Search for new scalar resonances using proton-proton collisions recorded by the CMS experiment at the Large Hadron Collider

doctoral thesis

• Title (croatian): Potraga za novim skalarnim rezonancijama u sudarima protona zabilježenima detektorom CMS na velikom hadronskom sudarivaču
• Author: Matej Roguljić
• Mentor: Devdatta Majumder (mentor)
• Committee member: Nikola Poljak (predsjednik povjerenstva)
• Committee member: Devdatta Majumder (član povjerenstva)
• Committee member: Sanjin Benić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2022-08-26, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

In this thesis, the search for the resonant production of a new massive scalar particle X decaying into a new light scalar particle Y and a standard model Higgs boson H through the process X → YH → bƃbƃ is presented. Data from CERN LHC proton-proton collisions at a centre-of-mass energy of 13 TeV are used, collected by the CMS detector in 2016–2018 and amounting to 138 fb^−1 of integrated luminosity. The search is performed in mass ranges of X (0.9–4 TeV) and Y (60–600 GeV) where both the Y and the H are highly Lorentz-boosted. In this kinematic regime, their b quark-antiquark daughter particles are collimated enough to allow the reconstruction of H and Y using single large area jets each. The mass of one of the large-area jets is required to be compatible with that of the Higgs boson, which is 125 GeV. A scan is performed in a two-dimensional plane spanned by the mass of the other jet, associated to Y, and the invariant mass of both large-area jets used to reconstruct X. The results are interpreted in the context of scalar resonances predicted in the next-to-minimal supersymmetric standard model and also in an extension of the standard model with two additional singlet scalar fields. Upper limits are placed on the production cross section of the process as a function of the masses of X and Y in the 0.1–150 fb range. This is the first search for this process using Lorentz-boosted event topologies and significantly extends the constraints on the studied model. A novel calibration method of the ParticleNet algorithm designed to recognize the decays of a massive particle into a pair of b quarks is also presented. The calibration consists of selecting events with a large-area jet with high momentum and measuring the strength of the Z boson peak on a smooth hadronic background. Previously used method performed the calibration on a set of jets originating from gluon fragmentation into bƃ, with special selection applied to make them more akin to the jets originating from the decays of a massive particle, such as Z or H. The presented measurement demonstrates the possibility of a direct calibration of the tagger and provides a validity test for previously used, indirect measurements.

Abstract (croatian)

U ovom je radu opisana potraga za kaskadnim raspadom teške skalarne čestice X u drugu skalarnu česticu Y i Higgsov bozon H gdje se obje dalje raspadaju u par b kvarkova. Raspadi čestice X u dvije skalarne čestice različitih masa predviđeni su u mnoštvu BSM teorija, ali su uglavnom neistraženi na LHC-u. Potraga je napravljena u rasponu masa 0.9–4 TeV za X i 60–600 GeV za Y, fokusirajući se na kinematički režim gdje Y i H imaju visoki pT te se produkti njihovih raspada mogu rekonstruirati kao jedinstveni široki mlazovi. Potraga koristi podatke proton-proton sudara na energiji od 13 TeV u sustavu centra mase, prikupljene CMS eksperimentom na LHC-u. Podaci su prikupljeni u 2016– 2018 i ukupna količina podataka iznosi 138 fb^−1. Od 260 različitih testiranih (M_X,M_Y) hipoteza, ona s najvećom značajnošću opažanja od 3.1σ je M_X = 1.6TeV i M_Y = 90GeV. Globalna značajnost takvog opažanja je 0.7σ. Također su postavljene i gornje granice na udarni presjek traženog procesa sa sigurnošću intervala pouzdanosti od 95% u rasponu 0.1–150 fb, ovisno oM_X iM_Y. Postavljene granice su uspoređene s najvećim dopuštenim udarnim presjecima u NMSSM i TRSM teorijama što dovodi do isključenja dijela faznog prostora teorija za određene M_X i M_Y. Za NMSSM, područje isključenja obuhvaća raspone M_X 1000–1150 GeV i 101–145 GeV za M_Y. Za TRSM, područje isključenja je unutar 950 < M_X < 1330GeV i 110 < M_Y < 132GeV. Gornje granice su također uspoređene s drugim relevantnim mjerenjima i pokazano je da predstavljeni rezultati značajno poboljšavaju prethodne rezultate. Također je u radu predstavljena i kalibracija ParticleNet algoritma za dvije radne točke. Kalibracija se sastoji od mjerenja Z rezonancije na pozadini QCD događaja. Ključna komponenta mjerenja su korekcije idućeg reda računa smetnje primijenjene na simulaciju Z i W događaja koja koristi najniži red računa smetnje. Rezultati dobiveni ovom, novom, metodom uspoređeni su s postojećim rezultatima koji koriste same QCD događaje za kalibraciju. Rezultati potvrđuju slaganje između dviju metoda.

• Keywords: LHC; CMS; standard model; Higgs boson; NMSSM; TRSM; boosted objects; heavy-flavor tagging; ParticleNet
• Keywords (english): Rad ne sadrži ključne riječi na drugom jeziku.
• Language: english
• URN:NBN: urn:nbn:hr:217:896236
• Promotion: 2022
• 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: 155 str.
• File origin: Born digital
• Access conditions: Open access
• Created on: 2022-10-25 10:17:54