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Une description améliorée de la forme de distribution de masse du boson Z, et une première analyse simultanée des paramètres électrofaibles et des fonctions de distribution des partons avec l'expérience ATLAS au LHC

Author: Tafoya Vargas Juan Salvador
Publication venue: HAL CCSD
Publication date: 13/10/2023
Field of study

The work reported in this thesis is presented in two parts. The first is focused on the study of a newly proposed calibration technique, while the second describes the results of the first combined mW+PDF fits performed in ATLAS.The Liquid Argon electromagnetic calorimeter is calibrated by looking at the mass lineshape of the Z-boson in the di-electron channel. Using the event samples corresponding to 2018 measurements, a newly proposed Monte Carlo calibration technique was implemented with the objective of absorbing the remaining disagreement between the mass lineshapes of the measured data and its simulation. The method consists of an electron-by-electron energy resolution correction, parametrised as functions of electron pseudorapidity and transverse momentum. The final fit improved the data-to-MC agreement, improving the χ² from 2323 to 122 for 100 degrees of freedom. The same calibration technique was later repeated with special low pile-up runs at 13 TeV, studying its impact in the measurement of the mass of the W-boson.As part of the minimisation studies implemented for the calibration, a new statistical method was proposed in order to treat discontinuities in a multidimensional χ² curve produced by the histogrammed comparison between two samples. The method is based on the parametrisation of the relevant histogram via a continuous functional form, which is used to estimate a χ² against the other histogram. This method allows to implement the fit of the 40 parameters used to define the resolution correction.The combined mW and PDF study aims primarily to determine the correlation between both measurements. The study revolves around the low pile-up inclusive W-boson cross sections as a function of lepton transverse momentum pT(lepton), used to probe the dependency of pT(lepton) as a function of mW; for the purpose of this study, pseudodata at mW(nom)=80.4 GeV was used instead of real unfolded measurements. The constraining of the PDF part was implemented as an extension of the ATLASpdf21 fit, which only uses ATLAS data (on top of HERA DIS cross sections). Combined studies revealed an important influence of the PDF on the fitted value of mW, leading to shifts of mW(fit) (with respect to mW(nom)) that depend on the shape of the pseudodata. The final combined mW+PDF fits on pseudodata (which includes all the relevant sources of systematic) retrieved a shift of Δ mW = (-7 ±23) MeV with a global correlation of ρ ≈ 0.39.Le travail rapporté dans cette thèse est présenté en deux parties. La première est centrée sur l'étude d'une technique d'étalonnage nouvellement proposée, tandis que la seconde décrit les résultats des premiers ajustements combinés mW+PDF effectués dans ATLAS. Le calorimètre électromagnétique à argon liquide est étalonné en examinant la distribution en masse du boson Z dans le canal diélectrons. En utilisant les échantillons d'événements correspondant aux mesures de 2018, une nouvelle technique d'étalonnage Monte Carlo a été mise en œuvre avec l'objectif d'absorber le désaccord restant entre les formes de masse des données mesurées et leur simulation. La méthode consiste en une correction de la résolution en énergie électron par électron, paramétrée en fonction de la pseudorapidité de l'électron et du moment transverse. L'ajustement final a permis d'améliorer l'accord entre les données et le MC, en faisant passer le χ² de 2323 à 122 pour 100 degrés de liberté. La même technique d'étalonnage a été répétée par la suite avec des passes spéciales à faible empilement à 13 TeV, pour étudier son impact sur la mesure de la masse du boson W.Dans le cadre des études de minimisation mises en œuvre pour l'étalonnage, une nouvelle méthode statistique a été proposée afin de traiter les discontinuités dans une courbe χ² multidimensionnelle produite par la comparaison entre les histogrammes de deux échantillons. La méthode est basée sur la paramétrisation de l'histogramme pertinent via une forme fonctionnelle continue, qui est utilisée pour estimer un χ² par rapport à l'autre histogramme. Cette méthode permet de mettre en œuvre l'ajustement des 40 paramètres utilisés pour définir la correction de la résolution.L'étude combinée mW et PDF vise principalement à déterminer la corrélation entre les deux mesures. L'étude porte sur les sections efficaces des bosons W mesurées par les données à faible empilement en fonction du moment transverse du lepton pT(lepton), utilisé pour sonder la dépendance de pT(lepton) en fonction de mW ; pour les besoins de cette étude, des pseudo-données à mW(nom)=80.4 GeV ont été utilisées à à la place des mesures réelles. La contrainte de la partie PDF a été mise en œuvre comme une extension de l'ajustement ATLASpdf21, qui utilise uniquement les données ATLAS (en plus des sections efficaces DIS de HERA). Les études combinées ont révélé une influence importante des PDF sur la valeur ajustée de mW, conduisant à des décalages de mW(fit) (par rapport à mW(nom)) qui dépendent de la forme des pseudo-données. Les ajustements combinés finaux de mW+PDF sur les pseudo-données (qui incluent toutes les sources systématiques pertinentes) ont montré un décalage de Δ mW = (-7 ±23) MeV avec une corrélation globale de ρ ≈ 0.39

HAL-IN2P3

Testbeam for the CMS ECAL upgrade for the High Luminosity phase of LHC

Author: Tafoya Vargas Juan Salvador
Publication venue
Publication date: 26/09/2018
Field of study

With the incoming High Luminosity phase of LHC some upgrades for the CMS ECAL are in line. As a part of the characterization process, data was taken using either a laser as light source or electrons at the testbeam H4. Dependance of gain in terms of voltage and temperature were studied, which allowed later to compare the measurements obtained with the VFEs working at 120 MHz and 160 MHz. Additionally, the electron beam used for the test was studied after the characterization was done, allowing for a ﬁrst reconstruction of energy

CERN Document Server

An improved description of the $Z$ -boson mass lineshape, and a first joint analysis of electroweak parameters and parton distribution functions with the ATLAS experiment at the LHC

Author: Tafoya Vargas Juan Salvador
Publication venue
Publication date: 08/03/2024
Field of study

The work reported in this thesis is presented in two parts. The first is focused on the study of a newly proposed calibration technique, while the second describes the results of the first combined

m_W

+PDF fits performed in ATLAS. The Liquid Argon electromagnetic calorimeter is calibrated by looking at the mass lineshape of the

Z

-boson in the di-electron channel. Using the event samples corresponding to 2018 measurements, a newly proposed Monte Carlo calibration technique was implemented with the objective of absorbing the remaining disagreement between the mass lineshapes of the measured data and its simulation. The method consists of an electron-by-electron energy resolution correction, parametrised as functions of electron pseudorapidity and transverse momentum. The final fit improved the data-to-MC agreement, improving the

\chi^2

from 2323 to 122 for 100 degrees of freedom. The same calibration technique was later repeated with special low pile-up runs at

13~\mathrm{TeV}

, studying its impact in the measurement of the mass of the

W

-boson. As part of the minimisation studies implemented for the calibration, a new statistical method was proposed in order to treat discontinuities in a multidimensional

\chi^2

curve produced by the histogrammed comparison between two samples. The method is based on the parametrisation of the relevant histogram via a continuous functional form, which is used to estimate a

\chi^2

against the other histogram. This method allows to implement the fit of the 40 parameters used to define the resolution correction. The combined

m_W

and PDF study aims primarily to determine the correlation between both measurements. The study revolves around the low pile-up inclusive

W

-boson cross sections as a function of lepton transverse momentum (

p_\mathrm{T}^\ell

), used to probe the dependency of

p_\mathrm{T}^\ell

as a function of

m_W

; for the purpose of this study, pseudodata at

m_W^\mathrm{nom}=80.4~\mathrm{GeV}

was used instead of real unfolded measurements. The constraining of the PDF part was implemented as an extension of the ATLASpdf21 fit, which only uses ATLAS data (on top of HERA DIS cross sections). Combined studies revealed an important influence of the PDF on the fitted value of

m_W

, leading to shifts of

m_W^\mathrm{fit}

(with respect to

m_W^\mathrm{nom}

) that depend on the shape of the pseudodata. The final combined

m_W

+PDF fits on pseudodata (which includes all the relevant sources of systematic) retrieved a shift of

\Delta m_W = (-7 \pm 23)~\mathrm{MeV}

with a global correlation of

\rho_{m_W} \approx 0.39

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