collected between 2015 and 2017. Our team was deeply involved in these analyses and also in b-jet identification performance studies and missing transverse energy reconstruction that are very important ingredients for such measurements. The results, which are among the most important in Run 2 of the LHC, are in good agreement with the theory, with uncertainties around 25%.

The goal of this thesis is to use the VH + X process, V->leptons, H->bb, and all the statistics of Run 2, which is almost twice as large as that used for H->bb observation, to study the properties of the Higgs boson and to verify that they correspond to the Standard Model predictions. Using all the data from Run 2, which ends at the end of 2018 but whose analysis will require an effort spread over several years to reduce the systematic uncertainties of this measure, the student will study in detail the decay H-> bb and the production of the Higgs boson in association with W or Z. The high number of signal events will allow the student to study the kinematic properties of this mode of production and to compare them with the predictions of the Standard Model or its extensions, to measure several properties of the Higgs boson as its couplings to the b quark (H->bb decay) and to the W and Z bosons (WH and ZH production), and to constrain theories “beyond the Standard Model”, through measurements of the differential spectrum of the transverse momentum of the V boson but also the number of jets produced with VH (in the context of the “simplified template cross sections” framework. The concept of simplified template cross sections (cross-sections measured in simple phase space fiducial volumes) has been developed in recent years to allow for more granular measurements than inclusive cross-section measurements, less affected by theoretical uncertainties, and that can allow the combination of the results of the analysis of the different modes of production of Higgs boson. This work has already begun in our team with a thesis ending in 2018 and another one in progress until autumn 2019; the candidate will take over and perform the measurement with all the data of Run 2, optimizing the selection criteria and the classification of events for an integrated luminosity of 140 fb-1 and incorporating in the measurement all the improvements on the reconstruction and identification of leptons, jets and b-jets that have been developed by the ATLAS collaboration. He / she will be asked to contribute to these improvements with work on the b-jet identification performance, during his pre-thesis internship and his first year of thesis. The final results will be combined by the candidate with the VH cross section measurements made by ATLAS using different Higgs boson decays, and will be used to measure (or put limits on) the operator coefficient values of an effective Lagrangian including beyond the Standard Model interactions. The thesis work will also include a work on b-jet identification performance optimization for the H->bb Run 3 (2021-2023) analysis, and the preparation of the VH, H->bb analysis on the same data. The first data of Run 3 (2021) will be used for a validation of the new algorithms developed and for a first measurement of the VH cross sections at 14 TeV.

Finally, the thesis work will also include an instrumental part on the future charged-particle tracker (“ITk”) planned for the “high-luminosity” phase (2026-) of ATLAS. The ITk is the main project of the ATLAS upgrade program. Its performance will beat those of the current ATLAS tracker in a much harsher radiation environment and with larger acceptance. The LPNHE group is involved in ITk since its conception, with a long R&D program on planar pixel sensors. It is also committed, together with the LAL and IRFU/CEA ATLAS groups, to build and test silicon pixel modules for the outer barrel part of the ITk. The student will participate to the qualification, in the clean room of the LPNHE, of the sensors produced for the upgrade of the ATLAS pixel detector and to the assembly and tests of sensor+read-out chip modules.

Contact : Giovanni Marchiori, 33 (0)1 44 27 21 43

Location : LPNHE, Paris

Trips : déplacements réguliers au CERN pour réunions de groupe d’analyse, réunions de collaboration, shifts de prise de données et travail de qualification.