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

doctoral thesis

• Title (croatian): Mjerenje udarnog presjeka zajedničke tvorbe W bozona i para b kvarkova detektorom CMS na Velikom hadronskom sudarivaču
• Author: Jelena Luetić
• Mentor: Vuko Brigljević (mentor)
• Committee member: Tatjana Šuša (predsjednik povjerenstva)
• Committee member: Vuko Brigljević (član povjerenstva)
• Committee member: Michele De Gruttola (član povjerenstva); strani drzavljanin: Nije dostupno
• Committee member: Krešimir Kumerički (član povjerenstva)
• Committee member: Mirko Planinić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2015-07-15, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

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 √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 aWboson 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.

Abstract (croatian)

Tema ove teze je mjerenje udarnog presjeka za proces pp -> W + bb + X koristeći podatke prikupljene CMS detektorom tijekom 2012 godine na energiji √s = 8 TeV. W bozon se raspada na elektron ili mion i odgovarajući neutrino. Prisutnost W bozona se čituje kroz postojanje izoliranog leptona visokog impulsa te nedostajuće energije koja dolazi od slabo interagirajućeg neutrina. Kvarkovi se u detektoru vide kao usmjereni mlazovi čestica te prolaze kroz posebne algoritme koji identificiraju mlazove nastale od b kvarkova. Složenost sudara protona proizlazi iz njihove unutarnje strukture. Iako je proton najvećim dijelom sastavljen od uud valentnih kvarkova, ostali kvakovi koje nazivamo kvarkovima mora, te gluoni također mogu sudjelovati u sudarima. Točna predviđanja konačnih stanja u sudarima protona uvelike ovise o kombinaciji teorijskih izračuna i eksperimentalnih rezultata. Opis takvih konačnih stanja se dijeli u nekoliko stupnjeva koji se odvijaju na različitim energijskim skalama. Na najvišim energijama se odvija tvrdi proces koji rezultira produkcijom teških ili visoko-energetskih čestica. Teške čestice se potom raspadaju na 106 Chapter 9. Strukturirani sažetak lakše. Ova dva procesa opisuju se matrični elementom. Pljusak partona je idući stupanj u opisu sudara protona i označava proces u kojem su izračene lagane čestice, fotoni i gluoni, koji su često kolinearni s originalnom česticom. Zadnji stupanj je hadronizacija tijekom koje partoni formiraju hadrone čijim se raspadima produciraju stabilne čestice opažene u detektoru. Prethodna mjerenja procesa koji u konačnom stanju sadrže W bozon i b kvarkove izvršena su na Tevatronu, eksperimentima CDF i D0 te na LHC-u, eksperimentima ATLAS i CMS. Prethodna mjerenja ne mogu se izravno usporedivati, budući da su proučavana različita konačna stanja u različitim faznim prostorima. Međutim, usporedba sa teorijskim predviđanjima ukazuje na slaganje.

• Keywords: LHC; CMS; Standard model; Wbb; cross section
• Keywords (croatian): LHC; CMS; Standardni model; Wbb
• Language: english
• URN:NBN: urn:nbn:hr:217:753986
• Study programme: Title: Physics; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika (doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika)
• Type of resource: Text
• Extent: 160 str.
• File origin: Born digital
• Access conditions: Open access
• Created on: 2017-08-21 12:53:01