Naslov
Measurement of the cross section for associated production of a W boson and two b quarks with the CMS detector at the Large Hadron Collider

Autori
Luetić, Jelena

Vrsta, podvrsta i kategorija rada
Ocjenski radovi, doktorska disertacija

Fakultet
Prirodoslovno-matematički fakultet

Mjesto
Zagreb

Datum
15.07

Godina
2015

Stranica
160

Mentor
Brigljević, Vuko

Ključne riječi
CMS ; LHC ; Standard model ; Electroweak process ; W boson ; b quarks

Sažetak
The goal of this thesis is a measurement of the cross section of the pp -> W + bb + X process using the data collected during 2012 at sqrt(s) = 8 TeV. The data is provided by the Large Hadron Collider (LHC) accelerator at CERN, Geneva and collected by the Compact Muon Solenoid (CMS). The production of a W boson in association with a pair of b quarks (Wbb) in proton collisions has been the topic of many theoretical calculations and simulations. It is however still not well described due to divergences which arise in theoretical calculations, in cases where b quarks are collinear or there is a low energy massless particle irradiated. A precise measurement of Wbb production will allow to further constrain theoretical predictions in the framework of perturbative quantum chromodynamics (pQCD). On the other hand, Wbb is a background in the measurements of different standard model processes as well as in several searches for physics beyond standard model. The presence of a W boson is identified through the detection of an energetic, isolated lepton (muon or electron) and a significant amount of missing energy, which indicates the presence of a neutrino. Jets in the detector are identified as collimated sprays of particles. Selected jets are required to be tagged as jets originating from b quarks. The cross section measurement was performed in the fiducial region defined by a presence of a lepton and exactly two 2 b-tagged jets. The result is quoted separately in the muon and electron channel. The measured values in the two channels are compatible, as predicted by the standard model. The theoretical cross section was derived using MCFM. The measured values are around one standard deviation higher than the predicted theoretical values. The uncertainty on the measured values is dominated by the systematic effects. The largest uncertainties are associated with the b tagging procedure, jet energy scale and jet energy resolution. Reducing these uncertainties in the future, would allow for more sensitive test of perturbative QCD calculation at next-to-leading order.

Izvorni jezik
Engleski

Znanstvena područja
Fizika

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