Spin correlations in top quark pair production at the Large Hadron Collider
(2022) FYTK02 20221Theoretical Particle Physics - Undergoing reorganization
- Abstract
- In this work we investigate the spin correlation effect of the interference between signal and irreducible background events present in top quark pair production at the LHC at LO in the dilepton channel. To simulate the high energy proton-proton collisions of the LHC, MadGraph5_aMC@NLO framework is used as an event generator. The spin correlations are explicitly measured via the lepton kinematics and analyzed through the spin-density formalism. Moreover, we also study the leptonic angular distributions. We find that there is a measurable effect on the spin correlations of top quark pairs from the interference between signal and irreducible background events present in top quark pair production on the order of percent level, but that there... (More)
- In this work we investigate the spin correlation effect of the interference between signal and irreducible background events present in top quark pair production at the LHC at LO in the dilepton channel. To simulate the high energy proton-proton collisions of the LHC, MadGraph5_aMC@NLO framework is used as an event generator. The spin correlations are explicitly measured via the lepton kinematics and analyzed through the spin-density formalism. Moreover, we also study the leptonic angular distributions. We find that there is a measurable effect on the spin correlations of top quark pairs from the interference between signal and irreducible background events present in top quark pair production on the order of percent level, but that there is no measurable difference for the leptonic angular distributions. (Less)
- Popular Abstract
- Atoms often come to mind when thinking of the smallest things in nature. However, atoms are in turn made up of a nucleus of protons and neutrons with orbiting electrons. The protons and neutrons in turn are made up of even smaller fundamental particles called quarks. Together with the leptons, which the electron is a family member of, they make up the fermions, the matter particles. Along with bosons, the particles responsible for the fundamental forces, they make up the Standard Model (SM), which describes most of what we see in our universe, except dark matter and dark energy, which is still unknown.
Arguably, the most peculiar particle in the SM is the top quark. Even though it is unimag- inable small, it’s about 175 times heavier than... (More) - Atoms often come to mind when thinking of the smallest things in nature. However, atoms are in turn made up of a nucleus of protons and neutrons with orbiting electrons. The protons and neutrons in turn are made up of even smaller fundamental particles called quarks. Together with the leptons, which the electron is a family member of, they make up the fermions, the matter particles. Along with bosons, the particles responsible for the fundamental forces, they make up the Standard Model (SM), which describes most of what we see in our universe, except dark matter and dark energy, which is still unknown.
Arguably, the most peculiar particle in the SM is the top quark. Even though it is unimag- inable small, it’s about 175 times heavier than the much bigger proton. This immense mass makes it stand out from the rest of the SM. Because of this property, the top quark plays an important role in fine tuning of the SM and can thus be used to probe the underlying physics of the SM. In practice the top quark is hard to produce since by Einsteins famous energy-mass relation, the higher the mass, the more energy is required. Because of this, top quarks can only be produced in high energy collisions. In Earth’s upper atmosphere this occurs naturally when cosmic rays collide with particles in the atmosphere. Though, here on earth only the Large Hadron Collider (LHC) at CERN can achieve sufficient energy, just like the Tevatron could in the past.
One property of the top quark is spin, since it can be measured relatively easily via the momentum, angles and directions of the decay products. Spin is an inherent property of all fundamental particles and when a top quark pair is produced their spins are correlated. Even though the pair decays so rapidly that it cannot be measured directly, the spin information is conserved in the decay products. In this way, the top quarks can be measured indirectly by measuring their decay products.
Top quarks are an invaluable tool since they serve as a gateway to understanding current and new physics. Determining the magnitude of the spin effects of the background for top quark pairs is therefore important for the ability to make predictions for observables related to top quarks which in turn can lead to a better understanding of physics. The spin correlation effects of top quark pair production have been measured on numerous levels, however the effect of the interference between signal and background has not yet been considered. The prevailing view is that the effect is negligible and the goal of this work is to confirm or disprove this.
The aim of this project is to quantitatively measure the effect that the interference be- tween signal and background has on the top quark pair’s spin correlation. By using the programme MadGraph5_aMC@NLO, proton-proton collisions at the LHC are simulated for the signal and background present in top quark pair production. The spin correlations are then reconstructed from the decay products. In this project we found that there is indeed a measurable effect from the interference between signal and background, however more research is needed in order to determine the origin of these interference effects and if they can be reduced by analysis cuts that select top quark pair production. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9085741
- author
- Ahlstedt, Simon LU
- supervisor
- organization
- course
- FYTK02 20221
- year
- 2022
- type
- M2 - Bachelor Degree
- subject
- keywords
- Top quarks, Top quark pair production, LHC, Spin correlations, MadGraph5_aMC@NLO, Dilepton decay channel, Leading order, Interference effects, Event generation.
- report number
- LU-TP 22-39
- language
- English
- id
- 9085741
- date added to LUP
- 2022-06-23 10:49:15
- date last changed
- 2022-06-29 15:29:19
@misc{9085741,
abstract = {{In this work we investigate the spin correlation effect of the interference between signal and irreducible background events present in top quark pair production at the LHC at LO in the dilepton channel. To simulate the high energy proton-proton collisions of the LHC, MadGraph5_aMC@NLO framework is used as an event generator. The spin correlations are explicitly measured via the lepton kinematics and analyzed through the spin-density formalism. Moreover, we also study the leptonic angular distributions. We find that there is a measurable effect on the spin correlations of top quark pairs from the interference between signal and irreducible background events present in top quark pair production on the order of percent level, but that there is no measurable difference for the leptonic angular distributions.}},
author = {{Ahlstedt, Simon}},
language = {{eng}},
note = {{Student Paper}},
title = {{Spin correlations in top quark pair production at the Large Hadron Collider}},
year = {{2022}},
}