ETD

Archivio digitale delle tesi discusse presso l'Università di Pisa

Tesi etd-04092021-212201


Tipo di tesi
Tesi di laurea magistrale
Autore
BRUSCHINI, DAVIDE
URN
etd-04092021-212201
Titolo
Production of photons from electroweak vector boson fusion in sqrt(s) = 13 TeV proton-proton collisions with the CMS experiment at CERN
Dipartimento
FISICA
Corso di studi
FISICA
Relatori
relatore Tonelli, Guido
relatore Azzurri, Paolo
Parole chiave
  • CERN
  • CMS
  • Vector Boson Fusion
  • Electroweak Physics
Data inizio appello
26/04/2021
Consultabilità
Non consultabile
Data di rilascio
26/04/2024
Riassunto
The object of this Master's thesis is the measurement of the Electroweak production of photons in association with two jets from proton collisions, with data collected by the CMS experiment at CERN during the ”Run II”. These events are important to test the SU(2)_LxU(1)_Y symmetry of the Electroweak interactions, in particular the self-couplings between the Gauge bosons resulting from the aforementioned symmetry. At the center-of-mass energy of 13 TeV reached at the LHC, the Electroweak production of the photon and the jets also involve Electroweak diagrams in which the photon is produced by the fusion of the massive Vector Bosons W radiated from each of the colliding partons, hence this process is more commonly referred to as Vector Boson Fusion, in particular Vector Boson Fusion to $\gamma$. The cross-section calculated including only the Vector Boson Fusion diagram diverges at high energies resulting in the violation of unitarity. The presence of other Electroweak same final state diagrams which interfere destructively prevents the cross-section from growing indefinitely. This Gauge cancellation mechanism must therefore be extremely precise for it to work, as even slight deviations from the Standard Model predictions would result in the divergence of the cross-section, thus hinting at the presence of new Physics which keeps the cross-section finite at high energies. Therefore this measurement, and more in general Vector Boson Fusion ones, allows to set stringent limits on Physics Beyond the Standard Model, often modelled as Anomalous Gauge Couplings.
To measure this process, events with a photon of p_T>220 GeV and two jets with m_{jj}>200 GeV are selected. For signal events, these jets are usually reconstructed using only the information on the electromagnetic and hadronic calorimeters, as they are often outside the acceptance of the tracking system. The photon is instead within the tracker acceptance.
This thesis starts with the description of the Standard Model with particular attention to SU(2)_LxU(1)_Y and its Gauge boson self-couplings (chapter 1). The Compact Muon Solenoid experiment at the Large Hadron Collider at CERN is presented in chapter 2.
The thesis work started from studies of the main features of the signal using Monte-Carlo simulations only including the Matrix Element calculations (chapter 3), and then moved to study the selection of the correct jets out of all those present in the events, using full Monte-Carlo simulations (chapter 4). Another important feature of Vector Boson Fusion events is the presence of a "rapidity gap" between the jets in which little additional hadronic activity is present. As already said, the jets are mostly outside of the tracking system acceptance, and the only central object is the photon, so few tracks are expected for these events. Because of this, the correct choice of the main Primary Vertex is studied, in particular the fraction of times in which the vertex is chosen correctly. Then characteristics of the vertices chosen correctly and incorrectly are studied, to better understand the causes for the wrong vertex choice. Then the impact of the vertex choice on the invariant mass is studied. This is described in chapter 5.
The analysis to measure Electroweak $\gamma+2$ jets is presented in chapter 6. The main background to this analysis is the mixed QCD/EW production of photon in association with jets. This is simulated using full Monte-Carlo simulation, whose predictions are validated by a Control Region with Z->l+ l- events and two jets with the same kinematic cuts. Another important source of background events is the pure QCD production of multijet events in which an object is mistakenly reconstructed as a single isolated photon, this objects are usually $\pi_0$s produced in jets. This source is instead validated using a data-driven approach using the $\sigma_{i\eta i\eta}$ and photon isolation variables. The analysis is performed first in the Control Region and a fit to measure the Electroweak Z-> l+l-+2 jets is performed on the output of a Boosted Decision Tree specifically built to discriminate Electroweak $\gamma+2$ jets events and its main background. This was done for all three years. Then this is repeated on the Signal Region for 2016 and the expected total uncertainty and significance were extracted. Both in Z+2 jets Control Region and the $\gamma+2$ jets Signal Region various sources of systematic uncertainties, Jet Energy Scale and Resolution and the variation of the QCD $\mu_R$ and $\mu_F$ scales.
Finally chapter 7 summarizes the results of the various chapters of this thesis and also discusses future directions for this work.
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